Bone union is classified into four distinct categories: union, non-union, delayed union, and malunion (2,5). Within the non-union classification, there are two subtypes: viable and non-viable non-union. Viable non-union subtypes exhibit physiological activity at the fracture site, indicated by bone reaction and callus formation, and are further categorized as hypertrophic or oligotrophic. In contrast, non-viable non-union subtypes lack adequate blood supply and the potential for callus formation, and are identified as either dystrophic, necrotic, defect, or atrophic non-union. The predominant cause of non-union is often unstable fixation of the fracture (2,5). Diagnosis is confirmed through radiographs taken at 4- to 8-week intervals. Non-union treatment strategies include the application of stronger fixation methods, the removal of existing implants, and bone graft techniques that may employ osteoinductive agents such as recombinant human bone morphogenetic protein 2 (rh-BMP2) (3,7-9,15).

To achieve strong fixation, titanium implants are often tailored to fit the defect site. Titanium bio-scaffolds have gained attention due to their superior biocompatibility, mechanical strength, and resistance to corrosion, making them ideal for providing structural support in load-bearing areas (1,10,12). The use of 3D printing techniques has allowed for precise customization of implants tailored to individual bone defects, improving surgical outcomes and reducing complications (1,10).

In addition to the mechanical stability provided by titanium scaffolds, bone graft materials play a crucial role in promoting bone regeneration. Traditional bone grafting techniques, such as autografts and allografts, are commonly employed to fill bone defects. Autografts, considered the gold standard, provide osteoconductive, osteoinductive, and osteogenic properties but are limited by donor site morbidity and the availability of harvestable bone (2,5). Allografts, derived from donors of the same species, offer an alternative source of bone material but lack the osteogenic capacity of autografts (2,5).

To further enhance bone regeneration, osteoinductive agents like recombinant human bone morphogenetic protein 2 (rh-BMP2) are frequently used in combination with scaffolds or graft materials (3,8,15). Unlike autografts or allografts, rh-BMP2 directly stimulates bone formation by recruiting mesenchymal stem cells and promoting their differentiation into osteoblasts (3,8). When integrated with bio-scaffolds or applied to the defect site, rh-BMP2 amplifies the regenerative process, ensuring successful graft integration and healing (3,15).

This report addresses a case diagnosed with viable hypertrophic non-union, which was stabilized using a titanium bio scaffold, rh-BMP2, and double plating.

A 2-year-old, 3.6 kg spayed female Maltese dog was presented to jeonbuk animal medical center (JAMC). The patient had sustained a left femoral fracture as a result of a fall. At a local hospital, intramedullary (IM) pin and cerclage wire were applied for fracture stabilization.

The patient was unable to bear weight on the left hindlimb, with observed muscle contracture and atrophy. Radiographic examination revealed a fracture line at the distal diaphysis of the left femur with a 1.6 mm gap. The fracture was identified as closed, simple, complete, and long oblique. There was callus formation and periosteal reaction around the fracture fragments, but no evidence of bridging, indicating a viable hypertrophic non-union.

To ensure a more accurate assessment, a computed tomography (CT) scan was recommended. However, due to potential artifacts caused by the existing implants, it was necessary to first remove the implants before performing the CT scan (slice thickness 1 mm, Alexion, TSX-034A, Toshiba Medical Systems, Tochigi, Japan). This approach allowed for a clearer evaluation of the fracture site and surrounding bone structures without interference from the metallic components. The CT scan revealed a sequestrum in the disto-medial region of the distal fragment, which was planned to be removed. CT data was converted to a DICOM file through INFINITT software (INFINITT healthcare, San Diego, USA). The DICOM file was reconstructed into a 3D bone model using Mimics software and saved as a standard triangle language (STL) file. The STL file was aligned by 3D CAD program (Autodesk Fusion 360, USA).

Novel patient-specific bio-scaffold was designed using CAD program (Fig. 1) and printed in Ti-alloy Ti6Al4V ELI (grade 23) (SLM Solution, Germany) using a SLM 125 3D printer (SLM Solutions, Germany) under the commission of the Interventional Mechano-Biotechnology Convergence Research Center at Jeonbuk National University. Additionally, novel 3D osteotomy guide was designed to facilitate the trimming of the bone fragment for secure anchoring of the bio-scaffold. The osteotomy guide was printed using a medical grade resin (ClearSG, ODS, Korea).

Figure 1. Design of the patient-specific 3D printed titanium bio-scaffold. The purpose of this design was to maximize the preservation of the bone fragment while ensuring sufficient length for the placement of screws for bio-scaffold fixation.

The second operation was performed. The fracture site was trimmed and cut to fit the shape of the bio-scaffold using a 3D osteotomy guide and an oscillating saw. A pre-contoured 2.0-mm locking reconstruction plate was used to stabilize the lateral side, and a pre-contoured 1.2-mm locking reconstruction plate was employed for the medial side. Two screws were inserted into the distal femur to securely fix the bio-scaffold (Fig. 2A). Subsequently, a collagen membrane (BCP/Collagen MEMBRANE, Ovis, Korea) that was trimmed at size of titanium bio-scaffold was placed beneath and above the bio-scaffold, and synthetic bone graft material along with rh-BMP2 (NOVOSIS, CGBIO, Korea) were applied into the scaffold and rest rh-BMP2 0.1 mL was applied into the scaffold using syringe (Fig. 2B).

Figure 2. (A) Image of the plate applied to the lateral side (white arrow), the plate applied to the medial side (black arrow), and the titanium bio-scaffold (white arrowhead). (B) Image of the application of the collagen membrane (black arrowhead) and rh-BMP2 (asterisk).

The surgical site healed without complications, and stitches were removed 10 days postoperatively. The patient was able to bear weight on the operated limb one month after surgery. However, the range of motion (ROM) in the operated limb was limited to 110-150 degrees compared to the unaffected limb, which had a ROM of 50-150 degrees. Six weeks post-surgery, radiographs revealed signs of secondary bone healing, indicated by the periosteal reaction observed around the plate. At six months post-surgery, a CT scan showed no evidence of implant migration or failure (Fig. 3). The X-ray taken 1year and 8month postoperatively revealed no implant-related complications and confirmed successful bone formation (Fig. 4). The operated limb weight bared but the ROM remained largely unchanged from immediately after surgery.

Figure 3. (A-C) CT scan and volume rendering of the reconstructed femur 6 months after surgery. The surgical device was maintained, and bone union was confirmed through bridging callus formation (black arrowhead).

Figure 4. (A) X-ray of the patient at the time of arrival at JAMC. Noticeable callus formation and a fracture gap indicate hypertrophic nonunion. (B) X-ray of the patient 1 year and 8 months after surgery. Significant callus formation (white arrow) around the titanium bio-scaffold indicates bone healing.

Non-union of bone fractures present a significant challenge in veterinary orthopedics. These are fractures where osteogenic activity has ceased, indicating that natural healing is unlikely without intervention (2). Traditional methods for addressing non-union, such as autograft, though effective, are hampered by increased surgical time, risk, and limitations on harvestable bone both in quantity and mechanical suitability (2,5). In this context, the use of rh-BMP2 combined with hydroxyapatite ceramic as a graft substitute has emerged as a promising alternative, mirroring the osteogenic outcomes of autografts, as demonstrated by prior research in both human and veterinary literature (3,7-9,15). Specifically, titanium bio-scaffolds designed with a porous structure are well-documented to effectively support bone graft filling, thereby facilitating a stable and robust bone healing process (1,12,16). Evidence of bone healing was confirmed by radiograph and CT scan taken 6 months after the surgery.

Precise placement of bio-scaffolds is crucial for the effective repair of bone defects not only for the effective repair of bone defects but also for ensuring postoperative stability, which is a key factor in treating non-union fractures (1,4,6,11, 14). To achieve this level of precision, a custom-designed, 3D-printed titanium bio-scaffold was utilized in this case. This scaffold was meticulously tailored to fit the unique contours and dimensions of the patient’s bone defect. Furthermore, it incorporated a step structure in the distal region, specifically designed to minimize bone loss and enhance structural stability. Additionally, a 3D printing osteotomy guide was employed to ensure the accurate and precise placement of the bio-scaffold in both the proximal and distal regions. This guide played a vital role in aligning the scaffold correctly, thereby optimizing its integration with the surrounding bone tissue. As reported in previous studies, the use of such guides has been shown to contribute significantly to reduced surgery time and improved osteotomy accuracy (4,6,14).

Expanding upon the stabilization strategies, this case also incorporated the use of double plating fixation methods. This technique was selected to augment the structural support provided by the titanium scaffold. Double plating, as evidenced by a previous comparison of single and double plate fixation methods for non-union (13), offers superior stabilization, which is critical for the healing of fractures with complex biomechanics or in areas prone to high stress.

Studies on maxillofacial and orthopedic applications have consistently demonstrated that 3D-printed titanium scaffolds possess exceptional mechanical properties, making them ideal for bearing physiological loads while maintaining structural integrity during the healing process (1,10,11). Their porous architecture not only facilitates mechanical stability but also promotes vascularization and cellular infiltration, which are critical for effective bone regeneration. When combined with osteoinductive agents like rh-BMP2, these scaffolds further enhance osteogenic activity by stimulating bone growth at the cellular level (15). This synergy between the scaffold’s structural support and rh-BMP2’s biological activity has been shown to accelerate healing and improve the integration of the graft with native bone tissue. Moreover, their adaptability for patient-specific designs allows for precise customization, further optimizing outcomes in complex cases such as non-union fractures or large bone defects in both veterinary and human medicine.

In conclusion, our comprehensive approach, blending the use of a patient-specific 3D printed titanium bio-scaffold, rh-BMP, and double plating fixation, represents a significant advancement in the surgical treatment of non-union and following bone defect. The success of this case provides a model for future interventions in similar non-union cases, highlighting the potential for customized implants and biological agents to reduce surgical risk and time while improving patient outcomes.

We would like to thank Editage (www.editage.co.kr) for English language editing, and JAMC surgery department for providing surgical facilities and assisting with postoperative care.

The authors have no conflicting interests.