Testing artificial hip. Equipment being used to test the torsional stability of an artificial hip. Torsion is a type of mechanical stress where force is applied to twist an object about an axis. This would be the force generated while twisting on one foot. The prosthesis is mostly inside the thigh bone (yellow), with the protruding ball part held by the clamp (upper centre). Four sensors are seen screwed into the bone. This is a cemented hip, intended for long-term use. Cemented hips are used in older people where the bone does not grow back around the prosthesis so easily and is weak, so a cement is used to glue it in place. Phototographed at the Rizzoli Institute, Bologna, Italy.