Biomechanical effects of structural optimization on the ball-and-socket lumbar artificial disc: A finite element study.

Lumbar disc degeneration is a major contributor to low back pain and disability. Lumbar total disc replacement (LTDR) has been introduced as a motion-preserving alternative to spinal fusion, and its biomechanical performance is strongly influenced by prosthesis geometry. This study investigated the effects of two key geometric parameters, curvature radius and radial clearance, on the biomechanical behavior of a ball-and-socket LTDR. Three-dimensional finite element models of the L3-L5 lumbar spine were developed, incorporating prostheses with curvature radii ranging from 7 to 10 mm and radial clearances from 0.1 to 0.3 mm. Segmental range of motion (ROM) and adjacent intervertebral disc biomechanics were evaluated under physiological loading conditions. Increasing curvature radius substantially reduced flexion and lateral bending ROM while producing radius-dependent effects on axial rotation. It also led to elevated annulus fibrosus (AF) von Mises stress across all motion modes. Nucleus pulposus (NP) von Mises stress and strain increased modestly during flexion and lateral bending but decreased during axial rotation. Total deformation of both NP and AF decreased progressively. In contrast, increasing radial clearance moderately enhanced flexion and lateral bending ROM while producing negligible changes in stress and slightly modulating strain and total deformation. These results indicate that prosthesis geometry plays a key role in modulating spinal kinematics and adjacent segment loading following lumbar total disc replacement. Curvature radius exerts a stronger influence than radial clearance, which mainly serves as a secondary parameter for fine-tuning segmental motion.