The purposes of this study were to investigate the expression pattern of pro-inflammatory cytokines during distraction osteogenesis and to compare these with expression during simple fracture healing.

Regenerated bones from the rat tibia subjected distraction osteogenesis and simple fracture healing models were harvested over three-week periods. Temporal expressions of mRNA of pro-inflammatory cytokines were investigated by RNase protection assay. Immunohistochemical studies for IL-6 were performed in postoperative day 7 and 9 tissue section specimens.

IL-1beta and IL-6 produced detectable signals, while IL-1alpha, TNFalpha and TNFbeta did not. The mRNA expressions of IL-1beta and IL-6 were markedly upregulated on postoperative day 1 and then subsided to the preoperative level. IL-1beta mRNA expression remained the same even when distraction began. However, IL-6 mRNA expression was reactivated during the distraction phase. Immunohistochemical study revealed the expressions of IL-6 not only at the transitional zone of the transchondroid ossification, in young osteoblasts lining newly formed trabeculae and in hematopoietic cells in the marrow but also in primitive mesenchymal cells at the distraction gap.

Distraction strain re-induced IL-6 expression during distraction osteogenesis, which suggests that well-controlled inflammatory reaction might contribute to active new bone formation in distraction osteogenesis.