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.

Radial–ulnar fractures in toy-breed dogs are difficult to manage because of their slender bone diameter and limited soft tissue coverage. Surgical fixation provides rigid stability but risks delayed or nonunion due to stress shielding and medullary occlusion, whereas conventional casting, though non-invasive, often fails to stabilize complete fractures and requires prolonged immobilization, reducing case quality of life. This study developed a case-specific, two-phase three-dimensional (3D) cast therapy tailored to each individual case. A rigid cast was applied during the inflammatory stage to ensure alignment and protection, followed by a slit-modified 3D cast during the early repair stage to allow controlled weight-bearing and functional rehabilitation. Sequential radiographs enabled radiographic visualization of the entire bone healing process, revealing distinct phenomena: rapid callus proliferation in young dogs (“Super-Healing”) and gradual, load-guided realignment of displaced fragments (“Slide-Healing”). These findings demonstrate that controlled mechanical loading under stable, flexible fixation can actively promote bone regeneration. This conservative, visualization-based approach achieved reliable bone union without surgery, minimized activity restriction, and markedly improved case welfare. Owing to its simplicity, safety, and reproducibility, 3D cast therapy has the potential to serve as a new standard of care for managing radial–ulnar fractures in toy and small-breed dogs, while also providing a translational model that bridges clinical orthopedics and regenerative medicine.