BACKGROUND Revascularization of a fracture depends on fracture stability and fracture gap conditions. The aim of the study was to determine quantitatively the revascularization and tissue differentiation in an animal model with different fracture gaps and controlled biomechanical conditions. METHODS The study was performed on ten sheep with an osteotomy on the right metatarsal. The fracture was stabilized by an external fixator that allowed adjustable axial interfragmentary movement. Two groups of five sheep each were adjusted to a medium sized gap (M, 2.1 mm) and a large gap (L, 5.7 mm) under comparable interfragmentary strain (30-32%). The animals were killed after 9 weeks, and the metatarsals were prepared for undecalcified histology and analysis of tissue differentiation and vessel distribution. RESULTS Group M showed significantly more revascularization (M=1.62, L=0.85 vessels/mm2), more bone formation (M=37.2%, L=13.9%) and less fibrocartilage tissue (M=18.1%, L=39.1%) than group L. Larger vessels (>40 microm) were found mainly in the medullary channel, and smaller vessels (<20 microm) mainly in the peripheral callus. Histologically, group M showed partial bony bridging of the osteotomy gap, and the group L had delayed healing. CONCLUSIONS A good reduction of a fracture with small interfragmentary gaps is important for its revascularization and healing.