Several techniques and systems have been developed for correcting and stabilizing damage or malformation of bones, especially the long bones and the spine. In one type of system, an elongated member such as a bendable rod is disposed longitudinally along a length of the bone(s). In spinal applications, the rod is preferably bent to correspond to the normal curvature of the spine in the particular region being instrumented. For example, the rod can be bent to form a normal kyphotic curvature for the thoracic region of the spine, or a lordotic curvature for the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along a length of the spinal column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of the vertebra and other bones. For instance, one such fixation element is a hook which is configured to engage the laminae of the vertebra. Another very prevalent fixation element is a screw that can be threaded into various parts of the vertebrae or other bones.
In one typical spinal procedure utilizing a bendable rod, the rod is situated on opposite sides of the spine or spinous processes. A plurality of bone screws are threaded into a portion of several vertebral bodies, very frequently into the pedicles of these vertebrae. The rods are affixed to this plurality of bone screws to apply corrective and stabilizing forces to the spine.
One example of a rod-type spinal fixation system includes elongated rods and a variety of hooks, screws, and bolts, all configured to create a segmental construct throughout the spine. In one aspect of the system, the spinal rod is connected to the various vertebral fixation elements using eyebolts. In this configuration, the fixation elements are engaged to the spinal rod laterally adjacent to the rod. In another aspect of the system, a variable angle screw may be engaged to the spinal rod with an eyebolt. The variable angle screw allows pivoting of the bone screw in a single plane parallel to the plane of the spinal rod. Details of this type of system can be found in U.S. Pat. No. 5,261,909 to Sutterlin et al., the disclosure of which is incorporated by reference herein. This type of system allows a surgeon to apply vertebral fixation elements, such as a spinal hook or a bone screw, to the spine in appropriate anatomic positions and also allows the surgeon to easily engage a bent spinal rod to each of the fixation elements for final tightening.
Another rod-type fixation system provides a variety of fixation elements for engagement between an elongated rod and the spine. In one aspect of the system, the fixation elements themselves include a body that defines a slot within which the spinal rod is received. The slot includes a threaded bore into which a threaded plug is engaged to clamp the rod within the body of the fixation element. The system uses hooks and bone screws having this “open-back” configuration. Details of this type of system can be found in U.S. Pat. No. 5,005,562, the disclosure of which is incorporated by reference herein.
The fixation elements of these types of systems are capable only of pivoting about the spinal rod to achieve variable angular positions relative to the rod. While this limited range of relative angular positioning may be acceptable for many spinal pathologies, many other cases require more creative orientation of a fixation element (such as a bone screw relative) to a spinal rod. While certain aspects of this problem may be addressed by the variable angle screw of the '909 patent type of system, there remains a need for a bone screw capable of angular orientation in multiple planes relative to the spinal rod, as well as multiple spherical head orientations. Preferably, the bone screw axis is capable of various three dimensional orientations with respect to the spinal rod. Screws of this type of angular orientation in multiple planes relative to the spinal rod have been referred to as poly-axial or multi-axial bone screws. The use of both angular orientations in multiple planes relative to the spinal rod allows for virtually unlimited axial angulations of the bone engaging screw member within the design parameters as well as an ultra-low profile of the said device utilizing a minimum of components without sacrificing the security of the interfaces of the invention components.
Others have approached the solution to this problem with various poly-axial screw designs. For example, U.S. Pat. No. 5,466,237 to Byrd et al., the disclosure of which is incorporated by reference herein, describes a bone screw which includes a spherical projection on the top of the bone screw. An externally threaded receiver member supports the bone screw and a spinal rod on top of the spherical projection. An outer nut is tightened onto the receiver member to press the spinal rod against the spherical projection to accommodate various angular orientations of the bone screw relative to the rod. While this particular approach utilizes a minimum of components, the security of the fixation of the bone screw to the rod is lacking. In other words, the engagement or fixation between the small spherical projection on the bone screw and the spinal rod is readily disrupted when the instrumentation is subjected to the high loads of the spine, particularly in the lumbar region.
Another approach is shown in U.S. Pat. No. 4,946,458 to Harms et al., the disclosure of which is incorporated by reference herein. A spherical headed bone screw is supported within separate halves of a receiver member. The bottoms of the halves are held together by a retaining ring. The top of the receiver halves are compressed about the bone screw by nuts threaded onto a threaded spinal rod. In another approach taken by Harms et al., in U.S. Pat. No. 5,207,678, the disclosure of which is incorporated by reference herein, a receiver member is flexibly connected about a partially spherical head of a bone screw. Conical nuts on opposite sides of the receiver member are threaded onto a threaded rod passing through the receiver. As the conical nuts are threaded toward each other, the receiver member flexibly compresses around the head of the bone screw to clamp the bone screw in its variable angular position. One detriment of the systems in the two Harms et al. patents is that the spinal rod must be threaded in order to accept the compression nuts. It is known that threading rods can tend to weaken the rods in the face of severe spinal loads. Moreover, the design of the bone screws in these patents requires a multiplicity of parts, which makes it fairly complicated to achieve complete fixation of the bone screw.
A further approach is illustrated in U.S. Pat. No. 5,797,911 to Sherman et al., the disclosure of which is incorporated by reference herein. A U-shaped holder is provided, through the top of which a bone fastener topped with a crown member is loaded. The holder accommodates a rod in a channel above the crown member and a compression member above the rod. The compression member presses on the rod and crown member to lock the fastener against the holder in any of a number of angles in three dimensions with respect to the rod. This approach has proven to be quite effective in addressing the above-identified problems. However, it does not permit bottom-loading of the fastener. Additionally, the holder is somewhat bulky in order to accommodate the other structural components.
Yet a further approach is shown in U.S. Pat. No. 5,733,285 to Errico et al., the disclosure of which is incorporated by reference herein. In this system, a holder is provided with a tapered and colletted portion at the bottom into which a bone fastener head is inserted. A sleeve is provided that slides down around the colletted portion to crush lock the colletted portion around the head of the bone fastener. This apparatus is believed to be relatively bulky and difficult to manipulate given the external sliding locking mechanism. It is further dependent on the fit of the external sleeve and the relative strength of the collet and its bending and crushing portions for secure locking of the bone fastener head.
There is therefore a need remaining in the industry for an ultra-low profile, multi-axial/double-locking bone anchor that can be readily and securely engaged to an elongated member of any configuration—i.e., smooth, roughened, knurled or even threaded—which achieves greatly improved angulations of the bone anchor, improved strength, and reduced size, including profile and bulk, of the components used to engage the bone anchor to the elongated member in any of a variety of angular orientations.