Effect of Axial and Coronal Malalignment on Polyethylene Stresses in Lumbosacral Total Joint Replacement: A Finite...

The purpose of this study was to analyze the sensitivity of polyethylene stresses and strains, as proxies for wear and surface damage, to coronal and axial misalignment for a novel lumbar total joint replacement (LTJR) implanted at L5-S1. We hypothesized that these stresses and strains would remain below the levels associated with worst-case impingement in a spine wear simulator. A finite element model (FEM) of the L4-S1 spine was developed using CT-based anatomy from a representative Investigational Device Exemption study patient. An appropriately sized LTJR was virtually implanted at L5-S1 in a standardized baseline orientation, and component positioning was altered to achieve eight independent misalignment scenarios, spanning axial convergence angle, anterior-posterior offset, and coronal tilt. Physiologic forward-bending loads were applied, and polyethylene contact pressure, von Mises stress, and effective strain were recorded. Peak values for the baseline scenario were 35.0 MPa, 19.9 MPa, and 4.8%, respectively. Convergence angle and anterior-posterior offset had minimal effect, whereas coronal tilt produced the largest changes. Contact pressure and von Mises stress varied by <20% due to misalignment, whereas effective strain was more sensitive, increasing from 4.8% at baseline to 9.9% with 20 degrees of coronal tilt. All values were comparable to prior L4-L5 findings and remained well below impingement levels. Within the bounds of reasonable misalignment, the device maintained bearing congruency and did not approach conditions associated with accelerated wear.