Background: This study aimed to investigate whether arithmetic hip-knee-ankle angle (aHKA) can be a surrogate for valgus stress radiography (VSR). Furthermore, we investigated the optimal cutoff values for preoperative radiographic parameters, thereby validating the appropriate inclusion criteria for prearthritic/kinematic unicompartmental knee arthroplasty (prearthritic-UKA).
Methods: We retrospectively analyzed 123 patients who underwent medial UKA. We measured the HKA on VSR, termed sHKA. The aHKA was calculated by subtracting the lateral distal femoral angle from the medial proximal tibial angle. Prearthritic-UKA was defined as a postoperative HKA angle within 3° of the aHKA. We divided the patients into the prearthritic-UKA and non-prearthritic-UKA groups. To assess whether the aHKA serves as a surrogate for VSR, Spearman's rank correlations were performed among the radiographic parameters. These radiographic parameters calculated the proper criteria for prearthritic-UKA using receiver operating characteristic (ROC) curve analysis and logistic regression analysis.
Results: Overall, 59 patients were classified into the prearthritic-UKA group. The aHKA was larger than the sHKA and exhibited no significant correlation with the sHKA. ROC analysis revealed that the cutoff values of aHKA for detecting overcorrected UKA (i.e., postoperative HKA - aHKA >3°) was -5.0°. Similarly, the cutoff values for detecting undercorrected UKA (i.e., postoperative HKA - aHKA <-3°) were -3°. Logistic regression analysis revealed that the aHKA was significantly associated with prearthritic-UKA.
Conclusion: The aHKA was not identified as a surrogate for the sHKA. Comprehensive preoperative radiographic assessment, including both aHKA and VSR, is crucial for optimizing UKA outcomes and minimizing risks of misalignment.
Keywords: Arithmetic hip–knee–ankle angle; Mechanical hip–knee–ankle angle; Unicompartmental knee arthroplasty; Valgus stress radiography.
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