Finite element analysis for mechanism of occasional anterior longitudinal ligament rupture with posterior correction procedure for adult spinal deformity using LLIF

Abstract

Abstract Background: There are several advantages of using lateral lumbar interbody fusion (LLIF) for correction surgeries for adult spinal deformity (ASD); however, we currently have unresolved new issues, including occasional anterior longitudinal ligament (ALL) rupture during posterior correction procedure. When LLIF was initially introduced, only less lordotic cages were available and we experienced ALL rupture more frequently than later periods when more lordotic cages were available. We performed finite element analysis (FEA) regarding the mechanism of ALL rupture during a posterior correction procedure. Methods: We introduced a spring (which mimics ALL) at the location of ALL in the FEA and employed a LLIF cage with two different lordotic angles: 6 and 12 degrees (6DC/12DC). To assess the extent of burden on the ALL, we measured the extension length of the spring during the correction procedure and examined the location of the rotation center. Results: We observed significantly higher degree of length extension of the spring during the correction procedure in the FEA model with 6DC compared with that of 12DC. We also observed that the location of the rotation center was shifted posteriorly in the FEA model with 6DC compared with that of 12DC. Conclusions: It is considered that the posterior and rostral edge of the less lordotic angle cage became a hinge, and the longer lever arm increased the burden on ALL as the principle of leverage. It is important to use a LLIF cage with a sufficient lordotic angle, that is compatible with the degree of posterior osteotomy in ASD correction.