Hemiarthroplasty of the hip is standard treatment of femoral neck fractures (hip fractures). Hemiarthroplasty means replacing the hip joint with a metal prosthesis. Unipolar prostheses has a one-piece design where the hip movement occurs between the prosthesis and the acetabulum (hip socket). A bipolar prosthesis has an additional artificial joint between the two components of the prosthesis. Both treatments are clinically proven and common around the world. No clinical trial has proven benefits of one or the other prosthesis design. The investigators want to measure the differences in acetabular wear using these two prostheses, using radiostereometric measurements.