Conventional mobile-bearing unicompartmental knee arthroplasty effectively achieves personalised alignment: A single surgeon series of 2472 knees

Abstract

Aims: This study aimed to determine the alignment achieved with a conventional technique of mobile-bearing unicompartmental knee arthroplasty (MB UKA) in a large single-surgeon series. Specifically, it investigated whether MB UKA restores constitutional alignment, correlated preoperative and postoperative alignment, and assessed the reliability of preoperative arithmetic HKA (aHKA) as a predictor of postoperative alignment.

Introduction: UKA effectively treats anteromedial osteoarthritis, but there is no unanimity regarding optimal postoperative alignment. Personalised alignment, aiming to restore pre-arthritic biomechanics, is gaining prominence. While robotic-assisted UKAs offer precision, a large series reporting conventionally performed mobile-bearing UKA alignment outcomes using full-length radiographs has been lacking.

Materials and methods: This retrospective analysis included prospectively collected data from 2472 consecutive cemented Oxford Phase 3 medial UKAs performed by a single surgeon. Patients were divided into two groups: Group 1 (n = 272) had unilateral UKA with an asymptomatic contralateral knee, and Group 2 (n = 2200) comprised bilateral UKAs or unilateral UKAs with an affected contralateral knee. Full-length hip-to-ankle radiographs assessed Hip-Knee-Ankle (HKA) angle, Knee Joint Line Obliquity (KJLO), and Mechanical Axis Deviation (MAD). Arithmetic HKA (aHKA) was also determined.

Results: In Group 1, the mean postoperative HKA angle (175.7°±2.8°) was not significantly different from the contralateral unaffected knee (175.4°±3.2°, p = 0.106). Postoperative HKA was within ±3° of the contralateral limb in 91 % of patients, with a strong positive correlation (r = 0.52, p < 0.001). KJLO was also similar between operated (91.6°±2.6°) and contralateral limbs (90.2°±2.8°), and 86.4 % were within ±3°. MAD distribution postoperatively was similar to the contralateral limb, with 54 % in zone 2, 30 % in zone 1, and 13 % in zone C. In Group 2, the mean preoperative HKA (170.7°±3.86°) significantly improved to postoperative HKA (176.2°±2.8°, p < 0.001). Postoperative MAD demonstrated improved alignment, with shifts from zone 0 towards zones 1 and 2, and a prevalence in zones 2, C, and 1. A strong negative correlation existed between ΔHKA and preoperative HKA (r = -0.695, p < 0.001). Preoperative aHKA showed only a weak correlation with postoperative HKA (r = -0.421, p < 0.001).

Discussion: This study demonstrates that conventional MB UKA effectively restores alignment close to the native or pre-arthritic state, consistent with personalised alignment principles. Preoperative mean varus of 9^o was corrected to approximately 4^o varus postoperatively. The strong correlation between postoperative HKA and the contralateral normal limb's HKA, similar KJLO and MAD distribution, support the restoration of physiological alignment. The study also highlights a strong correlation between the extent of preoperative varus deformity and the quantum of correction achieved. While alignment targets for UKA remain controversial, our findings show that restoration of native alignment occurs, which is associated with superior patient-reported outcomes. The limited reliability of aHKA for larger deformities was also observed.

Conclusions: Conventional mobile-bearing UKA consistently delivers personalised alignment, restoring the mechanical axis and joint line obliquity close to the patient's native state, most evident in cases with an unaffected contralateral limb. This large series supports that optimal alignment can be achieved without robotic assistance, so as to confer the potential benefits of alignment restoration on patient outcomes.

Keywords: Arthroplasty; Mechanical axis; Mobile-bearing UKA; Personalised alignment; Unicompartmental knee arthroplasty.