EFFECTS OF MECHANICAL FACTORS ON THE FRACTURE-HEALING PROCESS

An interdisciplinary study based on animal experiments, cell culture s tudies, and finite element models is presented. In a sheep model, the influence of the osteotomy gap size and interfragmentary motion on the healing success was investigated. Increasing gap sizes delayed the he aling process, Increasing movement stimulated callus formation but not tissue quality. Typical distributions of intramembranous bone, endoch ondral ossification, and connective tissue in the fracture gap are qua ntified. The comparison of the mechanical data determined by a finite element model with the histologic images allowed the attribution of ce rtain mechanical conditions to the type of tissue differentiation. Int ramembranous bone formation was found for strains smaller than approxi mately 5% and small hydrostatic pressure (<0.15 MPa). Strains less tha n 15% and hydrostatic pressure more than 0.15 MPa stimulated endochond ral ossification. Larger strains led to connective tissue. Cell cultur e studies on the influence of strain on osteoblasts supported these fi ndings. Proliferation and transforming growth factor beta production w as increased for strains up to 5% but decreased for larger strains. Os teoblasts under larger strains (>4%) turned away from the principal st rain axis and avoided larger deformations. It is hypothesized that gap size and the amount of strain and hydrostatic pressure along the calc ified surface in the fracture gap are the fundamental mechanical facto rs involved in bone healing.