B-twin expandable spinal spacer for posterior lumbar interbody stabilization: mechanical testing

Posterior lumbar interbody fusion is an accepted surgical technique to treat disabling lower back pain due to degenerative disc disease. In the techniques that prevail, installation of large fixed-size twin cages dictate the sacrifice of the posterior stabilizing structures. Moreover, excessive retraction of the dural sac imposes potential neurological hazard. The authors present a novel technique based on a spacer capable of threefold expansion once it has been installed in the disc space. The spacer was laboratory tested under controlled loading conditions. Strength and fatigue tests of an isolated spacer were performed using an artificial model. Pullout resistance and ultimate compression strength of the surgical construct were evaluated using a cadaveric specimen. The yield force and the ultimate force for the single spacer averaged 2660 +/- 483 N and 4313 +/- 420 N, respectively, while the endurance limit at 5 million cycles averaged 931 N. The single-spacer resistance to pullout averaged 556 +/- 207 N, while the ultimate compressive strength of bone-spacer interface averaged 3399 +/- 136 N for a pair of spacers. The results of the study indicate that the B-twin ESS is expected to withstand the loads imposed upon it during everyday activity and resist migration or significant subsidence until fusion is achieved.