Metacarpophalangeal joint implants. III. Roentgenographic study of the in vivo function

With tomographic examination of the metacarpophalangeal joint in the straight lateral projection with maximum active extension and flexion, performed after the implantation of 42 Swanson and 41 Niebauer finger joint implants, it was possible to study the in vivo function of these implants. A method was devised to determine the position of the proximal phalanx in relation to the metacarpal bone in the plane of flexion. This was correlated to the ideal position of the proximal phalanx as determined for each of the implants in experimental studies and defined only by the implant axis of rotation. A displacement of the phalanx in the proximal and volar direction was found. Usually, this was more pronounced with fracture of the implant, and could be about 7-8 mm in each direction. The hinge of the Niebauer implant was found to bend significantly less than expected, indicating movements taking place in other parts of the implant. The two blocks of the midsection were found to move in relation to the adjacent bones. Indicating an insufficient intramedullary fixation of the stems, these findings explained why cortical erosions appeared at the sites where the stems were in close contact with the bone: dorsally in the metacarpal bone and volarly in the proximal phalanx.