Cellular and molecular characterization of a murine non-union model.

PURPOSE: We have developed a method to study the molecular and cellular events underlying delayed skeletal repair in a model that utilizes distraction osteogenesis.
METHODS: The clinical states of delayed union and non-union were reproduced in this murine model by altering distraction parameters such as the inclusion and exclusion of a latency phase and variations in the rate and rhythm of distraction. Radiographic, cellular, and molecular analyses were performed on the distraction tissues.
RESULTS: Eliminating the latency period delayed bony union, but did not appreciably alter the extent of platelet endothelial cell adhesion marker (PECAM) immunostaining. Following elimination of a latency phase and a threefold increase in the rate of distraction, there was a further delay in bone regeneration and a higher rate of non-union (60%). Instead of bone, the distraction gap was comprised of adipose or fibrous tissue. Once again, despite the rigorous distraction protocol, we detected equivalent PECAM staining within the distraction gap. In a minority of cases, cartilage and osseous tissues occupied the distraction gap likely by a prolonged process of endochondral ossification.
CONCLUSIONS: Here, we have altered the mechanical environment in such a way to reproducibly create delays in skeletal regeneration. These delays in skeletal tissue regeneration appear to develop even in the presence of endothelial cells, which suggests that mechanisms other than a disruption to the vascular network can account for some cases of non-union.