Weight‐bearing alters the expression of collagen types I and II, BMP 2/4 and osteocalcin in the early stages ofdistraction osteogenesis

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Abstract
This study was performed to investigate the effect of loading on the biology of newly forming bone during limb lengthening. Unilateral 2.0 mm femoral lengthenings were performed in 20 male Sprague Dawley rats. Half (n = 10) of the animals were allowed to bear weight freely, while the other half were prevented from weight-bearing via an ipsilateral through-knee amputation. The animals in each group were sacrificed after one (n = 5) or four (n = 5) days of consolidation (post-operative days seven and 10, respectively). In situ hybridization for osteocalcin and collagen I, and antibody staining for collagen II and BMP 2/4 were used to evaluate the molecular influence of loading. There was more new bone in the distraction gap of the weight-bearing animals than there was in the non-weight-bearing animals. BMP 2/4 expression, and the messages for collagen I and osteocalcin, were more abundant in tissue from the weight-bearing animals; collagen II was higher in the non-weight-bearing animals. This suggests that early regenerate tissue is capable of responding to loading, and that weight-bearing appears to stimulate intramembranous ossification. These findings support the concept of early weight-bearing after limb lengthening.