Implants designed for finger joints. A roentgenorgraphic study and a study of implant wear and tear

Two types of flexible finger implants (one intramedullary movable, the Silastic Finger Joint Implant, Swanson design, and the other presumable intramedullary fixed, the Niebauer T-M Cutter Metacarpophalangeal Joint Prosthesis) were subjected to animal trials. Forty-four Swanson implants and 33 Niebauer implants were implanted in the knee joint of rabbits. Roentgenographic examinations were carried out immediately after surgery and then at monthly intervals. The joint was examined in the lateral projection in a flexed and in an extended position to obtain better information about the behaviour of the implant. The Swanson implant was assessed as being intramedullary movable. The Niebauer implant was found to bend within other parts than just the thin central part of the midsection as it is supposed to do. Intramedullary bone formation was seen along the distal stem of the Swanson implant. It developed in a very typical manner considered to be related to the motion of the stem. A similar phenomenon was seen along the distal stem of some of the Niebauer implants and was considered a sign of insufficient fixation of the stem. Cortical erosion and periosteal callus formation were found in the femur at the site at which the proximal stem of the Swanson implant was in direct contact with the bone. The implants were examined at autopsy both in situ and after removal. The Swanson implants showed consistently a wearing of the surface and a permanent deformation. Isolated cracks were seen in 8/44 implants, while fractures, either isolated or in combination with cracks and/or fragmentation, were seen in 22/44 implants. It was not possible to evaluate deformation of the Niebauer implants. Slight wearing was seen in 4/32 implants. Cracks, fragmentations and fractures were seen either isolated (except for the fragmentations) or in different combinations with no specific pattern in 25/32 implants. The combination of inelastic dacron fibres and elastic silicone rubber in one functional unit as in the Niebauer implant was considered the main cause of the damage.