Sensitivity of Lumbar Total Joint Replacement to Axial and Coronal Plane Misalignment Using Computational Modeling

During lumbar total joint replacement (LTJR), component misalignment during implantation may affect the bearing surface interaction. In this study, validated computational models of the lumbar spine were used to investigate a range of clinically relevant misalignment scenarios.

A finite element model (FEM) of the LTJR, exposed to mode I (normal wear) and mode IV (impingement) wear boundary conditions, was previously validated following the ASME V&V 40 standard. The LTJR FEM was virtually implanted into a previously validated FEM of the lumbar spine (L3-L5) at L4 to L5. The model included vertebrae, major spinal ligaments, erector muscle forces, and intervertebral discs. Misalignment was introduced by adjusting the bilateral implant axial plane convergence angle (20°-40°), anterior-posterior offset (0-4 mm), and coronal plane tilt (±20°). Analyses were conducted using LS-DYNA3D (ANSYS) under boundary conditions simulating bending at the waist. Contact pressures and von Mises stresses were evaluated for each misalignment scenario and compared with those developed during mode I and mode IV impingement scenarios.

Axial plane convergence angle had minimal impact on contact stress and von Mises stress magnitude and distribution. Increasing anterior-posterior offset led to higher stresses on the anteriorly shifted component but did not significantly alter the overall stress pattern. Coronal tilt had the most substantial effect on both stress magnitude and distribution.

Overall, polyethylene stresses in all misalignment scenarios remained below mode IV impingement levels. Contact areas remained within the intended spherical bearing surfaces without signs of impingement. LTJR contact stresses were found to be reasonably insensitive to misalignment under boundary conditions representing bending at the waist.

This work assesses the impact of clinically relevant implant misalignment scenarios on the polyethylene stresses associated with damage and wear for a novel LTJR and offers best practice guidelines for surgeons.