Background:Total hip arthroplasty (THA) is one of the most effective treatments for phase Ⅲ and Ⅳ hip arthrosis.Lower limb length balancing is one of the determining factors of a successful surgery,particularly in patients with developmental dysplasia of the hip (DDH).The purpose of this study was to evaluate the postoperative change in intra-pelvic obliquity (intra-PO) angle in the coronal plane and its effects on leg length discrepancy (LLD) within 2 years.Methods:A total of 78 patients (70 females,8 males) were enrolled in this study.All patients were suffering from DDH with varying degrees of LLD.Pelvic plain radiographs were collected before and after the operation.The intra-PO angles were measured 0,0.5,1 and 2 years after THA.At the same time,postoperative LLD was measured with blocking test.Results:PO changed significantly in the first year after THA surgery (0 year vs.0.5 year,P ＜ 0.01;0.5 year vs.1 year,P ＜ 0.01),and the changing value of intra-PO angle (△PO) slowed down substantially during the first 2 years after THA (0.5 year vs.0.5-1 year,P ＜ 0.01;0.5-1 year vs.1-2 years,P ＜ 0.01).With the change in intra-PO angle,LLD also got narrow within the 1st year (0 year vs.0.5 year,P ＜ 0.01;0.5 year vs.1 year,P ＜ 0.01).Elderly patients had a smaller intra-PO angle reduction (Group A vs.Group B,P =0.01;Group B vs.Group C,P＜ 0.01).Conclusions:Intra-PO angle and LLD gap narrowed with time after THA surgery.In particular,elderly patients had smaller change in intra-PO angle.