Our previous rat study demonstrated an ex vivo-created “Biomimetic Hematoma” (BH) that mimics the intrinsic structural properties of normal fracture hematoma, consistently and efficiently enhanced the healing of large bone defects at extremely low doses of rhBMP-2 (0.33 μg). The aim of this study was to determine if an extremely low dose of rhBMP-2 delivered within BH can efficiently heal large bone defects in goats.

Goat 2.5 cm tibial defects were stabilized with circular fixators, and divided into groups (n=2-3): 2.1 mg rhBMP-2 delivered on an absorbable collagen sponge (ACS); 52.5 μg rhBMP-2 delivered within BH; and an empty group. BH was created using autologous blood with a mixture of calcium and thrombin at specific concentrations. Healing was monitored with X-rays. After 8 weeks, femurs were assessed using microCT.

Using 2.1 mg on ACS was sufficient to heal 2.5 cm bone defects. Empty defects resulted in a nonunion after 8 weeks. Radiographic evaluation showed earlier and more robust callus formation with 97.5 % (52.5 μg) less of rhBMP-2 delivered within the BH, and all tibias were fully bridged at 3 weeks. The bone mineral density was significantly higher in defects treated with BH than with ACS. Defects in the BH group had smaller amounts of intramedullary and cortical trabeculation compared to the ACS group, indicating advanced remodeling.

The results confirm that the delivery of rhBMP-2 within the BH was much more efficient than on an ACS. Not only did the large bone defects heal consistently with a 40x lower dose of rhBMP-2, but the quality of the defect regeneration was also superior in the BH group. These findings should significantly influence how rhBMP-2 is delivered clinically to maximize the regenerative capacity of bone healing while minimizing the dose required, thereby reducing the risk of adverse effects.

The management of bone defects and impaired fracture healing remains one of the most challenging clinical problems. Several treatments exist to aid in the healing of large bone defects, including biologics such as recombinant human bone morphogenetic protein-2 (BMP-2), yet all have met with limited success. Regeneration of bone requires a coordinated network of molecular signals where the local mechanical environment plays a major role in the success of the healing process. The mechanical environment itself is determined by the stiffness of the implant used to stabilize the fracture and weight-bearing, and if fixation is either too flexible or too rigid the healing might fail. The hypothesis is that the healing of large-segmental bone defects and fractures can be accelerated by the imposition of an appropriate mechanical environment. An overview of the progress made in this research area on how the amount of rhBMP-2 could be reduced and its effectiveness increased by providing an optimized mechanical environment to achieve bone union will be presented. Additionally, the latest findings of improved fracture healing through the manipulation of fixation stability introducing a potential clinical strategy to improve the healing outcome of unstable fractures, particularly for non-unions through increased stabilization, will be discussed.