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Objective--To develop a technique for use in investigation of healing of long-bone defects by creation of a critical-size defect in the left metarsal III and IV bone (metatarsus) of sheep. Animals--18 healthy adult sheep. Procedure--Sheep were allocated to 4 groups (3, 3, 5, and 7 sheep in groups 1 to 4, respectively). An ostectomy with various segmental length-to-diaphyseal diameter ratios (0.5, 1.0, 2.0, and 2.0 for groups 1 to 4, respectively) was performed on the left metatarsus of each sheep. The defect was left empty in sheep of groups 1, 2, and 3, whereas the defect was filled with a massive corticocancellous bone autograft in sheep of group 4. Results--All sheep tolerated the surgical procedure well and were able to use the affected limb the day after surgery. Radiographic and histologic examinations conducted 16 weeks after surgery revealed nonunion in all sheep of groups 1, 2, and 3, whereas consistent bone healing with abundant bone formation was observed in all sheep of group 4. Conclusions and Clinical Relevance--Analysis of these findings suggests that the sheep metatarsal model is a critical-size defect model with low morbidity. It should allow the assessment of new technologies for bone regeneration in conditions closely mimicking the clinical setting. Impact for Human Medicine--Use of this technique in sheep should be of benefit for the preclinical study of osteoconductive, osteoinductive, or osteogenic biomaterials for use in humans. (Am J Vet Res 2004;65:1653-1657). [ABSTRACT FROM AUTHOR]