MODEL RELEASED. Studying artificial knee. X-rays of an artificial knee and a researcher studying it by radiostereometric analysis (RSA). RSA involves the use of X-rays to follow the motion of metal markers attached to the prosthesis inside the leg, seen labelled on the X-rays. Two X-rays are taken, one from below and one from the side. The computer will be used to build up a three-dimensional image of the knee. This is a high-precision technique that can track very small motions in the skeleton. This is part of research to study the stability of knee prostheses. Photographed at the Biomechanical Laboratory, Rizzoli Institute, Bologna, Italy.