Biomechanical Properties of Lumbar Vertebral Ring Apophysis Cage under Endplate Injury: A Finite Element Analysis

Abstract

Abstract Objective This study aimed to compare the biomechanical properties of lumbar interbody fusion involving two types of cages. The study evaluated the effectiveness of the cage spanning the ring apophysis, regardless of the endplate's integrity. Methods A finite element model of the normal spine was established and validated in this study. The validated model was then utilized to simulate Lateral Lumbar Interbody Fusion (LLIF) with posterior pedicle screw fixation without posterior osteotomy. Two models of interbody fusion cage were placed at the L4/5 level, and the destruction of the bony endplate caused by curetting the cartilaginous endplate during surgery was simulated. Four models were established, including Model 1 with an intact endplate and long cage spanning the ring apophysis, Model 2 with endplate decortication and long cage spanning the ring apophysis, Model 3 with an intact endplate and short cage, and Model 4 with endplate decortication and short cage. Analyzed were the ROM of the fixed and adjacent segments, screw rod system stress, interface stress between cage and L5 endplate, trabecular bone stress on the upper surface of L5, and intervertebral disc pressure (IDP) of adjacent segments. Results There were no significant differences in ROM and IDP between adjacent segments in each postoperative model. However, the ROM, the pressure of the cage-endplate contact surface, and the stress of the cancellous bone above L5 during the fixation stage tended to increase in the short cage model. This trend was more apparent under the condition of an incomplete endplate. Regardless of the integrity of the endplate, the long cage provided greater support strength compared to the short cage. The corresponding endplate, cancellous stress, and screw rod system stress exhibited a significantly lower trend with the long cage than with the short cage. Conclusions Caution should be exercised during endplate preparation and cage placement to maintain the endplate's integrity. The cage spanning the vertebral ring apophysis during lateral lumbar interbody fusion offers favorable biomechanical properties and prevents complications such as cage subsidence, internal fixation system failure, and screw and rod fracture.”