A Lateral Expandable Cage with Independently Adjustable Anterior and Posterior Heights Can Improve the Pressure Distribution at the Cage-Endplate Interface: A Biomechanics Study

Abstract

ObjectiveThis study investigated how the expansion trajectory of a lateral expandable cage affects pressure distribution at the cage-endplate interface under well-controlled biomechanical loading conditions. MethodsThree unique vertical height expansion trajectories used by clinically relevant lateral expandable cages were evaluated: craniocaudal, fixed-arc, and independently-adjustable anterior and posterior (IAP) height expansion. Two biomechanical loading scenarios were performed. The first scenario used custom bone foam test blocks to assess resultant pressure distribution at varying test block lordotic angles and expansion heights. The second scenario performed simulated expansion using synthetic spine units and compared the pressure distribution following expansion. ResultsFor an expandable cage with craniocaudal expansion, the pressure distribution at the cage-endplate interface was found to be heavily dependent on the lordotic angle of the test block (p<0.001) but not expansion height (p=0.634). The greatest maximum pressure occurred at higher test block lordotic angles. For an expandable cage with fixed-arc expansion, the pressure distribution shifted anteriorly throughout expansion. In the simulated expansion trials, an expandable cage with IAP expansion was found to improve the pressure distribution at the cage-endplate interface reducing the maximum pressure measurements by 22% and 14% compared to the craniocaudal and fixed-arc expansion respectively. ConclusionsOf the cage designs evaluated within this study, an expandable cage with independently adjustable anterior and posterior heights lowered the maximum pressure measured at the cage-endplate interface and alleviated the potential of cage edge loading, both important considerations fundamental for a successful fusion procedure and the mitigation of implant subsidence risk.