235. Evaluation of a nanosynthetic silicated calcium phosphate putty in a posterolateral rabbit spinal fusion model

Abstract

BACKGROUND CONTEXT Providing surgeons with an “off the shelf” bone graft substitute to aid in bone healing is advantageous providing the materials participate in and facilitate the desired goal of fusion. Synthetic calcium phosphate-based (CaP) materials are excellent candidates considering their comparable composition to bone mineral however other factors have been shown to play a role in osteogenic behavior of these materials including surface chemistry and surface topography. The preclinical rabbit posterolateral fusion developed by Professor Boden provides a robust means to evaluate the in vivo performance of graft materials in this challenging indication. PURPOSE This study aims to evaluate the in-vivo response of a nanosynthetic silicated CaP combined with autograft compared to autograft alone or a collagen-biphasic CaP combined with autograft in a rabbit spinal fusion model. STUDY DESIGN/SETTING Preclinical animal model. PATIENT SAMPLE N/A OUTCOME MEASURES N/A METHODS Following ethical approval, 70 skeletally mature female NZ white rabbits were divided into three groups: Group 1: Autograft alone (positive control); Group 2: Nanosynthetic Silicated CaP Putty + Autograft (50/50); Group 3: Collagen-Biphasic CaP Putty + Autograft (50/50). A single level posterolateral intertransverse process lumbar fusion was performed bilaterally after decortication of the transverse processes with a high-speed burr. Iliac crest autograft was harvested bilaterally in the positive control group and 2cc placed in the posterolateral gutters. The synthetic materials (1cc) were combined with autograft (1cc) and implanted. Fusion assessment at day 0 (n=2), 6, 9 weeks (n=5), 12 weeks (n=8) and 26 weeks (n=5) included harvest, manual palpation, radiographic assessment, micro-CT, non-destructive range of motion testing (12 weeks only) and decalcified paraffin and pmMA histology; no positive control group (autograft alone) was included at the 26-week time point. RESULTS The positive control group, autograft alone, performed consistent with this model in all endpoints. In the synthetic groups combined with autograft (Groups 2 and 3), radiographic incorporation was observed at the host transverse processes at 6 weeks and further into the fusion with time. Graft resorption and new bone formation was noted in the radiographs as well as the micro-computed tomography sections. Manual palpation was 7/8 at 12 weeks in the Nanosynthetic Silicated CaP Putty + Autograft group compared to 4/8 for the Collagen-Biphasic CaP Putty + Autograft. Histology showed normal healing and increased bone remodeling/formation with Nanosynthetic Silicated CaP Putty + Autograft at all timepoints compared to the Collagen-Biphasic CaP Putty + Autograft. New bone formed directly on the Nanosynthetic Silicated CaP material surface within the fusion mass while this was not a feature of the Collagen-Biphasic CaP material. By 26 weeks histology revealed consistent bridging of bone between the transverse processes in the Nanosynthetic Silicated CaP Putty + Autograft group, but this was not observed in the Collagen-Biphasic CaP group. CONCLUSIONS Developing nanosynthetic silicated CaP bone grafts that aim to mimic the size, structure and chemistry of bone mineral can eliminate the need for excessive autograft harvest and results from this preclinical study supports their effective use in spinal fusion. FDA DEVICE/DRUG STATUS Osteo 3 (the Nanosynthetic Silicated CaP Putty) (Investigational/Not approved)