Summary

Preoperative %MA less than 15% and MPTA less than 84° as significant risk factors for misalignment in Unicompartmental Knee Arthroplasty (UKA), emphasizing the need for careful preoperative planning and precise intraoperative adjustments to ensure optimal postoperative alignment and long-term success.

Abstract

Objective

In Unicompartmental Knee Arthroplasty (UKA), achieving good postoperative lower limb alignment is crucial for long-term success. Although Kennedy et al. reported that ideal postoperative weight-bearing line in medial UKA is recommended to pass through between the center of the medial tibial articular surface and the lateral intercondylar eminence (Zone K). However, the preoperative factors affecting the postoperative wight-bearing line are not fully understood. This study aims to identify preoperative factors that contribute to the appropriate weight-bearing line after UKA.

Methods

This study included 92 knees diagnosed with either isolated medial compartmental osteoarthritis or spontaneous osteonecrosis. All cases received primary medial fixed-bearing UKA (Partial Knee System; Zimmer Biomet). Patients were categorized into two groups: Group K, where the postoperative weight-bearing axis passed through zone K, and Group M, where it passed more medially. Preoperative patient characteristics (sex, age, BMI, range of motion) and radiographic parameters (%MA, MPTA, mLDFA, JLCA) were compared between the two groups.

Results

Group K included 50 knees, and Group M included 42 knees. No patient had detected postoperative weight-bearing line at lateral compartment. There were no significant differences in patient characteristics between the groups. Preoperative %MA (Group M: 7.3±11.5%, Group K: 23.5±12.3%), and MPTA (Group M: 82.1±3.4°, Group K: 84.3±2.5°) (p<0.01) were detected significant difference between the groups. Logistic regression analysis revealed that preoperative %MA and MPTA were significant factors, with cutoff values of 15.9 for %MA and 84.4° for MPTA identified by ROC curve analysis. Comparing the area under the curve, preoperative %MA was found to have the most significant impact (%MA: 0.83, MPTA: 0.69, p=0.034).

Discussion

The importance of %MA in achieving optimal alignment in UKA is similar to its role in around the knee osteotomy. %MA is a critical parameter because it is not influenced by skeletal structure such as bone length, bowing and individual height. The current study revealed that a preoperative %MA of less than 15% and an MPTA of less than 84° were associated with a higher risk of misalignment after UKA, which underscores the need for careful consideration of %MA during preoperative planning. In case of small MPTA, intraoperative adjustments become paramount. Specifically, fine tuning the amount of tibial resection and carefully selecting the thickness of the insert are essential in achieving the desired intra-articular correction. These adjustments help ensure that the postoperative limb alignment closely with zone K, which is crucial for the long-term success of UKA.