Abstract
An optical method to measure directly small displacements of the patella has been developed. Light from a helium-neon laser was reflected from a plane mirror taped to the skin over the patella to a glass plate which generated a set of fringes by interference of light reflected from the front and rear surfaces of the plate. The movement of the fringe system was detected using a photodiode. The optical measurements of displacement were compared with those deduced from an accelerometer attached to the same joint. The method was found to be capable of detecting patellar displacements in the range from 20 mu m to 180 mu m.