Prosthetic hip joint and Gruen zones. Diagram of the femoral component of a hip prosthesis with labels indicating the hip connection (upper left) and the seven Gruen zones. This component is implanted in the femur (thigh bone) after the head of the femur has been surgically removed. The other components (not shown) are a rounded ball on the insertion point to fit into the socket implanted in the patient's pelvis. Gruen zones are used to assess bone mineral density (BMD) after total hip replacement surgery. This is a Symax prosthesis. For a set of Gruen zone diagrams, see C016/6776 to C016/6781.