The purpose of this study is to compare the articular surface replacement (ASR) prosthesis placed by the conventional positioning method and the ASR prosthesis placed by 'computer assisted surgery' (CAS) in variation between pre planned position of the femoral component and the actual position of the femoral component of the prosthesis. The hypothesis is that this variation is larger with the conventional positioning method than when CAS is used.

For patients who suffer from end stage of osteoarthritis of the hip, a joint replacement could result in an obvious improvement of the quality of life. However, because of the life expectation and high level of activity of young and active patients, the conventional total hip replacement does not offer an optimal solution.

An alternative to a total hip replacement in young and active patients is the resurfacing hip prostheses. The articular surface replacement (ASR) hip prostheses can be placed by the conventional positioning method and by using 'computer assisted surgery' (CAS). The hypothesis is that the positioning of the femoral component by the conventional positioning method will show a larger variation between pre planned and actual position than when CAS is used.

This study will compare the results of those who had an ASR prosthesis placed by the conventional positioning method with those patients in whom the ASR prosthesis was placed using CAS.

Inclusion Criteria:

• Men aged ≤ 60 years, and women aged ≤ 55 years
• Presence of evident clinical and radiological osteoarthritis of the hip, indicated for a joint replacement surgery

Exclusion Criteria:

• Evident osteoporosis
• Active local or systemic infection
• Clinical manifestation of vascular deficiency of the lower extremity
• Pathological condition of the acetabulum
• Presence of evident clinical and radiological avascular necrosis of the femoral head, hip dysplasia, slipped capital femoral epiphysis, Legg-Calve- Perthes disease
• Rheumatoid arthritis
• Extreme varus position (neck-shaft angle < 110º)
• Presence of femoral cyst > 1 cm in diameter
• Previous hip surgery
• Presence of bilateral hip pathology, which may lead to joint replacement surgery within 1 year
• Presence of a THR at the contra lateral site less than 6 months post- operatively or patients with a THR at the contra lateral site with a poor function of the hip
• BMI> 30 kg/m2
• Renal deficiency (creatine > 115 μmol/l for men and > 90 μmol/l for women)
• Medically proven metal allergy
• Request of patient to correct an existing leg length discrepancy
• Head-neck ratio < 1
• Use of steroids, and/or immunosuppressive medication
• Alcoholism
• Patients from which it is not sure that they will be able to attend the follow-up measurements
• Insufficient command of the Dutch language, spoken and/of written