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 (<6 months, 6–12 months, 1–2 years, ≥2 years). Exclusion criteria included previous surgery, delayed presentation (>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 <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 < 0.0001). Age was a significant predictor of healing duration (β = 4.6 days/year; 95% CI, 3.1–6.2; p < 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.