Spinal fixation system

Abstract

A a spinal fixation system comprises an elongated fixation plate having a bottom side for facing the vertebrae, an opposite top side extending generally parallel to the bottom side, and an elongated slot defined therethrough which extends substantially along the length of the plate. The slot has a lower edge at the intersection of the slot and the bottom surface and an upper edge at the intersection of the slot and the top surface with a number of concave depressions formed at each edge. In one embodiment, the concave depressions at the lower and upper edges of the slot form comparable scalloped surfaces. In another embodiment, the lower edge of the slot forms a single concave surface extending around the entire perimeter of the lower edge of the slot. The system further includes a pedicle screw and a nut for load transmission between the vertebrae and the fixation plate. The pedicle screw includes a threaded nut engaging portion, a threaded bone engaging portion and an intermediate portion therebetween. The intermediate portion has a rigid upper arcuate surface facing toward the nut engaging portion which is substantially complementary to the concave depressions in lower edge of the slot. The nut includes a threaded head portion for engaging the nut engagement portion of the pedicle screw, and a rigid lower arcuate surface which is substantially complementary to the concave depressions at the upper edge of the slot. In the assembled configuration of the spinal fixation system, the fixation plate is supported by the upper arcuate surface of the pedicle screw and is clamped between the pedicle screw and the nut. The concave depressions and the upper and lower arcuate surfaces coact to permit angulation of the pedicle screw relative to the elongated plate while maintaining load transmitting contact over a wide range of angles of the pedicle screw relative to the fixation plate.