Patent Publication Number: US-2009228052-A1

Title: Break-off screw extensions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/561,455 filed on Nov. 20, 2006 and entitled “Break-Off Screw Extensions,” which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to tools for use in spinal surgery, and in particular to minimally invasive methods and devices for introducing a spinal connector to one or more spinal anchor sites within a patient&#39;s spine. 
     BACKGROUND OF THE INVENTION 
     For a number of known reasons, spinal fixation devices are used in orthopedic surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal connector, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the connector to various anchoring devices, such as hooks, bolts, wires, or screws. The connector can have a predetermined contour that has been designed according to the properties of the target implantation site, and once installed, the instrument holds the vertebrae in a desired spatial relationship, either until healing or spinal fusion has taken place, or for some longer period of time. 
     Spinal connectors can be anchored to specific portions of the vertebrae. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a threaded shank that is adapted to be threaded into a vertebra, and a head portion having a receiving element, usually in the form of a U-shaped head. A set-screw, plug, or similar type of fastening mechanism is used to lock the spinal connector, e.g., a spinal rod, into the receiving head of the pedicle screw. In use, the shank portion of each screw is threaded into a vertebra, and once properly positioned, a rod is seated through the receiving member of each screw and the rod is locked in place by tightening a cap or other fastener mechanism to securely interconnect each screw and the spinal rod. 
     Minimally invasive devices and methods for implanting spinal fixation devices are advantageous because such devices and methods utilize few incisions for introducing and implanting anchoring devices and spinal connectors at a target site within a patient&#39;s spine. Such procedures offer advantages over invasive techniques because they reduce the amount of damage to surrounding tissue and muscle and the amount of time required to complete the procedure. 
     Accordingly, there is a need for improved minimally invasive devices and methods for introducing a spinal connector into a patient&#39;s spine. 
     SUMMARY OF THE INVENTION 
     The present invention provides minimally invasive devices and methods for implanting a spinal anchor in bone, for percutaneously delivering various tools and implants to the spinal anchor through an extension coupled thereto, and for removing the extension coupled to the spinal anchor. In one embodiment, a spinal implant and access device is provided that includes a U-shaped receiver member, a bone-engaging member, and an extension member. The U-shaped receiver member can have a recess formed therein that is adapted to seat a spinal connector, such as a spinal rod. In one exemplary embodiment, the receiver member can have a maximum outer diameter that is substantially the same as a maximum outer diameter of the extension member. The bone-engaging member can extend distally from the receiver member and it can be adapted to engage bone to thereby mate the receiver member to bone. A variety of configurations are available for the bone-engaging member, for example, in one embodiment the bone-engaging member can be a threaded shank such as a bone screw. 
     As indicated above, the access device can include an extension member. The extension member can extend proximally from the receiver member and it can have a lumen extending therethrough. Although a variety of shapes and sizes are available for the extension member in an exemplary embodiment, the extension member has a length that is adapted to span through a skin surface in a patient to the U-shaped receiver member when the bone engaging member is implanted in a vertebra. The extension member can also include a frangible portion formed thereon that is adapted to break when a predetermined force is applied thereto thereby allowing at least a portion of the extension member to be separated from the receiver member. In one embodiment, the extension member can include a proximal end and a distal end that is coupled to the receiver member. The frangible portion can be formed adjacent to the distal end. 
     The extension member can also have a variety of configurations. For example, in one embodiment, the extension member can include first and second opposed extension arms that extend proximally from the U-shaped receiver member and that are separated by opposed slots that extend between the arms. The opposed slots can extend from the proximal end to the distal end or the first and second extension arms can be coupled to one another at least one location located proximal to the frangible portion. In another embodiment, the extension member can be a hollow elongate tube. The hollow elongate tube can include opposed slots that extend from a distal end of the tube and that terminate distal to a proximal end of the tube. Various configurations are also available for the frangible portion of the extension member. For example, in one embodiment, the frangible portion can include an annular groove formed around the extension member. 
     In another embodiment, a removal tool is provided. Although a variety of configurations are available for the removal tool, in an exemplary embodiment the tool can include an elongate member having a proximal portion that is adapted to be positioned adjacent to a skin incision and a distal portion that is adapted to be positioned adjacent to a vertebra and to be received within a lumen formed in an extension member of a spinal anchor. The distal portion can include an engagement mechanism that is adapted to engage an inner surface of the extension member of the spinal anchor and to apply a radially directed force to the extension member disposed therearound to break at least a portion of the extension member apart from the spinal anchor. The engagement mechanism can have a variety of configurations. For example, in one embodiment, the engagement mechanism can be a cam formed on a distal portion of the elongate member. In another embodiment, the engagement mechanism can be a set screw formed on a distal portion of the elongate member. The set screw can have a proximal threaded portion with an outer diameter that is greater than an outer diameter of a distal threaded portion. The removal tool can also include a handle disposed on a proximal portion that is adapted to rotate the elongate member to cause the engagement mechanism to apply a radially directed force to an extension member that is disposed therearound. 
     Exemplary spinal systems are also provided. In one embodiment, system can include a spinal anchor and a removal tool. The spinal anchor can be adapted to be implanted in bone and it can have a receiver member with a recess formed therein for seating a spinal connector. An extension member can be frangibly coupled to the receiver member and it can be adapted to span through a skin surface in a patient to the receiver member when the receiver member is coupled to a vertebra. A variety of configurations are available for the extension member and the removal tool, including those discussed above. 
     Methods for implanting a spinal anchor and removing a percutaneous access device from a spinal anchor are also provided. In one embodiment, the method can include percutaneously delivering a spinal anchor to a vertebra, advancing a spinal connector through an extension member that is frangibly attached to the spinal anchor, and separating the extension member from the spinal anchor. Advancing the spinal connector through the extension member can generally include advancing the connector in a first orientation substantially parallel to a longitudinal axis of the extension member and manipulating the connector to extend in a second orientation angled with respect to the first orientation to position the connector in relation to the spinal anchor. In one embodiment, the spinal connector can extend substantially parallel to a patient&#39;s spinal column in the second orientation. A variety of techniques can be used to separate the extension member from the spinal anchor. For example, removing the percutaneous access device from the spinal anchor can generally include inserting an elongate member into a lumen formed through the percutaneous access device and actuating the elongate member to cause the elongate member to apply a radially directed force to an inner surface of the access device to break at least a portion of the device away from the spinal anchor. In one exemplary embodiment, actuating the elongate member can include rotating the elongate member such that an engagement mechanism disposed on a distal portion of the member engages and applies the radially directed force to an inner surface of the access device. The elongate member can be rotated by applying a force to a handle disposed on a proximal end of the elongate member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1A  is a perspective view of a spinal implant and access device having an extension frangibly coupled to a spinal anchor according to one embodiment of the present invention; 
         FIG. 1B  is an enlarged perspective view of the frangible portion of the spinal implant and access device shown in  FIG. 1A ; 
         FIG. 1C  is a cross-sectional view of a portion of the spinal implant and access device of  FIG. 1A ; 
         FIG. 1D  is a perspective view of another embodiment of an extension member having a closed configuration for use with a spinal anchor and access device; 
         FIG. 1E  is a perspective view of another embodiment of an extension member having an open configuration for use with a spinal anchor and access device; 
         FIG. 1F  is a perspective view of another embodiment of a proximal end of an extension member having threads formed therein and flats formed on an external surface thereof; 
         FIG. 2A  is a perspective view of a removal tool according to one embodiment of the present invention; 
         FIG. 2B  is an enlarged perspective view of a distal end of the removal tool shown in  FIG. 2A ; 
         FIG. 2C  is a cross-sectional view of the tool shown in  FIG. 2B ; 
         FIG. 3A  is a perspective view of a removal tool according to another embodiment of the present invention; 
         FIG. 3B  is a cross-sectional view of the removal tool shown in  FIG. 3A  disposed within the extension member of the device of  FIG. 1A ; 
         FIG. 3C  is a cross-sectional view of the removal tool and extension member shown in  FIG. 3B  showing the tool applying a force to an inner surface of the extension member; 
         FIG. 4A  is a perspective view of another embodiment of a removal tool having first and second breaker arms, showing the tool coupled to the device of  FIG. 1A ; 
         FIG. 4B  is a perspective view of a distal portion of another embodiment of an extension member; 
         FIG. 5A  is an illustration showing the device of  FIG. 1A  implanted in adjacent vertebra, and showing a spinal rod about to be inserted therethrough; 
         FIG. 5B  is an illustration showing the spinal rod of  FIG. 5A  being passed through the slots in the extension members of the devices of  FIG. 5A ; 
         FIG. 5C  illustrates the rod of  FIG. 5B  positioned within the spinal anchor of the devices of  FIG. 5B ; 
         FIG. 6A  is an illustration showing a driver tool being used to apply a fastening element to the spinal anchor of  FIG. 5C  to lock the rod within the anchor; 
         FIG. 6B  is an illustration of one embodiment of a torque driver and a stabilizer to facilitate tightening of the fastening element of  FIG. 6A ; 
         FIG. 7A  is an illustration showing another embodiment of a technique for removing the extension arms from the spinal anchor of the device of  FIG. 1A , showing a removal tool in an initial position; 
         FIG. 7B  is an illustration showing the removal tool of  FIG. 7A  in a final position; 
         FIG. 8  is an illustration showing one embodiment of a jig and a targeting device in use with the spinal anchor and access device of  FIG. 1A ; 
         FIG. 9  is an illustration showing one embodiment of a distractor device in use with the spinal anchor and access device of  FIG. 1A ; and 
         FIG. 10  is an illustration showing one embodiment of a technique for using the spinal anchor and access device of  FIG. 1A  to correct a spinal deformity. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those of ordinary skill in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention. 
     The present invention provides minimally invasive methods and devices for introducing a spinal connector into a surgical site in a patient&#39;s spinal column. In general, a spinal implant and access device is provided for creating a minimally invasive pathway from a skin incision to a spinal anchor site for delivering various tools and devices to the spinal anchor site. In an exemplary embodiment, the access device can be in the form of an extension member that is mated to or formed on and extends proximally from a spinal anchor. The extension member can be used to implant the spinal anchor, and, once the anchor is implanted, to manipulate the spinal anchor from outside of the patient&#39;s body as well as to provide a pathway through a skin surface to the spinal anchor for introducing various tools and spinal system components, such as spinal connectors, fasteners, installation tools, and removal tools. The extension member can also be adapted to be easily detached from or broken off of the spinal anchor once the procedure is complete. Such a configuration eliminates the steps of assembling, disassembling, and cleaning which are currently required for conventional screw extensions. 
       FIG. 1A  illustrates one exemplary embodiment of a spinal implant and access device  10  having an extension member  12  that is mated to a spinal anchor  14 . As shown, the anchor  14  includes a U-shaped receiver member  16  and a bone-engaging member  14   b  that extends distally from the receiver member  16 . The U-shaped receiver member  16  can have a recess  16   a  formed therein that is adapted to seat a spinal connector, such as a spinal rod. The bone-engaging member  14   b  can extend distally from the receiver member  16  and it can be adapted to engage bone such that it is effective to mate the receiver member  16  to bone. The extension member  12  can include a distal end  12   b  that is mated to and extends proximally from the receiver member  16 . In use, the extension member  12  can define a pathway from a skin incision in a patient to the spinal anchor, which can be implanted in a vertebra of a spine. In an exemplary embodiment, the extension member  12  can be removably mated to the receiver member  16  to allow at least a portion of the extension member  12  to be separated from the receiver member  16 . 
     The anchor can have a variety of configurations. In the embodiment shown in  FIG. 1A , the anchor  14  is in the form of a spinal screw. As shown, the spinal screw  14  includes a distal bone-engaging member  14   b , e.g., a threaded shank, and a proximal U-shaped receiver member  16  that is adapted to seat a spinal connector, such as a rigid or dynamic spinal rod. The U-shaped receiver member  16  can include opposed arms  16   b ,  16   c  having cut-outs  16   d ,  16   e  formed therein that define the U-shaped recess  16   a . The bone-engaging member  14   b  can be fixedly attached to the receiver member  16  to form a monoaxial screw, or alternatively the bone-engaging member  14   b  can be configured as a polyaxial screw that is rotatably disposed through an opening formed in the distal end of the receiver member  16  to allow rotation of the bone-engaging member  14   b  with respect to the receiver member  16 . A variety of techniques known in the art can be used to allow rotation of the receiver member  16  with respect to the bone-engaging member  14   b . A person skilled in the art will appreciate that a variety of other anchors can be used with the devices and methods of the present invention, including, for example, spinal hooks, bolts, and wires. 
     The extension member  12  that is coupled to the anchor  14  can also have a variety of configurations. In the illustrated embodiment, the extension member  12  extends proximally from the receiver member  16  and defines a lumen  17  extending therethrough. In particular, a distal end of opposed arms  12   c ,  12   d  of the extension member  12  can be mated to or integrally formed with and can extend proximally from a proximal end of the opposed arms  16   b ,  16   c  of the receiver member  16 . As will be discussed in more detail below, this will allow opposed slots  13   a ,  13   b  in the extension member  12  to align with the opposed cut-outs  16   d ,  16   e  in the receiver member  16 . The inner lumen  17  of the extension member  12  can define a longitudinal axis L that extends between proximal and distal ends  12   a ,  12   b  of the extension member  12 . The size of the extension member  12  can vary depending on the intended use, but in an exemplary embodiment it has a length that allows the proximal end  12   a  of the extension member  12  to be positioned outside the patient&#39;s body, while the distal end  12   b  of the extension member  12  is coupled to the spinal anchor  14  which is implanted in a vertebra in a patient&#39;s spine. More particularly, the extension member  12  can have a length that allows the extension member  12  to span through the skin surface to the receiver member  16  when the bone engaging member is fully implanted in a vertebra, and the receiver member  16  is positioned adjacent to the vertebrae, e.g., the bottom surface of the receiver member  16  is in contact with or within about 1 mm to 2 mm of the vertebra. Such a configuration is particularly advantageous as it allows the extension member  12  to be used to introduce and facilitate positioning of various tools and devices therethrough, and it also allows the extension member  12  to be used to manipulate the anchor from outside of the patient&#39;s body. The length l e  of the extension member  12  is illustrated in  FIG. 1A , and is measured from the proximal-most end of the extension member  12  to the proximal end of the receiver member  16 , i.e., to a frangible portion  18  that couples the extension member to the U-shaped receiver member  16  as will be discussed in more detail below. In an exemplary embodiment, the length l e  is greater than about 25 mm, and more preferably is greater than about 55 mm. Thus, in such embodiments, the length of the extension member  12  and the receiver member  16  (which can be approximately 15 mm from the proximal-most end to the bottom or distal end), can be greater than about 40 mm, and more preferably greater than about 70 mm. Exemplary lengths l e  of the extension member  12  range from 85 mm to 185 mm (100 mm to 200 mm with the receiver member), and more preferably from 135 mm to 175 mm (150 mm to 190 mm with the receiver member). In certain exemplary embodiments, the extension member can have a length l e  of about 145 mm (160 mm with the receiver member) where the extension member has an open configuration, as will be discussed in more detail below, and a length l e  of about 165 mm (180 mm with the receiver member) where the extension member has a closed configuration. 
     The extension member  12  can also be adapted to provide a minimally invasive pathway for the delivery of various implants and devices. In an exemplary embodiment, shown in  FIG. 1C , the extension member  12  has inner and outer diameters d i , d o  that are substantially the same as the inner and outer diameters d ri , d ro  of the receiver member  16 . The matching inner diameters will allow spinal connectors, fasteners, and other implants and devices to be inserted therethrough and coupled to the receiver member  16 , and the matching outer diameters will allow a relatively small incision to be used to introduce the spinal implant and access device  10  into the body. It will also allow the extension member  12  and the anchor  14  to be introduced through a cannula or other access port, if necessary. 
     As indicated above, the extension member  12  can also include cut-outs or slots to facilitate the introduction of tools, spinal connectors, and other implants therethrough. A variety of configurations are available for the cut-outs. For example, as shown in  FIG. 1A , the extension member  12  can include opposed slots  13   a ,  13   b  that extend between and separate the first and second opposed extension arms  12   c ,  12   d  that extend proximally from the U-shaped receiver member  16 . The opposed slots  13   a ,  13   b  can extend from the proximal end  12   a  to the distal end  12   b , such that the extension member  12  has an open configuration, or the first and second extension arms  12   c ,  12   d  can be coupled to one another at one or more locations along the length thereof, such that the extension member  12  has a closed configuration.  FIGS. 1A and 1C  illustrate the extension having a closed configuration, in which the first and second extension arms  12   c ,  12   d  are coupled to one another by a web or connector  15  located distal to the midpoint of the extensions  12   c ,  12   d  and proximal to a frangible portion  18 , which will be discussed in more detail below. In use, the slot(s)  13   a ,  13   b  can be configured to allow various implants and devices to be passed therethrough. Thus, each slot  13   a ,  13   b  preferably has a width that is sufficient to accommodate the size of an implant or device to be passed therethrough. In another embodiment of an extension having a closed configuration, as shown in  FIG. 1D , the first and second extension arms  12   c ′,  12   d ′ can be coupled to one another at the proximal end  12   a ′ of the extension member  12 ′. Such a configuration resembles a hollow elongate tube having slots  13   a ′,  13   b ′ formed in the sidewalls thereof and extending proximally from the distal end  12   b ′ thereof and terminating distal to the proximal end  12   a ′ of the extension member  12 ′. 
     Alternatively, the extension member can have an open configuration. This is illustrated in  FIG. 1E , which shows an extension member  12 ″ having first and second extension arms  12   c ″,  12   d ″ that are separated by slots  13   a ″,  13   b ″ that extend from the distal end  12   b ″ of the extension member  12 ″ and that extend through the entire proximal portion of the extension member  12 ″ such that the proximal end  12   a ″ of each arm  12   c ″,  12   d ″ is separated from one another. Accordingly, the quantity, shape, and size of the slot(s)  13   a ,  13   b  can vary. A person skilled in the art will appreciate that the extension member  12  can include any number of sidewall openings having any shape that is sufficient to allow desired implants and devices to be passed therethrough. 
     The extension member  12  can also include other features to facilitate use of the device. For example, in another embodiment, shown in  FIG. 1F , the extension member  12 ′″ includes threads  12   t ′″ formed in a proximal portion of the extension member  12 ′″ for mating with corresponding threads formed on a driver tool, reduction device, or other delivery device. The threads  12   t ′″ will facilitate advancement of the delivery device through the extension member  12 ′″. For example, the threads  12   t ′″ can mate with corresponding threads on a rod reduction device to allow the rod reduction device to reduce a spinal rod or other spinal connector into the receiver member of the anchor. While  FIG. 1F  illustrates threads  12   t ′″ formed within the proximal end of the extension member  12 ′″, the threads  12   t ′″ can be formed at any location along the length of the extension member  12 ′″. For example, threads can be formed within the connector  15  ( FIG. 1C ) and around the circumference of the inner lumen of the extension member  12 . As further shown in  FIG. 1F , the extension member  12 ′″ can also or alternatively include one or more flats  12   f ″ formed on an external surface thereof for use with external devices, such as a rod stabilizer. A person skilled in the art will appreciate that the extension member can have various other configurations to facilitate use with other delivery tools and/or fastening elements. 
     Referring back to  FIGS. 1A-1C , as indicated above the extension member  12  can be removably coupled to the receiver member  16 . In an exemplary embodiment, the distal end  12   b  of the extension member  12  is integrally formed with the receiver member  16 , i.e., the extension member  12  and receiver member  16  have a unitary configuration. A frangible portion  18 , best shown in  FIG. 1A , can be formed on the extension member  12  to allow the extension member  12  to be frangibly separated from the receiver member  16 . The frangible portion can be formed at various locations along the extension member  12 , but in an exemplary embodiment the frangible portion  18  is located adjacent to the distal end  12   b  of the extension member  12 . The frangible portion  18  can be adapted to break when a predetermined force is applied thereto thereby allowing at least a portion of the extension member  12  to be separated from the receiver member  16 . A variety of techniques can be used to form a frangible portion  18 . For example, in one embodiment shown in  FIGS. 1A-1B , the frangible portion  18  can be formed by a reduced diameter region or thinned region of material formed between the extension member  12  and the receiver member  16 . Other configurations for the frangible portion  18  can include webbing, an annular grooved formed in an outer or inner surface of the extension member  12 , or other techniques known in the art. A variety of materials, including both plastics and metals, can be used for the frangible portion  18 , and the frangible portion  18  need not be made from the same material used for the anchor  14  or extension member  12 . By way of non-limiting example, the anchor  14  can be made from a metallic material, such as stainless steel, and the frangible portion  18  and extension member  12  can be formed from a plastic. Such a configuration allows the extension member  12  and frangible portion  18  to act as insulators during neuromonitoring. In use, the frangible portion  18  can provide a weak spot in the extension member  12  to allow the extension member  12  to be separated from the receiver member  16  when a predetermined force is applied thereto. 
     Several techniques can be used to apply the predetermined force to the frangible portion  18  of the extension member  12 . For example, a removal tool  20  can be inserted through a lumen  17  of the percutaneous access device  12  or extension member  12  and it can be actuated to apply a radially directed force to an inner surface  18   c  of the extension member  12  to break the extension member  12  away from the spinal anchor  14 . A variety of configurations are available for the removal tool  20 .  FIG. 2A  illustrates one exemplary removal tool  20  that generally includes an elongate member  22  having a proximal end  22   a  that is adapted to remain outside the body and a distal end  22   b  that is adapted to be inserted through the lumen  17  of the extension member  12  and to be positioned adjacent to the spinal anchor  14 . The distal portion  22   b  can include an engagement mechanism  24  that is adapted to engage an inner surface  18   c  of the extension member  12  and to apply an outward or radially directed force to the extension member  12  disposed therearound to break at least a portion of the extension member  12  apart from the spinal anchor  14 . The removal tool  20  can also include a handle  26  disposed on the proximal end  22   a  of the elongate member  22 . The handle  26  can be used to manipulate the device, as will be discussed in more detail below. 
     The engagement mechanism  24  can have a variety of configurations, and the size and shape of the engagement mechanism  24  can vary depending on the configuration of the extension member  12 . For example, in the embodiment shown in  FIGS. 2A-2C , the engagement mechanism  24  is in the form of a set screw or other threaded member formed on a distal portion  22   b  of the elongate member  22  and adapted to engage the complementary threads formed in the distal end of the extension portion  12 . As shown in  FIGS. 2B and 2C , the engagement mechanism  24  has a proximal threaded portion  24   a  with an outer diameter that is greater than an outer diameter of a distal threaded portion  24   b . In other words, the thread on the proximal portion is oversized relative to the thread on the distal portion. The proximal threaded portion  24   a  can be sized to engage threads formed within the extension portion  12 , and the distal threaded portion  24   b  can be configured to engage threads formed in the receiver member  16 .  FIG. 1B  illustrates threads  12   t  formed on inner surface of the extension member  12  just proximal to the frangible portion  18 , and threads  16   t  formed on an inner surface of the receiver member  16  just distal to the frangible portion  18 . The proximal threaded portion  24   a  on the engagement mechanism  24  can also have a size that is configured to apply a radially directed force to the threads  12   t  on the extension member  12  to break the frangible portion  18  and thereby separate the extension member  12  from the receiver member  16 . This can be achieved using various techniques. For example, a distal-most portion of the extension member  12 , i.e., the portion located adjacent to or at the frangible portion  18 , can be tapered such that the threaded portion  24   a  will apply a radially directed force thereto to cause the frangible portion  18  to break. Alternatively, the threads on the proximal threaded portion  24   a  can have an increasing diameter to cause the proximal threaded portion  24   a  to apply a radially directed force to the extension member  12 . The distal threaded portion  24   b , on the other hand, can have threads that complement the threads  16   t  within the receiver member  16  to allow the two portions to mate to one another. In use, the distal threaded portion  24   b  can thus function as a locking mechanism for locking a spinal connector, such as a spinal rod, within the receiver member  16 . 
     The engagement mechanism  24 , or a portion thereof, can also optionally be frangibly mated to the distal end  22   b  of the elongate member  22 . This will allow the elongate member  22  to be detached from the engagement mechanism  24 , thus allowing the engagement mechanism  24 , or a portion thereof, to remain mated to the receiver member  16  of the anchor  14 . The frangible portion can be formed at various locations on the removal tool  20 . For example,  FIG. 2C  illustrates a frangible portion  22   f  formed within the engagement mechanism  24  at a location that is between the proximal and distal threaded portions  24   a ,  24   b . In other embodiments, the proximal and distal threaded portions  24   a ,  24   b  can be frangibly mated to one another, thus allowing the proximal threaded portion  24   a  to be separated from the distal threaded portion  24   b  and removed from the body. 
     In use, the removal tool  20  can be actuated by inserting the elongate member  22  through the extension member  12  to position the engagement mechanism  24  within the distal end  12   b  of the extension member  12 . The distal threaded portion  24   b  can pass through the threads  12   t  formed within the distal end  12   b  of the extension member  12 , as they will have a size that is less than a size of the threads  12   t  formed within the extension member  12 . The tool  20  can then be rotated with respect to the extension member  12 , using, for example, the handle  26 . As the tool  20  is rotated, the proximal threaded portion  24   a  will engage the threads  12   t  formed in the extension portion while the distal threaded portion  24   b  engages the threads  16   t  formed within the receiver member  16 . Continued rotation will cause the proximal threaded portion  24   a  to apply a radially directed force to the extension member  12 , thereby breaking the frangible portion  18  formed between the extension member  12  and the receiver member  16 . The extension member  12  can thus be removed from the patient&#39;s body, leaving the spinal anchor  14  implanted in bone. 
     In another embodiment, the engagement mechanism can be in the form of a cam, wedge, or other inclined surface formed on a distal portion of an elongate shaft of a removal tool.  FIGS. 3A-3C  illustrate a distal end  22   b ′ of an elongate shaft  22 ′ of a removal tool having an engagement mechanism  24 ′ in the form of a cam having first and second fins  30   a ′,  30   b ′ disposed on opposed sides of and extending radially outward from the shaft  22 ′. Each fin  30   a ′,  30   b ′ can have a length l that is configured to correspond to a width of the slots  13   a ,  13   b  formed in the arms  12   c ,  12   d  of the extension member  12  to allow the fins  30   a ′,  30   b ′ to be slidably received therein during insertion of the removal tool through the extension member  12 . Each fin  30   a ′,  30   b ′ can also have height h that increases radially to form a ramped surface such that the fins  30   a ′,  30   b ′ can apply a radially directed force to the arms  12   c ,  12   d  of the extension member  12 . In particular, after the tool is inserted to position the fins  30   a ′,  30   b ′ within the slots  13   a ,  13   b  and adjacent to the distal end of the extension member  12 , as shown in  FIGS. 3A and 3B , the removal tool can be rotated to cause the fins  30   a ′,  30   b ′ to move in between and into alignment with the arms  12   c ,  12   d  of the extension member  12 , as shown in  FIG. 3C . The increasing height h of each fin  30   a ′,  30   b ′ will apply a radially directed force to an inner surface  18   c  of each arm  12   c ,  12   d , thereby causing the frangible portion to break thereby separating the extension member  12  from the receiver member  16  of the spinal anchor  14 . 
     In another embodiment, the removal tool shown in  FIGS. 3A-3C  can include first and second arm members (not shown) that extend along the length of the elongate shaft. The fins can be rotated with respect to the first and second arm members to cause the fins to move in between and into alignment with the arm members. Similar to the removal tool described above, the increasing height of each fin will apply a radially directed force to an inner surface of each arm member, thereby causing the first and second arm members to apply a radially directed force to an inner surface of the extension member. The frangible portion will thus break thereby separating the extension member from the receiver member of the spinal anchor. 
     In another embodiment, shown in  FIG. 4A , the removal tool  20 ″ can include first and second breaker arms  21   a ″,  21   b ″ that are configured to engage the proximal end of the first and second arms  12   c ,  12   d  of the extension member  12 . Various techniques can be used to engage the arms  12   c ,  12   d , but as shown in  FIG. 4A  the breaker arms  21   a ″,  21   b ″ each include a U-shaped member for receiving a proximal end of the arms  12   c ,  12   d  on the extension member  12 . Each breaker arm  21   a ″,  21   b ″ can also include a handle  23   a ″,  23   b ″ formed thereon, and once the breaker arms  21   a ″,  21   b ″ are engaged with the arms  12   c ,  12   d  on the extension member  12 , the handles  23   a ″,  23   b ″ can be rotated in opposite directions to break the arms  12   c ,  12   d  of the extension member  12  away from the receiver member  16  of the spinal anchor  14 . In another embodiment, in order to facilitate removal of the arms of the extension member, the arms can have complementary features formed therein that allow the arms to pass through one another. For example,  FIG. 4B  illustrates a distal end of first and second extension arms  11   a ,  11   b  having cut-outs  11   c  formed therein and offset from one another such that one arm, e.g., arm  11   a , can pass through the other arm, e.g., arm  11   b  as the arms  11   a ,  11   b  are rotated by the breaker arms  21   a ″,  21   b″.    
     Exemplary methods for implanting a spinal anchor  14  and delivering various tools and devices are also provided. In one exemplary embodiment, the procedure can begin by forming a minimally invasive percutaneous incision through tissue located adjacent to a desired implant site. The incision can be a relatively small incision having a length that is less than a diameter or width of the device being inserted therethrough, a spinal anchor and access device. Once the incision is made, a spinal anchor and access device, such as device  10 , can be delivered to the anchor site. In particular, the anchor  14  can be inserted through the incision with the extension member  12  frangibly attached thereto and extending through the skin incision and outside of the patient&#39;s body. The bone-engaging member  14   b  of the anchor  14  can be driven into bone using a tool, such as a driver, that is passed through the extension member  12 . While not shown, the extension member  12  can optionally include threads formed therein for mating with corresponding threads formed on the driver tool, thus facilitating advancement of the driver through the extension member  12  to drive the bone-engaging member  14   b  into bone. When the spinal anchor  14  is fully implanted, the bone-engaging member  14   b  will be fully disposed within bone and the receiver member  16  will be located adjacent to the bone such that it is either in contact with the bone or relatively close to the bone, i.e., 1 mm to 2 mm away from the bone. The extension member  12  will extend from the receiver member  16  and through the skin incision, thereby providing a pathway that spans through the skin incision to the anchor  14 . Additional spinal anchor and access devices can be implanted in adjacent vertebrae using the same technique, or using other techniques known in the art. 
     Once the anchor  14  is implanted, the extension member  12  can be used to deliver various spinal connectors, fasteners, and other tools and devices to the implant site. For example, a spinal connector, such as a spinal rod, can be introduced through the extension member and positioned within the spinal anchor  14 , and optionally within one or more additional spinal anchors implanted in adjacent vertebrae.  FIGS. 5A-5C  illustrate one exemplary method for inserting a spinal rod  40 . The rod  40  is introduced through the extension member  12  in a first orientation, shown in  FIG. 5A , in which the rod  40  is substantially parallel to the longitudinal axis of the extension member  12 . The spinal rod  40  can then be manipulated to extend at an angle with respect to the first orientation, such that the rod  40  extends in a direction substantially transverse to the longitudinal axis L of the extension member  12 , for example, in a direction that is substantially parallel to the patient&#39;s spine, as shown in  FIG. 5C . Since the length of the spinal rod  40  will necessarily be greater than the inner diameter of the extension member  12 , the slots  13   a ,  13   b  in the extension member  12  will allow the spinal rod  40  to pivot and pass therethrough while being transitioned from the first orientation to the second orientation, as shown in  FIG. 5B . A person skilled in the art will appreciate that the exact position of the spinal rod  40  with respect to the longitudinal axis L of the extension member  12  will of course vary depending on the configuration of the spinal rod  40  or other spinal connector. As further shown in  FIGS. 5A-5C , movement of the spinal rod  40  can be achieved using a manipulator or driver device  50 . The manipulator device can have a variety of configurations, but it should be effective to allow controlled movement of the rod  40 . 
     Once the spinal rod  40  is properly positioned and fully seated in the receiver member  16 , a closure mechanism or fastener can be inserted through the extension member  12  and applied to lock the rod  40  in place. A variety of closure mechanisms and tools for delivering closure mechanisms are known in the art, and they can be used with the present invention. By way of non-limiting example,  FIG. 6A  illustrates a driver tool  60  for delivering a set screw  62  to the spinal anchor  14  to lock the spinal rod  40  within the receiver member  16  of the spinal anchor  14 . As shown, the set screw  62  is coupled to the distal end of the driver tool  60 , which is passed through the inner lumen of the extension member  12 . The illustrated driver tool  60  includes threads  60   t  formed on the shaft thereof for mating with corresponding threads (not shown) formed within the extension member  12 . The threaded connection facilitates distal movement or advancement of the driver tool  60  through the extension member  12 , thereby allowing the set screw  62  to reduce the rod  50  into the U-shaped recess in the receiver member  16 . Final tightening of the set screw  62  can optionally be accomplished using a standard torque driver  64  along with a stabilizer  66  to prevent the receiver member  16  from rotating, as shown in  FIG. 6B . The stabilizer  66  can interface with flats formed on the external surface of the extension member  12 , as previously discussed with respect to  FIG. 1F . 
     While not shown, removal tool  20  previously described herein can optionally be used to deliver the engagement mechanism  24 , which includes a distal threaded portion  24   b  that functions as a set screw. Once the engagement mechanism  24  is mated to the receiver member  16 , the elongate member  22 , and optionally the proximal threaded portion  24   a  of the engagement mechanism  24 , can be frangibly detached from the distal threaded portion  24   b  to allow the removal tool  20  to be removed from the extension member  12  while the distal threaded portion  24   b  remains mated to the receiver member  16 . This step can be repeated for adjacent spinal anchor(s). A person skilled in the art will appreciate that the spinal connector does not need to be directly attached to each anchor, and that it can be indirectly attached to the anchors using, for example, a band clamp, a slotted offset connector, or other techniques known in the art. 
     Where the removal tool is not configured to simultaneously deliver a fastening element while breaking the extension member  12  away from the receiver member  16 , the extension member  12  can be detached from the anchor  14  by inserting a removal tool through the extension member  12  to apply a force to the frangible portion  18  formed between the extension member  12  and the spinal anchor  14 .  FIGS. 7A and 7B  illustrate one exemplary method for breaking the extension member  12  away from the receiver member  16  of the spinal anchor  14  using the cam-type engagement mechanism  24 ′ shown in  FIGS. 3A-3C . As shown in  FIGS. 7A and 7B , the removal tool  20 ′ is passed through the extension member  12  to position the engagement mechanism (not shown) within the distal end of the extension member  12 . The first and second fins on the engagement mechanism can be passed through the slots  13   a ,  13   b  formed in the extension member  12 . The tool  20 ′ is then rotated to cause the fins to pass between the arms  12   c ,  12   d  of the extension member  12 , thereby applying a radially directed force to the arms  12   c ,  12   d  to break the frangible portion  18 . The extension member  12  can thus be removed from the patient, leaving the spinal anchor  14  implanted in bone. As further shown in  FIGS. 7A and 7B , a stabilizing tool  76  can optionally be passed through at least a portion of the extension member  12  for stabilizing the extension member  12  while the removal tool  20 ′ is rotated. 
     In other embodiments, one or more spinal anchor and access devices can be implanted in adjacent vertebrae, and the extension member on each device can be used to perform other procedures, such as pedicle targeting, anchor alignment, distraction, deformity correction, etc. For example, the extension member  12  can also be used to manipulate the receiver member  16  coupled thereto. For example, the proximal end  12   a  of the extension member  12  can be grasped by a user and angularly oriented to thereby move the receiver member  16  relative to the bone-engaging member  14   b . Where the anchor  14  is a monoaxial bone screw, movement of the extension member  12  will move the vertebra that the anchor  14  is implanted in.  FIG. 8  illustrates a jig  70  mated to one or more extension members implanted in adjacent vertebrae. The jig  70  can be used, for example, to guide a pedicle targeting tool  72  and aid in the alignment of adjacent anchors. In yet another embodiment, as shown in  FIG. 9 , a distractor  80  can be mated to the extension member to facilitate the distraction of adjacent vertebrae. In another embodiment, shown in  FIG. 10 , multiple spinal anchor and access devices can be implanted in adjacent vertebrae and they can be used to correct a spinal deformity. For example, as shown in  FIG. 10 , the extension member of each device can aid in the placement of an extra long spinal rod  100 , and insertion of the rod  100  through each extension member can move the extension members to thereby move the adjacent vertebrae into a desired orientation relative to one another. Separate instruments can optionally be used to further distract and/or manipulate the adjacent vertebrae. A person skilled in the art will appreciate that a variety of other tools and devices can be mated to and used in conjunction with the extension member disclosed herein. 
     One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.