Patent Application
20060235385
Spinal fixation systems may be used in surgery to align, adjust and/or fix portions of the spinal column, i.e., vertebrae, in a desired spatial relationship relative to each other. Many spinal fixation systems employ a spinal rod for supporting the spine and for properly positioning components of the spine for various treatment purposes. Vertebral anchors, comprising pins, bolts, screws, and hooks, engage the vertebrae and connect the supporting rod to different vertebrae. The size, length and shape of the cylindrical rod depend on the size, number and position of the vertebrae to be held in a desired spatial relationship relative to each other by the apparatus.
Spinal fixation elements can be anchored to specific portions of the vertebra. 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 spinal fixation element-receiving element, which, in spinal rod applications, is usually in the form of a U-shaped slot formed in the head portion for receiving the rod. A set-screw, plug, cap or similar type of closure mechanism is used to lock the rod into the rod-receiving portion of the pedicle screw. In use, the shank portion of each screw is then threaded into a vertebra, and once properly positioned, a fixation rod is seated through the rod-receiving portion of each screw. The rod is locked into place by tightening a cap or similar type of closure mechanism to securely interconnect each screw and the fixation rod. Other anchoring devices also include hooks and other types of bone screws.
Polyaxial pedicle screws have been designed to allow angulation of one portion of the screw relative to another portion of the screw and the spinal fixation element coupled to one portion of the screw. For example, polyaxial pedicle screws allow for a shaft portion to pivot relative to a rod-receiving portion in all directions about a 360° arc around the rod-receiving portion. Polyaxial screws may be useful for positioning bone anchors on adjacent vertebrae, when the close proximity of adjacent vertebrae can result in interference between the bone anchors. Polyaxial screws allow for pivoting of the screws in any direction out of alignment with each other to avoid such interference.
An example of such a polyaxial pedicle screw assembly is described in detail in U.S. Patent Application Publication Number US 2004/0186473 entitled “Spinal Fixation Devices of Improved Strength and Rigidity”, U.S. Patent Application Publication Number US 2004/0181224 entitled “Anchoring Element for Use in Spine or Bone Surgery, Methods for Use and Production Thereof” and U.S. Patent Application Publication Number US 2003/0100896, entitled “Element With a Shank and a Holding Element Connected to It for Connecting to a Rod”, the contents of which are herein incorporated by reference.
Polyaxial and multi-axial screws, which allow the screw shank to pivot in all directions about the head portion, can have high profiles to accommodate the polyaxial mechanism and to provide the strength needed to secure the spinal rod to the vertebral body. However, high profile polyaxial screws are not desirable in areas where there is little distance between the vertebral body and the patient's skin such as in the posterior spine.
Disclosed herein are bone screw assemblies having a reduced profile that provide for polyaxial movement between an anchor portion and a rod-receiving portion of the bone screw assembly. The bone screw assemblies disclosed herein allow the anchor portion to pivot about the rod-receiving portion in one or more directions.
In accordance with one aspect, an exemplary embodiment of a bone anchor assembly may comprise a bone anchor having a distal shaft configured to engage bone and a hollow hemi-spherical proximal head defined by a convex outer surface and a concave inner surface, a receiving member for receiving a spinal fixation element and for engaging the head of the bone anchor, and a compression member positionable in the receiving member. The compression member, in the exemplary embodiment may have an upper portion configured to seat the spinal fixation element and a lower portion configured to engage the concave inner surface of the anchor head.
According to another exemplary embodiment, a bone anchor assembly may comprise a bone anchor having a distal shaft configured to engage bone and a hollow hemi-spherical proximal head defined by a convex outer surface and a concave inner surface, a receiving member for receiving a spinal fixation element and the head of the bone anchor, and a compression member positionable in the receiving member. In the exemplary embodiment, the convex outer surface and the concave inner surface may be spaced apart to form a wall having a thickness and the thickness of the wall may be approximately less than or equal to 33% of a radius of the convex outer surface. The compression member, in the exemplary embodiment, may have an upper portion including a groove to seat the spinal fixation element and a lower portion having a convex shape having a radius approximating a radius of the concave inner surface of the head of the bone anchor.
According with a further exemplary embodiment, a kit may comprise a spinal fixation element, a bone anchor assembly, and an instrument configured to engage a drive feature on the head of the bone anchor of the bone anchor assembly. The bone anchor assembly, in the exemplary embodiment, may comprise a bone anchor having a distal anchoring shaft and a proximal hollow hemispherical head defined by an outer convex surface and an inner concave surface, a rod-receiving member for receiving the spinal fixation element and the head of the bone anchor, and a compression member positionable within the rod-receiving member and configured to engage the hollow head of the bone anchor. The head of the bone anchor, in the exemplary embodiment, may have a drive feature positioned between the outer convex surface and the inner concave surface.
The foregoing and other objects, features and advantages of the bone anchor assemblies disclosed herein will be apparent from the following description and apparent from the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings illustrate principles of the invention and, although not to scale, show relative dimensions.
Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the bone anchor assemblies 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 bone anchor assemblies 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 articles “a” and “an” are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.
The terms “comprise,” “include,” and “have,” and the derivatives thereof, are used herein interchangeably as comprehensive, open-ended terms. For example, use of “comprising,” “including,” or “having” means that whatever element is comprised, had, or included, is not the only element encompassed by the subject of the clause that contains the verb.
During spinal surgeries, polyaxial and multi-axial screw assemblies may be used to fix spinal rods, cables or plates to the vertebral bodies at the pedicle. A polyaxial screw assembly having a low profile would be beneficial to the patient in reducing tissue irritation. Different exemplary embodiments of a low profile polyaxial screw assembly are illustrated in
The exemplary bone screw assemblies may be employed to engage one or more spinal fixation elements to bone. For example, a bone screw assembly may be employed to fix a spinal plate, rod, and/or cable to a vertebra of the spine. Although the exemplary bone screw assemblies described below are designed primarily for use in spinal applications, and specifically the pedicle region of a vertebra, one skilled in the art will appreciate that the structure, features and principles of the exemplary bone screw assemblies, as well as the other exemplary embodiments described below, may be employed to couple any type of orthopedic implant to any type of bone or tissue.
An exemplary embodiment of a low profile polyaxial bone screw assembly 100, as illustrated in
The bone anchor 114 has a proximal end and a distal end and a longitudinal axis 122 extending therebetween. The proximal head 116 is provided at the proximal end of the bone anchor 114. The exemplary anchor head 116 is hollow and has a generally hemi-spherical shape defined by an outer convex surface 111 and an inner concave surface 113. In the exemplary embodiment, the outer convex surface 111 and the inner concave surface 113 are generally spherical in shape. The convex outer surface has a radius RH as shown in
The distal shaft 118 may include one or more bone engagement mechanisms to facilitate gripping engagement of the bone anchor to bone. In the illustrated embodiment, the distal shaft 118 includes an external thread 124 extending along at least a portion of the shaft for engaging bone. In the illustrated embodiment, the external thread 124 is a single lead thread that extends from a distal tip 126 of the shaft to the anchor head 116, though one skilled in the art will recognize that the external thread may extend along any selected portion of the shaft and have any suitable number of leads. Other suitable bone engagement mechanisms include, but are not limited to, one or more annular ridges, multiple threads, dual lead threads, variable pitched threads and/or any conventional bone engagement mechanism.
The rod-receiving member 140 shown in
The U-shaped channel 145 of the receiving member 140 of the exemplary bone screw assembly 100 may be sized and shaped to receive a spinal rod 12 or another suitable spinal fixation element. The exemplary spinal rod 12 may be seated within the channel 145 by aligning the spinal rod 12 and the channel 145 and advancing the spinal rod through the top into the channel 145. The configuration of the channel 145 may be varied to accommodate any suitable spinal fixation element. A suitable configuration for the receiving member 140 is described in the U.S. Patent Application Publication Numbers US 2004/0186473, US 2004/0181224 and US 2003/0100896, the contents of which are herein incorporated by reference.
Continuing to refer to
The upper portion 182 of the compression member 180 of the exemplary bone anchor assembly 100 is generally disc-shaped having a circular cross-section or other cross section preferably corresponding to the axial bore 143 of the receiving member 140. The upper portion 182 may have a radius Rc extending from a center point of the upper portion 182 to the outer radial edge of the compression member 180. The radius Rp of the convex outer surface 188 of the lower portion 184 of the compression member 180 may be a percentage of the radius Rc of the upper portion 182 of the compression member 180. In one embodiment, for example, the radius Rp of the convex outer surface 188 of the lower portion 184 of the compression member 180 is greater than or equal to approximately 85% of the radius Rc of the upper portion 182 of the compression member 180. In another embodiment, for example, the radius Rp of the convex outer surface 188 of the lower portion 184 of the compression member 180 is greater than or equal to approximately 75% of the radius Rc of the upper portion 182 of the compression member 180. In another embodiment, for example, the radius Rp of the convex outer surface 188 of the lower portion 184 of the compression member 180 is greater than or equal to approximately 65% of the radius Rc of the upper portion 182 of the compression member 180.
The exemplary bone anchor assembly may further include a locking element to secure the fixation element relative to the receiving member and the bone anchor. In the exemplary embodiment, for example, a locking element 190 may fix the spinal rod 12 within the U-shaped channel 145 of the receiving member 140 and fix the position of the anchor head 116 with respect to the receiver member 140. In particular, the locking element 190 engages the spinal rod 12 and seats the rod 12 within the groove 186 of the compression member 180 and advances the lower portion 184 of the compression member 180 into fixed engagement with the proximal head 116 of the bone anchor 114. The locking element 190 can be in a threaded set screw, as in the illustrated embodiment, a twist-in cap, an external locking nut, a combination thereof or any other locking element known to one skilled in the art.
In the exemplary embodiment, the proximal head 116 of the bone anchor 114 may include a drive feature 169 positioned on the wall 167 formed between the inner concave surface 113 and the outer convex surfaces 111 of the proximal head 116. The drive feature 169 may be adapted to mate with an instrument to drive the screw assembly into bone. As shown in
As shown in
After pivoting the bone anchor portion 116 about an axis relative to the receiving portion 140, a user can lock the orientation of the bone anchor 114 relative to the receiving portion 140 by inserting the locking element 190. The locking element 190 secures a spinal rod 12 or other suitably configured spinal fixation element within the channel 145 of the receiving member 140 and locks the anchor head 116 in the selected orientation within and relative to the receiving member 140. In the illustrative embodiment, advancing the locking element 190 into engagement with the spinal rod 12 in the channel 145 seats the spinal rod 12 in the seat 186 of the compression member 180. The compression member 180 compresses against the inner concave surface 113 of the anchor head 116 to lock the bone anchor 114 in the selected orientation.
While the illustrative embodiment is a top-loading screw, one skilled in the art will recognize that the present invention encompasses a bottom-loading screw as well. A top-loading screw is assembled by inserting the shaft in a distal direction through the bottom opening, so that the anchor head is retained within a cavity in the receiving member. A bottom-loading screw is assembled by inserting the anchor head in a proximal direction through the bottom opening, and activating a securing means to prevent the anchor head from passing through the opening.
Another embodiment of the invention includes a bone anchor system. The system has at least one bone anchor having an anchor head 116, a shaft 118, a rod-receiving member 140 and a compression member 180. Also included in the system is an instrument 200 for driving the bone anchor assembly, a spinal fixation element 12, and a locking element 190 for securing the fixation element to the bone anchor. The individual components are as described above.
The components of the bone anchor assemblies described above may be manufactured from any suitable biocompatible material, including, but not limited to, metals and metal alloys such as titanium and stainless steel, polymers, ceramics, and/or composites thereof. The components may be manufactured from the same or different materials though manufacturing processes known in the art.
While the bone anchor assemblies and methods of the present invention have been particularly shown and described with reference to the exemplary embodiments thereof, those of ordinary skill in the art will understand that various changes may be made in the form and details herein without departing from the spirit and scope of the present invention. Those of ordinary skill in the art will recognize or be able to ascertain many equivalents to the exemplary embodiments described specifically herein by using no more than routine experimentation. Such equivalents are intended to be encompassed by the scope of the present invention and the appended claims.
It is also to be understood that the following claims are to cover all generic and specific features of the invention described herein, and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween.
