Laboratory evaluation of a metal-plastic type of metacarpophalangeal joint prosthesis

In the normal joint, the conformity of the joint surfaces can provide appreciable dorsal-volar shear stability when a compressive joint force acts. The centers of rotation were within 3 mm of the center of the metacarpal ball. Passive radial-ulna motion at 0 degrees flexion was close to 60 degrees, active motion being half of this; at 90 degrees flexion, there was 37 degrees passive and 23 degrees active motion. Studies of canal shapes and sizes showed that a transverse section deviated from circular by 1-2.5 mm; longitudinal sections were trumpet-shaped but had waves and ripples of about 0.5 mm. This is relevant to cement fixation. A metal-plastic prosthesis was tested in positions and with forces considered applicable to arthritics. It is possible that with plastic components, permanent deformation will progressively occur. The pull-out strength between cement and bone in cadaver bones was about 100 kg; metal stems in cement pulled out at about 50 kg, and plastic stems at 10-20 kg. Slippery stems pulled out at much lower values.