The adaptation of soleus and edl in a rat model of distraction osteogenesis: IGF-1 and fibrosis.

Abstract

The distraction rate of 0.5 mm/day produces good osteogenesis in small rodents; however, the effects of this distraction rate on muscle are not well documented. We evaluated the soleus and the extensor digitorum longus (EDL) after two weeks of lengthening distraction osteogenesis (DO) at 0.5 mm/day in skeletally mature rats. We found a modest but significant local increase of insulin like growth factor-1 (IGF-1) in the EDL, however, muscle growth indicated by developmental forms of myosin heavy chain (MHC) was not detected by mRNA (RT-PCR). To the contrary. the data suggested a decrease in cross-sectional area of the muscle fibers as well as a decrease in mRNA for slow MHC. Immunolabeling of fibronectin in cryosections of the EDL indicated fibrosis of the perimuscular connective tissue while assessment of the passive joint motion did not suggest a lack of excursion on the part of the dorsal flexors. While the literature suggests that IGF-1 facilitates muscle growth especially in young animals, excess of IGF-1 in muscle from adults may exacerbate DO-induced fibrosis.