Immediate loading of orthodontic mini-implants: a histomorphometric evaluation of tissue reaction.

Although immediate loading of orthodontic mini-implants can be clinically successful, a lack of histological data exists. The present investigation was performed to evaluate, in an animal model, tissue reaction to immediate loading. Fifty orthodontic titanium mini-implants were inserted in four adult male monkeys at four time intervals. Forty-two devices were loaded with 50 cN super-elastic coil springs immediately after insertion while eight were left unloaded and served as the controls. After euthanasia, the following histomorphometric parameters were evaluated: bone volume (BV/TV), bone-to-implant contact (BIC), mineralizing surface (MS/BS), and erosion surface (ES/BS). Statistical analysis was performed by means of non-parametric tests. Four devices were removed because of loss of stability. A wide variation between animals was found for all parameters. BV/TV: slightly higher values were found in the unloaded sample. Although no particular trend was observed, at 3 months higher values were found in the lower jaw. BIC: a trend to a decrease between 1 week and 1 month followed by a significantly progressive increase was observed. Implants that showed some sections with as little as 3 per cent BIC successfully resisted loading. MS/BS: higher values were found in the lower jaw. MS/BS increased significantly between 1 week and 1 month, followed by a progressive decrease. ES/BS: there was a decrease between 1 week and 1 month, followed by a progressive re-increase. BV/TV did not show any particular trend while BIC was a time-dependent factor. MS/BS and ES/BS demonstrated opposite trends during the healing period. Immediate loading with light forces did not negatively affect the bone healing pattern.