Differences in stiffness of the interface between a cementless porous implant and cancellous bone in vivo in dogs due to varying amounts of implant motion

To determine the mechanical properties of the interface between the tissue ingrowth into porous coatings and the implant, porous-coated cylindrical 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 of micromotion (0, 20, 40 and 150 microns). Applied torques and resulting displacements were recorded. The torsional resistance per unit angular displacement (TR/AD), reflecting the stiffness of the bone-porous coating interface, was 0.88 +/- 0.25 N-M/deg immediately after implantation in the 20-micron displacement group. It increased with time after surgery, reaching a maximum of 1.25 +/- 0.60 N-M/deg at 6 weeks. The TR/AD was lower initially (0.77 +/- 0.43 N-M/deg) in the 40-micron group and gradually decreased with time after surgery, reaching a maximum of 0.54 +/- 0.13 N-M/deg at 6 weeks. The TR/AD was even lower (0.24 +/- 0.10 N-M/deg) in the 150-micron group initially and remained the same (0.16 +/- 0.09 N-M/deg) with time after surgery. Histologic evaluation showed bone ingrowth in continuity with the surrounding bone in the 20-micron group consistent with the high stiffness values at sacrifice. In contrast, a mixture of fibrocallus and bone were found at the bone-porous coating interface in the 40-micron group, consistent with the intermediate stiffness values. In contrast, despite the fact that bone was found in the depth of the porous coating in the dogs in the 150-micron group, the low stiffness values were a reflection of fibrous tissue formation at the interface in that group, because of the large motion disrupting bony ingrowth at the bone-porous coating interface. By monitoring the torsional resistance per unit of angular displacement dynamically in vivo, it was possible to evaluate the mechanical properties of the bone-porous coating interface as tissue ingrowth proceeded. Twenty microns of oscillating displacement was compatible with stable bone ingrowth with high interface stiffness, whereas 40 and 150 microns of motion was not.