Flexibility analysis of posterolateral fusions in a New Zealand white rabbit model.

Abstract

Biomechanics of posterolateral spinal fusion were studied in an in vivo rabbit model. To determine the extent of stabilization produced by posterolateral lumbar fusion and to test the hypothesis that motions are not completely eliminated after successful fusion. Previous human cadaveric studies, clinical studies, and animal studies have attempted to characterize the biomechanics of posterolateral fusion. Such studies have been limited by either methods of fusion modeling or methods of stability testing. No previous study has examined biologic fusion with a physiologic biomechanical testing technique. Ten adult New Zealand white rabbits underwent L5-L6 intertransverse process fusion using autogenous iliac crest bone graft. Rabbits were killed 5 weeks after surgery. Only one time point was studied. This time point was chosen because previous pull-apart studies have shown plateauing of rabbit fusion mass strength and stiffness around this time. Spines were then harvested and evaluated with manual palpation and an established flexibility testing protocol. Resulting data were compared with previously acquired, nonoperative spine flexibility data. Two animals were excluded because of complications. Of those that were fused (n = 5), biomechanical testing revealed significant decreases in flexion (81%), extension (61%), and right and left lateral bending (67% and 83%, respectively) (P < 0.01). These findings define the amount of motion reduction that can be expected with posterolateral fusions in the rabbit model at 5 weeks. These results suggest that motion was significantly decreased but was not eliminated.