Beyond valgus stress radiography: arithmetic HKA angle (aHKA) as a superior predictor of limb alignment after UKA.

Abstract

Purpose: Accurate prediction of postoperative coronal alignment is essential for successful outcomes following medial unicompartmental knee arthroplasty (UKA). Traditionally, valgus stress femorotibial angle (FTA) has been used to estimate the correctability of varus deformity; however, its reliability is limited by dependence on soft tissue behavior and examiner technique. In contrast, the arithmetic hip-knee-ankle angle (aHKA), calculated from bony anatomy, offers an objective and reproducible measure of constitutional limb alignment. While early studies suggest aHKA correlates well with postoperative alignment, direct comparison with valgus stress FTA has been lacking. This study aimed to compare the predictive accuracy of aHKA and valgus stress FTA for postoperative alignment and alignment correction (ΔHKA) in medial UKA.

Methods: This retrospective study included 125 knees undergoing medial fixed-bearing UKA. Preoperative aHKA was calculated from long-leg radiographs, and valgus stress FTA was measured using a Telos arthrometer. Postoperative hip-knee-ankle angle (HKA) was obtained from standardized radiographs. Correlation analyses were performed between postoperative HKA and both aHKA and 360°-valgus stress FTA. Similarly, correlations were assessed between ΔHKA and (aHKA-preoperative HKA) as well as (360°-valgus stress FTA-preoperative HKA). Fisher's Z-test was used to assess differences in correlation strengths.

Results: Postoperative HKA showed stronger correlation with aHKA (R² = 0.5003, P < 0.001) than with 360°-valgus stress FTA (R² = 0.1157, P < 0.001), with a statistically significant difference (Z = -4.12, P < 0.001). ΔHKA was more strongly associated with aHKA-preoperative HKA (R² = 0.3805, P < 0.001) than with 360°-valgus stress FTA-preoperative HKA (R² = 0.1111, P < 0.001) (Z = -2.92, P = 0.0036).

Conclusion: aHKA demonstrated superior predictive accuracy for both postoperative alignment and alignment correction compared to valgus stress FTA. As a bone-based and examiner-independent parameter, aHKA is a valuable tool for preoperative planning in medial UKA and may reduce the need for stress radiography. Video Abstract.