A morphometric and biomechanic comparison of titanium implants inserted in rabbit cortical and cancellous bone

The removal torques for screw-shaped pure titanium implants inserted in rabbit tibia and the femoral part of the knee joint and the tissue response to these implants, as quantitated with light microscopic morphometry on ground sections, were compared after 6 weeks, 3 months, and 6 months. The bone surrounding the femoral intra-articular implants was mostly cancellous, while cortical bone was formed around the tibial implants. The torque needed to remove the intra-articular implants increased with time, but there was no such increase for the tibial implants. At 6 weeks, significantly less torque was needed to remove the intra-articular implants in spite of the fact that significantly more bone was found in the threads of these implants as compared with the tibial implants. When calculating the amount of bone in threads situated in the cortical and subchondral passage, more was found in the threads of the tibial implants, which corresponded to the higher removal torque. Additional light microscopic observations on implants unscrewed after 12 months in rabbit tibia indicated that rupture occurred between the implant surface and calcified bone. Findings indicate that the resistance to unscrewing is dependent on the amount of compact bone surrounding a titanium implant.