Bone Healing of Canine Radius–Ulna Fractures Treated with a Novel 3D Cast: Age-Based Radiographic Evaluation and Healing Time Analysis in 191 Limbs (179 Dogs) — A Retrospective Clinical Study

Fractures of the distal radius and ulna with marked displacement are particularly challenging in toy-breed dogs because of their extremely small bone diameter, where both surgical fixation and conventional casting often lead to delayed or failed union. Building on our previous development of a patient-specific three-dimensional (3D) cast that enables reliable union through natural callus formation and functional loading, this large-scale clinical study aimed to clarify age-related differences in fracture healing and to establish a predictive model for time to union.A total of 191 limbs (179 dogs) treated between 2019 and 2025 were retrospectively analyzed and classified into four age groups (lt;6 months, 6–12 months, 1–2 years, ≥2 years). Exclusion criteria included previous surgery, delayed presentation (gt;7 days), comminuted or isolated fractures, and loss to follow-up. Healing duration was evaluated using Kaplan–Meier analysis and box-and-whisker plots, and linear regression was used to assess the effects of age and body weight.Median time to union increased with age: 38 days 31–41 in dogs lt;6 months, 50 days 42–60 in 6–12 months, 62 days 55–75 in 1–2 years, and 74 days 67–92 in ≥2 years (log-rank p lt; 0.0001). Age was a significant predictor of healing duration (β = 4.6 days/year; 95% CI, 3.1–6.2; p lt; 0.001), whereas body weight showed no significant effect (β = 1.0 days/kg; 95% CI, –2.3–4.3; p = 0.54). All fractures achieved union, with an overall median of 50 days 39–67, demonstrating faster and more physiological recovery compared with conventional methods.Sequential radiographic observations supported these statistical findings, visually confirming age-dependent differences in callus formation and remodeling. This study provides the first large-scale, evidence-based framework for predicting fracture healing in toy-breed dogs, highlighting 3D cast therapy as a practical and biologically sound alternative to conventional fixation.