INTRODUCTION: Spinal fusion is used to treat disabling back pain arising from a failed motion segment. Recently, transfacet pedicle screws (TFPS) of the facet joint have been investigated as an alternative to bilateral pedicle screw (BPS) and rod fixation systems in the lumbar spine; 1) as an adjunctive posterior surgery following interbody fusion; and 2) as a stand‐alone form of fixation to traditional posterior techniques.
METHODS: Two groups of seven fresh frozen human cadaver lumbar spines (L3‐S1) were tested by applying pure moments of ± 6 Nm. Each specimen in both groups was tested in a primary and circumferential fusion (ALIF spacer, spacer+plate) environment. Both TFPS and BPS were used as posterior fixation (Fig. 1). Motion (ROM) was obtained at L4‐L5 for single‐level constructs in flexion‐extension, lateral bending and axial rotation modes.
RESULTS: In primary fixation, TFPS and BPS statistically reduced motion below intact levels. Fixation with BPS restricted motion more than TFPS in all modes, except for flexion‐extension, but often with minimal differences. In circumferential fixation, incorporating an anterior spacer and plate with posterior instrumentation proved to be more beneficial for pedicle screw fixation than facet fixation. The interpretation is explained by the location of fixation with respect to the center‐of‐rotation of the vertebral bodies. In lateral bending and axial rotation, BPS constructs were stiffer, though not statistically. (cont'd)
DISCUSSION: In flexion‐extension, TFPS screws provided similar fixation to bilateral pedicle screws in primary and circumferential fixation (Fig. 2). Results suggest that TFPS may be an equivalent alternative in either primary fusion or circumferential fusion which incorporates an anterior plate, as the differences in ROM and ultimate stability between facet and pedicle screws are minimized. Biomechanically, facet screws represent a stable alternative to pedicle screw systems.