Biomechanical consequences of minor medial joint line elevation after medial unicompartmental knee arthroplasty

Medial joint line elevation (JLE) commonly occurs during unicompartmental knee arthroplasty (UKA), but its biomechanical effects remain unexplored. This study addresses this gap by evaluating the biomechanical consequences of global JLE across different range of flexion angles.

A three-dimensional finite element model of the knee was constructed from CT and MRI data and validated against published experimental data. Medial fixed-bearing UKA was simulated with three joint line conditions (native, + 1 mm, and + 2 mm global elevation) at four flexion angles (0°, 30°, 60°, 90°), resulting in 12 models. All models were subjected to physiological axial loads simulating daily activities for comparative biomechanical analysis.

The biomechanical impact of JLE was profoundly dependent on flexion angle and was mediated by induced coronal plane alignment changes. At full extension (0°), JLE triggered a compensatory valgus shift, increasing lateral compartment stress. During mid‑flexion (30° and 60°), a varus alignment trend led to a paradoxical "biphasic effect"-lateral cartilage stress decreased, while medial polyethylene insert stress increased, peaking with a 12.4% rise at 60° under 2 mm elevation. A critical finding was a sharp "stress concentration peak" in the tibial cancellous bone beneath the prosthesis at 60° flexion with 2 mm JLE, where stress surged by 55.5%. At 90° flexion, the pattern shifted again, with JLE re‑inducing a valgus trend and elevating lateral stress toward physiological levels.

This study reveals a novel, angle-dependent biomechanical cascade triggered by minor JLE, driven by dynamic alignment changes. Precise joint line restoration and avoidance of repetitive 60° flexion weight-bearing are suggested as potential considerations for surgical practice and postoperative rehabilitation, although this remains a hypothesis that requires clinical validation.