A joint endoprosthesis system has first and second prosthetic components. The second prosthetic component has a bearing surface. The interface of the bearing surface and the first prosthetic component defines the joint articulation. A signal source and sensor are affixed on opposite sides of the joint...http://www.google.co.uk/patents/US7195645?utm_source=gb-gplus-sharePatent US7195645 - In vivo joint space measurement device and method
