Objective The influence of knee alignment on the rate of hip joint degeneration remains unclear. This study aimed to clarify the relationship between pre- and postoperative knee alignment in total knee arthroplasty (TKA) and ipsilateral hip joint space narrowing (mm/year), and to assess the utility of a novel index incorporating acetabular morphology. Methods A retrospective observational study was performed on patients who underwent TKA at our institution from 2012 to 2020. Those with Kellgren–Lawrence grade II or higher hip osteoarthritis, rheumatoid arthritis, or prior hip surgery were excluded. A total of 345 patients (690 hips) were analyzed. The hip–knee–ankle angle (HKA) and center-edge (CE) angle were measured on pre- and postoperative full-length standing radiographs. The change in hip joint space width during postoperative follow-up was recorded. To account for radiographic magnification and body size, the width was normalized by the femoral head diameter, and the annualized change was defined as the normalized joint space narrowing rate (nJSNR). Results Preoperative HKA, HKA change, and relative postoperative HKA showed no correlation with ipsilateral hip nJSNR. However, the absolute postoperative HKA correlated significantly with nJSNR (P = 0.037, β = 0.11). The standing CE angle was also correlated (P = 0.0003, β = 0.130). A combined index, “Combined CE angle,” defined as CE angle plus postoperative HKA (varus positive/valgus negative), demonstrated the strongest correlation (P < 0.0001, β = 0.219). Conclusion Greater postoperative varus or valgus alignment may accelerate ipsilateral hip joint space narrowing. The Combined CE angle, integrating knee alignment and acetabular morphology, may serve as a practical indicator for quantifying hip degeneration risk. Optimization of lower limb alignment after TKA could help preserve hip joint integrity.
Toshiyuki Kawai (Thu,) studied this question.