Morphological and biomechanical effects of distraction rate and Insulin-like Growth Factor-1 in distraction osteogenesis of the rabbit mandible
The effects of the rate of distraction and of local infusion of IGF-1 upon bone deposition during mandibular distraction osteogenesis was studied in a rabbit model. Five groups of rabbits were studied. All rabbits, except sham operated controls, underwent distraction to 15 mm. The variables studied were the rate of distraction (0.5 mm twice a day versus 1.5 mm twice a day) and the effects of local IGF-1 infusion via osmotic infusion pumps. Analysis by DEXA scanning and three point bending 28 days after the end of distraction demonstrated no difference in density or strength of bone between the experimental groups. Histological examination demonstrated non-union across the distraction gap to be more common in rapidly distracted rabbits. Histomorphometric analysis demonstrated higher mineral apposition rates and less un-mineralised osteoid with slow as opposed to rapid distraction (p = 0.0001). Infusion of exogenous IGF-1 also resulted in a small increase in mineral apposition rate which was significant at slow but not a rapid distraction. Bone densitometry and three point bending results did not reveal any effect of distraction rate or IGF-1 infusion other than greater stiffness associated with IGF-1 infusion during rapid distraction (p = 0.01). It seems probable that the overwhelming stimulus to new bone formation produced by distraction renders the anabolic effects of IGF-1 less significant making it detectable by only the more sensitive analysis. This may be due to maximal stimulation of IGF-1 production by slow distraction thus rendering the administration of exogenous IGF-1 relatively superfluous. Rapid distraction may produce less stimulus to growth factor synthesis or the level of production maybe insufficient to cope with a higher requirement. The complete union of rapidly distracted rabbits who received exogenous IGF-1 may be as a result of levels being restored to optimal.