BIOMECHANICAL ANALYSIS OF FACET AND GRAFT LOADING IN A SMITH-ROBINSONTYPE 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 post erior elements of the same motion segment. Study Design. The study uti lizes a strain gauge technique for the measurement of facet joint load ing and a subminiature load cell for the measurement of graft loads. S ummary of Background Data. A number of cases of Smith-Robinson procedu res have had some form of collapse of the interspace and graft materia l after surgery. Some patients with collapse of the graft go on to hav e prolonged sclerotomal-type pain or pseudarthrosis. The appropriate a mount of distraction is not well defined in the literature and may aff ect the outcome. Methods. Cervical spines (C5-C6) were instrumented by placing strain gauges bilaterally on the pedicles of C6 (to measure t he forces across the posterior elements). A miniature load cell with m atching 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 compare d as the disc space was distracted. Results. The ratio of posterior el ement load to graft load with increasing disc space distraction signif icantly decreased from 1.06 +/- 0.65 (1.4 mm distraction) to 0.30 +/- .13 (4.6 mm distraction) (P < .03). The posterior element load decreas ed 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 m m disc heights) disc space distraction in excess of 3.0 mm from preope rative height caused a significant decrease in both the ratio of poste rior element to graft loading and posterior element loads. These findi ngs may help explain recent clinical reports of a limit of effective d isc space distraction.