Age-Dependent Time to Radiographic Union and Bone-Healing Dynamics in Distal Radius and Ulna Fractures in Small Dogs Managed with an Anatomically Molded Cast

This study investigated age-related differences in time to radiographic union and healing morphology in longitudinally displaced distal radius and ulna fractures in small dogs. Medical records and serial radiographs of 107 dogs (107 limbs) with radiographically confirmed union following anatomically molded cast (AMC) treatment between 2019 and 2025 were retrospectively analyzed. Dogs were classified into four age groups: <6 months, 6–<12 months, 1–<2 years, and ≥2 years. Time to radiographic union, bone diameter expansion, and a radiographic index of interfragmentary distance reduction (IDR) were evaluated. Median times to union were 40.0, 52.0, 64.0, and 81.0 days across the four age groups, respectively (P < 0.001). After adjustment for age group, each 1-kg decrease in body weight was associated with a 6.2% longer time to union (P = 0.014). Bone diameter expansion and IDR differed significantly among age groups (both P < 0.001), with the greatest expansion and lowest IDR in dogs aged <6 months. The age-related trend in union time persisted in a sensitivity analysis including cases lost to follow-up. Healing morphology was characterized by callus-dominant envelopment in younger dogs, whereas more skeletally mature dogs tended to show fracture-end reshaping and reduced interfragmentary distance, consistent with a convergence-dominant healing pattern. These findings may help anticipate age-related healing trajectories, inform owners, and guide treatment planning.