[Imageless computer navigation of hip resurfacing arthroplasty].

Objective: Precise implantation of hip resurfacing arthroplasty by imageless computer navigation. Hence a malalignment of the femoral component, leading to early loss of the implant, can safely be avoided.

Indications: Coxarthrosis in patients with normal bone mineral density; only minor deformity of the femoral head that enables milling around the femoral neck without notching.

Contraindications: Osteoporosis; large necrosis of the femoral head; metal allergy; small acetabular seat and corresponding wide femoral neck, leading to needless acetabular bone loss; pregnancy, lactation.

Surgical Technique: Hip joint exposure by a standard surgical approach, bicortical placement of a Schanz screw for the navigation array in the lesser trochanter. Referencing of the epicondyles, the four planes around the femoral neck and head by use of the navigation pointer. Planning of the desired implant position on the touchscreen of the navigation device; a guide wire is inserted into the femoral head and neck using the navigated drill guide; navigated depth drilling is performed. The femoral head is milled using the standard instruments. The acetabular bone stock is prepared with the conventional instrumentation; high-viscosity cement is finger-packed on the reamed head and the femoral component is inserted. Hammer blows should be avoided to prevent microfractures. Verification of the implant position by the navigation device; displacement of the Schanz screw; joint reposition and closure of the wound.

Postoperative Management: Standard postoperative management after hip arthroplasty.

Results: The comparison of 40 navigated and 32 conventionally implanted ASR prostheses resulted in a significant reduction of outliers by use of computer navigation (navigated procedures: one outlier, conventional procedure: nine outliers; p<0.001). Accuracy of the navigation device was tested by analysis of planned and verified implant position: CCD angle accuracy was 1 degrees , antetorsion accuracy was 1 degrees , and offset accuracy was 1.5 mm. An ongoing computed tomography-based anatomic study proved a varus-valgus accuracy of the navigation device of 1 degrees .