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Dual-energy X-ray absorptiometry (DXA) was used to quantitate the localized densitometric changes that occur early (0-16 weeks) in a tibial ostectomy model of three different gap widths in 15 dogs. Dogs were divided into three equal groups. A 5-mm (group 1), 15-mm (group 2), or 25-mm (group 3) unilateral tibial ostectomy was performed and stabilized with a unilateral external skeletal fixator in each dog. DXA of the gap tissue was performed at 0, 14, 30, 60, 90, and 120 days after surgery. Regions of interest (ROIs) included the entire gap (groups 1, 2, 3) and ROIs within the gap a defined distance from the proximal or distal cortical bone ends: 0-2.5 mm (groups 1, 2, 3); 2.5-5.0 mm (groups 2, 3), 5.0-7.5 mm (groups 2, 3), 7.5-10.0 mm (group 3), and 10.0-12.5 mm (group 3). Bone mineral density (BMD) significantly changed over time in all three groups (P0.0001). The BMD of the 5-mm gap increased over the 4-month study period and reached normal middiaphysial tibial BMD by 90 days after surgery. The BMD of the 15-mm gap also increased after surgery but reached a plateau at a BMD of approximately 0.45 g/cm2 (48% of middiaphyseal BMD) at 60 days after surgery. The BMD of the 25-mm gap increased to a small extent during the first 30 days after surgery and then gradually decreased during the study period. Overall, the 5-mm gap had the highest BMD, followed by the 15-mm gap and the 25-mm gap (P0.0001).