Successful fracture fixation in osteoporotic individuals requires a better understanding of the interplay between implant insertion, bone remodelling and mechanical regulation. Here, we characterise the mechanical regulation of cortical bone remodelling following implantation. We performed ovariectomy on C57BL/6 mice (OVX, n=9) to induce bone loss, whereas in SHM (n=8) the ovaries were not removed. Implants were inserted into the sixth caudal vertebrae and the bone-implant system was scanned weekly using in vivo micro-computed tomography for 6 weeks. Cortical bone was divided into a peri-implant region (close to the implant) and a reference region (far from the implant). Periosteal and endosteal bone remodeling were assessed by overlapping consecutive scans. Micro-finite element models were generated to compute the strain energy density (SED) at formed, resorbed, and quiescent sites. Following implantation, peri-implant bone formation rate was transiently increased on periosteal and endosteal surfaces in SHM and OVX. However, on endosteal surface, bone resorption rate (BRR) in SHM was elevated soon after implantation while in OVX higher values were measured at later time points; periosteal BRR was significantly increased only in SHM. Even in the presence of the implant, periosteal bone remodelling was mechanically regulated as shown by SED at formed/resorbed sites being higher/lower than at quiescent sites. On endosteal surface far from the implant OVX showed a prevailing bone resorption independently of SED, while close to the implant the normal mechanical regulation pattern was detected. Our results indicated an initial fast peri-implant bone forming phase, in which formation and resorption were strongly mechanically coupled only in SHM. Endosteal remodeling seems to be less mechanically controlled than periosteal remodeling and implant insertion restored locally the mechanical control also on endosteal bone. Our approach may be used to develop smart implants attaining long-term stability by manipulating the remodeling process to favour bone formation over resorption.
Disclosure: The authors declared no competing interests. DR kindly acknowledges support from ECTS Postdoctoral Fellowship 2010.