Residual Hindfoot Valgus Alignment Detected by True Mechanical Axis Is Associated with Poorer Clinical Outcomes after Total Knee Arthroplasty: The Role of Ankle Osteoarthritis.

Background: Lower-limb alignment should be accurately assessed to achieve favorable outcomes after total knee arthroplasty (TKA). Compared to conventional alignment methods, the recently adopted hip-to-calcaneus (HTC) axis better reflects actual weight-bearing alignment by including hindfoot alignment. However, whether discrepancies between two mechanical alignments vary according to ankle osteoarthritis (OA) and whether such discrepancies are clinically meaningful remains unclear.

Methods: Data from 145 knees from 107 patients who underwent primary TKA for knee OA were retrospectively analyzed. Postoperative alignment was assessed using conventional hip-to-talus (HTT) and HTC methods. Patients were grouped by ankle OA and residual hindfoot valgus alignment, defined as more valgus on HTC than HTT. Clinical outcomes were evaluated using Western Ontario and McMaster University Osteoarthritis Index (WOMAC) scores. Logistic regression analyses determined factors associated with residual valgus alignment and clinical outcomes. Sobel tests assessed whether residual valgus alignment mediated the relationship between ankle OA and postoperative clinical outcomes.

Results: Patients who have ankle OA were significantly more likely to exhibit residual valgus alignment as measured by the HTC method (odds ratio = 2.59; 95% confidence interval, 1.18 to 5.72; P = 0.018). In multivariate analyses, residual hindfoot valgus alignment (P = 0.030) and older age (P = 0.033) were independently associated with worse postoperative WOMAC scores. However, ankle OA did not impact WOMAC outcomes (P = 0.94). Hindfoot valgus alignment may have a mediating effect, although this was not statistically significant (P = 0.093).

Conclusion: Ankle OA was significantly associated with residual hindfoot valgus alignment based on HTC. Although ankle OA did not directly affect clinical outcomes, residual valgus alignment and older age were independently associated with worse postoperative WOMAC scores.