Bone Healing Response of Two Different Silicate-Containing Calcium Phosphate Synthetic Bone Grafts in a Rabbit Femoral Critical-Sized Defect Model

Abstract

Synthetic materials based on calcium phosphates (CaP) are frequently used as bone graft substitutes when natural bone grafts are not available or not suitable. One approach used to enhance their performance is the incorporation of ions such as silicate ions, which are known to support osteointegration by stimulating bone formation. The present study aimed to evaluate the bone healing response of two different synthetic silicate-containing CaP bone graft materials, one with a nanoscale primary structure (OssDsign Catalyst) and the other with a micron-sized primary structure (Actifuse ABX) using an established distal femur rabbit model. The two bone graft materials were implanted into critical-sized bone defects created in the femoral condyles of rabbits for implantation times of 4, 8, 12, and 26 weeks. Bone formation was assessed by radiographs and microcomputed tomography (μCT) imaging, complemented by histology and histomorphometry. Additionally, both materials were characterized regarding their calcium ion release and protein adsorption in vitro. Both materials formed new woven bone after 4 weeks, found on both the surfaces of, and in the spaces between, granules. However, the amount of bone formed with OssDsign Catalyst plateaued at approximately 30% at 4 weeks, whereas this occurred after 8 weeks for the Actifuse ABX. After 4, 8, 12 and 26 weeks, the graft material remaining decreased by 32%, 43%, 49% an 60%, respectively for OssDsign Catalyst, whereas for the Actifuse ABX the decrease was 15%, 35%, 39%, and 26%. Furthermore, the amount of calcium ions released into saline solution and the quantity of protein adsorbed in vitro were significantly higher in granules of OssDsign Catalyst than in granules of Actifuse ABX. In this preclinical study, a nanoscale silicate-containing CaP bone graft (OssDsign Catalyst) showed a continuous progression of graft remodeling throughout the 4-to-26-week time course coupled with new bone formation. In contrast, a micron-scale silicate-containing calcium phosphate (Actifuse ABX) remained almost unchanged from 4 through to 26 weeks, exhibiting minimal graft remodeling. Differences in material properties of synthetic bone grafts influence the healing response in critical-sized defects, including the ability of the graft to be remodeled in a sustained manner while not compromising bone repair.