To investigate the influence of the stability of an osteotomy fixation on the local vascularization and tissue differentiation in callus healing, a transverse osteotomy of the right metatarsal with a gap size of 2 mm was performed in 10 sheep and stabilized with an external fixator. This fixator permitted a defined axial movement. Two groups of 5 sheep were each operated upon to allow 0.2 mm (group A) or I turn (group B) of axial movement. Nine weeks after Surgery, the callus was dissected and histological sections prepared. The type of tissue and the vessel distribution were determined. Larger interfragmentary movements led to significantly more fibrocartilage (small axial movement A: 6.2%, large axial movement B: 21.6%) and significantly less bone formation (A: 38.2%, B: 26.3%). On average, and particularly close to the periosteum the number of vessels in the callus healing area was greater in the group With smaller movements than in the group with larger movements. There was a significant difference between the distribution of small (<20 mum) and large (>40 mum) vessels across the whole healing area for both groups. Whereas the large vessels showed maximum density in the medullary cavity, the small vessels showed the highest frequency in the peripheral part of the periosteal callus. (C) 2002 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.