Load sharing and kinematics of threaded cages for lumbar interbody fusion.

The load-sharing characteristics of threaded interbody cages before and after cyclic loading are poorly understood. In the current study, lumbar interbody cages were filled with epoxy, sectioned longitudinally, and pressure sensors were placed between halves of the cages to measure the distribution of loads between and within the cages. Human lumbar spine segments were instrumented anteriorly with bilateral cages and subjected to cyclic compression loads combined with flexion and extension moments. Sagittal plane motion between vertebrae on either side of the cages also was measured during application of cyclic compression and flexion loads. A small but statistically significant asymmetry was found in the distribution of load between the left and right cages, and the extent of asymmetry varied during compression and flexion loading. With ligament tension only, 66% of the load was supported by the posterior regions of the cages, whereas during peaks in the combined compression and flexion loading, only 33% of the load was supported by the posterior regions. The cages reduced intervertebral motion 78% during forward flexion, whereas extension resulted in a 100% increase in motion. Surgeons should recognize that bilateral cages may not share loads equally, and the results of this study suggest that early extension should be restricted after the placement of anterior lumbar interbody cages.