This study was initiated to investigate the impact of the computer based image free navigation on the precision of the cup implantation. The implantation of the acetabular component (cup) is known to be a critical step in hip endoprosthetics. The cup position influences significantly the function, the absence of pain and the longevity of the artificial hip joint. The image free navigation system OrthoPilot is used to control the positioning of the cup during surgery.

Inclusion Criteria:

• Indication for primary cementless total hip endoprosthesis implantation with Bicontact stem and Plasmacup
• Diagnoses: primary or secondary coxarthrosis, rheumatoid arthritis, femoral head necrosis
• Age ≥ 50 Years
• ASA score <4
• Patient signed the informed consent

Exclusion Criteria:

• Excessive damage to the hip joint (e.g. dysplasia)
• Severe deformities of the pelvis, femoral bone or knee
• Unfeasibility of landmark palpation (e.g. due to adiposity)
• Acute or chronic infection
• Pregnancy
• Patients not available for follow-up-examination at the center
• Patients exceeding 10 mSv effective radiation dose because of previous scientific or clinical exposition during the past 10 years