DIFFERENCES IN STIFFNESS OF THE INTERFACE BETWEEN A CEMENTLESS POROUSIMPLANT AND CANCELLOUS BONE IN-VIVO IN DOGS DUE TO VARYING AMOUNTS OFIMPLANT MOTION

To determine the mechanical properties of the interface between the ti ssue ingrowth into porous coatings and the implant, porous-coated cyli ndrical implants were inserted into the distal femur in 20 mature dogs and oscillated in vivo 8 hours per day for 6 weeks at fixed amounts o f micromotion (0, 20, 40, and 150 mu m) Applied torques and resulting displacements were recorded. The torsional resistance per unit angular displacement (TRIAD), reflecting the stiffness of the bone-porous coa ting interface, was 0.88 +/- 0.25 N-M/deg immediately after implantati on in the 20-mu m displacement group. It increased with time after sur gery, reaching a maximum of 1.25 +/- 0.60 N-M/deg at 6 weeks. The TRIA D was lower initially (0.77 +/- 0.43 N-M/deg) in the 40-mu m group and gradually decreased with time after surgery, reaching a maximum of 0. 54 +/- 0.13 N-M/deg at 6 weeks. The TRIAD was even lower (0.24 +/- 0.1 0 N-M/deg) in the 150-mu m group initially and remained the same (0.16 +/- 0.09 N-M/deg) with time after surgery. Histologic evaluation show ed bone ingrowth in continuity with the surrounding bone in the 20-mu m group consistent with the high stiffness values at sacrifice. In con trast, a mixture of fibrocallus and bone were found at the bone-porous coating interface in the 40-mu m group, consistent with the intermedi ate stiffness values. In contrast, despite the fact that bone was foun d in the depth of the porous coating in the dogs in the 150-mu m group , the low stiffness values were a reflection of fibrous tissue formati on at the interface in that group, because of the large motion disrupt ing bony in growth at the bone-porous coating interface. By monitoring the torsional resistance per unit of angular displacement dynamically in viva, it was possible to evaluate the mechanical properties of the bone-porous coating interface as tissue ingrowth proceeded. Twenty mi crons of oscillating displacement was compatible with stable bone ingr owth with high interface stiffness, whereas 40 and 150 mu m of motion was not.