Is Anterior Longitudinal Ligament Rupture During Posterior Corrective Surgery for Adult Spinal Deformity a Phenomenon Unique to When Combined with Lateral Lumber Interbody Fusion? -Finite Element Analysis with Comparison to When Combined with Posterior Lumber Interbody Fusion.

Background: The occurrence of ALL rupture during posterior correction of adult spinal deformity (ASD) was rare before the introduction of lateral lumbar interbody fusion (LLIF) but has become more frequent recently. It remains unclear whether this phenomenon is unique to LLIF-combined procedures or primarily related to enhanced corrective ability. Methods: The research method used in this study is finite element analysis (FEA). Using preoperative computed tomography images, LLIF cage (L group) or posterior lumbar interbody fusion (PLIF) cage (P group) were placed in the disc space with identical lordotic angles and distances from the anterior vertebral body edge for the same patients' samples. Finite element simulations of corrective procedures were conducted. A spring simulating the ALL was introduced into the FEA, and the load on the ALL was evaluated with either LLIF or PLIF cage placement. Spring elongation directly measured the load on the ALL, while the location of the rotation center served as an indirect evaluation. Two different types of corrective procedures were created, one of which is mimicking ASD correction. For both procedures, the load to ALL was measured using abovementioned parameters when either LLIF cage (L group) or PLIF cage (P group) was used. The load to ALL was compared between L group and P group. Results: The degree of spring elongation during the simulation of a corrective procedure significantly decreased in the L group compared to the P group only in the model which is mimicking ASD correction (p = 0.006, Cohen's d = 2.33, Power (1-β) = 0.956). The rotation center was significantly more posteriorly located in the P group than that in the L group in both models. These differences were more obvious in the model mimicking ASD correction (p = 0.0013, Cohen's d = 2.00, Power (1-β) = 0.891). Conclusions: Our findings suggest that the use of a PLIF cage, which has a longer anterior-posterior cage length, caused the posterior edge of the cage to act as a pivot point. This configuration places greater leverage on the ALL, potentially leading to rupture during posterior correction procedures. This phenomenon, ALL rupture during posterior correction for ASD, is thought to be associated with increased corrective capabilities rather than being specific to the geometry of the LLIF cage.