Unicompartmental knee arthroplasty cannot restore the functional flexion axis of a living knee to normal

Abstract

Purpose: The purpose of this study was to investigate the hypothesis that a medial unicompartmental knee arthroplasty might restore the functional flexion axis of a knee to normal. The flexion axis can be indirectly identified by tracking the vertical translation of anatomic landmarks that basically move around the flexion axis during a knee motion. If a unicompartmental knee could help restore the normal flexion axis, the anatomic landmarks after the arthroplasty would show the vertical translation similar to those of normal knees during a knee flexion.

Methods: While performing a squatting motion, the kinematics of 17 knees were determined before and after a medial unicompartmental arthroplasty to calculate the vertical translation of a clinical epicondylar axis, using a three- to two-dimensional registration technique through a single-plane fluoroscopic system incorporating a biplanar static radiography. The results were compared with a normal data, and a statistical analysis including a two-way repeated-measured analysis of variance was performed.

Results: For the medial end, from 10° to 100° knee flexion, normal, osteoarthritic, and unicompartmental knees had the average superior vertical translation of 7.3 ± 4.2, 4.3 ± 7.2, and 2.4 ± 3.1 mm, respectively, with statistical significance between normal and unicompartmental knees (p < 0.001). The vertical translation did not return to normal post-implantation.

Conclusions: A unicompartmental knee could not reproduce the normal flexion axis. As for clinical relevance, the changes of the implant design and surgical procedure may be necessary to obtain the normal flexion axis reproducing a normal motion.

Level of evidence: IV.

Keywords: 3D to 2D registration; Clinical epicondylar axis; Functional flexion axis; Osteoarthritis and normal knees; Unicompartmental knee arthroplasty; Vertical translation.