This is a prospective, multicentric study that aim to assess stability of a tapered porous coated stem and a cementless hemispherical acetabular component.

Main Translation and rotation around the x., y-, and z-axis of the femoral and cup component by radiostereometric analysis (RSA) at immediate post op (within 5 days and before weight bearing), 6 months, 1year and 2 years

Aim of this study is to assess stem and cup migration over the time in patient subjected to a total hip arthroplasty. Medical Device Research Australia will perform image analysis services and reporting for model based radiostereometric analysis (RSA) of cementless fixation of the femoral and acetabular cup components. Migration of the components will be measured with model-based RSA (MBRSA) at immediate post op (within 5 days and before weight bearing), 6 months, 1year and 2 years A subgroup of 26 patients will be invited to be evaluated also by additional RSA analysis: the first 13 patients for each investigator who will accept.

The results of this work will establish the mechanical stability of Masterloc femoral component (Medacta International SA) and MPACT acetabular component (Medacta International SA), provide a general estimate of survivorship, and establish clinical safety and efficacy as measured by RSA and clinical outcomes.

Inclusion Criteria:

• Primary osteoarthritis of the hip necessitating primary hip replacement,
• Suitability for straight cementless stem,
• Adult male and female under 70years old,
• Ability to give informed consent,
• Patients who understand the conditions of the study and are willing to participate for the length of the prescribed follow-up.

Exclusion Criteria:

• Post-traumatic deformity in the affected hip,
• Patient suffering by rheumatoid arthritis,
• Patient suffering by congenital or developmental deformity,
• Severe osteoporosis,
• Earlier surgery in the hip to be operated on,
• Perioperative fracture,
• Personal disorders (dementia, alcohol or drug abuse etc) suspected of making completion of the trial uncertain,
• Patients with a history of active infection
• Pregnant women or those seeking to become pregnant

26 patients who entirely fulfil the inclusion/exclusion criteria. The patients will be recruited in a period of 12 months.