The present invention relates generally to the field of spinal implant systems of the type which employ spinal rods connected at various locations along the spinal column by various fixation elements including spinal screws and, more particularly, to a spinal screw fixation element which provides variable angle adjustability.
Several systems have been developed for use in correcting and stabilizing spinal curves and facilitating spinal fusion. In one system, a bendable rod is longitudinally disposed adjacent the vertebral column and is fixed to various vertebrae along the length of the column by way of a number of fixation elements. A variety of fixation elements can be provided, such as hooks or bone screws, which are configured to engage specific portions of the vertebra.
An example of one such system is the TSRH.TM. spinal system of Danek Medical, Inc. In this system, the hooks or screws are engaged to the spinal rod by way of eyebolts. As is well known in the art, the eyebolts are received over the spinal rod and captured within a yoke means formed on the head of the fixation hook or screw. A nut is threaded onto a threaded post of the eyebolt to clamp the yoke and to provide a three-point shear clamp force positively locking the hook or screw element to the spinal fixation rod. Details of the TSRH spinal implant system are disclosed in the "Surgical Technique Manual" provided by Danek Medical, Inc., published in 1990, which disclosure is incorporated herein by reference.
It is the goal of the surgeon using such spinal implant systems to apply the vertebral fixation elements (hooks and/or screws) to the spine in the appropriate anatomic position, and then to engage each fixation element to the spinal rod. Once the spinal implant system is assembled it is then possible to correct anatomical deformities and stabilize the spine. In order to perform this procedure with a minimum of patient trauma, it is important that the system used be relatively easy to install. Further, it is highly desirable that the system permit three dimensional adjustment of the bone screw fixation elements in order to take advantage of the most desirable fixation sites in the spinal column. Ideally, a mechanism providing such adjustment should be designed in a manner which does not create undue difficulty in installing the spinal rod or detract from the overall rigidity of the system.
Various mechanisms have been employed in spinal implant systems to provide a three dimensional adjustment capability. U.S. Pat. No. 4,946,458 to Harms et al. discloses a pedicle screw provided with a ball and socket type arrangement for permitting angulation of the bone screw relative to the receiver.
U.S. Pat. No. 4,662,365 to Gotzen el al describes an external bone fixator employing radially splined interdigitation elements to vary the angular orientation of bone screws. However, the arrangement is far too cumbersome and complex for internal fixation to the spinal column.
U.S. Pat. No. 4,987,892 to Krag et al. discloses a spinal fixation system employing pedicle screws having radially splined heads interdigitating with radial splines or teeth integrally formed upon articulating clamps which in turn clamp about a stabilizing rod. The articulating clamp arrangement requires the radial splines be offset relative to the stabilizing rod. This offset creates alignment problems which make it difficult, if not impossible, to permit the screws to be angularly rotated in a plane perpendicular to the axis of the stabilizing rod. Furthermore, the Krag et al. system requires a specially formed articulating clamp for the pedicle screw. If hooks, crosslinks or other fixation elements are to be connected to the stabilizing rod, differently configured rod attachment elements would be needed. This undesirably complicates the overall structure and installation method. Another difficulty of the Krag et al. system is that it is not an open design. In other words, the articulating clamp must be engaged on the rod prior to insertion. On the contrary, a more beneficial design would permit dropping the spinal rod into the screw connector.
German Patent No. 3,219,575 C2 to Kluger discloses a spinal implant system which provides angular adjustment of the bone screws by relative movement of interdigitating disk contact surfaces on corner pieces within which each bone screw is mounted. The system is mechanically complex, in that separate adjusting means are needed to angularly adjust the orientation of the bone screws and to fix the position of the bone screws relative to the corner pieces and to one another. Any rotational adjustment of the bone pins in a plane normal to the spinal rods changes the distance between the pins, requiring further adjustment of the sleeve nut. Further, the system is limited to situations where only two pins or other fixation elements are required on the same rod.