In the conversion of an arthrodesed hip to a total hip arthroplasty, the osteotomy of the femoral neck and the placement of the acetabular socket are difficult procedures as anatomical abnormalities hamper identification of the femoral neck and of the original center of the acetabulum. A 59-year-old woman who had a hip arthrodesis for dysplastic osteoarthritis at 21 years of age underwent total hip arthroplasty for relief of back pain, achievement of good gait function, and improvement of activities of daily living. In this report, we introduce a technical solution, using a computed tomography-based navigation system to determine the site and direction of the femoral neck osteotomy and the positioning of the acetabular socket.