Patent Publication Number: US-8968366-B2

Title: Method and apparatus for flexible fixation of a spine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 10/728,566, entitled “A Method And Apparatus For Flexible Fixation Of A Spine,” filed Dec. 5, 2003, which claims the benefit of priority under 35 U.S.C. §119(a) to Korean Application Ser. No. 2003-0066108, entitled “Dynamic Spinal Fixation Device,” filed on. Sep. 24, 2003, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and system for fixing and stabilizing a spinal column and, more particularly, to a method and system of spinal fixation in which one or more screw type fixing members are implanted and fixed into a portion of a patient&#39;s spinal column and flexible, semi-rigid rods or plates are connected and fixed to the upper ends of the fixing members to provide dynamic stabilization of the spinal column. 
     2. Description of the Related Art 
     Degenerative spinal column diseases, such as disc degenerative diseases (DDD), spinal stenosis, spondylolisthesis, and so on, need surgical operation if they do not take a turn for the better by conservative management. Typically, spinal decompression is the first surgical procedure that is performed. The primary purpose of decompression is to reduce pressure in the spinal canal and on nerve roots located therein by removing a certain tissue of the spinal column to reduce or eliminate the pressure and pain caused by the pressure. If the tissue of the spinal column is removed the pain is reduced but the spinal column is weakened. Therefore, fusion surgery (e.g., ALIF, PLIF or posterolateral fusion) is often necessary for spinal stability following the decompression procedure. However, following the surgical procedure, fusion takes additional time to achieve maximum stability and a spinal fixation device is typically used to support the spinal column until a desired level of fusion is achieved. Depending on a patient&#39;s particular circumstances and condition, a spinal fixation surgery can sometimes be performed immediately following decompression, without performing the fusion procedure. The fixation surgery is performed in most cases because it provides immediate postoperative stability and, if fusion surgery has also been performed, it provides support of the spine until sufficient fusion and stability has been achieved. 
     Conventional methods of spinal fixation utilize a rigid spinal fixation device to support an injured spinal part and prevent movement of the injured part. These conventional spinal fixation devices include: fixing screws configured to be inserted into the spinal pedicle or sacral of the backbone to a predetermined depth and angle, rods or plates configured to be positioned adjacent to the injured spinal part, and coupling elements for connecting and coupling the rods or plates to the fixing screws such that the injured spinal part is supported and held in a relatively fixed position by the rods or plates. 
     U.S. Pat. No. 6,193,720 discloses a conventional spinal fixation device, in which connection members of a rod or plate type are mounted on the upper ends of at least one or more screws inserted into the spinal pedicle or sacral of the backbone. The connection units, such as the rods and plates, are used to stabilize the injured part of the spinal column which has been weakened by decompression. The connection units also prevent further pain and injury to the patient by substantially restraining the movement of the spinal column. However, because the connection units prevent normal movement of the spinal column, after prolonged use, the spinal fixation device can cause ill effects, such as “junctional syndrome” (transitional syndrome) or “fusion disease” resulting in further complications and abnormalities associated with the spinal column. In particular, due to the high rigidity of the rods or plates used in conventional fixation devices, the patient&#39;s fixed joints are not allowed to move after the surgical operation, and the movement of the spinal joints located above or under the operated area is increased. Consequently, such spinal fixation devices cause decreased mobility of the patient and increased stress and instability to the spinal column joints adjacent to the operated area. 
     It has been reported that excessive rigid spinal fixation is not helpful to the fusion process due to load shielding caused by rigid fixation. Thus, trials using load sharing semi-rigid spinal fixation devices have been performed to eliminate this problem and assist the bone fusion process. For example, U.S. Pat. No. 5,672,175, U.S. Pat. No. 5,540,688, and U.S. Pub No 2001/0037111 disclose dynamic spine stabilization devices having flexible designs that permit axial load translation (i.e., along the vertical axis of the spine) for bone fusion promotion. However, because these devices are intended for use following a bone fusion procedure, they are not well-suited for spinal fixation without fusion. Thus, in the end result, these devices do not prevent the problem of rigid fixation resulting from fusion. 
     To solve the above-described problems associated with rigid fixation, non-fusion technologies have been developed. The Graf band is one example of a non-fusion fixation device that is applied after decompression without bone fusion. The Graf band is composed of a polyethylene band and pedicle screws to couple the polyethylene band to the spinal vertebrae requiring stabilization. The primary purpose of the Graf band is to prevent sagittal rotation (flexion instability) of the injured spinal parts. Thus, it is effective in selected cases but is not appropriate for cases that require greater stability and fixation. See, Kanayarna et al, Journal of Neurosurgery 95(1 Suppl):5-10, 2001, Markwalder &amp; Wenger, Acta Neurochrgica 145(3):209-14.). Another non-fusion fixation device called “Dynesys” has recently been introduced. See Stoll et al, European Spine Journal 11 Suppl 2:S170-8, 2002, Schmoelz et al, J of spinal disorder &amp; techniques 16(4):418-23, 2003. The Dynesys device is similar to the Graf band except it uses a polycarburethane spacer between the screws to maintain the distance between the heads of two corresponding pedicle screws and, hence, adjacent vertebrae in which the screws are fixed. Early reports by the inventors of the Dynesys device indicate it has been successful in many cases. However, it has not yet been determined whether the Dynesys device can maintain long-term stability with flexibility and durability in a controlled study. Because it has polyethylene components and interfaces, there is a risk of mechanical failure. Furthermore, due to the mechanical configuration of the device, the surgical technique required to attach the device to the spinal column is complex and complicated. 
     U.S. Pat. Nos. 5,282,863 and 4,748,260 disclose a flexible spinal stabilization system and method using a plastic, non-metallic rod. U.S. patent publication no. 2003/0083657 discloses another example of a flexible spinal stabilization device that uses a flexible elongate member. These devices are flexible but they are not well-suited for enduring long-term axial loading and stress. Additionally, the degree of desired flexibility vs. rigidity may vary from patient to patient. The design of existing flexible fixation devices are not well suited to provide varying levels of flexibility to provide optimum results for each individual candidate. For example, U.S. Pat. No. 5,672,175 discloses a flexible spinal fixation device which utilizes a flexible rod made of metal alloy and/or a composite material. Additionally, compression or extension springs are coiled around the rod for the purpose of providing de-rotation forces on the vertebrae in a desired direction. However, this patent is primarily concerned with providing a spinal fixation device that permits “relative longitudinal translational sliding movement along [the] vertical axis” of the spine and neither teaches nor suggests any particular designs of connection units (e.g., rods or plates) that can provide various flexibility characteristics. Prior flexible rods such as that mentioned in U.S. Pat. No. 5,672,175 typically have solid construction with a relatively small diameter in order to provide a desired level of flexibility. Because they are typically very thin to provide suitable flexibility, such prior art rods are prone to mechanical failure and have been known to break after implantation in patients. 
     Therefore, conventional spinal fixation devices have not provided a comprehensive and balanced solution to the problems associated with curing spinal diseases. Many of the prior devices are characterized by excessive rigidity, which leads to the problems discussed above while others, though providing some flexibility, are not well-adapted to provide varying degrees of flexibility. Additionally, existing flexible fixation devices utilize non-metallic components that are not proven to provide long-term stability and durability. Therefore, there is a need for an improved dynamic spinal fixation device that provides a desired level of flexibility to the injured parts of the spinal column, while also providing long-term durability and consistent stabilization of the spinal column. 
     Additionally, in a conventional surgical method for fixing the spinal fixation device to the spinal column, a doctor incises the midline of the back to about 10-15 centimeters, and then, dissects and retracts it to both sides. In this way, the doctor performs muscular dissection to expose the outer part of the facet joint. Next, after the dissection, the doctor finds an entrance point to the spinal pedicle using radiographic devices (e.g., C-arm flouroscopy), and inserts securing members of the spinal fixation device (referred to as “spinal pedicle screws”) into the spinal pedicle. Thereafter, the connection units (e.g., rods or plates) are attached to the upper portions of the pedicle screws in order to provide support and stability to the injured portion of the spinal column. Thus, in conventional spinal fixation procedures, the patient&#39;s back is incised about 10˜15 cm, and as a result, the back muscle, which is important for maintaining the spinal column, is incised or injured, resulting in significant post-operative pain to the patient and a slow recovery period. 
     Recently, to reduce patient trauma, a minimally invasive surgical procedure has been developed which is capable of performing spinal fixation surgery through a relatively small hole or “window” that is created in the patient&#39;s back at the location of the surgical procedure. 
     Through the use of an endoscope, or microscope, minimally invasive surgery allows a much smaller incision of the patient&#39;s affected area. Through this smaller incision, two or more securing members (e.g., pedicle screws) of the spinal fixation device are screwed into respective spinal pedicle areas using a navigation system. Thereafter, special tools are used to connect the stabilizing members (e.g., rods or plates) of the fixation device to the securing members. Alternatively, or additionally, the surgical procedure may include inserting a step dilator into the incision and then gradually increasing the diameter of the dilator. Thereafter, a tubular retractor is inserted into the dilated area to retract the patient&#39;s muscle and provide a visual field for surgery. After establishing this visual field, decompression and, if desired, fusion procedures may be performed, followed by a fixation procedure, which includes the steps of finding the position of the spinal pedicle, inserting pedicle screws into the spinal pedicle, using an endoscope or a microscope, and securing the stabilization members (e.g., rods or plates) to the pedicle screws in order to stabilize and support the weakened spinal column. 
     One of the most challenging aspects of performing the minimally invasive spinal fixation procedure is locating the entry point for the pedicle screw under endoscopic or microscopic visualization. Usually anatomical landmarks and/or radiographic devices are used to find the entry point, but clear anatomical relationships are often difficult to identify due to the confined working space. Additionally, the minimally invasive procedure requires that a significant amount of the soft tissue must be removed to reveal the anatomy of the regions for pedicle screw insertion. The removal of this soft tissue results in bleeding in the affected area, thereby adding to the difficulty of finding the correct position to insert the securing members and causing damage to the muscles and soft tissue surrounding the surgical area. Furthermore, because it is difficult to accurately locate the point of insertion for the securing members, conventional procedures are unnecessarily traumatic. 
     Radiography techniques have been proposed and implemented in an attempt to more accurately and quickly find the position of the spinal pedicle in which the securing members will be inserted. However, it is often difficult to obtain clear images required for finding the corresponding position of the spinal pedicle using radiography techniques due to radiographic interference caused by metallic tools and equipment used during the surgical operation. Moreover, reading and interpreting radiographic images is a complex task requiring significant training and expertise. Radiography poses a further problem in that the patient is exposed to significant amounts of radiation. 
     Although some guidance systems have been developed which guide the insertion of a pedicle screw to the desired entry point on the spinal pedicle, these prior systems have proven difficult to use and, furthermore, hinder the operation procedure. For example, prior guidance systems for pedicle screw insertion utilize a long wire that is inserted through a guide tube that is inserted through a patient&#39;s back muscle and tissue. The location of insertion of the guide tube is determined by radiographic means (e.g., C-arm flouroscope) and driven until a first end of the guide tube reaches the desired location on the surface of the pedicle bone. Thereafter, a first end of the guide wire, typically made of a biocompatible metal material, is inserted into the guide tube and pushed into the pedicle bone, while the opposite end of the wire remains protruding out of the patient&#39;s back. After the guide wire has been fixed into the pedicle bone, the guide tube is removed, and a hole centered around the guide wire is dilated and retracted. Finally, a pedicle screw having an axial hole or channel configured to receive the guide wire therethrough is guided by the guide wire to the desired location on the pedicle bone, where the pedicle screw is screw-driven into the pedicle. 
     Although the concept of the wire guidance system is a good one, in practice, the guide wire has been very difficult to use. Because it is a relatively long and thin wire, the structural integrity of the guide wire often fails during attempts to drive one end of the wire into the pedicle bone, making the process unnecessarily time-consuming and laborious. Furthermore, because the wire bends and crimps during insertion, it does not provide a smooth and secure anchor for guiding subsequent tooling and pedicle screws to the entry point on the pedicle. Furthermore, current percutaneous wire guiding systems are used in conjunction with C-arm flouroscopy (or other radiographic device) without direct visualization with the use of an endoscope or microscope. Thus, current wire guidance systems pose a potential risk of misplacement or pedicle breakage. Finally, because one end of the wire remains protruding out of the head of the pedicle screw, and the patient&#39;s back, this wire hinders freedom of motion by the surgeon in performing the various subsequent procedures involved in spinal fixation surgery. Thus, there is a need to provide an improved guidance system, adaptable for use in minimally invasive pedicle screw fixation procedures under endoscopic or microscopic visualization, which is easier to implant into the spinal pedicle and will not hinder subsequent procedures performed by the surgeon. 
     As discussed above, existing methods and devices used to cure spinal diseases are in need of much improvement. Most conventional spinal fixation devices are too rigid and inflexible. This excessive rigidity causes further abnormalities and diseases of the spine, as well as significant discomfort to the patient. Although some existing spinal fixation devices do provide some level of flexibility, these devices are not designed or manufactured so that varying levels of flexibility may be easily obtained to provide a desired level of flexibility for each particular patient. Additionally, prior art devices having flexible connection units (e.g., rods or plates) pose a greater risk of mechanical failure and do not provide long-term durability and stabilization of the spine. Furthermore, existing methods of performing the spinal fixation procedure are unnecessarily traumatic to the patient due to the difficulty in finding the precise location of the spinal pedicle or sacral of the backbone where the spinal fixation device will be secured. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention addresses the above and other needs by providing an improved method and system for stabilizing an injured or weakened spinal column. 
     To overcome the deficiencies of conventional spinal fixation devices, in one embodiment, the inventor of the present invention has invented a novel flexible spinal fixation device with an improved construction and design that uses metal or metal-synthetic hybrid components to provide a desired level of flexibility, stability and durability. 
     As a result of long-term studies to reduce the operation time required for minimally invasive spinal surgery, to minimize injury to tissues near the surgical area, in another embodiment, the invention provides a method and device for accurately and quickly finding a position of the spinal column in which securing members of the spinal fixation device will be inserted. A novel guidance/marking device is used to indicate the position in the spinal column where the securing members will be inserted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of a spinal fixation device in accordance with one embodiment of the invention. 
         FIG. 2  illustrates a perspective view of spinal fixation device in accordance with another embodiment of the invention. 
         FIG. 3  illustrates an exploded view of the coupling assembly  14  of the pedicle screw  2  of  FIGS. 1 and 2 , in accordance with one embodiment of the invention. 
         FIG. 4  illustrates a perspective view of a flexible rod connection unit in accordance with one embodiment of the invention. 
         FIG. 5  illustrates a perspective view of a flexible rod connection unit in accordance with another embodiment of the invention. 
         FIG. 6  illustrates a perspective view of a flexible rod connection unit in accordance with a further embodiment of the invention. 
         FIG. 7  illustrates a perspective view of a pre-bent flexible rod connection unit in accordance with one embodiment of the invention. 
         FIG. 8  illustrates a perspective, cross-sectional view of a flexible portion of connection unit in accordance with one embodiment of the invention. 
         FIG. 9  illustrates a perspective, cross-sectional view of a flexible portion of connection unit in accordance with another embodiment of the invention. 
         FIG. 10  illustrates a perspective, cross-sectional view of a flexible portion of connection unit in accordance with a further embodiment of the invention. 
         FIG. 11  illustrates a perspective view of a flexible rod connection unit in accordance with one embodiment of the invention. 
         FIG. 12A  illustrates a perspective view of a flexible connection unit having one or more spacers in between two end portions, in accordance with one embodiment of the invention. 
         FIG. 12B  illustrates an exploded view of the flexible connection unit of  FIG. 12A . 
         FIG. 12C  provides a view of the male and female interlocking elements of the flexible connection unit of  FIGS. 12A and 12B , in accordance with one embodiment of the invention. 
         FIG. 13  shows a perspective view of a flexible connection unit, in accordance with a further embodiment of the invention. 
         FIG. 14  illustrates a perspective view of a spinal fixation device in accordance with another embodiment of the invention. 
         FIG. 15  illustrates an exploded view of the spinal fixation device of  FIG. 14 . 
         FIG. 16A  shows a perspective view of a flexible plate connection unit in accordance with one embodiment of the invention. 
         FIG. 16B  illustrates a perspective view of a flexible plate connection unit in accordance with a further embodiment of the invention. 
         FIG. 16C  shows a side view of the flexible plate connection unit of  FIG. 16A . 
         FIG. 16D  shows a top view of the flexible plate connection unit of  FIG. 16A . 
         FIG. 16E  illustrates a side view of the flexible plate connection unit of  FIG. 16A  having a pre-bent configuration in accordance with a further embodiment of the invention. 
         FIG. 17  is a perspective view of a flexible plate connection unit in accordance with another embodiment of the invention. 
         FIG. 18  illustrates a perspective view of a flexible plate connection unit in accordance with another embodiment of the invention. 
         FIG. 19  illustrates a perspective view of a hybrid rod-plate connection unit having a flexible middle portion according to a further embodiment of the present invention. 
         FIG. 20  is a perspective view of a spinal fixation device that utilizes the hybrid rod-plate connection unit of  FIG. 19 . 
         FIG. 21  illustrates a perspective view of the spinal fixation device of  FIG. 1  after it has been implanted into a patient&#39;s spinal column. 
         FIGS. 22A and 22B  provide perspective views of spinal fixation devices utilizing the plate connection units of  FIGS. 16A and 16B , respectively. 
         FIG. 23A  illustrates a perspective view of two pedicle screws inserted into the pedicles of two adjacent vertebrae at a skewed angle, in accordance with one embodiment of the invention. 
         FIG. 23B  illustrates a structural view of a coupling assembly of a pedicle screw in accordance with one embodiment of the invention. 
         FIG. 23C  provides a perspective view of a slanted stabilizing spacer in accordance with one embodiment of the invention. 
         FIG. 23D  illustrates a side view of the slanted stabilizing spacer of  FIG. 23C . 
         FIG. 23E  is a top view of the cylindrical head of the pedicle screw of  FIG. 23 . 
         FIG. 24  illustrates a perspective view of a marking and guiding device in accordance with one embodiment of the invention. 
         FIG. 25  is an exploded view of the marking and guidance device of  FIG. 24 . 
         FIG. 26A  provides a perspective, cross-section view of a patient&#39;s spine after the marking and guiding device of  FIG. 24  has been inserted during surgery. 
         FIG. 26B  provides a perspective, cross-section view of a patient&#39;s spine as an inner trocar of the marking and guiding device of  FIG. 24  is being removed. 
         FIGS. 27A and 27B  illustrate perspective views of two embodiments of a fiducial pin, respectively. 
         FIG. 28  is a perspective view of a pushing trocar in accordance with a further embodiment of the invention. 
         FIG. 29A  illustrates a perspective, cross-sectional view of a patient&#39;s spine as the pushing trocar of  FIG. 28  is used to drive a fiducial pin into a designate location of a spinal pedicle, in accordance with one embodiment of the invention. 
         FIG. 29B  illustrates a perspective, cross-sectional view of a patient&#39;s spine after two fiducial pins have been implanted into two adjacent spinal pedicles, in accordance with one embodiment of the invention. 
         FIG. 30  is a perspective view of a cannulated awl in accordance with one embodiment of the invention. 
         FIG. 31  is a perspective, cross-sectional view of a patient&#39;s spine as the cannulated awl of  FIG. 30  is being used to enlarge an entry hole for a pedicle screw, in accordance with one embodiment of the invention. 
         FIG. 32  provides a perspective view of fiducial pin retrieving device, in accordance with one embodiment of the invention. 
         FIG. 33  is a perspective view of a pedicle screw having an axial cylindrical cavity for receiving at least a portion of a fiducial pin therein, in accordance with a further embodiment of the invention. 
         FIG. 34  is a perspective, cross-sectional view of a patient&#39;s spine after one pedicle screw has been implanted into a designated location of a spinal pedicle, in accordance with one embodiment of the invention. 
         FIG. 35  is a perspective, cross-sectional view of a patient&#39;s spine after two pedicle screws have been implanted into designated locations of two adjacent spinal pedicles, in accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is described in detail below with reference to the figures wherein like elements are referenced with like numerals throughout. 
       FIG. 1  depicts a spinal fixation device in accordance with one embodiment of the present invention. The spinal fixation device includes two securing members  2  (designated as  2 ′ and  2 ″), and a flexible fixation rod  4  configured to be received and secured within a coupling assembly  14 , as described in further detail below with respect to  FIG. 3 . Each securing member  2  includes a threaded screw-type shaft  10  configured to be inserted and screwed into a patient&#39;s spinal pedicle. As shown in  FIG. 1 , the screw-type shaft  10  includes an external spiral screw thread  12  formed over the length of the shaft  10  and a conical tip at the end of the shaft  10  configured to be inserted into the patient&#39;s spinal column at a designated location. Other known forms of the securing member  2  may be used in connection with the present invention provided the securing member  2  can be inserted and fixed into the spinal column and securely coupled to the rod  4 . 
     As described above, the spinal fixation device is used for surgical treatment of spinal diseases by mounting securing members  2  at desired positions in the spinal column. In one embodiment, the rod  4  extends across two or more vertebrae of the spinal column and is secured by the securing members  2  so as to stabilize movement of the two or more vertebrae. 
       FIG. 2  illustrates a perspective view of a spinal fixation device in accordance with a further embodiment of the present invention. The spinal fixation device of  FIG. 2  is similar to the spinal fixation device of  FIG. 1  except that the rod  4  comprises a flexible middle portion  8  juxtaposed between two rigid end portions  9  of the rod  4 . 
       FIG. 3  provides an exploded view of the securing member  2  of  FIGS. 1 and 2  illustrating various components of the coupling assembly  14 , in accordance with one embodiment of the invention. As shown in  FIG. 3 , the coupling assembly  14  includes: a cylindrical head  16  located at a top end of the screw-type shaft  10 , a spiral thread or groove  18  formed along portions of the inner wall surface of the cylindrical head  16 , and a U-shaped seating groove  20  configured to receive the rod  4  therein. The coupling assembly  14  further comprises an outside-threaded nut  22  having a spiral thread  24  formed on the outside lateral surface of the nut  22 , wherein the spiral thread  24  is configured to mate with the internal spiral thread  18  of the cylindrical head  16 . In a further embodiment, the coupling assembly  14  includes a fixing cap  26  configured to be mounted over a portion of the cylindrical head  16  to cover and protect the outside-threaded nut  22  and more securely hold rod  4  within seating groove  20 . In one embodiment an inner diameter of the fixing gap  26  is configured to securely mate with the outer diameter of the cylindrical head  16 . Other methods of securing the fixing cap  26  to the cylindrical head, such as correspondingly located notches and groove (not shown), would be readily apparent to those of skill in the art. In preferred embodiments the components and parts of the securing member  2  may be made of highly rigid and durable bio-compatible materials such as: stainless steel, iron steel, titanium or titanium alloy. As known in the art, and used herein, “bio-compatible” materials refers to those materials that will not cause any adverse chemical or immunological reactions after being implanted into a patient&#39;s body. 
     As shown in  FIGS. 1 and 2 , in preferred embodiments, the rod  4  is coupled to the securing means  2  by seating the rod  4  horizontally into the seating groove  20  of the coupling means  14  perpendicularly to the direction of the length of the threaded shaft  10  of securing member  2 . The outside threaded nut  22  is then received and screwed into the cylindrical head  16  above the rod  4  so as to secure the rod  4  in the seating groove  20 . The fixing cap  26  is then placed over the cylindrical head  16  to cover, protect and more firmly secure the components in the internal cavity of the cylindrical head  16 .  FIGS. 4-7  illustrate perspective views of various embodiments of a rod  4  that may be used in a fixation device, in accordance with the present invention.  FIG. 4  illustrates the rod  4  of  FIG. 1  wherein the entire rod is made and designed to be flexible. In this embodiment, rod  4  comprises a metal tube or pipe having a cylindrical wall  5  of a predefined thickness. In one embodiment, in order to provide flexibility to the rod  4 , the cylindrical wall  5  is cut in a spiral fashion along the length of the rod  4  to form spiral cuts or grooves  6 . As would be apparent to one of ordinary skill in the art, the width and density of the spiral grooves  6  may be adjusted to provide a desired level of flexibility. In one embodiment, the grooves  6  are formed from very thin spiral cuts or incisions that penetrate through the entire thickness of the cylindrical wall of the rod  4 . As known to those skilled in the art, the thickness and material of the tubular walls  5  also affect the level of flexibility. 
     In one embodiment, the rod  4  is designed to have a flexibility that substantially equals that of a normal back. Flexibility ranges for a normal back are known by those skilled in the art, and one of ordinary skill can easily determine a thickness and material of the tubular walls  5  and a width and density of the grooves  6  to achieve a desired flexibility or flexibility range within the range for a normal back. When referring to the grooves  6  herein, the term “density” refers to tightness of the spiral grooves  6  or, in other words, the distance between adjacent groove lines  6  as shown in  FIG. 4 , for example. However, it is understood that the present invention is not limited to a particular, predefined flexibility range. In one embodiment, in addition to having desired lateral flexibility characteristics, the rigidity of the rod  4  should be able to endure a vertical axial load applied to the patient&#39;s spinal column along a vertical axis of the spine in a uniform manner with respect to the rest of the patient&#39;s natural spine. 
       FIG. 5  illustrates the rod  4  of  FIG. 2  wherein only a middle portion  8  is made and designed to be flexible and two end portions  9  are made to be rigid. In one embodiment, metal end rings or caps  9 ′, having no grooves therein, may be placed over respective ends of the rod  4  of  FIG. 4  so as make the end portions  9  rigid. The rings or caps  9 ′ may be permanently affixed to the ends of the rod  4  using known methods such as pressing and/or welding the metals together. In another embodiment, the spiral groove  6  is only cut along the length of the middle portion  8  and the end portions  9  comprise the tubular wall  5  without grooves  6 . Without the grooves  6 , the tubular wall  5 , which is made of a rigid metal or metal hybrid material, exhibits high rigidity. 
       FIG. 6  illustrates a further embodiment of the rod  4  having multiple sections, two flexible sections  8  interleaved between three rigid sections  9 . This embodiment may be used, for example, to stabilize three adjacent vertebrae with respect to each other, wherein three pedicle screws are fixed to a respective one of the vertebrae and the three rigid sections  9  are connected to a coupling assembly  14  of a respective pedicle screw  2 , as described above with respect to  FIG. 3 . Each of the flexible sections  8  and rigid sections  9  may be made as described above with respect to  FIG. 5 . 
       FIG. 7  illustrates another embodiment of the rod  4  having a pre-bent structure and configuration to conform to and maintain a patient&#39;s curvature of the spine, known as “lordosis,” while stabilizing the spinal column. Generally, a patient&#39;s lumbar is in the shape of a ‘C’ form, and the structure of the rod  4  is formed to coincide to the normal lumbar shape when utilized in the spinal fixation device of  FIG. 2 , in accordance with one embodiment of the invention. In one embodiment, the pre-bent rod  4  includes a middle portion  8  that is made and designed to be flexible interposed between two rigid end portions  9 . The middle portion  8  and end portions  9  may be made as described above with respect to  FIG. 5 . Methods of manufacturing metallic or metallic-hybrid tubular rods of various sizes, lengths and pre-bent configurations are well-known in the art. Additionally, or alternatively, the pre-bent structure and design of the rod  4  may offset a skew angle when two adjacent pedicle screws are not inserted parallel to one another, as described in further detail below with respect to  FIG. 23A . 
     Additional designs and materials used to create a flexible tubular rod  4  or flexible middle portion  8  are described below with respect to  FIGS. 8-10 .  FIG. 8  illustrates a perspective, cross-sectional view of a flexible tubular rod  4 , or rod portion  8  in accordance with one embodiment of the invention. In this embodiment, the flexible rod  4 ,  8  is made from a first metal tube  5  having a spiral groove  6  cut therein as described above with respect to  FIGS. 4-7 . A second tube  30  having spiral grooves  31  cut therein and having a smaller diameter than the first tube  5  is inserted into the cylindrical cavity of the first tube  5 . In one embodiment, the second tube  30  has spiral grooves  31  which are cut in an opposite spiral direction with respect to the spiral grooves  6  cut in the first tube  5 , such that the rotational torsion characteristics of the second tube  30  offset at least some of the rotational torsion characteristics of the first tube  5  The second flexible tube  30  is inserted into the core of the first tube to provide further durability and strength to the flexible rod  4 ,  8 . The second tube  30  may be made of the same or different material than the first tube  5 . In preferred embodiments, the material used to manufacture the first and second tubes  5  and  30 , respectively, may be any one or combination of the following exemplary metals: stainless steel, iron steel, titanium, and titanium alloy. 
       FIG. 9  illustrates a perspective, cross-sectional view of a flexible rod  4 ,  8  in accordance with a further embodiment of the invention. In this embodiment, the flexible rod  4 ,  8  includes an inner core made of a metallic wire  32  comprising a plurality of overlapping thin metallic yarns, such as steel yarns, titanium yarns, or titanium-alloy yarns. The wire  32  is encased by a metal, or metal hybrid, flexible tube  5  having spiral grooves  6  cut therein, as discussed above. The number and thickness of the metallic yarns in the wire  32  also affects the rigidity and flexibility of the rod  4 ,  8 . By changing the number, thickness or material of the yarns flexibility can be increased or decreased. Thus, the number, thickness and/or material of the metallic yarns in the wire  32  can be adjusted to provide a desired rigidity and flexibility in accordance with a patient&#39;s particular needs. Those of ordinary skill in the art can easily determine the number, thickness and material of the yarns, in conjunction with a given flexibility of the tube  5  in order to achieve a desired rigidity v. flexibility profile for the rod  4 ,  8 . 
       FIG. 10  shows yet another embodiment of a flexible rod  4  wherein the flexible tube  5  encases a non-metallic, flexible core  34 . The core  34  may be made from known biocompatible shape memory alloys (e.g., NITINOL), or biocompatible synthetic materials such as: carbon fiber, Poly Ether Ether Ketone (PEEK), Poly Ether Ketone Ketone Ether Ketone (PEKKEK), or Ultra High Molecular Weight Poly Ethylene (UHMWPE). 
       FIG. 11  illustrates a perspective view of another embodiment of the flexible rod  35  in which a plurality of metal wires  32 , as described above with respect to  FIG. 9 , are interweaved or braided together to form a braided metal wire rod  35 . Thus, the braided metal wire rod  35  can be made from the same materials as the metal wire  32 . In addition to the variability of the rigidity and flexibility of the wire  32  as explained above, the rigidity and flexibility of the braided rod  35  can be further modified to achieve desired characteristics by varying the number and thickness of the wires  32  used in the braided structure  35 . For example, in order to achieve various flexion levels or ranges within the known flexion range of a normal healthy spine, those of ordinary skill in the art can easily manufacture various designs of the braided wire rod  35  by varying and measuring the flexion provided by different gauges, numbers and materials of the wire used to create the braided wire rod  35 . In a further embodiment each end of the braided metal wire rod  35  is encased by a rigid metal cap or ring  9 ′ as described above with respect to  FIGS. 5-7 , to provide a rod  4  having a flexible middle portion  8  and rigid end portions  9 . In a further embodiment (not shown), the metal braided wire rod  35  may be utilized as a flexible inner core encased by a metal tube  5  having spiral grooves  6  cut therein to create a flexible metal rod  4  or rod portion  8 , in a similar fashion to the embodiments shown in  FIGS. 8-10 . As used herein the term “braid” or “braided structure” encompasses two or more wires, strips, strands, ribbons and/or other shapes of material interwoven in an overlapping fashion. Various methods of interweaving wires, strips, strands, ribbons and/or other shapes of material are known in the art. Such interweaving techniques are encompassed by the present invention. In another exemplary embodiment (not shown), the flexible metal rod  35  includes a braided metal structure having two or more metal strips, strands or ribbons interweaved in a diagonally overlapping pattern. 
       FIG. 12A  illustrates a further embodiment of a flexible connection unit  36  having two rigid end portions  9 ′ and an exemplary number of rigid spacers  37 . In one embodiment, the rigid end portions  9 ′ and spacers can be made of bio-compatible metal or metal-hybrid materials as discussed above. The connection unit  36  further includes a flexible wire  32 , as discussed above with respect to FIG.  9 ′, which traverses an axial cavity or hole (not shown) in each of the rigid end portions  9 ′ and spacers  37 .  FIG. 12B  illustrates an exploded view of the connection unit  36  that further shows how the wire  32  is inserted through center axis holes of the rigid end portions  9 ′ and spacers  37 . As further shown in  FIG. 12B , each of the end portions  9 ′ and spacers  37  include a male interlocking member  38  which is configured to mate with a female interlocking cavity (not shown) in the immediately adjacent end portion  9 ′ or spacer  37 .  FIG. 12  C illustrates an exploded side view and indicates with dashed lines the location and configuration of the female interlocking cavity  39  for receiving corresponding male interlocking members  38 . 
       FIG. 13  shows a perspective view of a flexible connection unit  40  in accordance with another embodiment of the invention. The connection  40  is similar to the connection unit  36  described above, however, the spacers  42  are configured to have the same shape and design as the rigid end portions  9 ′. Additionally, the end portions  9 ′ have an exit hole or groove  44  located on a lateral side surface through which the wire  32  may exit, be pulled taut, and clamped or secured using a metal clip (not shown) or other known techniques. In this way, the length of the flexible connection unit  36  or  40  may be varied at the time of surgery to fit each patient&#39;s unique anatomical characteristics. In one embodiment, the wire  32  may be secured using a metallic clip or stopper (not shown). For example, a clip or stopper may include a small tubular cylinder having an inner diameter that is slightly larger than the diameter of the wire  32  to allow the wire  32  to pass therethrough. After the wire  32  is pulled to a desired tension through the tubular stopper, the stopper is compressed so as to pinch the wire  32  contained therein. Alternatively, the wire  32  may be pre-secured using known techniques during the manufacture of the rod-like connection units  36 ,  40  having a predetermined number of spacers  37 ,  42  therein. 
       FIG. 14  depicts a spinal fixation device according to another embodiment of the present invention. The spinal fixation device includes: at least two securing members  2  containing an elongate screw type shaft  10  having an external spiral thread  12 , and a coupling assembly  14 . The device further includes a plate connection unit  50 , or simply “plate  50 ,” configured to be securely connected to the coupling parts  14  of the two securing members  2 . The plate  50  comprises two rigid connection members  51  each having a planar surface and joined to each other by a flexible middle portion  8 . The flexible middle portion  8  may be made in accordance with any of the embodiments described above with respect to  FIGS. 4-11 . Each connection member  51  contains a coupling hole  52  configured to receive therethrough a second threaded shaft  54  ( FIG. 15 ) of the coupling assembly  14 . 
     As shown in  FIG. 15 , the coupling assembly  14  of the securing member  2  includes a bolt head  56  adjoining the top of the first threaded shaft  10  and having a circumference or diameter greater than the circumference of the first threaded shaft  10 . The second threaded shaft  54  extends upwardly from the bolt head  56 . The coupling assembly  14  further includes a nut  58  having an internal screw thread configured to mate with the second threaded shaft  54 , and one or more washers  60 , for clamping the connection member  51  against the top surface of the bolt head  56 , thereby securely attaching the plate  50  to the pedicle screw  2 . 
       FIGS. 16A and 16B  illustrate two embodiments of a plate connection unit  40  having at least two coupling members  51  and at least one flexible portion  8  interposed between and attached to two adjacent connection members  51 . As shown in  FIGS. 16A and 16B , the flexible middle portion  8  comprises a flexible metal braided wire structure  36  as described above with respect to  FIG. 11 . However, the flexible portion  8  can be designed and manufactured in accordance with any of the embodiments described above with respect to  FIGS. 4-11 , or combinations thereof.  FIGS. 16C and 16D  illustrate a side view and top view, respectively, of the plate  50  of  FIG. 16A . The manufacture of different embodiments of the flexible connection units  50  and  58  having different types of flexible middle portions  8 , as described above, is easily accomplished using known metallurgy manufacturing processes. 
       FIG. 16E  illustrate a side view of a pre-bent plate connection unit  50 ′, in accordance with a further embodiment of the invention. This plate connection unit  50 ′ is similar to the plate  50  except that connection members  51 ′ are formed or bent at an angle θ from a parallel plane  53  during manufacture of the plate connection unit  50 ′. As discussed above with respect to the pre-bent rod-like connection unit  4  of  FIG. 7 , this pre-bent configuration is designed to emulate and support a natural curvature of the spine (e.g., lordosis). Additionally, or alternatively, this pre-bent structure may offset a skew angle when two adjacent pedicle screws are not inserted parallel to one another, as described in further detail below with respect to  FIG. 23A . 
       FIG. 17  illustrates a perspective view of a plate connection unit  60  having two planar connection members  62  each having a coupling hole  64  therein for receiving the second threaded shaft  44  of the pedicle screw  2 . A flexible middle portion  8  is interposed between the two connection members  62  and attached thereto. In one embodiment, the flexible middle portion  8  is made in a similar fashion to wire  32  described above with respect to  FIG. 9 , except it has a rectangular configuration instead of a cylindrical or circular configuration as shown in  FIG. 9 . It is understood, however, that the flexible middle portion  8  may be made in accordance with the design and materials of any of the embodiments previously discussed. 
       FIG. 18  illustrates a perspective view of a further embodiment of the plate  60  of  FIG. 17  wherein the coupling hole  64  includes one or more nut guide grooves  66  cut into the top portion of the connection member  62  to seat and fix the nut  58  ( FIG. 15 ) into the coupling hole  64 . The nut guide groove  66  is configured to receive and hold at least a portion of the nut  58  therein and prevent lateral sliding of the nut  58  within the coupling hole  64  after the connection member  62  has been clamped to the bolt head  56  of the pedicle screw  2 . 
       FIG. 19  illustrates a perspective view of a hybrid plate and rod connection unit  70  having a rigid rod-like connection member  4 ,  9  or  9 ′, as described above with respect to  FIGS. 4-7 , at one end of the connection unit  70  and a plate-like connection member  51  or  62 , as described above with respect to  FIGS. 14-18 , at the other end of the connection unit  70 . In one embodiment, interposed between rod-like connection member  9  ( 9 ′) and the plate-like connection member  52  ( 64 ) is a flexible member  8 . The flexible member  8  may be designed and manufactured in accordance with any of the embodiments discussed above with reference to  FIGS. 8-13 . 
       FIG. 20  illustrates a perspective view of a spinal fixation device that utilizes the hybrid plate and rod connection unit  70  of  FIG. 19 . As shown in  FIG. 20 , this fixation device utilizes two types of securing members  2  (e.g., pedicle screws), the first securing member  2 ′ being configured to securely hold the plate connection member  42 ( 64 ) as described above with respect to  FIG. 15 , and the second securing member  2 ″ being configured to securely hold the rod connection member  4 ,  9  or  9 ′, as described above with respect to  FIG. 3 . 
       FIG. 21  illustrates a perspective top view of two spinal fixation devices, in accordance with the embodiment illustrated in  FIG. 1 , after they are attached to two adjacent vertebrae  80  and  82  to flexibly stabilize the vertebrae.  FIGS. 22A and 22B  illustrate perspective top views of spinal fixation devices using the flexible stabilizing members  50  and  58  of  FIGS. 16A and 16B , respectively, after they are attached to two or more adjacent vertebrae of the spine. 
       FIG. 23A  illustrates a side view of a spinal fixation device after it has been implanted into the pedicles of two adjacent vertebrae. As shown in this figure, the pedicle screws  2  are mounted into the pedicle bone such that a center axis  80  of the screws  2  are offset by an angle θ from a parallel plane  82  and the center axes  80  of the two screws  2  are offset by an angle of approximately 2θ from each other. This type of non-parallel insertion of the pedicle screws  2  often results due to the limited amount of space that is available when performing minimally invasive surgery. Additionally, the pedicle screws  2  may have a tendency to be skewed from parallel due to a patient&#39;s natural curvature of the spine (e.g., lordosis). Thus, due to the non-parallel nature of how the pedicle screws  2  are ultimately fixed to the spinal pedicle, it is desirable to offset this skew when attaching a rod or plate connection unit to each of the pedicle screws  2 . 
       FIG. 23B  illustrates a side view of the head of the pedicle screw in accordance with one embodiment of the invention. The screw  2  includes a cylindrical head  84  which is similar to the cylindrical head  16  described above with respect to  FIG. 3  except that the cylindrical head  84  includes a slanted seat  86  configured to receive and hold a flexible rod  4  in a slanted orientation that offsets the slant or skew θ of the pedicle screw  2  as described above. The improved pedicle screw  2  further includes a slanted stabilizing spacer  88  which is configured to securely fit inside the cavity of the cylindrical head  84  and hold down the rod  4  at the same slant as the slanted seat  86 . The pedicle screw  2  further includes an outside threaded nut  22  configured to mate with spiral threads along the interior surface (not shown) of the cylindrical head  84  for clamping down and securing the slanted spacer  88  and the rod  4  to the slanted seat  86  and, hence, to the cylindrical head  84  of the pedicle screw  2 . 
       FIG. 23C  shows a perspective view of the slanted spacer  88 , in accordance with embodiment of the invention. The spacer  88  includes a circular middle portion  90  and two rectangular-shaped end portions  92  extending outwardly from opposite sides of the circular middle portion  90 .  FIG. 23D  shows a side view of the spacer  88  that further illustrates the slant from one end to another to compensate or offset the skew angle θ of the pedicle screw  2 .  FIG. 23E  illustrates a top view of the cylindrical head  84  configured to receive a rod  4  and slanted spacer  88  therein. The rod  4  is received through two openings or slots  94  in the cylindrical walls of the cylindrical head  84 , which allow the rod  4  to enter the circular or cylindrical cavity  96  of the cylindrical head  84  and rest on top of the slanted seat  86  formed within the circular or cylindrical cavity  94 . After the rod  4  is positioned on the slanted seat  86 , the slanted stabilizing spacer  88  is received in the cavity  96  such that the two rectangular-shaped end portions  92  are received within the two slots  94 , thereby preventing lateral rotation of the spacer  88  within the cylindrical cavity  96 . Finally, the outside threaded nut  22  and fixing cap  26  are inserted on top of the slanted spacer  88  to securely hold the spacer  88  and rod  4  within the cylindrical head  84 . 
       FIG. 24  illustrates a perspective view of a marking and guidance device  100  for marking a desired location on the spinal pedicle where a pedicle screw  2  will be inserted and guiding the pedicle screw  2  to the marked location using a minimally invasive surgical technique. As shown in  FIG. 24 , the marking device  100  includes a tubular hollow guider  52  which receives within its hollow an inner trocar  104  having a sharp tip  105  at one end that penetrates a patient&#39;s muscle and tissue to reach the spinal pedicle. The inner trocar  104  further includes a trocar grip  106  at the other end for easy insertion and removal of the trocar  104 . In one embodiment, the marking and guidance device  100  includes a guider handle  108  to allow for easier handling of the device  100 . 
     As shown in  FIG. 25 , the trocar  104  is in the form of a long tube or cylinder having a diameter smaller than the inner diameter of the hollow of the guider  102  so as to be inserted into the hollow of the tubular guider  102 . The trocar  104  further includes a sharp or pointed tip  105  for penetrating the vertebral body through the pedicle. The trocar  104  further includes a trocar grip  106  having a diameter larger than the diameter of the hollow of the guider tube  102  in order to stop the trocar  104  from sliding completely through the hollow. The trocar grip  106  also allows for easier handling of the trocar  104 . 
       FIGS. 26A and 26B  provide perspective views of the marking and guidance device  100  after it has been inserted into a patient&#39;s back and pushed through the muscle and soft tissue to reach a desired location on the spinal pedicle. The desired location is determined using known techniques such as x-ray or radiographic imaging for a relatively short duration of time. After the marking and guidance device  100  has been inserted, prolonged exposure of the patient to x-ray radiation is unnecessary. As shown in  FIG. 26B , after the guidance tube  102  is positioned over the desired location on the pedicle, the inner trocar  104  is removed to allow fiducial pins (not shown) to be inserted into the hollow of the guidance tube  102  and thereafter be fixed into the pedicle. 
       FIGS. 27A and 27B  illustrate perspective views of two embodiments of the fiducial pins  110  and  112 , respectively. As mentioned above, the fiducial pins  110  and  112  according to the present invention are inserted and fixed into the spinal pedicle after passing through the hollow guider  102 . The pins  110  and  112  have a cylindrical shape with a diameter smaller than the inner diameter of the hollow of the guider tube  102  in order to pass through the hollow of the guider  102 . An end of each fiducial pin is a sharp point  111  configured to be easily inserted and fixed into the spinal pedicle of the spinal column. In one embodiment, as shown in  FIG. 27B , the other end of the fiducial pin incorporates a threaded shaft  114  which is configured to mate with an internally threaded tube of a retriever (not shown) for extraction of the pin  112 . This retriever is described in further detail below with respect to  FIG. 32 . 
     The fiducial pins  110 ,  112  are preferably made of a durable and rigid biocompatible metal (e.g., stainless steel, iron steel, titanium, titanium alloy) for easy insertion into the pedicle bone. In contrast to prior art guide wires, because of its comparatively shorter length and more rigid construction, the fiducial pins  110 ,  112  are easily driven into the spinal pedicle without risk of bending or structural failure. As explained above, the process of driving in prior art guidance wires was often very difficult and time-consuming. The insertion of the fiducial pins  110 ,  112  into the entry point on the spinal pedicle is much easier and convenient for the surgeon and, furthermore, does not hinder subsequent procedures due to a guide wire protruding out of the patient&#39;s back. 
       FIG. 28  shows a cylindrical pushing trocar  116  having a cylindrical head  118  of larger diameter than the body of the pushing trocar  116 . The pushing trocar  116 , according to the present invention, is inserted into the hollow of the guider  102  after the fiducial pin  110  or  112  has been inserted into the hollow of the guider  102  to drive and fix the fiducial pin  110  or  112  into the spinal pedicle. During this pin insertion procedure, a doctor strikes the trocar head  118  with a chisel or a hammer to drive the fiducial pin  110  and  112  into the spinal pedicle. In preferred embodiments, the pushing trocar  116  is in the form of a cylindrical tube, which has a diameter smaller than the inner diameter of the hollow of the guider tube  112 . The pushing trocar  116  also includes a cylindrical head  118  having a diameter larger than the diameter of the pushing trocar  116  to allow the doctor to strike it with a chisel or hammer with greater ease. Of course, in alternative embodiments, a hammer or chisel is not necessarily required. For example, depending on the circumstances of each case, a surgeon may choose to push or tap the head  118  of the pushing trocar  116  with the palm of his or her hand or other object. 
       FIG. 29A  illustrates how a hammer or mallet  120  and the pushing trocar  116  may be used to drive the pin  110 ,  112  through the hollow of the guider tube  102  and into the designated location of the spinal pedicle.  FIG. 29B  illustrates a perspective cross-sectional view of the spinal column after two fiducial pins  110 ,  112  have been driven and fixed into two adjacent vertebrae. 
     After the fiducial pins  110  or  112  have been inserted into the spinal pedicle as discussed above, in one embodiment, a larger hole or area centered around each pin  110 ,  112  is created to allow easer insertion and mounting of a pedicle screw  2  into the pedicle bone. The larger hole is created using a cannulated awl  122  as shown in  FIG. 30 . The cannulated awl  122  is inserted over the fiducial pin  110 ,  112  fixed at the desired position of the spinal pedicle. The awl  122  is in the form of a cylindrical hollow tube wherein an internal diameter of the hollow is larger than the outer diameter of the fiducial pins  110  and  112  so that the pins  110 ,  112  may be inserted into the hollow of the awl  122 . The awl  122  further includes one or more sharp teeth  124  at a first end for cutting and grinding tissue and bone so as to create the larger entry point centered around the fiducial pin  110 ,  112  so that the pedicle screw  2  may be more easily implanted into the spinal pedicle.  FIG. 31  illustrates a perspective cross-sectional view of a patient&#39;s spinal column when the cannulated awl  122  is inserted into a minimally invasive incision in the patient&#39;s back, over a fiducial pin  110 ,  112  to create a larger insertion hole for a pedicle screw  2  (not shown). As shown in  FIG. 31 , a retractor  130  has been inserted into the minimally invasive incision over the surgical area and a lower tubular body of the retractor  130  is expanded to outwardly push surrounding tissue away from the surgical area and provide more space and a visual field for the surgeon to operate. In order to insert the retractor  130 , in one embodiment, the minimally invasive incision is made in the patient&#39;s back between and connecting the two entry points of the guide tube  102  used to insert the two fiducial pins  110 ,  112 . Before the retractor  130  is inserted, prior expansion of the minimally invasive incision is typically required using a series of step dilators (not shown), each subsequent dilator having a larger diameter than the previous dilator. After the last step dilator is in place, the retractor  130  is inserted with its lower tubular body in a retracted, non-expanded state. After the retractor  130  is pushed toward the spinal pedicle to a desired depth, the lower tubular portion is then expanded as shown in  FIG. 31 . The use of step dilators and retractors are well known in the art. 
     After the cannulated awl  122  has created a larger insertion hole for the pedicle screw  2 , in one embodiment, the fiducial pin  110 ,  112  is removed. As discussed above, if the fiducial pin  112  has been used, a retrieving device  140  may be used to remove the fiducial pin  112  before implantation of a pedicle screw  2 . As shown in  FIG. 32 , the retriever  140  comprises a long tubular or cylindrical portion having an internally threaded end  142  configured to mate with the externally threaded top portion  114  of the fiducial pin  112 . After the retriever end  142  has been screwed onto the threaded end  114 , a doctor my pull the fiducial pin  112  out of the spinal pedicle. In another embodiment, if the fiducial pin  110  without a threaded top portion has been used, appropriate tools (e.g., specially designed needle nose pliers) may be used to pull the pin  110  out. 
     In alternate embodiments, the fiducial pins  110 ,  112  are not extracted from the spinal pedicle. Instead, a specially designed pedicle screw  144  may be inserted into the spinal pedicle over the pin  110 ,  112  without prior removal of the pin  110 ,  112 . As shown in  FIG. 33 , the specially designed pedicle screw  144  includes an externally threaded shaft  10  and a coupling assembly  14  ( FIG. 3 ) that includes a cylindrical head  16  ( FIG. 3 ) for receiving a flexible rod-shaped connection unit  4  ( FIGS. 4-13 ). Alternatively, the coupling assembly  14  may be configured to receive a plate-like connection unit as shown in  FIGS. 14-20 . The pedicle screw  144  further includes a longitudinal axial channel (not shown) inside the threaded shaft  10  having an opening  146  at the tip of the shaft  10  and configured to receive the fiducial pin  110 ,  112  therein. 
       FIG. 34  illustrates a perspective cross-sectional view of the patient&#39;s spinal column after a pedicle screw  2  has been inserted into a first pedicle of the spine using an insertion device  150 . Various types of insertion devices  150  known in the art may be used to insert the pedicle screw  2 . As shown in  FIG. 34 , after a first pedicle screw  2  has been implanted, the retractor  130  is adjusted and moved slightly to provide space and a visual field for insertion of a second pedicle screw at the location of the second fiducial pin  110 ,  112 . 
       FIG. 35  provides a perspective, cross sectional view of the patient&#39;s spinal column after two pedicle screws  2  have been implanted in two respective adjacent pedicles of the spine, in accordance with the present invention. After the pedicle screws  2  are in place, a flexible rod, plate or hybrid connection unit as described above with respect to  FIGS. 4-20  may be connected to the pedicle screws to provide flexible stabilization of the spine. Thereafter, the retractor  130  is removed and the minimally invasive incision is closed and/or stitched. 
     Various embodiments of the invention have been described above. However, those of ordinary skill in the art will appreciate that the above descriptions of the preferred embodiments are exemplary only and that the invention may be practiced with modifications or variations of the devices and techniques disclosed above. Those of ordinary skill in the art will know, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such modifications, variations and equivalents are contemplated to be within the spirit and scope of the present invention as set forth in the claims below.