RELATIVE MOTION AT THE BONE PROSTHESIS INTERFACE

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 and work). It is generally recognized that little a ctual bone ingrowth occurs. The best clinical results report between 1 0 and 20% of the total prosthetic surface in contact with bone will fe ature good bone ingrowth. One inhibiting factor is the relative motion of the bone with respect to the implant during load-bearing. This stu dy investigated mathematically the interface micromotion (transverse r eversible relative motion) between a flat metal tibial prosthetic surf ace of a prototype implant, and the bone at the resection site. The ai m was to assess the effect of perimeter fixation versus midcondylar pi n fixation and the effect of plate thickness and plate stiffness. Resu lts showed that in the prototype design the largest reversible relativ e bone motion occurred at the tibial eminence. By design, the skirt fi xation at the perimeter would prevent bone motion. A PCA (Howmedica In c.) prosthesis has been widely used clinically and was chosen for a co ntrol because its fixation by two pegs beneath the condyles is a commo n variation on the general design of a relatively thick and stiff meta l tibial support tray with pegs in each condylar area. The PCA tibial prosthesis showed the largest bone motion at the perimeter along the m idcondylar mediolateral line, while being zero at the pegs. Maximum re lative bone motion for the prototype was 37 mu m and for the control w as 101 mu m. Averaged values showed the prototype to have 38% of the r elative reversible bone motion of the control (PCA). Relevance This wo rk attempts to minimize (or control) the reversible bone motion that o ccurs at the implant-tissue interface by containing the lateral motion of the resected tibia while at the same time allowing the gross physi ological bone motion of the proximal tibia. This is clinically relevan t as the small bone-implant interface motion is more conductive to bon e ingrowth and therefore improves fixation in biologically anchored im plants.