Bone ingrowth in porous surfaces of human joint implants is a desired condition for long-term fixation in patients who are physically active (such as in sport or work). It is generally recognized that little actual bone ingrowth occurs. The best clinical results report between 10 and 20% of the total prosthetic surface in contact with bone will feature good bone ingrowth. One inhibiting factor is the relative motion of the bone with respect to the implant during load-bearing. This study investigated mathematically the interface micromotion (transverse reversible relative motion) between a flat metal tibial prosthetic surface of a prototype implant, and the bone at the resection site. The aim was to assess the effect of perimeter fixation versus midcondylar pin fixation and the effect of plate thickness and plate stiffness. Results showed that in the prototype design the largest reversible relative bone motion occurred at the tibial eminence. By design, the skirt fixation at the perimeter would prevent bone motion. A PCA (Howmedica Inc.) prosthesis has been widely used clinically and was chosen for a control because its fixation by two pegs beneath the condyles is a common variation on the general design of a relatively thick and stiff metal tibial support tray with pegs in each condylar area. The PCA tibial prosthesis showed the largest bone motion at the perimeter along the midcondylar mediolateral line, while being zero at the pegs. Maximum relative bone motion for the prototype was 37 ¿m and for the control was 101 ¿m. Averaged values showed the prototype to have 38% of the relative reversible bone motion of the control (PCA).