Developmental dysplasia of the hip is common in patients undergoing total hip arthroplasty under 50 years of age

Introduction: Developmental dysplasia of the hip (DDH) refers to congenital and/or developmental hip instability that can result in hip joint subluxation or dislocation. When detected neonatally, conservative treatment with hip bracing can restore normal hip anatomy. Missed detection of DDH in the neonatal period or late development of DDH often requires surgical intervention to correct the abnormal anatomy. Furthermore, despite surgical intervention, residual sequelae may persist leading to early osteoarthritis of the hip joint requiring joint replacement surgery. Aim: This study investigates the prevalence of hip dysplasia in patients undergoing total hip arthroplasty (THA) under 50 years of age. Methods: The hip arthroplasty database at a national referral centre was investigated from January 2014 to December 2020. In patients under 50 years of age, those with an adequate pre-operative anteroposterior pelvic radiograph without previous hip arthroplasty were included, while those with inadequate radiographs were excluded. The following measurements were made on the contralateral non-operated hip: (1) lateral centre-edge angle (LCEA), (2) Tonnis angle, (3) acetabular version, (4) acetabular depth, (5) femoral head lateralisation, (6) femoral head extrusion index, and (7) acetabular depth-to-width ratio. Results: In total, 451 patients were included in this study. Twenty two percent of the patients had hip dysplasia, based on a LCEA of <25 & DEG; and 42.6% of patients had hip dysplasia, based on a Tonnis angle of > 10 & DEG;. The mean LCEA and Tonnis angle were 31.47 & PLUSMN; 9.64 and 9.82 & PLUSMN; 6.79 & DEG;, respectively. Conclusion: Hip dysplasia is common in patients undergoing THA under the age of 50 years with over 40% having dysplasia according to the Tonnis angle. Classification of primary and secondary osteoarthritis in the joint registries will benefit our knowledge on the prevalence of DDH in the adult population.