Hip malformation is a very common finding in young patients scheduled for total hip arthroplasty.

<bold>Introduction: </bold>In Denmark, 20% of all registered total hip arthroplasties (THA) from 1995 to 2014 has been patients younger than 60 years with primary idiopathic osteoarthritis (OA). It is speculated that hip malformations may be a major contributor to early OA development. It has been shown that hip malformation may compromise implant position and, therefore, identifying and knowing the incidence of malformations is important. Our aim was to assess the prevalence and type of hip malformations in a cohort of younger patients undergoing THA.<bold>Materials and Methods: </bold>In this prospective two center cohort study, 95 consecutive patients (106 hips) met the inclusion criteria. One observer performed radiographic measurements for malformations and radiographic OA. Inter- and intraobserver variability was assessed.<bold>Results: </bold>From 95 patients (male n = 52 and female n = 43) age ranged from 35 to 59 years and prevalences of hip malformations were; CAM-deformity 50.9 and 25.5%, coxa profunda 33 and 27.4%, acetabular retroversion 33 and 29.2%, and acetabular dysplasia 10.4 and 3.8%. All patients showed minimum of one malformation. Prevalences of Tönnis grade 0-1 were 22.6% and 2-3 were 77.4%.<bold>Conclusion: </bold>All patients showed malformations, especially high prevalences were found for CAM-deformity, coxa profunda and acetabular retroversion. Identifying these malformations is fairly simple and recognizing the high prevalence may help surgeons avoid pitfalls during implant positioning in THA surgery. Further, focus on hip malformations may facilitate correct referral to joint-preserving surgery before OA develops. [ABSTRACT FROM AUTHOR]