BIOMECHANICAL ANALYSIS OF FACET AND GRAFT LOADING IN A SMITH-ROBINSON TYPE CERVICAL-SPINE MODEL

Objective. This study determined the effect of change in graft height on the forces across a Smith-Robinson graft as well as across the posterior elements of the same motion segment. Study Design. The study utilizes a strain gauge technique for the measurement of facet joint loading and a subminiature load cell for the measurement of graft loads. Summary of Background Data. A number of cases of Smith-Robinson procedures have had some form of collapse of the interspace and graft material after surgery. Some patients with collapse of the graft go on to have prolonged sclerotomal-type pain or pseudarthrosis. The appropriate amount of distraction is not well defined in the literature and may affect the outcome. Methods. Cervical spines (C5-C6) were instrumented by placing strain gauges bilaterally on the pedicles of C6 (to measure the forces across the posterior elements). A miniature load cell with matching metallic shims was used to measure the force across the graft site and to distract the segment. Forces across the posterior elements and the graft site were measured, during flexion loading, and compared as the disc space was distracted. Results. The ratio of posterior element load to graft load with increasing disc space distraction significantly decreased from 1.06 +/- 0.65 (1.4 mm distraction) to 0.30 +/- .13 (4.6 mm distraction) (P < .03). The posterior element load decreased significantly after the same distraction, from 46.1 +/- 22.0 to 18.7 +/- 9.7 N/Nm (P < .05). Conclusions. In spondylotic specimens (4-5 mm disc heights) disc space distraction in excess of 3.0 mm from preoperative height caused a significant decrease in both the ratio of posterior element to graft loading and posterior element loads. These findings may help explain recent clinical reports of a limit of effective disc space distraction.