Patent Publication Number: US-2013253518-A1

Title: System and method for creating a bore and implanting a bone screw in a vertebra

Description:
CLAIM TO PRIORITY 
     This application claims the benefit of priority to U.S. Provisional Patent Application No. 61/725,771, filed Nov. 13, 2012, entitled “SYSTEM AND METHOD FOR IMPLANTING A BONE SCREW IN A VERTEBRA”; and 
     This application claims the benefit of priority to and is a continuation-in-part of: 
     U.S. patent application Ser. No. 13/434,652, filed Mar. 29, 2012, entitled “SYSTEM AND METHOD FOR SECURING AN IMPLANT TO A BONE CONTAINING BONE CEMENT”; and 
     U.S. patent application Ser. No. 13/434,674, filed Mar. 29, 2012, entitled “SYSTEM AND METHOD FOR SECURING AN IMPLANT TO A BONE CONTAINING BONE CEMENT”; and which claims the benefit of priority to: 
     U.S. Provisional Application No. 61/615,639, filed Mar. 26, 2012, entitled “SYSTEM AND METHOD FOR SECURING AN IMPLANT TO A BONE CONTAINING BONE CEMENT” which all of the above applications are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND OF INVENTION 
     Back pain is a significant clinical problem and the costs to treat it, both surgical and medical, are estimated to be over $2 billion per year. One method for treating a broad range of degenerative spinal disorders is spinal fusion. Implantable medical devices designed to fuse vertebrae of the spine have developed rapidly over the last decade. However, spinal fusion has several disadvantages including reduced range of motion and accelerated degenerative changes adjacent the fused vertebrae. Alternative devices and treatments have been developed for treating degenerative spinal disorders while preserving motion. These devices and treatments offer the possibility of treating degenerative spinal disorders without the disadvantages of spinal fusion. 
     Devices for treating the spine, including those used in spinal fusion and spinal stabilization with motion preservation, are typically secured to the spine using screws which penetrate the bone. Such screws are designed to engage the structure of the bone. However, such screws are poorly adapted for use in bones which have been previously treated with bone cement. Consequently, there is a need for new and improved devices and methods for securing spinal implants to vertebrae that have previously been treated with bone cement. 
     SUMMARY OF INVENTION 
     Systems and methods of the embodiments of the present invention include a bone cutting tool that can be used to create a bore in a vertebral body in order to implant a bone screw with the aid of bone cement. Embodiments of the bone cutting tool of the invention include at least an outer bone cutting blade and an inner rod, preferably, an outer tube with first and second bone cutting blades and an inner rod. Movement of the inner rod causes the first and second bone cutting blades to expand. Rotating the tool causes bone to be cut and a bore in which the tool is placed to expand. Continued expansion of the bone cutting blades and rotation of the tool cause the bore to expand. The expanded bore can be cylindrical due to a cylindrical shape of the bone cutting blades. Once the bore has a desired size, the bone cutting blades can be retracted and the tool removed from the bore. 
     Embodiments of the invention use the bone cutting tool to create a bore of the desired size. After the bone cutting tool is removed, a bone screw is inserted and bone cement is used to affix the bone screw into the vertebra. The bone cement may be applied between the bone screw and the bore. Alternatively and/or additionally, the bone cement may be applied through a bore and channels in the bone screw and exit through a port in the bone screw to fill the space between the bone screw and the bore in the vertebra. 
     The present invention includes a bone anchor system and methods that can secure a spinal implant to a vertebra that has previously been treated with bone cement. Embodiments of the invention include polyaxial bone anchors; dynamic bone anchors; bone screws adapted to engage bone and hardened bone cement in a bone, and methods of implantation. 
     An aspect of embodiments of the invention is the ability of the bone anchor system to engage both bone and hardened bone cement with a single anchor. Another aspect of embodiments of the invention is the ability to provide a kit of versatile components suitable for particular bones of the patient and which may be customized to the anatomy and needs of a particular patient and procedure. Another aspect of the invention is to facilitate the process of implantation of the bone anchor and minimize disruption of the bone and hardened bone cement during implantation. 
     Thus, the present invention provides new and improved systems, devices and methods for treating degenerative spinal disorders by providing and implanting a bone anchor system adapted to engage bone and hardened bone cement in a bone. These and other objects, features and advantages of the invention will be apparent from the drawings and detailed description which follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are front and back perspective views of a bone anchor according to an embodiment of the present invention. 
         FIG. 1C  is a sectional view of the bone anchor of  FIGS. 1A and 1B . 
         FIGS. 1D ,  1 E, and  1 F are enlargements of portions of  FIG. 1C . 
         FIGS. 2A-2D  illustrate steps in the implantation of the bone anchor of  FIGS. 1A and 1B  into a vertebra according to an embodiment of the invention. 
         FIGS. 2E-2I  illustrate steps in the implantation of a bone anchor into a vertebra according to alternative embodiments of the invention. 
         FIGS. 3A-3H  show illustrative views of alternative bone anchors according to embodiments of the present invention. 
         FIGS. 4A-4F  illustrative views of alternative bone anchor cross-sections according to embodiments of the present invention. 
         FIGS. 5A-5D  show illustrative views of alternative tips of bone anchors according to embodiments of the present invention 
         FIGS. 6A-6F  show illustrative views of bone anchor heads which can be combined with the shaft of the bone anchors shown in  FIGS. 1A-5D . 
         FIGS. 7A-7C  show views of a dynamic bone anchor head in combination with the shaft of the bone anchor shown in  FIGS. 1A-1F . 
         FIGS. 8A-8D  show illustrative views of alternative bone anchors which can be combined with the shaft of the bone anchors shown in  FIGS. 1A-5D . 
         FIGS. 9A-9F  show illustrative views of alternative bone anchors having heated tips which can be combined with the shaft of the bone anchors shown in  FIGS. 1A-5D . 
         FIGS. 10A-10B  show perspective views of a bone cutting tool in a non-expanded mode and an expanded mode of an embodiment of the invention. 
         FIG. 10C  shows a perspective view of the first and second cutting blade of the embodiment of the invention. 
         FIGS. 11A-11B  show side views of a bone cutting tool in a non-expanded mode and an expanded mode of an embodiment of the invention. 
         FIG. 12A  shows a cross-sectioned view of the bone cutting tool of an embodiment of the invention as depicted in  FIG. 10A . 
         FIG. 12B  shows a perspective view of the proximal end of the bone cutting tool of an embodiment of the invention. 
         FIG. 12C  shows a close-up side view of the first and second cutting blade of an embodiment of the invention in an unexpanded configuration. 
         FIG. 12D  shows a close-up of an alternative embodiment of the invention of the first and second cutting blade having a different unexpanded configuration. 
         FIG. 13  shows a side view of the cutting blades of the bone cutting tool of an embodiment of the invention expanded into a cylindrical shape. 
         FIG. 14  shows a cross sectional view of an embodiment of the handle of the bone cutting tool of an embodiment of the invention tool is substantially perpendicular to a longitudinal axis. 
         FIGS. 15A-15B  show flow charts of an embodiment of the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Devices for treating the spine, including those used in spinal fusion and spinal stabilization with motion preservation, are typically secured to the spine using screws which penetrate the bone. Such screws are designed to engage the structure of the bone. However, such bones may have been treated with bone cement in a prior procedure. For example, in a kyphoplasty or vertebroplasty procedure, bone cement is injected percutaneously into a fractured or degenerated vertebra with the goal of ameliorating vertebral compression fractures. The bone cement is injected into the bone where it fills natural or surgically created voids in the cancellous bone material within the bone. 
     A commonly used bone cement is polymethyl methacrylate or PMMA. Bone cements may include a powder (i.e., pre-polymerized PMMA and or PMMA or MMA co-polymer beads and/or amorphous powder, radio-opacifier, initiator) and a liquid (MMA monomer, stabilizer, inhibitor). Bone cements are typically provided as two-components which are mixed shortly before use. When the two components are mixed polymerization of the monomer begins. As polymerization continues the bone cement viscosity changes from a runny liquid into a dough-like state and then finally hardens into solid hardened material. The setting time can be tailored to provide suitable viscosity for implantation and help the physician safely apply the bone cement into the bone. A wide variety of bone cement formulations are known in the art. 
     Bone cement is implanted into bones in a variety of procedures using a variety of methods. For example, in kyphoplasty and vertebroplasty the bone cement is injected into the vertebra through a needle/cannula while liquid. In some procedures, the liquid bone cement is restrained to a particular portion of the bone using a barrier or barrier technique. In other procedures the liquid bone cement migrates through and fills natural voids in the cancellous bone. The net result is a bone that comprises portions of natural cancellous bone, and portions of cancellous bone embedded with bone cement. 
     Bone cement is a reliable anchorage and reinforcement material. It is easy to use in clinical practice and has a proven long survival rate with cemented-in prostheses. Moreover, the development of minimally invasive bone reinforcement procedures such as kyphoplasty and vertebroplasty has resulted in an increase of its use to reinforce the spine both as an adjunct to spinal stabilization procedures and as a therapy on its own. However, although bone cement is a hard stable material, it has properties different than the bone in which it resides. In particular, bone cement can be prone to fracture if disturbed after hardening/curing. 
     A situation that is arising with increasing frequency is the need to perform a spinal stabilization procedure (e.g. a spinal fusion or dynamic stabilization) on a spine in which the one or more vertebrae have been treated with bone cement. In such spinal stabilization procedures a spinal implant is anchored to two or more adjacent vertebrae. The spinal implant is designed to hold the adjacent vertebrae in fixed positions relative to one another to allow fusion or to stabilize and constrain the relative movement of the vertebrae and share the load between the vertebrae in dynamic stabilization. The implant is typically anchored to the vertebrae utilizing bone anchors, for example, bone screws which penetrate the bone. The bone screws are designed to engage and be secured to the natural bone structure including cortical and cancellous bone. However, bone screws are poorly adapted for use in bones which have been previously treated with bone cement. In particular, the use of bone screws in hardened bone cement can fracture the bone cement preventing the bone anchor from adequately securing the implant and degrading the reinforcing properties of the bone cement. Moreover, removing the hardened bone cement prior to the installing the anchor (and replacing with uncured bone cement) is time consuming and damaging to the integrity of the bone. Consequently, there is a need for new and improved devices and methods for securing spinal implants to vertebrae that have previously been treated with bone cement. 
     In embodiments of the present invention, a bone anchor, in the form of a bone screw, is provided which has different thread characteristics on the distal shaft adjacent the tip as compared to the proximal shaft adjacent the head. The thread on the proximal shaft is designed to engage and secure the anchor to natural cancellous and cortical bone. The thread on the distal shaft is designed to engage and secure the anchor to bone cement embedded within the bone. 
     In particular embodiments the bone anchor has more threads on the distal shaft than on the proximal shaft. The threads on the distal shaft merge into the thread(s) of the proximal shaft at the transition between the proximal and distal shafts. The increased number of threads on the distal shaft allows the depth of the thread to be reduced to a suitable depth for engaging bone cement without fracture while maintaining sufficient surface area for the distal threads to engage and secure the anchor to the bone cement. 
     The pitch of the threads on the distal shaft (distance between adjacent threads) and pitch of the thread(s) on the proximal shaft are selected to be consistent with the lead of the screw (the distance the screw advances along its axis during one complete turn). Thus, in one embodiment, the bone anchor has two distal threads on the distal shaft and one proximal thread on the proximal shaft. The thread pitch on the proximal shaft is equal to the lead. The thread pitch on the distal shaft is half of the thread pitch on the proximal shaft and, thus, equal to half of the lead. The reduced thread depth and thread pitch on the distal shaft results in thread characteristics similar to that of a machine screw on the distal shaft while maintaining thread characteristics on the proximal shaft more typical of a bone screw. 
     During implantation, a pilot bore is made into the vertebra passing through the natural cancellous and cortical bone and into the bone cement at the position at which the bone anchor is to be implanted. The pilot bore is made, for example, by a bone drill. The size of the pilot bore includes a distal bore sized to receive the distal shaft and a proximal bore sized to receive the proximal shaft (equal or typically larger in diameter than the distal shaft). The bone anchor is then inserted into the pilot bore such that the multiple distal threads engage the distal bore drawing the bone anchor into the pilot bore. Turning the bone anchor through one complete turn advances the bone anchor into the bore by a distance equivalent to the lead. As the bone anchor advances, the bone cement of the distal bore is engaged by the two threads on the distal shaft which have characteristics suitable for securing the distal shaft to the bone cement without fracturing it. The natural cancellous and cortical bone of the proximal bore is engaged by the single thread of the proximal shaft which has characteristics suitable for securing the proximal shaft to the bone. 
     Thus, embodiments of the invention provide a bone anchor shaft design suitable for anchoring an implant into a bone including bone cement. The shaft design can be applied to any type of bone anchor useful for bone surgery where it is to be used in a bone comprising bone cement including, but not limited to, lag screw, bone screws, pedicle screws, adjustable pedicle screws, polyaxial pedicle screws, dynamic bone screws, and Steffee screws. 
     These and other objects, features and advantages of the invention will be further apparent from the drawings and description of particular embodiments below. Common reference numerals are used to indicate like elements throughout the drawings and detailed description; therefore, reference numerals used in a drawing may or may not be referenced in the detailed description specific to such drawing if the associated element is described elsewhere. The first digit in a three digit reference numeral indicates the series of figures in which the referenced item first appears. Likewise, the first two digits in a four digit reference numeral. 
     The terms “vertical” and “horizontal” are used throughout the detailed description to describe general orientation of structures relative to the spine of a human patient that is standing. This application also uses the terms proximal and distal in the conventional manner when describing the components of the spinal implant system. Thus, proximal refers to the end or side of a device or component closest to the hand operating the device, whereas distal refers to the end or side of a device furthest from the hand operating the device. For example, the tip of a bone screw that enters a bone would conventionally be called the distal end (it is furthest from the surgeon) while the head of the screw would be termed the proximal end (it is closest to the surgeon). 
     Bone Anchor 
       FIGS. 1A-1F  illustrate a bone anchor  100  in the form of a bone screw adapted to engage bone and bone cement present in the bone.  FIGS. 1A and 1B  are front and back perspective views of a bone anchor  100  according to an embodiment of the present invention.  FIG. 1C  is a sectional view of the bone anchor  100  of  FIGS. 1A and 1B .  FIGS. 1D ,  1 E, and  1 F are enlargements of portions of  FIG. 1C  illustrating the thread profile. 
     Referring first to  FIGS. 1A and 1B  which show front and back perspective views of a bone anchor  100  according to an embodiment of the present invention. Bone anchor  100  includes a head  102 , at the proximal end and a tip  104  at the distal end. A shaft  106  extends between head  102  and tip  104  and includes a proximal shaft  120  and a distal shaft  140 . Proximal shaft  120  bears on its outside surface a single proximal thread  122 . Distal shaft  140  bears on its outside surface first and second distal threads  142   a  and  142   b . First and second distal threads  142   a  and  142   b  begin on opposite sides of distal shaft  140  adjacent tip  104 . First distal thread  142   a  begins at first start  144   a  shown in  FIG. 1A . Second distal thread  142   b  begins at second start  144   b  shown in  FIG. 1B . First and second distal threads  142   a  and  142   b  merge together and connect to single proximal thread  122  at transition  146  between distal shaft  140  and proximal shaft  120 . The proximal thread  122  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  112  on the proximal shaft  120  is equal to the lead  110 . The distal threads  142   a  and  142   b  have a thread depth and threadform suitable for engaging bone cement, and the distal thread pitch  114  on the distal shaft  140  is half of the proximal thread pitch  112  on the proximal shaft, and, thus, equal to half of the lead  110 . The reduced thread depth and thread pitch on the distal shaft  140  results in thread characteristics similar to that of a machine screw while maintaining thread characteristics on the proximal shaft  120  more typical of a bone screw. 
     Head  102  is illustrated as a simple countersunk head having an internal hex socket  108 . Hex socket  108  is adapted to be engaged by a driver to turn bone anchor  100  during implantation. In alternative embodiments head  102  is replaced by any other bone anchor head including, but not limited to, Steffee heads, hex heads, hex socket heads, Torx heads, breakaway heads, fixed heads, polyaxial heads, pedicle screw heads, angled heads, dynamic bone anchor heads or other heads desired to be securely mounted to a bone containing hardened bone cement. 
     Note that in alternative embodiments, the number and pitch of the proximal and distal threads may be varied. For example, a bone anchor shaft can comprise two proximal threads having pitch P and two pairs of distal threads having pitch P/2 where each pair of distal threads merges into one of the proximal threads at the transition between the distal shaft and the proximal shaft. Alternatively, a bone anchor shaft can comprise one proximal threads having pitch P and three distal threads having pitch P/3 where the three distal threads merge into the proximal thread at the transition between the distal shaft and the proximal shaft. In general, the distal shaft is provided with a greater number of threads having a smaller pitch (and typically a smaller thread depth) than the proximal shaft where the pitch of the proximal threads and distal threads is calculated to be consistent with the lead of the bone anchor (the distance the bone anchor advances per rotation). 
     Referring now to  FIGS. 1C ,  1 D,  1 E and  1 F which show sectional views of bone anchor  100 .  FIG. 1C  shows a longitudinal section of the entire bone anchor  100 .  FIG. 1D  shows an enlarged view of portion  1 D of  FIG. 1C  and illustrates the threadform of proximal thread  122 .  FIG. 1E  shows an enlarged view of portion  1 E of  FIG. 1C  and illustrates the threadform of first distal thread  142   a .  FIG. 1F  shows an enlarged view of portion  1 F of  FIG. 1C  and illustrates the threadform of second distal thread  142   b . As illustrated in  FIG. 1C , the proximal thread pitch  112  on the proximal shaft  120 —the distance between adjacent crests of proximal thread  122 —is equal to the lead  110 . The distal thread pitch  114  on the distal shaft  140 —the distance between the crest of first distal thread  142   a  and an adjacent crest of second distal thread  142   b —is equal to half of lead  110 . 
     As shown in  FIG. 1D , the proximal thread  122  has a thread depth and threadform suitable for engaging bone. Proximal thread  122  has a buttress threadform and has a proximal thread depth (distance from crest to root)  123  suitable for engaging bone. 
     As shown in  FIG. 1E , the first distal thread  142   a  has a thread depth and threadform suitable for engaging bone cement. First distal thread  142   a  has a triangular or V-shaped threadform. First distal thread  142   a  has a first distal thread depth (distance from crest to root)  143   a  suitable for engaging bone cement. In embodiments, first distal thread depth  143   a  is less than proximal thread depth  123 . First distal thread depth  143   a  can be, for example, 75%, 60%, 50% 40% or less of proximal thread depth  123 . 
     As shown in  FIG. 1F , the second distal thread  142   b  has a thread depth and threadform suitable for engaging bone cement. Second distal thread  142   b  has a triangular or V-shaped threadform. Second distal thread  142   b  has a second distal thread depth (distance from crest to root)  143   b  suitable for engaging bone cement. In embodiments, second distal thread depth  143   b  is less than proximal thread depth  123 . Second distal thread depth  143   b  can be, for example, 75%, 60%, 50% 40% or less of proximal thread depth  123 . In the embodiment illustrated in  FIGS. 1A-1F , second distal thread depth  143   b  is approximately 70% of proximal thread depth  123  whereas the first distal thread depth  143   a  is approximately 40% of proximal thread depth  123 . However, in alternative embodiments, second distal thread depth  143   b  is greater than, less than or the same as first distal thread depth  143   a.    
     The distal threads  142   a  and  142   b  have a thread depth and threadform suitable for engaging bone cement, and the distal thread pitch  114  on the distal shaft  140  is half of the proximal thread pitch  112  on the proximal shaft, and, thus, equal to half of the lead  110 . The reduced thread depth and thread pitch on the distal shaft  140  results in thread characteristics similar to that of a machine screw while maintaining thread characteristics on the proximal shaft  120  more typical of a bone screw. 
     Referring again to  FIG. 1C , the proximal thread  122  has a proximal major diameter  125  equal to maximum diameter of the proximal thread  122  (crest to crest measured perpendicular to the longitudinal axis of the bone anchor) and a proximal minor diameter  127  (root to root measured perpendicular to the longitudinal axis of the bone anchor). The proximal minor diameter  127  can be conceived as the diameter of the proximal shaft  120 . The proximal major diameter is generally equal to the proximal minor diameter plus twice the proximal thread depth  123 . 
     Referring again to  FIG. 1C , the first distal thread  142   a  has a first distal major diameter  145   a  equal to the maximum diameter of the first distal thread  142   a  (crest to crest measured perpendicular to the longitudinal axis of the bone anchor) and a distal minor diameter  147  (root to root measured perpendicular to the longitudinal axis of the bone anchor). The distal minor diameter  147  can be conceived as the diameter of the distal shaft  140 . The first distal major diameter  145   a  is generally equal to the distal minor diameter  147  plus twice the first distal thread depth  143   a.    
     Referring again to  FIG. 1C , the second distal thread  142   b  has a second distal major diameter  145   b  equal to the maximum diameter of the second distal thread  142   a  (crest to crest measured perpendicular to the longitudinal axis of the bone anchor) and a distal minor diameter  147  (root to root measured perpendicular to the longitudinal axis of the bone anchor). The second distal major diameter  145   b  is generally equal to the distal minor diameter  147  plus twice the second distal thread depth  143   b . Because the root of the first distal thread  142   a  connects with the root of the second distal thread  142   b , the first distal thread  142   a  and second distal thread  142   b  have the same distal minor diameter  147  which can be conceived as the diameter of the distal shaft  140 . In this embodiment having different first and second distal major diameters reduces and/or redirects stress placed on the bone cement during implantation thereby reducing the risk of fracturing the bone cement. High and low distal threads, as shown, can serve to redirect stress along the axis of the bone anchor rather than outwardly from the bore into the bone cement thereby minimizing cracking or splitting of the bone cement. 
     It should be noted that, in the embodiment shown in  FIGS. 1A-1F , the distal minor diameter  147  is substantially constant along the length of distal shaft  140 . Likewise, the proximal minor diameter  127  is substantially constant along the length of proximal shaft  120 . In alternative embodiments, one or both of the proximal shaft  120  and distal shaft  140  are conical such that the proximal minor diameter  127  and/or distal minor diameter  147  increases going from the tip  104  towards the head  102 . In the embodiment shown in  FIGS. 1A-1F , the proximal minor diameter  127  is greater than the distal minor diameter  147 , but less than the second distal major diameter  145   b . In alternative embodiments, the major diameter of the distal threads may be selected to be less than the minor diameter of the proximal threads such that the distal threads do not engage the proximal bore of a pilot bore during implantation. 
     The lengths and diameters of bone anchors are selected as appropriate for the anatomy of the bones into which they are implanted. In the particular case of pedicle screws, the screws are typically manufactured with a variety of shaft lengths in the range from 30 mm to 60 mm long and shaft diameters in the range from 5 mm to 8.5 mm suitable for the size of the vertebra and pedicle into which they are implanted. The thread depth, threadform, lead and pitch is selected such that the threads defined thereby are suitable for engaging bone and/or bone cement as required. For example, in a range of pedicle screw embodiments of the bone anchor  100 , the proximal shaft has a length between about 10 and about 50 mm and a proximal minor diameter (proximal shaft diameter) between about 5 and about 8.5 mm, the proximal thread has a proximal thread depth between about 1 mm and about 2.5 mm, the distal shaft has a length between about 10 and about 50 mm and a distal minor diameter (distal shaft diameter) between about 5 mm and about 8.5 mm, the first distal thread has a first distal thread depth between about 0.4 mm and about 1.5 mm, the second distal thread has a second distal thread depth between about 0.4 mm and about 1.5 mm, the lead is between about 2 mm and about 5 mm, the proximal pitch is the same as the lead and the distal pitch is half of the lead. In a particular pedicle screw embodiment of the bone anchor  100 , the proximal shaft has a length of 20 mm and a proximal minor diameter (proximal shaft diameter) of 5.2 mm, the proximal thread has a proximal major diameter of 8 mm (proximal thread depth is 1.4 mm), the distal shaft has a length of 20 mm and a distal minor diameter (distal shaft diameter) of 4.4 mm, the first distal thread has a first distal major diameter of 5.6 mm (first distal thread depth is 0.6 mm), the second distal thread has a second distal major diameter of 6.4 mm (second distal thread depth is 1.0 mm), the lead is 3.2 mm, the proximal pitch is 3.2 mm and the distal pitch is 1.6 mm. 
     Method for Implanting Bone Anchor 
     The implantation of a bone anchor/bone screw into a vertebra is preferably performed in a minimally invasive manner and, thus, tools are provided to facilitate installation and assembly through cannulae. These tools can also be used in open procedures. One suitable minimally invasive approach to the lumbar spine is the paraspinal intermuscular approach. This approach is described, for example, in “The Paraspinal Sacraspinalis-Splitting Approach to the Lumber Spine,” by Leon L. Wiltse et al.,  The Journal of Bone  &amp;  Joint Surgery , Vol. 50-A, No. 5, July 1968, which is incorporated herein by reference. In general, the patient is positioned prone. Incisions are made posterior to the vertebrae to be stabilized. The dorsal fascia is opened and the paraspinal muscle is split to expose the facet joints and lateral processes of the vertebra. Either a cannula is inserted to provide for port access (minimally invasive) or a larger incision is made with tissue refraction to expose the vertebra (open procedure). 
     Once the access to the implantation location on the vertebra has been obtained, a bore is made in the vertebra to receive the bone anchor. Where the bone anchor is a pedicle screw, the bore is placed lateral to the facet joints and angled in towards the vertebral body. The diameter and profile of the bore is selected to be compatible with the shaft of the bone anchor to be implanted. For example, the distal bore is sized to receive and be engaged by the distal shaft of the bone anchor, and the proximal bore is sized to receive and be engaged by the proximal shaft of the bone anchor. The bore is, in some cases, formed using a single device having the desired size and profile. In alternative embodiments, the distal bore is formed with a first device and then the proximal bore is enlarged with a second device. The diameter and length of the proximal and distal bore is selected based on the anatomy of the patient and the bone screw selected. In preferred embodiments one or more twist drills are utilized in conjunction with suction in order to remove bone cement and bone material cut by the drill. After forming the proximal and distal bore, the drill is removed. 
     The bone anchor is inserted into the proximal bore. A driver connected to the head of the bone anchor is then used to turn the bone anchor such that the distal threads engage the distal bore and the proximal threads engage the proximal bore. For each complete turn of the bone anchor, the bone anchor advances by a distance along its axis equal to the lead. The distal threads engage the distal bore without fracturing the bone cement. The bone anchor is turned until the head of the bone anchor is at the desired position relative to the surface of the bone and the distal shaft is engaged and secured to the bone cement surrounding the distal shaft and the proximal shaft is engaged and secured to the bone surrounding the proximal shaft. After implantation of the bone anchor, the driver is disconnected from the head of the bone anchor. Other components of a spinal implant system, for example spinal rods, can then be mounted to the vertebra by securing them to the head of the bone anchor. 
       FIGS. 2A-2D  show steps in the implantation of a bone anchor into a vertebra previously treated with bone cement. Referring first to  FIG. 2A , the patient is positioned prone. Incisions are made posterior to the vertebrae  200 . The dorsal fascia is opened and the paraspinal muscle is split to expose the facet joints  202  and lateral processes  204  of the vertebra  200 . As shown, a cannula  220  is inserted to provide for port access. Alternatively, a larger incision is made with tissue retraction to expose the vertebra (open procedure). As shown, the vertebra  200  includes harder cortical bone  210  at the surface, spongy cancellous bone  212  in the interior, and hardened bone cement  214  within the vertebral body  208 . Note that although bone cement  214  is shown for illustrative purposes as a homogenous mass, bone cement  214  may be distributed no-homogenously interspersed with regions including or consisting of cancellous bone. 
     Once the access to the implantation location on the vertebra  200  has been obtained, a bore is made in the vertebra  200  to receive to bone anchor. Where the bone anchor is a pedicle screw, the bore is placed lateral to the facet joints  202  and angled in towards the vertebral body  208 . As shown in  FIG. 2A , in one embodiment, a drill  222  having a stepped profile is inserted through the cannula  220  and advanced into the vertebra  200  through the cancellous bone  212  of the pedicle  206  and into the bone cement  214  within vertebral body  208 . In alternative embodiments, two devices/drills are used in separate steps—the distal bore is formed with a first device and then the proximal bore is enlarged with a second device. Alternatively, the proximal bore is formed first with a first device (such as a blunt probe) through cancellous bone and the distal bore is created as an extension of the proximal bore into bone cement with an appropriate tool. In preferred embodiments, one or more low speed twist drills are utilized in conjunction with suction in order to remove bone cement and bone material cut by the drill. After forming the proximal and distal bore, the drill is removed. 
     In an alternative preferred embodiment a blunt probe is inserted through the pedicle to create the proximal bore. The probe can be passed through the pedicle without excessive force until it contacts bone cement. The probe compresses cancellous bone (enhancing bone density) rather than cutting and removing the bone. The length of probe in the pedicle, when it contacts the bone cement, can be assessed with fluoroscopy/radiographic imaging or markings on the probe or a gauge. The distal bore is then created using a twist drill which cuts away and removes bone cement from the distal bore. Suction is used to clean cut bone cement from the operative site prior to implantation of the screw. Radiographic imaging and/or a gauge is utilized to select the correct length of distal shaft. The length of the proximal bore and the length of the distal bore are assessed and used to select a bone anchor having a proximal shaft and distal shaft of the correct length for the patient&#39;s anatomy from a kit containing a variety of configurations of bone anchors. 
     The bone anchors are preferably provided in the form of a kit which includes a range of bone anchors having different lengths including different lengths of the proximal and distal shafts. Thus, a screw having a particular length of proximal shaft and distal shaft is selected as appropriate for the anatomy of the patient and the distribution of bone cement within the target vertebra. In some cases, imaging of the vertebra and bone cement within it may be used to preoperatively assess configurations of the bone anchor shaft (diameters and shaft length) suitable for implantation in order to ensure that a suitable variety of bone anchors are available for the procedure. In preferred embodiments, the kit and/or a separate toolkit includes a range of installation/implantation tools (as, for example, described herein) suitable for creation of the bore in a bone containing hardened bone cement and for implantation of the bone anchor in the bore thereby created. 
     As shown, in  FIG. 2B , after forming the bore  230 , the drill  222  is removed. The diameter and profile of the bore  230  is selected to be compatible with the patient&#39;s anatomy and the shaft of the bone anchor to be implanted. As shown in  FIG. 2B , for example, the distal bore  234  within bone cement  214  is of a lower diameter sized to receive and be engaged by the distal shaft of the bone anchor, and the proximal bore  232  within cancellous bone  212  and cortical bone  210  is of a larger diameter sized to receive and be engaged by the proximal shaft of the bone anchor. 
     In embodiments, the relative lengths of the proximal and distal bore are selected based on the patient&#39;s anatomy and the position of the bone cement  214  within the vertebra  200 . The position of the bone cement  214  within the vertebra  200  and the size of vertebra  200  are, in some cases, assessed using imaging during preoperative planning in order to select a bone anchor having appropriate characteristics, and, thus, determine the proper characteristics for the proximal bore  232  and distal bore  234 . Alternatively, the size of the vertebra and position of the bone cement is assessed by the surgeon during the procedure using appropriate tools. 
     As shown, in  FIG. 2C , the bone anchor  100  is inserted into the proximal bore  232 . A driver  224  connected to the head  102  of the bone anchor  100  is then used to turn the bone anchor  100  such that the distal threads engage the distal bore  234  and the proximal threads engage the proximal bore  232 . For each complete turn of the bone anchor  100 , the bone anchor  100  advances by a distance along its axis equal to the lead. The distal threads engage the distal bore  234  without fracturing the bone cement  214 . The bone anchor  100  is turned until the head  102  of the bone anchor  100  is at the desired position relative to the surface of the vertebra  200  and the distal shaft  140  is engaged and secured to the bone cement  214  surrounding the distal bore  234  and the proximal shaft  120  is engaged and secured to the cancellous bone  212  and cortical bone  210  surrounding the proximal bore  232 . 
     As shown in  FIG. 2D , after implantation of the bone anchor  100  into the vertebra  200  the driver is disconnected from the head  102  of the bone anchor  100 . Other components of a spinal implant system, for example, spinal rods, can then be mounted to the vertebra by securing them to the head  102  of the bone anchor  100 . 
     Alternative Implantation Procedures 
     As illustrated above in  FIG. 2A , and described in the accompanying text a bore  230  (including a proximal bore  232  and a distal bore  234 ) is created in a vertebra to receive a bone anchor. One way of creating the bore  230  is with one or more drills or with a blunt probe and a drill. However, the distal bore  234  in the bone cement can be created using a variety of techniques and devices. The most common bone cement, PMMA, is a hard glass-like polymer which can be prone to fracture when drilled or machined. However, because of the particular properties of bone cement/PMMA, a bore can be made in PMMA using a number of techniques unsuitable for creating a bore in bone. Thus, in some embodiments, the distal bore  234  is created using a different method and apparatus than used to create the proximal bore  232 . For example, the glass transition temperature of PMMA ranges from 85° C. to 165° C. or more depending upon the formulation. PMMA may safely be heated above its glass transition temperature before, during and/or after manipulation to soften and/or melt the PMMA in order to reduce the risk of fracture. 
     In one method, a heated probe is used to melt the PMMA. The melted PMMA can be displaced or removed during insertion of the heated probe. The probe can be heated electrically, ultrasonically, mechanically or using electromagnetic radiation such as, for example, a laser. Alternatively, the distal bore is created using a mechanical tool such as a rotating burr that mechanically heats the PMMA and softens/melts the PMMA during creation of the bore. Alternatively, the distal bore is created using a drill and then the bone cement surrounding the distal bore is heat treated before or during bone anchor implantation to anneal/fuse any fractures that may have been formed during the cutting of the distal bore. Alternatively, an ultrasound probe can be used to heat and soften the bone cement during creation of the distal bore. 
       FIG. 2E  illustrates an alternative method for creating distal bore  234 . As before, the proximal bore  232  is created using conventional methods for creating a bore in a vertebra, e.g. a blunt probe or drill. For example, a probe can be passed through the pedicle without excessive force until it contacts bone cement. When the probe contacts bone cement, it is removed and a heated probe  240  is inserted through cannula  220 . Heated probe  240  includes a shaft  242  and heated tip  244 . A power/temperature controller  246  is coupled to heated tip  244  through shaft  242 . The power/temperature controller  246  provides one of electrical, ultrasonic or electromagnetic energy to heat heated tip  244 . In some embodiments, heated probe  240  is inserted through a hollow sleeve (not shown). The hollow sleeve is inserted into and engages the proximal bore  232 , aligns the heated tip  244  with the distal bore  234 , and insulates the bone adjacent proximal bore  232  from heating by heated tip  244 . 
     In use, the physician operates power/temperature controller  246  to raise the temperature of heated tip  244  above the glass transition temperature of bone cement  214 . The physician utilizes shaft  242  to drive heated tip  244  into bone cement  214 . Bone cement  214  flows away from heated tip  244  as heated tip  244  is introduced creating distal bore  234  (dotted lines). Heated probe  240  is, in some embodiments, provided with channels and/or grooves which allow melted bone cement  214  to flow towards the proximal bore  232 . When a distal bore  234  having a desired length as been created, heated probe  240  is removed. Heated tip  244  and bone cement  214  may be allowed to cool prior to removal of heated probe  240  in order that melted bone cement  214  does not flow into distal bore  234  after removal of heated probe  240 . 
     In an alternative embodiment heated probe  240  is inserted through a cannulated bone anchor (see e.g.  FIG. 3F ) such that heated tip  244  extends beyond the distal end of the bone anchor (See, e.g.  FIGS. 8A-8C ). In this procedure heated tip  244  is used to melt the bone cement during implantation of the bone anchor thereby reducing the possibility of fracture. In this embodiment, distal bore  234  may be formed simultaneously with the implantation of the bone anchor. 
       FIG. 2F  illustrates an alternative method for creating distal bore  234 . As before, the proximal bore  232  is created using conventional methods for creating a bore in a vertebra, e.g. a blunt probe or drill. For example, a probe can be passed through the pedicle without excessive force until it contacts bone cement. When the probe contacts bone cement, it is removed and a rotary probe  250  is inserted through cannula  220 . Rotary probe  250  includes a shaft  252  and burr tip  254 . A driver  256  (for example, an electrical motor) is coupled to burr tip  254  through shaft  252 . The driver  256  rotates shaft  252  and burr tip  254  at high speed. In some embodiments, rotary probe  250  includes a hollow sleeve  253  through which shaft  252  passes. The hollow sleeve  253  is inserted into and engages the proximal bore  232 , aligns the burr tip  254  with the distal bore  234 , and prevents contact between shaft  252  and the bone adjacent proximal bore  232 . 
     In use, the physician operates driver  256  to rotate the burr tip  254  at high speed. Friction between burr tip  254  and bone cement  214  raises the temperature of burr tip  254  and bone cement  214  above the glass transition temperature of bone cement  214 . The physician utilizes shaft  252  to drive burr tip  254  into bone cement  214 . Bone cement  214  flows away from burr tip  254  as burr tip  254  is introduced creating distal bore  234  (dotted lines). Rotary probe  250  is, in some embodiments, provided with channels and/or grooves which allow melted bone cement  214  to flow towards the proximal bore  232 . When a distal bore  234  having a desired length as been created, rotary probe  250  is removed. Burr tip  254  and bone cement  214  may be allowed to cool prior to removal of rotary probe  250  in order that melted bone cement  214  does not flow into distal bore  234  after removal of rotary probe  250 . 
     In an alternative embodiment rotary probe  250  is inserted through a cannulated bone anchor (see e.g.  FIG. 3F ) such that burr tip  254  extends beyond the distal end of the bone anchor (See, e.g.  FIGS. 8A-8C ). In this procedure burr tip  254  is used to melt the bone cement during implantation of the bone anchor thereby reducing the possibility of fracture. In this embodiment, distal bore  234  may be formed simultaneously with the implantation of the bone anchor. 
       FIG. 2G  illustrates an alternative method for creating distal bore  234 . As before, the proximal bore  232  is created using conventional methods for creating a bore in a vertebra, e.g. a blunt probe or drill. For example, a probe can be passed through the pedicle without excessive force until it contacts bone cement. When the probe contacts bone cement, it is removed and an ultrasonic probe  260  is inserted through cannula  220 . Ultrasonic probe  260  includes a shaft  262  and ultrasonic tip  264 . An ultrasonic transducer  266  is coupled to ultrasonic tip  264  through shaft  262 . The ultrasonic transducer  266  provides ultrasonic vibrations through shaft  262  to ultrasonic tip  264 . In some embodiments, ultrasonic probe  260  includes a hollow sleeve  263  through which shaft  262  passes. The hollow sleeve  263  is inserted into and engages the proximal bore  232 , aligns the ultrasonic tip  264  with the distal bore  234 , and prevents contact between shaft  262  and the bone adjacent proximal bore  232 . 
     In use, the physician operates ultrasonic transducer  266  to vibrate the ultrasonic tip  264  at high frequency. High frequency vibration where the ultrasonic tip  264  contacts bone cement  214  raises the temperature of ultrasonic tip  264  and bone cement  214  above the glass transition temperature of bone cement  214 . The physician utilizes shaft  262  to drive ultrasonic tip  264  into bone cement  214 . Bone cement  214  flows away from ultrasonic tip  264  as ultrasonic tip  264  is introduced—creating distal bore  234  (dotted lines). Ultrasonic probe  260  is, in some embodiments, provided with channels and/or grooves which allow melted bone cement  214  to flow towards the proximal bore  232 . When a distal bore  234  having a desired length has been created, ultrasonic probe  260  is removed. Ultrasonic tip  264  and bone cement  214  may be allowed to cool prior to removal of ultrasonic probe  260  in order that melted bone cement  214  does not flow into distal bore  234  after removal of ultrasonic probe  260 . 
     In an alternative embodiment ultrasonic probe  260  is inserted through a cannulated bone anchor (see e.g.  FIG. 3F ) such that ultrasonic tip  264  extends beyond the distal end of the bone anchor (see, e.g.  FIGS. 8A-8D ). In this procedure, ultrasonic tip  264  is used to melt the bone cement during implantation of the bone anchor thereby reducing the possibility of fracture. In this embodiment, distal bore  234  may be formed simultaneously with the implantation of the bone anchor. 
     The tools for creating the proximal and/or distal bore are, in some embodiments, cannulated such that they are adapted to be received over a guide wire to facilitate proper location of the tools relative to the bone during bore formation. In such a procedure a wire, for example a k-wire or other guidewire, is positioned at the target position on or in the bone, the cannulated bore creation tool is then directed over the guidewire to the target position. The guidewire is received in the central bore of the cannulated bore creation tool. The cannulated bore creation tool is then used to create and/or extend the bore. The guidewire is advanced with or incrementally ahead of the bore creation tool as the bore is created and/or extended. When a bore of the desired size has been created, the cannulated bore creation tool is withdrawn leaving the guidewire in place. If necessary or desirable, additional tools may be inserted over the guidewire to prepare the bore for implantation of a bone anchor and removed subsequent to use while maintaining the guidewire within the bore. When the desired bore has been prepared, a cannulated bone anchor is inserted over the guidewire and thereby directed to the bore for implantation. The guidewire is removed after the bone anchor is implanted at the correct position. 
     Maintaining the guidewire at the target location and within the bore facilitates the implantation procedure by ensuring a consistent location and orientation of the tool(s) and bone anchor during the procedure. This is particularly useful where the procedure is minimally invasive and/or percutaneous where the physician may not have direct visualization of the bone. Radiographic/fluoroscopic imaging can be used during initial placement of the guidewire. Thereafter, the placement of the guidewire is maintained and used to orient the tools and bone anchor, and, thus, the need for additional radiographic/fluoroscopic imaging during subsequent steps is reduced and/or eliminated thereby reducing procedure time and/or physician exposure to radiation. 
     Each of the tools for bore creation described herein can be cannulated in order to allow for use of a guidewire including, but not limited to, a heated probe, ultrasound probe, blunt probe, drill, stepped drill, burr probe, thermoelectric probe, or laser heated probe.  FIGS. 2H and 2I  illustrate two of the steps in the use of guidewire to guide implantation of a bone anchor. As shown in  FIG. 2H , a guidewire  278  is positioned relative to vertebra  200  and aligned with the longitudinal axis of bore  230 . A cannulated bore creation tool  270  having a cannulated shaft  272  is received over guidewire  278 . The guidewire  278  is received in a central bore of the cannulated bore creation tool  270  which can slide along the guidewire  278 . The driver  276  (for example, a motor) is used to operate head  274  (for example, a burr tip or drill) via the cannulated shaft  272  to create (in this step) distal bore  234  by extending proximal bore  232 . The guidewire  278  is advanced with the cannulated bore creation tool  270  as the bore is extended. (The proximal bore can be created the same way.) If necessary or desirable, the cannulated bore creation tool  270  may be exchanged with a different cannulated bore creation tool  270  to prepare the bore  230  while maintaining the guidewire  278  in place within the bore  230 . For example, a cannulated thread tapping tool (not shown) may be used to create threads in the bore  230 —the tap may be inserted over the guidewire, used to create threads in the bore  230 , and then removed, leaving the guidewire  278  in place within the bore  230 . 
     When the desired bore  230  has been prepared, the cannulated bore creation tool(s)  270  is/are removed leaving the guidewire  278  in position and aligned with the bore  230  as shown in  FIG. 2I . A cannulated bone anchor  280  (see e.g.  FIG. 3F ) is then placed on guidewire  278 . The physician slides cannulated bone anchor  280  along guidewire  278  which directs the cannulated bone anchor  280  to bore  230  and aligns cannulated bone anchor  280  with bore  230 . The cannulated bone anchor  280  is then implanted in the bore  230  using a driver appropriate to the cannulated bone anchor  280  (the driver is, in some embodiments, also received over guidewire  278 ). The guidewire is removed after the cannulated bone anchor  280  is implanted at the correct position with bore  230  and vertebra  200 . In some embodiments, the guidewire may also be used to guide installation of additional spinal components by guiding connection of the components to the head of cannulated bone anchor  280 . 
     Variations of Bone Anchor Shaft 
       FIGS. 3A-3H  illustrate variations of the shaft of the bone anchor  100  of  FIGS. 1A-1F . As previously described, the shaft of the bone anchor including proximal shaft  120  and distal shaft  140  is designed/selected to be compatible with the anatomy of the bone into which it is to be implanted and the relative positions and extent of bone cement and natural bone material within the bone. In preferred embodiments, both the proximal and distal shafts are cylindrical with the proximal shaft having a larger diameter than the distal shaft. In alternative embodiments one or more of the proximal shaft and distal shaft is tapered/conical. The thread depth can also be varied over the length of one or more of the proximal shaft and distal shaft. Moreover, the relative lengths of the proximal shaft and distal shaft and the overall length of the bone anchor are varied so as to be suitable for bones of different sizes and having different positions and extent of bone cement and natural bone material within the bone. The bone anchors may be provided in the form of a kit which includes a range of bone anchors having different features and different lengths including different lengths of the proximal and distal shafts. The physician is, thus, able to select from the kit, during the procedure, bone anchors suitable for the particular anatomy of the bone in which a bone anchor is desired to be implanted. 
       FIG. 3A  shows a perspective view of a bone anchor  300   a  according to an alternative embodiment of the present invention. Bone anchor  300   a  includes a head  302   a , at the proximal end and a tip  304   a  at the distal end. A shaft  306   a  extends between head  302   a  and tip  304   a  and includes a proximal shaft  320   a  and a distal shaft  340   a . Proximal shaft  320   a  bears on its outside surface a single proximal thread  322   a . Distal shaft  340   a  bears on its outside surface first and second distal threads  342   a . First and second distal threads  342   a  merge together and connect to single proximal thread  322   a  at the transition  346   a  between the distal shaft  340   a  and proximal shaft  320   a . The proximal thread  322   a  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  312   a  on the proximal shaft  320   a  is equal to the lead  310   a . The distal threads  342   a  have a thread depth and threadform suitable for engaging bone cement and the distal thread pitch  314   a  on the distal shaft  340   a  is half of the proximal thread pitch  312   a  on the proximal shaft  320   a , and, thus, equal to half of the lead  310   a . In the alternative embodiment shown in  FIG. 3A , the length of proximal shaft  320   a  is reduced and the length of distal shaft  340   a  is increased relative to the embodiment of  FIGS. 1A-1F . The alternative bone anchor  300   a  of  FIG. 3A  is, thus, suited to implantation in a bone having a larger extent of bone cement in which distal shaft  340   a  is to be secured. 
       FIG. 3B  shows a perspective view of a bone anchor  300   b  according to an alternative embodiment of the present invention. Bone anchor  300   b  includes a head  302   b , at the proximal end and a tip  304   b  at the distal end. A shaft  306   b  extends between head  302   b  and tip  304   b  and includes a proximal shaft  320   b  and a distal shaft  340   b . Proximal shaft  320   b  bears on its outside surface a single proximal thread  322   b . Distal shaft  340   b  bears on its outside surface first and second distal threads  342   b . First and second distal threads  342   b  merge together and connect to single proximal thread  322   b  at the transition  346   b  between the distal shaft  340   b  and proximal shaft  320   b . The proximal thread  322   b  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  312   b  on the proximal shaft  320   b  is equal to the lead  310   b . The distal threads  342   b  have a thread depth and threadform suitable for engaging bone cement and the distal thread pitch  314   b  on the distal shaft  340   b  is half of the proximal thread pitch  312   b  on the proximal shaft  320   b , and, thus, equal to half of the lead  310   b . In the alternative embodiment shown in  FIG. 3B , the length of proximal shaft  320   b  is increased and the length of distal shaft  340   b  is reduced relative to the embodiment of  FIGS. 1A-1F . The alternative bone anchor  300   b  of  FIG. 3A  is, thus, suited to implantation in a bone having a smaller extent of bone cement in which distal shaft  340   b  is to be secured. 
       FIG. 3C  shows a sectional view of a bone anchor  300   c  according to an alternative embodiment of the present invention. Bone anchor  300   c  includes a head  302   c , at the proximal end and a tip  304   c  at the distal end. A shaft  306   c  extends between head  302   c  and tip  304   c  and includes a proximal shaft  320   c  and a distal shaft  340   c . Proximal shaft  320   c  bears on its outside surface a single proximal thread  322   c . Distal shaft  340   c  bears on its outside surface first and second distal threads  342   c . First and second distal threads  342   c  merge together and connect to single proximal thread  322   c  at the transition  346   c  between the distal shaft  340   c  and proximal shaft  320   c . The proximal thread  322   c  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  312   c  on the proximal shaft  320   c  is equal to the lead  310   c . The distal threads  342   c  have a thread depth and threadform suitable for engaging bone cement and the distal thread pitch  314   c  on the distal shaft  340   c  is half of the proximal thread pitch  312   c  on the proximal shaft  320   c  and, thus, equal to half of the lead  310   c . In the alternative embodiment shown in  FIG. 3C , distal shaft  340   c  is tapered/conical in that the minor diameter of the distal shaft increases going from tip  304   c  towards transition  346   c . In the embodiment shown, the thread depth of the distal threads remains constant over the distal shaft  340   c , thus, the major diameter of the distal shaft also increases going from tip  304   c  towards transition  346   c . Proximal shaft  320   c  is, in this embodiment, cylindrical, and has a minor diameter greater than or equal to the maximum minor diameter of the distal shaft  340   c . In alternative embodiments, proximal shaft  320   c  is also conical in shape. 
       FIG. 3D  shows views of a bone anchor  300   d  according to an alternative embodiment of the present invention. Bone anchor  300   d  includes a head  302   d , at the proximal end and a tip  304   d  at the distal end. A shaft  306   d  extends between head  302   d  and tip  304   d  and includes a proximal shaft  320   d  and a distal shaft  340   d . Proximal shaft  320   d  bears on its outside surface a single proximal thread  322   d . Distal shaft  340   d  bears on its outside surface first and second distal threads  342   d . First and second distal threads  342   d  merge together and connect to single proximal thread  322   d  at the transition  346   d  between the distal shaft  340   d  and proximal shaft  320   d . The proximal thread  322   d  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  312   d  on the proximal shaft  320   d  is equal to the lead  310   d . The distal threads  342   d  have a thread depth and threadform suitable for engaging bone cement, and the distal thread pitch  314   d  on the distal shaft  340   d  is half of the proximal thread pitch  312   d  on the proximal shaft  320   d , and, thus, equal to half of the lead  310   d . In the alternative embodiment shown in  FIG. 3D , the proximal shaft  320   d  is conical/tapered and increases in minor diameter going from transition  346   d  towards head  302   d . Note however, that the major diameter of proximal shaft  320   d  is substantially constant such that as the shaft increases in diameter going towards head  302   d , the thread depth of the proximal thread is reduced. The conical design of proximal shaft  320   d  serves to compress cancellous and cortical bone surrounding the proximal bore which can assist the engagement between proximal thread  322   d  and the bone. 
       FIG. 3E  shows a section view of a bone anchor  300   e  according to an alternative embodiment of the present invention. Bone anchor  300   e  includes a head  302   e , at the proximal end and a tip  304   e  at the distal end. A shaft  306   e  extends between head  302   e  and tip  304   e  and includes a proximal shaft  320   e  and a distal shaft  340   e . Proximal shaft  320   e  bears on its outside surface a single proximal thread  322   e . Distal shaft  340   e  bears on its outside surface first and second distal threads  342   e . First and second distal threads  342   e  merge together and connect to single proximal thread  322   e  at the transition  346   e  between the distal shaft  340   e  and proximal shaft  320   e . The proximal thread  322   e  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  312   e  on the proximal shaft  320   e  is equal to the lead  310   e . The distal threads  342   e  have a thread depth and threadform suitable for engaging bone cement and the distal thread pitch  314   e  on the distal shaft  340   e  is half of the proximal thread pitch  312   e  on the proximal shaft  320   e , and, thus, equal to half of the lead  310   e . In the alternative embodiment shown in  FIG. 3E , the thread depth and threadform of both of distal threads  342   e  is identical. Moreover, the major diameter of distal shaft (crest to crest) is less than the minor diameter of the proximal shaft. Thus, distal shaft  340   e  can pass through a proximal bore of suitable diameter for proximal shaft  320   e  without distal threads  342   e  engaging the wall of the proximal bore thereby facilitating implantation of bone anchor  300   e.    
       FIG. 3F  which shows views of a bone anchor  300   f  according to an alternative embodiment of the present invention. Bone anchor  300   f  includes a head  302   f , at the proximal end and a tip  304   f  at the distal end. A shaft  306   f  extends between head  302   f  and tip  304   f  and includes a proximal shaft  320   f  and a distal shaft  340   f . Proximal shaft  320   f  bears on its outside surface a single proximal thread  322   f . Distal shaft  340   f  bears on its outside surface first and second distal threads  342   f . First and second distal threads  342   f  merge together and connect to single proximal thread  322   f  at the transition  346   f  between the distal shaft  340   f  and proximal shaft  320   f . The proximal thread  322   f  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  312   f  on the proximal shaft  320   f  is equal to the lead  310   f . The distal threads  342   f  have a thread depth and threadform suitable for engaging bone cement and the distal thread pitch  314   f  on the distal shaft  340   f  is half of the proximal thread pitch  312   f  on the proximal shaft  320   f , and, thus, equal to half of the lead  310   f.    
     In the alternative embodiment shown in  FIG. 3F , bone anchor  300   f  is cannulated in that a continuous bore  350  extends through head  302   f , proximal shaft  320   f , distal shaft  340   f  and tip  304   f . The continuous bore  350  can be sized to receive a guidewire such that bone anchor  300   f  can be guided to a target location over a guidewire. Also, continuous bore  350  can be utilized for the injection of bone cement into the bone to strengthen bone and/or connection between the bone and the bone anchor  300   f  after implantation. Bone cement injected through head  302   f  passes through continuous bore  350  and passes out of bone anchor  300   f  into the bone through one or more proximal aperture  352 , distal aperture  354  or tip aperture  356  which communicate with continuous bore  350 . The location and number of apertures can be varied to achieve a desired distribution of bone cement. In embodiments, only the proximal aperture  352 , or the distal aperture  354  or the tip aperture  356  are present. For example, where the continuous bore  350  is to be used only for insertion of a guidewire or other tool, only tip aperture  356  is required to allow the guidewire to extend from tip  304   f.    
       FIG. 3G  shows a perspective view of a bone anchor  300   g  according to an alternative embodiment of the present invention. Bone anchor  300   g  includes a head  302   g , at the proximal end and a tip  304   g  at the distal end. A shaft  306   g  extends between head  302   g  and tip  304   g  and includes a proximal shaft  320   g  and a distal shaft  340   g . Proximal shaft  320   g  bears on its outside surface a single proximal thread  322   g . Distal shaft  340   g  bears on its outside surface first and second distal threads  342   g . First and second distal threads  342   g  merge together and connect to single proximal thread  322   g  at the transition  346   g  between the distal shaft  340   g  and proximal shaft  320   g . The proximal thread  322   g  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  312   g  on the proximal shaft  320   g  is equal to the lead  310   g . The distal threads  342   g  have a thread depth and threadform suitable for engaging bone cement, and the distal thread pitch  314   g  on the distal shaft  340   g  is half of the proximal thread pitch  312   g  on the proximal shaft  320   g , and, thus, equal to half of the lead  310   g . In the alternative embodiment shown in  FIG. 3G , a longitudinal groove  360  has been cut into the distal shaft  340   g  and distal threads  342   g . Although a single groove  360  is shown, a number of grooves, for example, 1, 2, 3 or 4, grooves  360  can be spaced around the distal shaft  340   g . The distal threads  342   g  are segmented by groove  360 , however, the segments are aligned as if part of a contiguous thread. Because the thread in this embodiment only intermittently engages the bore around the perimeter of the shaft, the thread places less stress on the bore reducing the chance of fracture. Moreover, any bone cement which is displaced during implantation can collect in a void formed by the groove  360  rather than being compressed and possibly causing facture. (See, also  FIGS. 4C-4E  and accompanying text.) 
       FIG. 3H  shows a perspective view of a bone anchor  300   h  according to an alternative embodiment of the present invention. Bone anchor  300   h  includes a head  302   h  (which is in this case a polyaxial head), at the proximal end and a tip  304   h  at the distal end. A shaft  306   h  extends between head  302   h  and tip  304   h  and includes a proximal shaft  320   h  and a distal shaft  340   h . Proximal shaft  320   h  bears on its outside surface a single proximal thread  322   h . Distal shaft  340   h  bears on its outside surface first and second distal threads  342   h . First and second distal threads  342   h  merge together and connect to single proximal thread  322   h  at the transition  346   h  between the distal shaft  340   h  and proximal shaft  320   h . The proximal thread  322   h  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  312   h  on the proximal shaft  320   h  is equal to the lead  310   h . The distal threads  342   h  have a thread depth and threadform suitable for engaging bone cement, and the distal thread pitch  314   h  on the distal shaft  340   h  is half of the proximal thread pitch  312   h  on the proximal shaft  320   h , and, thus, equal to half of the lead  310   h . In the alternative embodiment shown in  FIG. 3H , the distal shaft  340   h  has the same major diameter as the proximal shaft  320   h  along most of its length. Adjacent tip  304   h , the distal shaft  340   h  tapers rapidly to form a conical segment  370 . Note also, that a self-tapping groove  372  is made in the surface of distal shaft  340   h  and distal threads  342   h  in the region of conical segment  370 . Conical segment  370  and self tapping groove  372  serve to facilitate implantation of bone anchor  300   h  without fracturing bone cement. A self tapping groove  372  or conical segment  370  may be incorporated into any of the other distal shaft designs described herein. 
     Variations of Bone Anchor Shaft Cross-Section 
     In the Figures, the shafts of the bone anchors are illustrated as having a generally circular solid cross-section in a plane perpendicular to the longitudinal axis of the shaft. Thus, the shafts are shown as generally cylindrical or conical/truncated conical.  FIG. 4A  schematically represents the cross-section of a circular shaft  400   a  having one or more threads  402   a . For clarity of shaft shape, the position of thread(s)  402   a  is shown schematically as the projection of the threads into the plane of the section—the section of the thread is not shown. Although this is the most commonly used cross-section for a bone screw, alternative cross-sections are used in some embodiments.  FIGS. 4B-4F  illustrate alternative shaft cross-sections which can be utilized in place of the circular cross-section shown in any of the shaft embodiments illustrated herein. The proximal and distal shafts may have the same or different of the cross-sections shown in  FIGS. 4A-4F . In particular embodiments, the proximal shaft of the bone anchor has the cross-section shown in  FIG. 4A , and the distal shaft has one of the cross-sections illustrated in  FIGS. 4B-4E . 
       FIG. 4B  illustrates a shaft  400   b  having a tri-lobular or generally triangular shape. The thread  402   b  is illustrated as also triangular. The thread only engages the wall of the bore into which it is placed at the maximum radius from the center of the shaft. Essentially the thread will only engage the bore at the tips of the triangle. In alternative embodiments, the thread need not be continuous along the walls of the shaft. Because the thread  402   b  only intermittently engages the bore around the perimeter of the shaft  400   b , the force required to drive the bone anchor is reduced thereby placing less stress on the bore reducing the chance of fracture. Moreover, any bone cement which is displaced by the thread  402   b  during implantation can collect in voids between the apices of the triangle rather than being compressed and possibly causing facture. Furthermore, cold flow of PMMA into the void after implantation can serve to lock in the bone anchor—reducing the chance that it will back out of the bore. 
       FIG. 4C  illustrates a shaft  400   c  have generally circular shape from which two segments/grooves  404   c  have been cut. The thread  402   c  is generally circular. The thread only engages the wall of the bore into which it is placed at the maximum radius from the center of the shaft. The thread has been removed between the apices during formation of the two grooves  404   c . Consequently, the thread is segmented along the perimeter of the shaft although the segments are aligned as if the thread remained contiguous. Similarly,  FIG. 4D  illustrates a generally circular shaft  400   d  having a thread  402   d  and in which three grooves  404   d  have been cut thereby segmenting thread  402   d  into three segments. Likewise  FIG. 4E  illustrates a generally circular shaft  400   e  having a thread  402   e  and in which four grooves  404   e  have been cut thereby segmenting thread  402   e  into four segments. Because the thread in these embodiments only intermittently engages the bore around the perimeter of the shaft, the force required to drive the bone anchor is reduced thereby placing less stress on the bore reducing the chance of fracture. Moreover, any bone cement which is displaced during implantation can collect in voids formed by the grooves rather than being compressed and possibly causing facture. Furthermore, cold flow of bone cement into the void(s) after implantation can serve to lock in the bone anchor—reducing the chance that it will back out of the bore. 
       FIG. 4F  similarly represents the cross-section of a circular shaft  400   f  having one or more threads  402   f  wherein the shaft is cannulated and has a central bore  410  (as previously described). A central bore  410  may similarly be provided in each of the other shaft cross-sections shown in  FIGS. 4A-4E  if desired for receiving a guidewire or tool, or for the injection of bone cement. 
     Variations Of Bone Anchor Tip 
     The inventive bone anchor shaft described herein is useful for anchoring a variety of orthopedic implants in the situation where a bone screw must be implanted in a bone which has been previously treated with bone cement, and, therefore, contains hard bone cement material. Although a blunt tip is shown in many of the figures, in alternative embodiments a different bone anchor tip suitable for a particular application may be used in combination with any one of the disclosed shafts including, but not limited to: a self-tapping tip; rounded tip; blunt tip; blunt self-tapping tip; trocar tip; tapered tip; or corkscrew tip.  FIGS. 5A-5B  show alternative tip embodiments which can replace the tips shown in the otherwise disclosed embodiments. 
       FIG. 5A , illustrates a variation  500   a  of bone anchor  100  of  FIGS. 1A-1F  having a self tapping tip. A blunt tip, as shown in  FIG. 1A , allows for good accuracy of implantation in a pre-drilled bore. However, the blunt tip displaces bone cement cut by the threads in the distal bore. As shown in  FIG. 5A , the bone anchor  500   a  can be provided with one or more flutes  502  cut into the distal threads  142   a ,  142   b  adjacent the tip  104  to allow cuttings created during the formation of threads in the bore to escape. The provision of flutes  502  prevents or reduces the accumulation of cuttings ahead of the screw tip which might lead to fracture of the bone cement. The sharpness, number, and geometry of flute(s)  502  are selected to be effective to avoid facture of the bone cement material. (See, also  FIGS. 3G ,  3 H,  4 B- 4 F and accompanying text.) 
       FIG. 5B  illustrates a variation  500   b  of bone anchor  100  of  FIGS. 1A-1F  having a trocar tip  504 . Trocar tip  504  is sharper and more tapered than rounded tip  104  of  FIG. 1A . Trocar tip  504  is, in an alternative embodiment, provided with one or more flutes. 
       FIG. 5C  illustrates a variation  500   c  of bone anchor  100  of  FIGS. 1A-1F  having a sharp tapered tip  506 . Tip  506  facilitates implantation of bone anchor  500   c  into bone cement. Tip  506  tapers rapidly from the minor diameter of the distal shaft  140  to a sharp point  510 . Sharp point  510  enables sharp tapered tip  506  to penetrate bone cement during implantation. 
       FIG. 5D  illustrates a variation  500   d  of bone anchor  100  of  FIGS. 1A-1F  having a drill tip  508 . Drill tip  508  facilitates implantation of bone anchor  500   d  into bone cement. The drill tip  508  can form the distal bore simultaneously with implantation of bone anchor  500   d . Alternatively, a pilot bore is predrilled and drill tip  508  enlarges the bore during implantation of bone anchor  500   d . Drill tip  508  includes one or more sharp cutting lips  520  and one or more flutes  522 . During operation lips  520  cut into bone cement thereby forming the distal bore in all or in part. 
     Variations of Bone Anchor Head 
     The bone anchor shaft described herein is useful for anchoring a variety of orthopedic implants in the situation where a bone screw must be implanted in a bone which has been previously treated with bone cement, and, therefore, contains hard bone cement material. The head of the bone anchor is selected to be suitable for the secure connection of a spinal prosthesis component, and, thus, the spinal prosthesis to the bone anchor whereby the spinal prosthesis is effectively secured to the bone in which the bone anchor is implanted. Although a simple head is shown in many of the figures, in alternative embodiments a different bone anchor head suitable for a particular application may be used in combination with any one of the disclosed shafts. In embodiments, the bone anchor head is selected from: Steffee heads; hex heads; hex socket heads; Torx heads; breakaway heads; fixed heads; polyaxial heads, pedicle screw heads; angled heads; dynamic bone anchor heads; and other heads desired to be securely mounted to a bone containing hardened bone cement. In principle, any conventional or future-developed bone anchor head can be combined with the shaft of this invention where it is desired to secure the head to a bone which has been previously treated with bone cement. 
       FIGS. 6A-6F  show alternative heads which can replace the heads shown in the otherwise disclosed embodiments.  FIG. 6A , illustrates a variation  600   a  of bone anchor  100  of  FIGS. 1A-1F  having a Steffee type head  610  suitable for mounting a plate or other spinal implant component to a bone. As shown in  FIG. 6A , head  610  includes a base  612  having a hexagonal external surface  614  which can be engaged by a wrench for turning bone anchor  600   a  during implantation. A threaded post  616  extends proximally from base  612 . At the proximal end of threaded post  616  is a hex end  618  which can also be engaged by a wrench. In use, bone anchor  600   a  is implanted in a bone, and a plate (not shown) is, thereafter, secured to threaded post  616  with one or more nuts (not shown). 
       FIG. 6B , illustrates a variation  600   b  of bone anchor  100  of  FIGS. 1A-1F  having a pedicle screw head  620  suitable for mounting a spinal rod or other spinal implant component to a bone. As shown in  FIG. 6B , head  620  includes a body  622  having a hexagonal external surface  624  which can be engaged by a wrench for turning bone anchor  600   b  during implantation. Body  622  has a fixed relationship to shaft  106 . A threaded bore  626  extends into body  622 . Threaded bore  626  receives a threaded set screw  627 . Threaded bore  626  is slotted  628  such that a rod can be inserted across threaded bore  626 . In use, bone anchor  600   b  is implanted in a bone, and a rod (not shown) is, thereafter, inserted through slots  628  across threaded bore  626 . Set screw  627  is then tightened to secure the rod to head  620 . 
       FIG. 6C , illustrates a variation  600   c  of bone anchor  100  of  FIGS. 1A-1F  having a polyaxial pedicle screw head  630  suitable for mounting a spinal rod or other spinal implant component at an adjustable angle to a bone. As shown in  FIG. 6C , head  630  includes a body  632 . Body  632  has a socket  633  which is mounted to a coupling  635  formed at the end of shaft  106 . Socket  633  is mounted to coupling  635  such that body  632  can be arranged at a variable angle and/or rotation relative to shaft  106 . A threaded bore  636  extends into body  632 . Threaded bore  636  receives a threaded set screw  637 . Threaded bore  636  is slotted  638  such that a rod can be inserted across threaded bore  636 . In use, bone anchor  600   c  is implanted in a bone—in some embodiments coupling  635  includes a hex socket which can be engaged by a wrench for turning bone anchor  600   c  during implantation. After implanting bone anchor  600   c , a rod (not shown) is, thereafter, inserted through slots  638  across threaded bore  636 . Set screw  637  is then tightened to secure the rod to head  630  and lock the angle of body  632  relative to shaft  106 . Polyaxial screw head  630  is shown in simplified form and may include one or more elements not shown. A wide variety of polyaxial heads is known in the art and is suitable for combining with shaft  106 . The term polyaxial head is meant to encompass all of the various polyaxial heads known in the art. 
       FIG. 6D  illustrates a variation  600   d  of bone anchor  100  of  FIGS. 1A-1F  having a dynamic head  640  suitable for mounting a spinal rod or other spinal implant component to a bone in a manner which allows motion preservation and load sharing. As shown in  FIG. 6D , dynamic head  640  includes a body  642 . Body  642  has a socket  643  and a cap  644 . Body  642  has surface feature  645  which can be engaged by a wrench for turning bone anchor  600   d  during implantation. A deflectable post  646  includes a distal coupling  647  received in socket  643  and extending through cap  644 . Coupling  647  is mounted and retained in socket  643  such that deflectable post  646  can deflect through a range of angles and/or rotate relative to shaft  106  even after a spinal rod or other spinal implant component is mounted to deflectable post  646 . Movement of deflectable post  646  is constrained by contact with cap  644 . Deflectable post  646  includes a threaded mount  648  to which a spinal rod or other spinal implant component can be secured with a nut without locking the angle of deflectable post  646  relative to shaft  106 . Threaded mount  648  includes one or more features  649  (e.g. a hex extension and/or hex socket) which allow deflectable post  646  to be engaged by a wrench during the securing of a rod or other spinal implant component to deflectable post  646 . In use, bone anchor  600   d  is implanted in a bone. After implanting bone anchor  600   d , a rod (not shown) is thereafter inserted over deflectable post  646  and secured to threaded mount  648  with a nut (not shown). Note that dynamic head  640  is designed such that it secures the rod or other spinal component to the shaft  106  while still permitting constrained movement of the rod or other spinal component relative to the shaft  106  in a manner which allows motion preservation and load sharing. A wide variety of dynamic heads is taught in U.S. patent application Ser. No. 13/352,882 entitled “Low Profile Spinal Prosthesis Incorporating A Bone Anchor Having A Deflectable Post And A Compound Spinal Rod” filed Jan. 18, 2012, which is hereby incorporated by reference in its entirety. These dynamic heads are suitable for combining with shaft  106 . The term “dynamic stabilization head” is meant to encompass all of the various dynamic heads disclosed in U.S. patent application Ser. No. 13/352,882. 
       FIG. 6E , illustrates a variation  600   e  of bone anchor  100  of  FIGS. 1A-1F  having a post type head  650  suitable for mounting a rod or other spinal implant component to a bone. As shown in  FIG. 6E , head  650  includes a post  652 . Post  652  has at its proximal end a hexagonal socket  654  which can be engaged by a wrench for turning bone anchor  600   e  during implantation. In use, bone anchor  600   e  is implanted in a bone, and a rod is thereafter secured to post  652  with a coupling (not shown). 
       FIG. 6F , illustrates a variation  600   f  of bone anchor  100  of  FIGS. 1A-1F  having a hex type head  660  suitable for mounting a plate or other spinal implant component to a bone. As shown in  FIG. 6F , head  660  has a hexagonal exterior surface  662  which can be engaged by a wrench for turning bone anchor  600   f  during implantation. 
     Dynamic Bone Anchor 
       FIGS. 7A-7C  illustrate a dynamic bone anchor  700  incorporating the shaft of the bone anchor  100  of  FIGS. 1A-1F  in conjunction with one embodiment of a dynamic stabilization head.  FIG. 7A  shows an exploded view of bone anchor  700 .  FIG. 7B  shows a perspective view of bone anchor  700 , as assembled.  FIG. 7C  shows a sectional view of bone anchor  700 . Referring first to  FIG. 7A , bone anchor  700  includes, in this embodiment, three components: bone screw  720 , deflectable post  740 , and cap  710 . Bone screw  720  comprises a threaded shaft  106  (which is the same as shaft  106  described in  FIGS. 1A-1F ) with a housing  730  at one end. Housing  730  may, in some embodiments, be cylindrical, and, is, in some embodiments, provided with splines/flutes. Housing  730  is preferably formed in one piece with threaded shaft  106 . Housing  730  has a cavity  732  oriented along the axis of threaded shaft  106 . Cavity  706  is open at the proximal end of housing  730  and is configured to receive deflectable post  740 . 
     In a preferred embodiment, deflectable post  740  is a titanium post 5 mm in diameter. Deflectable post  740  has a retainer  742  at one end. At the other end of deflectable post  740 , is a mount  744 . Retainer  742  is a ball-shaped or spherical structure in order to form part of a linkage connecting deflectable post  740  to bone screw  720 . Mount  744  is a low profile mount configured to connect deflectable post  740  to a spinal rod (not shown). Mount  744  comprises a threaded cylinder  746  to which the vertical rod component may be secured. Mount  744 , in some embodiments, also comprises a polygonal section  745  to prevent rotation of a component relative to mount  744 . 
     Mount  744  includes a male hex extension  748  which may be engaged by a tool to hold stationary mount  744  during attachment to a vertical rod. At the proximal end of male hex extension  748  is a nipple  749  for securing male hex extension  748  into a tool. Hex extension  748  is a breakaway component. Between hex extension  748  and threaded cylinder  746  is a groove  747 . Groove  747  reduces the diameter of deflectable post  740  such that hex extension  748  breaks away from threaded cylinder  746  when a desired level of torque is reached during attachment of a vertical rod. The breakaway torque is determined by the diameter of remaining material and the material properties. In a preferred embodiment, the breakaway torque is approximately 30 foot pounds. Thus, hex extension  748  breaks away during implantation and is removed. Nipple  749  is engaged by the tool in order to remove hex extension  748 . Deflectable post  740  is also provided with flats  743  immediately adjacent mount  744 . Flats  743  allow deflectable post  740  to be engaged by a tool after hex extension  748  has been removed. 
     Referring again to  FIG. 7A , a cap  710  is designed to perform multiple functions including securing retainer  742  in cavity  732  of bone screw  720 . Cap  710  has a central aperture  712  for receiving deflectable post  740 . In the embodiment of  FIG. 7A , cap  710  has surface features  714 , for example, splines or flutes, adapted for engagement by an implantation tool or mounting a component, e.g. an offset connector. Surface features  714  may be, for example, engaged by a driver that mates with surface features  714  for implanting bone anchor  700  in a bone. As shown in  FIG. 7A , cap  710  comprises a cylindrical shield section  718  connected to a collar section  716 . Shield section  718  is designed to mate with cavity  732  of housing  730 . Shield section  718  is threaded adjacent collar section  716  in order to engage threads at the proximal end of cavity  732  of housing  730 . The distal end of shield section  718  comprises a flange  719  for securing retainer  742  within cavity  732  of housing  730 . 
     Bone anchor  700  is assembled prior to implantation in a patient.  FIG. 7B  shows a perspective view of bone anchor  700  as assembled. When assembled, deflectable post  740  is positioned through cap  710 . Cap  710  is then secured to the threaded end of cavity  732  (see FIGS.  7 A and  7 C) of housing  730  of bone screw  720 . Cap  710  has surface features  714  for engagement by a wrench to allow cap  710  to be tightened to housing  730 . For example, cap  710  may be hexagonal or octagonal in shape or may have splines and/or flutes and/or other registration elements. Cap  710  may alternatively, or additionally, be laser welded to housing  730  after installation. Cap  710  secures deflectable post  740  within cavity  732  of bone screw  720 . Deflectable post  740  extends out of housing  730  and cap  710  such that mount  744  is accessible for connection to a vertical rod. Bone anchor  700  is implanted in a bone in the configuration shown in  FIG. 7B  and prior to attachment of a vertical rod or other spinal rod. A special tool may be used to engage the surface features  714  of cap  710  during implantation of bone anchor  700  into a bone. 
     As previously described, threaded shaft  106  includes a tip  104  at the distal end. Shaft  106  extends between housing (head)  730  and tip  104  and includes a proximal shaft  120  and a distal shaft  140 . Proximal shaft  120  bears on its outside surface a single proximal thread  122 . Distal shaft  140  bears on its outside surface first and second distal threads  142   a ,  142   b . First and second distal threads  142   a ,  142   b  merge together and connect to single proximal thread  122  at the transition between the distal shaft  140  and proximal shaft  120 . The proximal thread  122  has a thread depth and threadform suitable for engaging bone and the proximal thread pitch  112  on the proximal shaft  120  is equal to the lead  110 . The distal threads  142   a ,  142   b  have a thread depth and threadform suitable for engaging hardened bone cement, and the distal thread pitch  114  on the distal shaft  140  is half of the proximal thread pitch  112  on the proximal shaft  120 , and, thus, equal to half of the lead  110 . In conjunction with threaded shaft  106 , dynamic bone anchor  700  can be utilized to provide dynamic stabilization of a vertebra previously treated with bone cement. 
       FIG. 7C  shows a sectional view of a bone anchor  700 . Retainer  742  fits into a hemispherical pocket  739  in the bottom of cavity  732  of housing  730 . The bottom edge of cap  710  includes the curved flange  719  which secures ball-shaped retainer  742  within hemispherical pocket  739  while allowing ball-shaped retainer  742  to pivot and rotate. Accordingly, in this embodiment, a ball-joint is formed.  FIG. 7C  also illustrates deflection of deflectable post  740  shown in dashed lines. Applying a force to mount  744  causes deflection of deflectable post  740  of bone anchor  700 . Deflectable post  740  pivots about a pivot point  703  indicated by an X. Deflectable post  740  may pivot about pivot point  703  in any direction, as shown by arrow  750 . Concurrently or alternatively, deflectable post  740  can rotate, as shown by arrow  752 , about the long axis of deflectable post  740  (which also passes through pivot point  703 ). In this embodiment, pivot point  703  is located at the center of ball-shaped retainer  742 . 
     Dynamic bone anchor  700  is designed such that deflectable post  740  remains deflectable after the mounting of a spinal rod or other spinal implant to deflectable post  740 . In this way, dynamic bone anchor stabilizes the spine while still permitting relative movement of vertebrae of the spine within constraints imposed by the limits of deflection of deflectable post  740 . In a preferred embodiment, deflectable post  740  may deflect from 0.5 mm to 2 mm in any direction before making contact with limit surface  713 . More preferably, deflectable post  740  may deflect approximately 1 mm before making contact with limit surface  713 . After a fixed amount of deflection, deflectable post  740  comes into contact with limit surface  713  of cap  710 . Limit surface  713  is oriented such that when deflectable post  740  makes contact with limit surface  713 , the contact is distributed over an area to reduce stress on deflectable post  740 . In this embodiment, the deflectable post  740  contacts the entire sloping side of the conically-shaped limit surface  713 . In another embodiment, the deflectable post may only contact a limit ring that is located distally from the flange  719  of cap  710 . After deflectable post  740  comes into contact with limit surface  713 , further deflection requires deformation (bending) of deflectable post  740 . 
     The configuration and materials of the dynamic head may be selected to create a deflection assembly having stiffness/deflection characteristics suitable for the needs of a patient. By selecting appropriate dimensions and materials, the deflection characteristics of the deflectable post can be configured to approach the natural dynamic motion of the spine of a particular patient, while giving dynamic support to the spine in that region. It is contemplated, for example, that the spinal prosthesis utilizing the bone anchor having a dynamic head can be made in stiffness that can replicate a 70% range of motion and flexibility of the natural intact spine, a 50% range of motion and flexibility of the natural intact spine and a 30% range of motion and flexibility of the natural intact spine. 
     In alternative embodiments, a compliant member/sleeve/ring can be added to the bone anchor  700  positioned within housing  730 , cap  710 , and/or deflectable post  740 . The compliant member is positioned such that it is compressed by deflection of deflectable post  740  away from alignment with the longitudinal axis of shaft  106 . As a result of such compression, the compliant member exerts a restoring force upon deflectable post  740  pushing it back into alignment with the longitudinal axis of shaft  106 . The compliant member can be, for example, a metal, superelastic, nitinol, or polymer member. The material of the compliant member/sleeve/ring is, in some embodiments, a biocompatible and implantable polymer having the desired deformation characteristics. The sleeve may, for example, be made from PEEK or a polycarbonate urethane (PCU) such as Bionate®. If the sleeve is comprised of Bionate®, a polycarbonate urethane or other hydrophilic polymer, the sleeve can also act as a fluid-lubricated bearing for rotation of the deflectable post relative to the longitudinal axis of the deflectable post. 
     Movement of the deflectable post relative to the bone anchor provides load sharing and dynamic stabilization properties to the dynamic stabilization assembly. As described above, deflection of the deflectable post deforms the material of the sleeve. The characteristics of the material of the sleeve in combination with the dimensions of the components of the deflection rod assembly affect the force-deflection curve of the deflection rod. By changing the dimensions of the deflectable post, sleeve and the shield, the deflection characteristics of the deflection rod assembly can be changed. The stiffness of components of the deflection rod assembly can be, for example, increased by increasing the diameter of the deflectable post and/or by decreasing the diameter of the inner surface of the shield. Additionally, decreasing the diameter of the deflectable post will decrease the stiffness of the deflection rod assembly while decreasing the diameter of the post and/or by increasing the diameter of the inner surface of the shield will decrease the stiffness of the deflection rod. Alternatively and/or additionally, changing the materials which comprise the components of the deflection rod assembly can also affect the stiffness and range of motion of the deflection rod. For example, making the sleeve out of stiffer and/or harder material reduces deflection of the deflectable post. 
     Particular embodiments of dynamic bone anchors, deflectable posts with and without compliant members/sleeves/rings, and dynamic spinal stabilization systems are disclosed in U.S. patent application Ser. No. 13/352,882 entitled “Low Profile Spinal Prosthesis Incorporating A Bone Anchor Having A Deflectable Post And A Compound Spinal Rod” filed Jan. 18, 2012, which is hereby incorporated by reference in its entirety. The embodiments of bone anchor shafts and tips and installation tools and methods described in the present patent application can be utilized with any of the bone anchor embodiments disclosed in patent application Ser. No. 13/352,882 by replacing/modifying the bone anchor shafts and tips and installation tools and methods disclosed in patent application Ser. No. 13/352,882 with those described in the present patent application for use in situations where implantation is required in a vertebra including hardened bone cement. 
     Alternative Bone Anchor Implantation Tools 
     As described with respect to  FIGS. 2E ,  2 F, the distal bore  234  is, in some procedures, created by heating of the bone cement  214  before or during implantation of a bone anchor. To form distal bore  234  during implantation of a bone anchor, the bone anchor is provided with means for melting the bone cement during implantation. In one method, a heated probe inserted through a cannulated bone anchor is used to melt the PMMA adjacent the tip of the bone anchor. The melted PMMA can be displaced or removed during insertion of the bone anchor. Alternatively, the tip of the bone anchor itself is heated rather than a separate probe. The probe or anchor tip can be heated electrically, ultrasonically or using electromagnetic radiation, for example, an infrared laser. Alternatively, the distal bore is created using a mechanical tool such as a rotating burr inserted through a cannulated bone anchor that mechanically heats the PMMA above its melting temperature during implantation of the bone anchor. Alternatively, the distal bore is created using a drill and then the bone cement surrounding the distal bore is heat treated before or during bone anchor implantation to fuse any fractures that may have been formed during the cutting of the distal bore. 
       FIG. 8A  illustrates a cannulated bone anchor  300   f  as previously described with respect to  FIG. 3F  in conjunction with a heated probe  840  which includes a shaft  842  and heated tip  844 . A power/temperature controller  846  is coupled to heated tip  844  through shaft  842 . The power/temperature controller  846  provides one of electrical, ultrasonic or electromagnetic energy to heat heated tip  844 . Heated probe  840  is inserted through a channel  802  in a wrench  800  having a head  804  adapted to engage the head  302   f  of bone anchor  300   f  in order to turn bone anchor  300   f  during implantation. Heated probe  840  may be fixed in wrench  800  or removable. Shaft  842  extends beyond head  804  through continuous bore  350  and out of tip aperture  356  of bone anchor  300   f . Shaft  842  has a length selected such that heated tip  844  protrudes beyond the tip  304   f  of bone anchor  300   f.    
     In use, the physician operates power/temperature controller  846  to raise the temperature of heated tip  844  above the glass transition temperature of bone cement. The physician utilizes wrench  800  to drive bone anchor  300   f  into the vertebra. Heated tip  844  heats the bone cement adjacent the tip  304   f  of bone anchor  300   f . Melted bone cement flows away from heated tip  844  as heated tip  844  is introduced with bone anchor  300   f  creating the distal bore simultaneous with implantation. Heated probe  840  and/or bone anchor  300   f  are, in some embodiments, provided with channels and/or grooves which allow melted bone cement to flow towards the proximal bore  232  during implantation. When the bone anchor has been implanted to its desired position in the bone, heated probe  840  and wrench  800  are removed. In this embodiment the distal bore is formed simultaneously with the implantation of the bone anchor. 
       FIG. 8B  illustrates a cannulated bone anchor  300   f  similar to that previously described with respect to  FIG. 3F  in conjunction with a heating system  850  for heating the tip  304   f  of bone anchor  300   f . As shown in  FIG. 8B , continuous bore  350  extends from head  302   f  but terminates just before the surface of tip  304   f . Bone anchor  300   f  has, in this embodiment, no tip aperture. A power/temperature controller  856  is coupled to tip  854  through fiber  852 . The power/temperature controller  856  provides one of electrical, ultrasonic or electromagnetic energy to heat the tip  304   f  of bone anchor  300   f . In some embodiments, fiber  852  is inserted through a channel  802  in a wrench  800  having a head  804  adapted to engage the head  302   f  of bone anchor  300   f  in order to turn bone anchor  300   f  during implantation. Fiber  852  may be fixed in wrench  800  or removable. Fiber  852  extends beyond head  804  through continuous bore  350  of bone anchor  300   f . Fiber  852  has a length selected such that tip  854  is just proximal of the distal end of continuous bore  350 . Tip  854  is designed to deliver heat energy to tip  304   f  of bone anchor  300   f  thereby raising the temperature of tip  304   f  of bone anchor  300   f.    
     In one embodiment fiber  852  is an optical fiber which transmits laser light from power/temperature controller  856  to tip  854 . Tip  854  is designed to emit the laser light such that it is incident upon and heats the tip  304   f  of bone anchor  300   f . Power/temperature controller  856  monitors tip temperature by assessing electromagnetic radiation returned through fiber  852 . In this way, closed-loop temperature control of the tip  304   f  of bone anchor  300   f  can be achieved. 
     In use, the physician operates heating system  850  to raise the temperature of tip  304   f  above the glass transition temperature of bone cement. The physician utilizes wrench  800  to drive bone anchor  300   f  into the vertebra. Heated tip  304   f  heats the bone cement adjacent the tip  304   f  of bone anchor  300   f . Melted bone cement flows away from heated tip  304   f  creating the distal bore simultaneous with implantation. Bone anchor  300   f  is, in some embodiments, provided with channels and/or grooves which allow melted bone cement to flow towards the proximal bore during implantation. When the bone anchor  300   f  has been implanted to its desired position in the bone, heating system  850  and wrench  800  are removed. In this embodiment, the distal bore is formed simultaneously with the implantation of the bone anchor. However, the heated tip of a bone anchor may also be used to anneal/fuse the walls of a pre-drilled/preformed distal bore. 
       FIG. 8C  illustrates an alternative method for creating a distal bore in conjunction with implantation of a bone anchor. As before, the proximal bore is created using conventional methods for creating a bore in a vertebra, e.g. a blunt probe or drill. For example, a probe can be passed through the pedicle without excessive force until it contacts bone cement. When the probe contacts bone cement, it is removed and a rotary probe  860  is inserted through a channel  802  in a wrench  800  having a head  804  adapted to engage the head  302   f  of bone anchor  300   f  in order to turn bone anchor  300   f  during implantation. Rotary probe  860  includes a shaft  862  and burr tip  864 . A driver  866  (for example, an electrical motor) is coupled to burr tip  864  through shaft  862 . The driver  866  rotates shaft  862  and burr tip  864  at high speed. Rotary probe  860  may be fixed in wrench  800  or removable. Shaft  862  extends beyond head  804  through continuous bore  350  and out of tip aperture  356  of bone anchor  300   f . Shaft  862  has a length selected such that burr tip  864  protrudes beyond the tip  304   f  of bone anchor  300   f.    
     In use, the physician operates driver  866  to rotate the burr tip  864  at high speed. Friction between burr tip  864  and bone cement adjacent tip  304   f  raises the temperature of burr tip  864  and the bone cement above the glass transition temperature of the bone cement. The burr tip advances through the bone cement as the physician utilizes wrench  800  to rotate bone anchor  300   f . The bone cement flows away from burr tip  864  as burr tip  864  is introduced, creating the distal bore simultaneous with implantation. Bone anchor  300   f  is, in some embodiments, provided with channels and/or grooves which allow melted bone cement to flow towards the proximal bore. When the bone anchor has been implanted in the desired position, rotary probe  860  and wrench  800  are removed. In this procedure burr tip  864  is used to melt the bone cement during implantation of the bone anchor thereby reducing the possibility of fracture. In this embodiment distal bore may be formed simultaneously with the implantation of the bone anchor. 
       FIG. 8D  illustrates an alternative method for creating a distal bore in conjunction with implantation of a bone anchor. As before, the proximal bore is created using conventional methods for creating a bore in a vertebra, e.g. a blunt probe or drill. For example, a probe can be passed through the pedicle without excessive force until it contacts bone cement. When the probe contacts bone cement, it is removed and an ultrasonic probe  870  is inserted through a channel  802  in a wrench  800  having a head  804  adapted to engage the head  302   f  of bone anchor  300   f  in order to turn bone anchor  300   f  during implantation. Ultrasonic probe  870  includes a shaft  872  and ultrasonic tip  874 . An ultrasonic transducer  876  is coupled to ultrasonic tip  874  through shaft  872 . The ultrasonic transducer  876  provides ultrasound vibrations through shaft  872  to ultrasonic tip  874 . Ultrasonic probe  870  may be fixed in wrench  800  or removable. Shaft  872  extends beyond head  804  through continuous bore  350  and out of tip aperture  356  of bone anchor  300   f . Shaft  872  has a length selected such that ultrasonic tip  874  protrudes beyond the tip  304   f  of bone anchor  300   f.    
     In use, the physician operates ultrasonic transducer  876  to vibrate the ultrasonic tip  874  at high frequency. High frequency vibration at the region of contact between ultrasonic tip  874  and bone cement adjacent tip  304   f  raises the temperature of ultrasonic tip  874  and the bone cement above the glass transition temperature of the bone cement. The ultrasonic tip  874  advances through the bone cement as the physician utilizes wrench  800  to rotate bone anchor  300   f . The bone cement flows away from ultrasonic tip  874  as ultrasonic tip  874  is introduced—creating the distal bore simultaneous with implantation. Bone anchor  300   f  is, in some embodiments, provided with channels and/or grooves which allow melted bone cement to flow towards the proximal bore. When the bone anchor has been implanted in the desired position, ultrasonic probe  870  and wrench  800  are removed. In this procedure ultrasonic tip  874  is used to melt or soften the bone cement during implantation of the bone anchor thereby reducing the possibility of fracture. In this embodiment distal bore may be formed simultaneously with the implantation of the bone anchor. 
     Heated Tip Bone Anchors 
     In alternative embodiments of the present invention, the bone anchor is provided with an integrated heated tip which is adapted to heat the bone cement adjacent the heated tip thereby softening and/or melting the bone cement to facilitate implantation of the bone anchor into bone cement without fracturing the bone cement. The heated tip can be utilized to entirely create the distal bore simultaneous with implantation. Alternatively, the distal bore (or a pilot bore) can be created in a preliminary step and the heated tip can be used to fuse and/or anneal the bone cement adjacent the bore preventing propagation of any fractures. The integrated heated tip can be, for example, a thermoelectrically heated tip, ultrasonically heated tip, or mechanically heated tip. 
     A thermoelectric heated tip converts electrical energy into heat energy which is then transmitted by conduction into the bone cement to soften and/or melt the bone cement adjacent the tip of the bone anchor during implantation. The thermoelectric tip can be blunt, tapered, or sharp, and can include the screw tip features previously disclosed including, but not limited to, one or more of threads, flutes, grooves, self tapping, drill, and a distal aperture. In preferred embodiments, two electrical conductors pass along the length of the bone anchor to the tip. The bone anchor shaft is used as one of the two conductors in some embodiments. The two conductors are coupled to a power supply which supplies electrical current to the thermoelectric tip which converts electrical energy into heat energy which heats the thermoelectric tip and the bone cement with which it is in contact. The thermoelectric tip may include one or more resistive heating elements which produce heat in response to an electrical current. The resistive heating elements can be formed from a material having a higher resistivity than the conductors and/or in a shape and size that has a higher resistance than the conductors such that heat is generated in the resistive elements rather than the conductors. If the material of the resistive element is not biocompatible the resistive elements are preferably encased or enclosed in a biocompatible material, for example, stainless steel or titanium. In preferred embodiments, the temperature of the thermoelectric tip is regulated such that it remains at a temperature suitable for softening and/or melting bone cement during implantation of the bone anchor without damaging surrounding tissues or burning the bone cement. 
       FIG. 9A  illustrates a variation  910  of the cannulated bone anchor  300   f  previously described with respect to  FIG. 3F  in which the tip  304   f  is replaced and/or augmented with an integrated thermoelectric tip  914 . A pair of conductors  912  (for example, insulated wires) pass through continuous bore  350  from thermoelectric tip  914  to head  302   f . An electrical connector  916  provides for releasable connection of conductors  912  to a power supply  900 . Power supply  900  is, thus, coupled to thermoelectric tip  914  via electrical connector  916  by conductors  912 . The power supply  900  provides electrical energy to heat thermoelectric tip  914 . Integrated thermoelectric tip  914  converts electrical energy into heat energy which is then transmitted by conduction into the bone cement to soften and/or melt the bone cement adjacent the tip of the bone anchor during implantation. For example, in one embodiment thermoelectric tip  914  includes one or more resistive heating elements. Power supply  900  drives an electrical current through the one or more resistive heating elements which generate heat in response. For example, in one embodiment, thermoelectric tip  914  includes one or more resistive heating elements. Power supply  900  drives an electrical current through the one or more resistive heating elements which generate heat in response. In an embodiment, the connection between conductors  912  and thermoelectric tip  914  are releasable such that the conductors  912  can be disconnected from thermoelectric tip  914  by pulling the proximal end of conductors  912  such that conductors  912  are removed from bone anchor  910  after implantation and are, therefore, not permanently implanted in the patient. 
       FIG. 9B  illustrates a variation  920  of the cannulated bone anchor  300   f  previously described with respect to  FIG. 3F  in which the tip  304   f  is replaced and/or augmented with an integrated thermoelectric tip  924 . Thermoelectric tip  924  can be blunt, tapered, or sharp, and can include the screw tip features previously disclosed including, but not limited to, one or more of threads, flutes, grooves, self tapping, drill, and a distal aperture. A single conductor  922  (for example, a titanium or stainless rod) passes through continuous bore  350  from thermoelectric tip  924  to head  302   f . Conductor  922  may be spaced from shaft  306   f  by an air gap  928  to prevent short circuit. Alternatively, a sleeve made from an insulating biocompatible material (e.g. PEEK) is used to surround conductor  922 . An electrical connector  926  provides for releasable connection of conductor  922  and shaft  306   f  to a power supply  900 . Power supply  900  is, thus, coupled to thermoelectric tip  924  via electrical connector  916  through shaft  306   f  and conductor  922 . The power supply  900  provides electrical energy to heat thermoelectric tip  924 . Integrated thermoelectric tip  924  converts electrical energy into heat energy which is then transmitted by conduction into the bone cement to soften and/or melt the bone cement adjacent the tip  924  of the bone anchor  300   f  during implantation. For example, in one embodiment thermoelectric tip  924  includes one or more resistive heating elements. Power supply  900  drives an electrical current through the one or more resistive heating elements which generate heat in response. In an embodiment, the connection between conductor  922  and thermoelectric tip  924  is releasable such that the conductor  922  can be disconnected from thermoelectric tip  924  by pulling the proximal end of conductor  922  such that conductor  922  is removed from bone anchor  920  after implantation and is, therefore, not permanently implanted in the patient. 
       FIG. 9C  illustrates a variation  930  of the polyaxial pedicle screw  660   c  previously described with respect to  FIG. 6C  in which the tip  104  is replaced and/or augmented with an integrated thermoelectric tip  934 . One or more conductors  932  (for example, insulated wire(s)) pass through shaft  106  from thermoelectric tip  934  to a rotary electrical connector  936 . Rotary electrical connector  936  provides for releasable connection of conductor(s)  932  to a power supply  900 . Rotary electrical connector  936  is designed to rotate independent of shaft  106  while maintaining an electrical connection with conductor(s)  932  thereby allowing bone anchor  930  to be turned during implantation without interference from the connection to power supply  900 . Power supply  900  is, thus, coupled to thermoelectric tip  934  via rotary electrical connector  936  through conductor(s)  932 . The power supply  900  provides electrical energy to heat thermoelectric tip  934 . Integrated thermoelectric tip  934  converts electrical energy into heat energy which is then transmitted by conduction into the bone cement to soften and/or melt the bone cement adjacent the tip of the bone anchor during implantation. For example, in one embodiment thermoelectric tip  934  includes one or more resistive heating elements. Power supply  900  drives an electrical current through the one or more resistive heating elements which generate heat in response. In an embodiment, the connection between rotary electrical connector  936  and shaft  106  is releasable such that the rotary electrical connector  936  can be disconnected from shaft  106  after implantation. 
       FIG. 9D  illustrates a variation  940  of the cannulated bone anchor  300   f  previously described with respect to  FIG. 3F  in which the tip  304   f  has no tip aperture but is augmented with an integrated thermoelectric element  944 . Tip  304   f  can be blunt, tapered, or sharp, and can include the screw tip features previously disclosed including, but not limited to, one or more of threads, flutes, grooves, self tapping, drill, and a distal aperture. A single conductor  942  (for example, a titanium rod, a stainless rod, or a copper wire) passes through continuous bore  350  from thermoelectric element  944  to head  302   f . Conductor  942  may be spaced from shaft  306   f  by an air gap  948  to prevent short circuit. Alternatively, a sleeve made from an insulating biocompatible material (e.g. PEEK) is used to surround conductor  942 . An electrical connector  946  provides for releasable connection of conductor  942  and shaft  306   f  to a power supply  900 . Power supply  900  is, thus, coupled to thermoelectric element  944  via electrical connector  946  through shaft  306   f  and conductor  942 . The power supply  900  provides electrical energy to heat thermoelectric element  944  which thereby heats tip  304   f . Thermoelectric element  944  converts electrical energy into heat energy which is then transmitted by conduction through tip  304   f  into bone cement to soften and/or melt the bone cement adjacent the tip of the bone anchor during implantation. For example, in one embodiment, thermoelectric element  944  is a high resistivity material, for example, Nichrome 80/20, Kanthal, Cupronickel alloy, Molybedenum disilicide, or PTC ceramic. Power supply  900  drives an electrical current through the high resistivity material which generates heat in response. In an embodiment, the connection between conductor  942  and thermoelectric element  944  is releasable such that the conductor  942  can be disconnected from thermoelectric element  944  by pulling the proximal end of conductor  942  such that conductor  942  is removed from bone anchor  940  after implantation and is, therefore, not permanently implanted in the patient. 
     In using a bone anchor having a thermoelectric tip as disclosed, for example, in  FIGS. 9A-9D , the physician connects the power supply to the electrical connector of the bone anchor. The physician then operates the power supply  900  to raise the temperature of the thermoelectric tip to a temperature suitable for softening and/or melting bone cement. The physician utilizes a wrench to drive the bone anchor into the bone cement while the thermoelectric tip is maintained at the desired temperature. The thermoelectric tip heats the bone cement adjacent the thermoelectric tip. Melted/softened bone cement flows away from the thermoelectric tip as the thermoelectric tip is driven into the bone thereby creating a bore simultaneous with implantation. The thermoelectric tip and/or shaft of the bone anchor are, in some embodiments, provided with channels and/or grooves which allow softened/melted bone cement to flow away from the thermoelectric tip during implantation of the bone anchor. When the bone anchor has been implanted to its desired position in the bone, the power supply  900  is disconnected from the electrical connector. 
     Power supply  900  can be a conventional surgical power supply commonly available in an operating room, for example, a bovie or cautery power supply. However, in a preferred embodiment, the temperature of the thermoelectric tip is monitored and regulated by power supply  900  such that thermoelectric tip achieves, and remains at a temperature suitable for softening and/or melting bone cement during implantation of the bone anchor without damaging surrounding tissues or burning the bone cement. For example, in the thermoelectric tip can include one or more resistive heating elements. Power supply  900  drives an electrical current through the one or more resistive heating elements which generate heat in response. Power supply  900  can preferably monitor the resistance of the resistive heating elements in order to assess the temperature of the thermoelectric tip and modulate the supplied current in order to achieve and regulate a desired temperature of the thermoelectric tip. The temperature necessary for melting bone cement is variable dependent upon the composition of the bone cement. Thus, in some embodiments, the power supply  900  includes a control for selecting the temperature to which the thermoelectric tip is raised—for example, between 100° C. and 200° C. 
       FIG. 9E  illustrates a variation  950  of the cannulated bone anchor  300   f  previously described with respect to  FIG. 3F  in which the tip  304   f  is replaced with an integrated burr tip  954 . Burr tip  954  can be blunt, tapered, or sharp, and can include the screw tip features previously disclosed including, but not limited to, one or more of threads, flutes, grooves, self tapping, drill, and a distal aperture. A shaft  952  (for example, a titanium rod or stainless steel rod) passes through continuous bore  350  from burr tip  954  to head  302   f . The proximal end of shaft  952  includes a mechanical power coupling  953  (for example, a square or hex socket or shaft end). Shaft  952  can be formed in one piece with burr tip  954  from titanium. A snap-ring/bushing  955  secures burr tip  954  and shaft  952  within bone anchor  300   f  and/or reduces the friction between coupling  953  and head  302   f . Another bushing  957  optionally reduces friction between the distal end of shaft  306   f  and burr tip  954 . Burr tip  954 , shaft  952  and coupling  953  rotate as one unit and can rotate independently of shaft  306   f.    
     During implantation, the physician utilizes a wrench  960  which has a head  964  adapted to engage socket  308   f  of bone anchor  300   f  in order to turn bone anchor  300   f . Wrench  960  includes a motor  969  coupled to drive shaft  962  which has at its distal end mechanical power coupling  968  designed to engage the mechanical power coupling  953  of bone anchor  950 . When engaged motor  969  can be operated to rotate the burr tip  954  at high speed, friction between burr tip  954  and bone cement adjacent burr tip  954  raises the temperature of burr tip  954  and the bone cement softening and/or melting the bone cement. The burr tip  954  advances through the bone cement as the physician utilizes wrench  960  to rotate bone anchor  950  independent of the rotation of burr tip  954 . The bone cement flows away from burr tip  954  as burr tip  954  is introduced, creating the distal bore simultaneous with implantation. Bone anchor  950  is, in some embodiments, provided with channels and/or grooves which allow melted bone cement to flow away from burr tip  954 . When the bone anchor has been implanted in the desired position, wrench  960  is removed. In this procedure burr tip  954  is used to soften and/or melt the bone cement during implantation of the bone anchor thereby reducing the possibility of fracture. 
       FIG. 9F  illustrates a variation  970  of the cannulated bone anchor  300   f  previously described with respect to  FIG. 3F  in which the tip  304   f  is replaced with an integrated ultrasound tip  974 . Ultrasound tip  974  can be blunt, tapered, or sharp, and can include the screw tip features previously disclosed including, but not limited to, one or more of threads, flutes, grooves, and a distal aperture. A shaft  972  (for example, a titanium rod or stainless steel rod) passes through continuous bore  350  from ultrasound tip  974  to head  302   f . The proximal end of shaft  972  includes an ultrasound coupling  973 , for example, a socket or shaft end. Shaft  972  can be formed in one piece with ultrasound tip  974  from titanium. A snap-ring/bushing  975  secures ultrasound tip  974  and shaft  972  within bone anchor  300   f  and/or vibrationally isolates coupling  973  from head  302   f . Another bushing  977  optionally vibrationally isolates the distal end of shaft  306   f  and ultrasound tip  974 . Ultrasound tip  974 , shaft  972  and coupling  973  can vibrate ultrasonically independent of vibration of shaft  306   f.    
     During implantation, the physician utilizes a wrench  980  which has a head  984  adapted to engage socket  308   f  of bone anchor  300   f  in order to turn bone anchor  300   f . Wrench  980  includes an ultrasound transducer  989  coupled to shaft  982  which has at its distal end an ultrasound coupling  988  designed to engage the ultrasound coupling  973  of bone anchor  970 . When engaged, ultrasound transducer  989  can be operated to send ultrasound vibrations to ultrasound tip  974  via shaft  982 . (In an alternative embodiment, ultrasound frequency vibrations are induced directly in ultrasound coupling  973  by a device located in the head  984  of wrench  980 .) Friction caused by high frequency vibration between ultrasound tip  974  and bone cement adjacent ultrasound tip  974  raises the temperature of ultrasound tip  974  and/or the bone cement softening and/or melting the bone cement. The ultrasound tip  974  advances through the bone cement as the physician utilizes wrench  980  to rotate bone anchor  970 . The bone cement flows away from ultrasound tip  974  as ultrasound tip  974  is introduced, creating the distal bore simultaneous with implantation. Bone anchor  970  is, in some embodiments, provided with channels and/or grooves which allow melted bone cement to flow away from ultrasound tip  974 . When the bone anchor has been implanted in the desired position, wrench  980  is removed. In this procedure, ultrasound tip  974  is used to soften and/or melt the bone cement during implantation of the bone anchor thereby reducing the possibility of fracture of the bone cement. 
       FIGS. 10A and 10B  depict perspective views of an embodiment of the bone cutting tool  1000  of the invention with the first cutting blade  1002  and the second cutting blade  1004  in the non-expanded and expanded positions, respectively. Further,  FIGS. 11A ,  11 B and  12 C depict side views of an embodiment of the bone cutting tool  1000  of the invention with the first cutting blade  1002  and the second cutting blade  1004  in the non-expanded and expanded positions, respectively. It is to be understood that an alternative embodiment of the invention can include a single cutting blade that works, for example, like the first cutting blade or the second cutting blade. For all the embodiments described herein, the edges of the blades, such as blades  1002  and  1004  can be sharpened or tapered in order to enhance the cutting ability of the blades. 
     The first and second blades are formed in an outer tube  1006  which has a distal end  1008  and a proximal end  1010 . As the first and second cutting blade  1002 ,  1004  are formed from a tube, in a preferred embodiment, in a plane perpendicular to and over the longitudinal axis, the blades are curved. The proximal end  1010  of the outer tube  1006  is secured to a handle  1011 . An inner rod  1012  is positioned in the outer tube  1006 . The inner rod  1012  includes a distal end  1014  which is secured to the distal end  1008  of the outer tube  1006  and a proximal end  1016  (shown in  FIG. 12A ) which is secured relative to the proximal end of the outer tube  1002  to the handle  1011 . The inner rod  1012  also has a longitudinal axis  1015  which serves additionally as the longitudinal axis of the tool  1000 . The outer tube and the inner rod can be stainless steel, or a superelastic material such Nitinol (Niti), or titanium. Alternatively, the tube and cutting blades can be made of a superelastic material and the inner rod can be made of stainless steel or titanium. In a preferred embodiment of the invention, the cutting blades are made of a superelastic material such as Nitinol so that the cutting blades can flexibly expand and contract. 
     As can be seen, for example, in the combination of  FIGS. 10A ,  10 B,  11 A,  11 B and  FIG. 12A , the handle  1011  includes a first part  1018  which is secured to the proximal end  1016  of the inner rod  1012 . The handle  1011  also includes a second part  1020  and a third part  1022 . The second part  1020  of the handle  1011  is secured to the third part  1022  of the handle  1011  by a ring  1024  that fits into grooves shown in the second part  1020  and the third part  1022  of the handle  1011 . Further, the second part  1020  of the handle  1011  can rotate relative to the third part  1022  of the handle  1011  due to the ring  1024 . As indicated above, the first part  1018  of the handle  1011  is secured to said proximal end  1016  of the inner rod  1012 . The third part  1022  of the handle  1011  includes a first bore  1026  (shown in  FIG. 12 ) which slidingly receives a distal end  1030  of the first part  1018  of the handle  1011  and a second bore  1028  which receives the proximal end of the inner rod  1012 . The first bore  1026  communicates with the second bore  1028 . The second part  1020  of the handle  1011  includes a threaded third bore  1034  which receives a threaded proximal end  1032  of the first part  1018  of the handle  1011 . 
     Accordingly, rotation of the second part  1020  of the handle  1011  relative to the third part  1022  of the handle  1011  causes the first part  1018  of the handle  1011  to move causing the rod  1012  to move relative to the outer tube  1006 . Rotation of the second part  1020  of the handle  1011  in one direction causes the inner rod to move distally relative to the outer tube and rotation of the second part of the handle in the opposite direction causes the inner rod to move proximally relative to the outer tube. 
     As can be seen in  FIGS. 10B and 11B , movement of the inner rod in a proximal direction towards the handle  1011  causes the first blade  1002  and the second blade  1004 , respectively, to move to an expanded configuration. As depicted in  FIGS. 10A and 11A  movement of the inner rod distally relative to the outer tube causes the first and second cutting blades to contrast to the original shape of the tube. 
     In a preferred embodiment of the invention, the cutting blades are made of a superelastic material such as Nitinol so that the cutting blades can flexibly expand and contract. 
     As can be seen in  FIG. 14 , the first part  1018  of the handle  1011  includes “D” shaped ends  1036  that fit into a corresponding shaped first bore  1026  of  FIG. 12A  of the third part  1022  of the handle  1011  to prevent the first part  1018  and the inner rod  1002  from rotating when the second part of the handle rotates relative to the first part of the handle  1011 . Other features such as the use of a single “D” shaped end and a longitudinal key (not shown) could prevent the rod from rotating relative to the outer tube. 
     As can be seen in  FIGS. 12A ,  12 B and  13 , the first and second cutting blades  1002 ,  1004  include recesses or weakened portions or sections that promote bending of one portion of each of the first and second cutting blades relative to another portion of the respective first and second cutting blades. First cutting blade  1002  includes end recesses  1038   a  and  1038   b  as well as middle recesses  1038   c  and  1038   d . Second cutting blade  1004  includes end recesses  1040   a  and  1040   b  as well as middle recesses  1040   c  and  1040   d . Due to these recesses and as seen in  FIG. 13 , when the inner rod  1012  is moved relative to the outer tube  1006  in order to expand the cutting blades  1002  and  1004 , the portion of the first cutting blade  1002  between middle recesses,  1038   c  and  1038   d  and the portion of the second cutting blade  1004  between middle recesses  1040   c  and  1040   d  expand substantially in a manner to remain parallel to the longitudinal axis of the inner rod  1012 . Thus, the cutting blades take on a cylindrical shape in order to cut a cylindrical bore in the bone. This is in contrast to the more curved or somewhat parabolic shaping that the expanded bone cutting blades can take in other embodiments of the invention as shown in  FIGS. 10B and 11B  where, by way of example only, the blades are made of superelastic material. As can be seen in  FIGS. 12D and 13 , a third bone cutting blade  1005  can be formed in the outer tube  1006 . In a preferred embodiment, the blades are formed in the tube of superelastic material such as Nitinol using laser cutting techniques. 
     In an alternative embodiment, the unexpanded first and second cutting blade  1002 ,  1004  in  FIG. 12D  have a modified structure, with a broader, wider and/or flatter middle portion  1003 ,  1005  cut into the outer tube. The geometry of this cut influences the expanded shape of the first and second cutting blade  1002 ,  1004 . With a broader or wider or flatter middle section, the middle section tends to stay more flat and parallel to the longitudinal axis, than do the parabolic shaped expanded blades  1002 ,  1004  of  FIG. 10C . 
     In yet another alternative embodiment, the cutting blades can be spiral in shape and also have teeth cut into the edge of the blades or the edges of the blades can be serrated. 
     In the embodiments of the invention, it is to be understood that the outer tube with the one or more cutting blades and the inner rod can be selectively connected to the handle so that the outer tube and the inner rod can be replaceable with the reusable handle. A release mechanism for selectively connecting the outer tube with the one or more cutting blades and the inner rod to the handle are well known in the art. 
     As can be seen in  FIG. 15A , an embodiment of the method of the invention includes the following steps. At step  1060 , a bore is created in the bone or an opening is identified in the bone. At step  1062 , the bone cutting tool  1000  is inserted into the bore or the identified opening. The tool may be rotated to remove or cut away bone. At step  1064 , the first and second blades are expanded and the tool is further rotated to remove bone. At step  1066 , the first and second blades are further expanded and rotated and this is continued until the bore in the bone achieves the desired size. At step  1068 , the bone cutting tool is removed from the bore. Such a removal step may require the cutting blades to be contracted using the handle  1011 . At step  1070 , a bone screw is introduced into the bore and either one or both of bone cement is introduced into the bore between the bore and the bone screw and/or the bone cement is introduced through channels, bores and ports formed in the screw (see  FIG. 3F ) and into the bore. The bone cement is allowed to flow into the porous bone to dry, thereby securing the bone screw to the bone. It is to be understood that in practice, the bone screw will have threads thereof engage some portions of the bore, but not other portions, as the bore is formed in porous bone. Thus, the bone cement will ensure that the voids in the porous bone are filled and that the thread of the bone screw will engage the bone cement if bone is not available. 
     As can be seen in  FIG. 15B , an embodiment of the method of the invention includes the following steps. At step  1160 , a bore is created in the bone or an opening is identified in the bone. At step  1162 , the bone cutting tool  1000  is inserted into the bore or the identified opening. The tool may be rotated to remove or cut away bone. At step  1164 , the first and second blades are expanded and the tool is further rotated to remove bone. At step  1166 , the first and second blades are further expanded and rotated and this is continued until the bore in the bone achieves the desired size. At step  1168 , the bone cutting tool is removed from the bore. Such a removal step may require the cutting blades to be contracted using the handle  1011 . At step  1170 , the bore is filled with bone cement and the bone cement is allowed to dry. At step  1172 , a bore is drilled or created in the dried bone cement. At step  1174 , a bone screw is inserted into the bore in the bone cement. 
     It is also to be understood that the system and method of embodiments of the invention can be used to create and expand bores in the other tissue of the body in addition to creating and expanding bores in the bone of the vertebral body. For example, the system and method of embodiments of the invention can be used to create and expand bores in the disks that are located between the vertebral bodies of the spine. Further embodiments of the inventions can be used to create and expand bores in other soft tissue and bone of the body. 
     Materials 
     The bone anchor, implantation tools, deflectable post, spinal rods, spinal plates, and other spinal implant components are preferably made of biocompatible and/or implantable metals. The bone anchor and implantation tools can, for example, be made of titanium, titanium alloy, cobalt chrome alloy, a shape memory metal, for example, Nitinol (NiTi) or stainless steel. In preferred embodiments, the bone anchor is made of titanium alloy; however, other materials, for example, stainless steel may be used instead of, or in addition to, the titanium\titanium alloy components. Typically, the tip, proximal shaft, distal shaft, and head (or at least that portion of the head attached to the proximal shaft) are formed in one piece from titanium\titanium alloy\stainless steel. The bone anchor may be cast and/or molded in one piece and/or machined from a block of metal using methods known in the art. In alternative embodiments one or more elements of the bone anchor are formed separately and then joined to the other components during manufacturing. 
     The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. 
     The particular bone anchor embodiments shown herein are provided by way of example only. The bone anchors have been described with particular reference to spinal stabilization, however, the invention disclosed herein and bone anchors embodying it may find application in any bone or orthopedic application where a bone anchor/bone screw is desired to be secured in a bone which includes hardened bone cement. It is an aspect of preferred embodiments of the present invention that a range of bone anchors are provided (for example, in a kit) and that different of the bone anchors have different combinations of the shafts, tips, heads and other features disclosed herein. Particular bone anchors may incorporate any combination of the shafts, tips, heads and other features disclosed herein, and in the application incorporated by reference, and standard spinal stabilization and/or fusion components, for example, screws, pedicle screws, polyaxial screws and rods. Additionally, any of the implantation tools and methods described herein and in the related application incorporated by reference can be used or modified for use with such bone anchors. It is intended that the scope of the invention be defined by the claims and their equivalents.