BackgroundCotyloplasty is a technique that involves making a perforation of the medial wall of a shallow acetabulum and then inserting an acetabular cup with the medial aspect of its dome beyond the Kohler line. The purpose of this study was to evaluate the results of cementless total hip arthroplasty (THA) using cotyloplasty with focusing on the amount of medial cup protrusion.MethodsSixteen hips with insufficient acetabulum in sixteen patients were treated by cementless THA using cotyloplasty. The average patient age was 47 years. The diagnoses included dysplastic hip (12) and infection sequelae (4). All the patients were followed up for at least 2 years. Clinically, the Harris hip scores were assessed and radiologically, the amount of cup protrusion, the hip center movement and cup fixation were evaluated.ResultsThe average Harris hip score improved from 57 to 94 postoperatively. The average proportion of cup surface beyond the Kohler line was 44.1% and the hip centers were medialized by an average of 23 mm. Stable fixation of the acetabular cup was achieved in all the cases except one. In this one case, migration of the cup was detected 2 weeks postoperatively and a reoperation was performed.ConclusionsUsing cotyloplasty, good coverage of the acetabular cup was obtained without a block bone graft, and the hip joint centers were medialized. However, the safety margin for the amount of protrusion should be established.