Summary

Valgus stress radiographs and long-leg films are extremely useful for preventing overcorrection in medial unicompartmental knee arthroplasty. It represents a very valuable contribution to the understanding of unicompartmental knee arthroplasty principles, which can serve to extend its indications, and increase reproducibility of the surgical technique.

Abstract

Objectives: Unicompartmental knee arthroplasty (UKA) is a procedure that could be done in near 50% of knee replacements, but it is known to be less reproducible than total knee arthroplasty. Avoiding overcorrection is the golden rule in the implementation of medial UKA, and restoring the primitive alignment is one of the main objectives. The aim of this study was to develop a strategy to detect the postoperative risk of overcorrection when implanting a medial UKA using preoperative radiographic data based on long leg films, valgus stress views and the joint line convergence angle (JLCA). Additionally, the study aimed to identify intraoperative protective factors that could help prevent overcorrection.

Methods

This retrospective study involved radiographic measurements of consecutive patients who underwent image-assisted robotic medial UKA between February 2022 and February 2023 at a single high-volume center. Postoperative overcorrection was defined as an HKA angle more or similar to 180°. Receiver Operating Characteristic (ROC) curves were constructed to evaluate the predictive ability of three parameters: "preoperative HKA" (preHKA) measured on long-leg films, "preoperative stress HKA" (sHKA) (HKA measured on preoperative valgus stress x-rays), and the "estimated HKA" (eHKA) (HKA + JLCA). The area under the curve (AUC) was calculated to determine the best parameter for predicting the risk of overcorrection. Cut-off points, along with their respective sensitivity (Se) and specificity (Sp), were determined using the Youden method. Subsequently, a subgroup analysis was performed on patients at risk to determine if any specific cut-off value of residual laxity (sHKA - postoperative HKA) measured intraoperatively could be identified as a protective factor against overcorrection.

Results

The study included ninety-five patients, of whom eight cases of overcorrection (8.4%, 8/95) were recorded with a mean postoperative HKA of 180.6° +- 0.5 [180.1-181.3]. The best predictor of overcorrection was preHKA (AUC=0.96) with a cut-off point of 176° (Se=100%, Sp=86%). This was followed by eHKA (AUC=0.94) with a cut-off point of 179.7° (Se=100%, Sp=86%), and sHKA (AUC=0.81) with a cut-off point of 181.1° (Se=100%, Sp=85%). In the subgroup of 20 patients with preHKA ≥176°, maintaining a residual laxity >2.6° was found to be a protective factor against overcorrection (0/8 in the overcorrected group versus 6/12 in the control group, p<.04).

Conclusions

PreHKA ≥176° was the best predictor of overcorrection. In this population, maintaining a residual laxity >2.6° was identified as a protective factor against overcorrection. SHKA and eHKA were also significative predictive factors, with a cut-off point >181.3º and 179.7º respectively. It represents a very valuable contribution to the understanding of UKA principles, which can serve to extend its indications, and increase reproducibility of the surgical technique.