Effects of total hip arthroplasty for secondary coxarthrosis due to developmental dysplasia of the hip on the knee joint

Objective: This study aims to evaluate the impact of total hip arthroplasty (THA) performed for secondary coxarthrosis due to developmental dysplasia of the hip (DDH) on lower extremity biomechanics, coronal plane alignment, and knee joint function. Postoperative biomechanical changes in the knee were assessed both radiologically and functionally over a mid-to-long-term follow-up. Methods: The study included 108 hips, with 74 undergoing unilateral and 34 undergoing bilateral THA. Hips were categorized based on Crowe classification (low-grade: types 1–2; high-grade: types 3–4) and the type of femoral prosthesis used. The mean follow-up period was approximately 4 years (range: 1–10 years). Radiological assessments included mechanical and anatomical lateral distal femoral angle (mLDFA, aLDFA), mechanical medial proximal tibial angle (mMPTA), joint line convergence angle (JLCA), pelvic tilt (PT), leg length discrepancy (LLD), Q angle, femoral offset (FO), hip-knee-ankle angle (HKA), knee mechanical axis deviation ratio (KMADR), femoral mechanical-anatomical axis angle (FMAA), Kellgren-Lawrence (KL) grading for knee osteoarthritis, and tibiotalar angle (TTA). Functional outcomes were assessed using the Visual Analog Scale (VAS), Harris Hip Score (HHS), Knee Society Score (KSS), Oxford Knee Score, Knee Injury and Osteoarthritis Outcome Score (KOOS), and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Results: The study evaluated 57 hips with low-grade and 51 with high-grade dysplasia. Postoperatively, the high-grade dysplasia group exhibited significantly greater mMPTA, LLD and its change, Q angle, HKA angle and its change, KMADR, and knee KL grade progression, while preoperative and postoperative FMAA values were significantly lower. Among prosthesis types, the lowest mLDFA and FMAA and the highest HKA angle and PT were observed in conical stems, whereas the highest mLDFA, FO, and FMAA and the lowest PT and HKA angle were found in wedge stems. However, no significant differences in clinical outcomes were observed based on Crowe classification or femoral stem type. Postoperatively, all patients exhibited a significant reduction in hip and knee VAS, along with improvements in HHS, KSS, KOOS, Oxford Knee Score, and WOMAC scores. Conclusion: Following THA, valgus alignment of the knee increases in high-grade dysplasia patients, whereas in low-grade dysplasia cases, this tendency normalizes over time. Conical femoral stems may contribute to increased valgus alignment, while wedge stems provide more stable mechanical axis alignment. Nevertheless, postoperative radiological changes in the knee do not significantly impact mid- to long-term functional outcomes. Key Words: Developmental Dysplasia of the Hip; Coxarthrosis; Total Hip Arthroplasty; Knee Biomechanics; Lower Extremity Coronal Plane Alignment; Femoral Prosthesis; Knee Joint; Valgus Alignment; Hip-Knee-Ankle Angle; Radiologic Assessment; Functional Outcomes