Because a hip replacement typically has a smaller bearing than your natural hip joint. The natural hip joint only rarely dislocates with extreme trauma (a fall from a second story , or a severe motor vehicle accident where the knee is driven against the dashboard). However , a traditional total hip replacement carries a 5-7% lifetime risk of dislocation , without significant trauma. This is because a total hip replacement relies on a mechanically unstable small bearing. To provide a plastic bearing surface and a socket that is thick enough to last , the femoral bearing size needs to be reduced to a much smaller size (average size 28 mm) , than the natural hip (size 44-56 mm). Similar small bearing sizes are also required for ceramic on ceramic implants. This leads to a 5-7% risk of dislocation over the lifetime of the implant. However , very durable modern metal on metal bearings can be manufactured with very thin (3-6 mm) acetabular components. This allows the use of a large femoral component (either THR or HSR) that allow retaining the natural stability of the hip joint. Large bearing metal on metal TKR and HSR rarely if ever dislocate.