Aims. The goal of closed reduction (CR) in the treatment of developmental dysplasia of the hip (DDH) is to achieve and maintain concentricity of the femoral head in the acetabulum. However, concentric reduction is not immediately attainable in all hips and it remains controversial to what degree a non-concentric reduction is acceptable. This prospective study is aimed at investigating the dynamic evolution of the hip joint space after CR in DDH using MRI. Methods. A consecutive series of patients with DDH who underwent CR since March 2014 were studied. Once the safety and stability were deemed adequate intraoperatively, reduction was accepted regardless of concentricity. Concentricity was defined when the superior joint space (SJS) and medial joint space (MJS) were both less than 2 mm, based on MRI. A total of 30 children, six boys and 24 girls, involving 35 hips, were recruited for the study. The mean age at CR was 13.7 months (3.5 to 27.6) and the mean follow-up was 49.5 months (approximately four years) (37 to 60). The joint space was evaluated along with the interval between the inverted and everted limbus. Results. Only three hips (8.6%) were fully concentric immediately after CR. During follow-up, 24 hips (68.6%) and 27 hips (77.1%) became concentric at six months and one year, respectively. Immediate SJS after CR decreased from 3.51 mm to 0.79 mm at six months follow-up (p = 0.001). SJS in the inverted group decreased from 3.75 mm to 0.97 mm at six months follow-up. SJS or MJS in the everted group were less than those in the inverted group at each time of follow-up (p = 0.008, p = 0.002). Conclusion. A stable, safe but non-concentric reduction achieved before the age of two years appears to improve over time with nearly 80% of hips becoming fully concentric by one year. Cite this article: Bone Joint J 2020;102-B(5):618–626

Aims: To establish whether the Ôidealñ concentric positioning of acetabular components within the acetabular cement mantle is achieved during routine cemented total hip arthroplasty. In vitro studies recommend concentric placement of the acetabular component. There are however no in vitro studies on acetabular component positioning. Methods: Radiographs of 100 primary cemented total hip replacements were studied. Acetabular component positioning and cement mantles were assessed with respect to implant type, grade of surgeon and operated side. Results: 78% of the components were eccentrically placed, with increasing cement mantle thickness from zones 1 to 3. Concentricity occurred in only 13% of the Charnley Ogee, and 28% of the IP Lubinus components. The Charnley Ogee was more superiorly eccentric than the IP Lubinus (p< 0.001). The IP Lubinus was generally more open than the Charnley Ogee and Stanmore (p=0.053). Surgical grade affected neither cement mantle nor opening angle. Conclusions: This study indicates that the ideal component position in the acetabulum is very difþcult to achieve consistently in most patients, using the two commonly used cemented prostheses in this study. The practical difþculty of concentric component positioning while obtaining simultaneous pressurisation is illustrated