Author(s): Shigeo Ishijima
Correspondence: ishijima2013@gmail.com
From “Fixation-Based Reduction” to “Regeneration-Based Union” —
Our hospital has developed a 3D Cast Therapy that maximizes the body’s natural healing potential for treating radial–ulnar fractures in dogs.
Through this innovative approach, we observed two new modes of biological bone healing — Slide-Healing and Super-Healing — in which the fractured bone spontaneously reconstructs itself under controlled functional loading.
The first paper reports the radiographic visualization of bone regeneration and the sequential documentation of the entire healing process.
The second paper establishes an age-based predictive model of healing time based on clinical data from 191 limbs.
Statistical analysis demonstrated that younger dogs heal significantly faster, and that healing duration increases progressively with age.
All cases achieved complete bone union without surgery, confirming that 3D cast therapy is a safe, biological, and minimally invasive treatment.
This research represents a paradigm shift in fracture management — from mechanical fixation to physiological regeneration.
Below is a summary of the two research papers by Dr. Shigeo Ishijima.
Visualizing Bone Healing in Canine Radial–Ulnar Fractures: A Novel 3D Cast Technique.
Abstract
DOI: 10.20944/preprints202510.1026.v1
Summary
Summary
Fractures of the radius and ulna in small or toy-breed dogs are considered challenging to treat because of the slender bones and limited soft tissue coverage.
Although surgical fixation with metal implants provides stability, it can cause complications such as delayed union or nonunion due to stress shielding and marrow cavity obstruction.
Conventional casting, while minimally invasive, often results in deformity if the fixation is insufficient, or bone atrophy if it is excessively rigid.
To address these issues, we developed a two-phase cast therapy for canine radial–ulnar fractures.
During the inflammatory phase, a rigid cast was applied to maintain alignment, followed by the application of a 3D castin the early reparative phase to allow walking and controlled weight-bearing.
This functional loading induced callus formation and promoted natural bone healing.
Sequential radiographs enabled visualization of the healing process, revealing two distinct biological phenomena:
Slide-Healing: Gradual, spontaneous realignment of bone fragments under light mechanical loading.
Super-Healing: Rapid and exuberant callus formation observed in young dogs.
Case: Toy poodle, 9 months old, weight 1.4kg
This case involved a distal oblique fracture of the radius and ulna in a low-body-weight dog, in which conventional casting or surgical fixation would normally be considered difficult.
With 3D cast therapy, the displaced fracture line gradually slid into correct alignment (Slide-Healing), and both the radius and ulna thickened and achieved complete union.
Case 2: Toy Poodle, 5 months old, 3.1 kg
This case represents “Super-Healing”, characterized by rapid callus formation observed in young dogs.
Within two weeks after initiating 3D cast therapy, the fracture site had more than doubled in thickness due to vigorous callus formation.
By approximately one month, bridging union was confirmed radiographically, and complete healing was achieved.
Additional examples presented in this study include Slide-Healing, where displaced fractures realigned spontaneously under controlled loading, and successful healing in comminuted fractures.
Discussion
This study demonstrated that 3D cast therapy effectively treats radial–ulnar fractures in toy-breed dogs while minimizing activity restriction.
Sequential radiographs objectively visualized the process of indirect bone healing and revealed two distinct, load-responsive phenomena:
(1) rapid and exuberant callus formation in juveniles (“Super-Healing”), and
(2) spontaneous realignment of displaced fragments under functional loading (“Slide-Healing”).
These findings indicate that controlled axial loading combined with stable yet flexible immobilization can activate mechanobiological pathways, accelerating both fracture union and alignment correction.
Conventional osteosynthesis offers rigid fixation but is frequently associated with complications such as stress shielding, medullary canal obstruction, and implant failure.
Direct bone healing typically shows minimal callus and often relies on subjective radiographic interpretation.
Conversely, traditional casting is less invasive but often fails to maintain stability in displaced fractures, leading to deformity or disuse atrophy.
In contrast, 3D cast therapy provides a stable yet elastic fixation that transmits axial load during gait, thereby stimulating callus formation.
This concept is consistent with experimental evidence showing that controlled mechanical stimulation during the reparative phase enhances bone regeneration, in accordance with Wolff’s law.
Recent mechanobiological studies further emphasize that interfragmentary strain and load-dependent mechanotransduction are key determinants of tissue differentiation during fracture healing.
The phenomena observed in this study provide clear visual evidence of biological repair mechanisms that have rarely been captured using conventional fixation methods.
3D cast therapy also offers a unique clinical model for observing physiological bone regeneration.
Controlled functional loading allows the intrinsic healing potential of bone tissue to emerge without the biological disruption associated with surgery.
This approach embodies the core principle of regenerative medicine—stimulating natural repair mechanisms rather than replacing them artificially.
Continuous radiographic observation demonstrated how appropriate mechanical cues orchestrate cellular differentiation, angiogenesis, and tissue remodeling in vivo.
Thus, 3D cast therapy serves not only as a minimally invasive orthopedic treatment but also as a translational model bridging clinical orthopedics and regenerative biology.
In adult dogs, satisfactory bone union can also be achieved with 3D cast therapy; however, delayed presentation after fracture may result in early callus hardening, making manual reduction difficult.
Therefore, early presentation and initiation of treatment are crucial for achieving accurate alignment and optimal cosmetic outcomes.
In large-breed dogs, the structural limitations of the 3D cast make initial alignment maintenance difficult.
A two-stage strategy, involving surgical reduction followed by transition to 3D cast therapy once partial union is achieved, may be a more practical and physiological approach.
This staged method combines the precision of surgical fixation with the biological benefits of functional loading, promoting a more natural healing environment.
The implications of this research extend beyond fracture management into regenerative medicine.
Future studies integrating imaging analysis with molecular and cellular investigations may clarify how mechanical stimulation interacts with stem cell–mediated osteogenesis, further linking orthopedic practice with regenerative biology.
Conclusions
Sequential radiographic observation in this study successfully visualized the entire bone healing process, enabling objective and reproducible evaluation of fracture union.
Beyond its clinical utility, this visualization-based method provides a novel in vivo model for studying bone regeneration mechanisms and elucidates the essential role of mechanical stimulation in the healing process.
Owing to its simplicity, safety, and reproducibility, 3D cast therapy has the potential to become the standard of care for managing radial–ulnar fractures, particularly in toy and small-breed dogs.
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
Author(s): Shigeo Ishijima
Abstract
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.
Radiographic Findings
Sequential radiographic observations visually confirmed the age-dependent differences in callus formation and remodeling rate, supporting the results of statistical analysis.
Representative age-based radiographs obtained in this study are provided in the main article for reference.
Conclusion
This large-scale clinical study is the first to establish a predictive model for fracture healing in small-breed dogs such as Toy Poodles.
The findings demonstrated that 3D cast therapy serves as a practical and biologically superior alternative to conventional surgical fixation for managing radial–ulnar fractures.
Discussion
This large-scale clinical study revealed clear age-dependent differences in fracture healing among toy-breed dogs.
Healing was extremely rapid in dogs younger than six months, remained efficient up to twelve months, and gradually slowed between one and two years of age before stabilizing thereafter.
Sequential radiographs visually corroborated these quantitative trends.
In dogs under one year, callus formation was highly active and remodeling was completed within weeks.
Between one and two years, callus formation decreased and the healing pattern transitioned to a more mature, stable process—representing a physiological transition rather than a pathological decline.
Notably, refractures (2.6%) occurred primarily during this transitional stage, coinciding with the radiographic reduction in callus production.
This overlap suggests that structural resilience may temporarily decrease during late adolescence, a phenomenon consistent with developmental remodeling changes reported in other species.
The statistical results and radiographic findings were highly consistent, strongly supporting the reliability of this model.
By combining numerical analysis with direct visual evidence, the study achieved a level of clarity and clinical relevance unattainable through purely statistical or descriptive approaches.
Quantitative and visual understanding of bone healing provides veterinarians with a deeper insight into patient recovery and a rational, evidence-based means of predicting healing duration.
Regression analysis identified age, but not body weight, as an independent determinant of healing time.
This indicates that slender bones are not biologically predisposed to delayed union; rather, the fracture susceptibility of toy breeds arises from mechanical and structural characteristics.
Fractures were most frequently observed in Toy Poodles, Pomeranians, and Italian Greyhounds—breeds known to possess narrow medullary canals and limited periosteal coverage, resulting in low resistance to bending and torsional stress.
Micro-CT analysis further revealed that these breeds have sparse, thin trabeculae with a high cortical-to-trabecular ratio, suggesting reduced internal energy absorption and greater vulnerability to fracture under normal loading.
Under the stable, physiological loading conditions provided by 3D cast therapy, even slender bones exhibited equivalent regenerative potential, overturning the long-held assumption that “small means fragile.”
This finding enables prognosis to be guided primarily by patient age, providing a simple, objective indicator directly applicable to clinical decision-making.
Although this was a single-center, retrospective study limited to toy breeds (≤10 kg), the combination of a standardized treatment protocol, consistent radiographic follow-up, and large sample size allowed robust age-based analysis.
This approach, for the first time in real-world clinical practice, clearly depicted how bone healing capacity evolves from the highly regenerative state of juveniles to the steady maintenance phase of adults.
Overall, integrating statistical analysis with sequential radiographic visualization established a practical and reproducible model for predicting fracture healing time in toy-breed dogs.
This model enhances clinical planning and communication with owners, while providing important biological insight into age-dependent bone regeneration.
Future prospective and cellular-level studies focusing on the regenerative potential of bone marrow–derived mesenchymal stem cells (MSCs) in young dogs are warranted.
The clinical evidence presented here provides a translational link between radiographic healing dynamics and the cellular biology of bone regeneration, paving the way for future research on stem cell function in dogs.
Conclusion
This study established an age-stratified, evidence-based model for predicting fracture healing in toy-breed dogs treated with patient-specific 3D cast therapy.
By integrating large-scale statistical analysis with continuous radiographic visualization, it clarified the quantitative and visual dynamics of age-dependent healing.
The results elucidated the physiological transition from the highly regenerative juvenile phase to the stable adult phase and presented practical prognostic indicators directly applicable to clinical practice.