Relative effects of age on implant integration in a rat model: A longitudinal in vivo microct study.

The effect of age on implant fixation in bone is not always considered during the design of preclinical models. The decision on animal's age is often related to practical or historical reasons, which ultimately may affect the reproducibility of results. This study aimed to quantify the effect of age by monitoring the fixation of contrast-enhanced PEEK screws in rats, hypothesizing that the kinetics of fixation is impaired in older animals but that age effects are less severe than osteoporotic effects. The time course of implant fixation was investigated in healthy rats at 24, 40, and 60 weeks of age; and in ovariectomized rats. Implant fixation was monitored using in-vivo microCT and dynamic histomorphometry during 1 month. The rats were euthanized 28 days post screw insertion. The data was analyzed both in absolute value and after normalization to baseline bone mass. In absolute terms, greater age had a detrimental effect on bone implant contact, bone fraction, implant stiffness, and bone remodeling but less than ovariectomy. Interestingly, once data was normalized to baseline bone mass this effect disappeared, suggesting that the physiologic response to implant placement was not affected by age. In conclusion, implant fixation kinetics is less affected by age than by baseline bone mass in this rat model. Animals of different ages can therefore be compared but data must be construed relatively to baseline bone mass and not in absolute terms.