The hip joint is formed by the femoral head and acetabulum. Osteoarthritis of the hip develops when the cartilage covering the femoral head and acetabulum wears down. In a THR, the femoral head and its worn cartilage are removed and replaced with a ball attached to a stem inserted into the proximal femur. The stem can be cemented or uncemented (press-fit) into the proximal femur. The bone grows into the coating on a press-fit stem over time providing the long-term stability. Initial stability is achieved by inserting a femoral stem slightly larger than the prepared space in the bone. Most femoral stems for a primary hip replacement are press-fit.
Loosening results from the failure of the bond between a bone and an implant. Loosening can be caused by an infection in the bone of a replaced joint or can develop in the absence of infection. Loosening in the absence of an infection, i.e., aseptic loosening, is one of the more problematic complications of a THR because it often leads to another surgery. A loose implant will cause pain and can lead to progressive bone loss around the implant, potentially making revision surgery more difficult with higher rates of complications. A loose femoral stem can be diagnosed on an x-ray by a change in the position of the femoral stem or radiolucencies around the implant, i.e., darker lines at the interface between the bone and the implant.
A loose implant can be seen early after surgery or develop decades after the joint replacement. Early cases of loosening can occur from inadequate initial fixation or from mechanical loss of fixation over time. When loosening develops many years after surgery, microscopic wear particles from motion of the prosthetic joint accumulate in the surrounding tissues. These wear particles cause an inflammatory immune system response leading to osteolysis (bone resorption) around the implant, and loosening.