Changes in Coronal Lower-Limb Alignment and Knee Phenotypes After Total Hip Arthroplasty in Patients Who Have Crowe IV Developmental Dysplasia of the Hip

Abstract Background: Total hip arthroplasty (THA) can alter coronal lower-limb alignment in patients who have developmental dysplasia of the hip (DDH), but changes in hip–knee–ankle angle (HKA), femoral mechanical angle (FMA), and tibial mechanical angle (TMA) phenotypes and Coronal Plane Alignment of the Knee (CPAK) types after THA for Crowe IV DDH remain incompletely characterized. We evaluated these changes, residual alignment deviations, and associated factors.Methods: We retrospectively reviewed 86 patients who underwent primary THA for unilateral Crowe IV DDH. Preoperative and final follow-up standing full-length lower-extremity radiographs were compared. We determined HKA, FMA, and TMA phenotypes and CPAK types and assessed alignment deviations using reference values from a Chinese population without knee osteoarthritis. Paired tests, Bowker tests of symmetry, and multivariable regression analyses were performed.Results: After THA, FMA and HKA shifted toward varus by 2.68° and 2.44°, respectively (both P < 0.001), whereas TMA did not change significantly (*P* = 0.436). Phenotype changes occurred in 74 and 62 patients (86.0 and 72.1%) for FMA and HKA, respectively, with transitions predominantly toward varus (both Bowker P < 0.001); the TMA phenotype changed in only 11 patients (12.8%). A change in CPAK type occurred in 40 patients (46.5%), whereas the number of patients who had at least one deviant or aberrant angle decreased from 45 (52.3%) preoperatively to 21 (24.4%) postoperatively. Multivariable analysis showed that change in femoral offset was independently associated with change in HKA (*B* = −0.083°/mm; 95% confidence interval, −0.155 to −0.010; P = 0.026).Conclusions: In patients who had Crowe IV DDH, changes in coronal alignment after THA occurred predominantly on the femoral side, whereas tibial alignment remained largely stable. Although marked alignment deviations decreased after THA, residual alignment deviations persisted in some patients. FMA and HKA functional phenotypes shifted predominantly toward varus, and changes in CPAK type were common. Therefore, coronal lower-limb alignment and knee phenotypes should be reassessed after THA in patients who may subsequently undergo total knee arthroplasty to inform preoperative planning.