P1. Efficacy of a calcium phosphate collagen matrix bone graft with needle-shaped submicron surface topography in a rabbit posterolateral lumbar spine fusion model

Abstract

<h3>BACKGROUND CONTEXT</h3> Synthetic bone grafts are a promising alternative for the treatment of degenerative spine conditions and are available without the added drawbacks associated autografts including limited availability and donor-site morbidity. Of these synthetic materials, calcium phosphate (CaP) with a characteristic needle-shaped submicron surface topography (BCP<µm) has attracted much attention due to its unique bone-forming ability which is essential for repairing critical-size bone defects such as those found in the posterolateral spine [1,2]. Previous in vitro and ex-vivo data presented by van Dijk LA [3] and Yuan H [4] at NASS, 2020 demonstrated that these specific surface characteristics drive a favorable response from the innate immune system. <h3>PURPOSE</h3> This study was to evaluate the efficacy of a novel synthetic bone graft using a clinically-relevant "Boden" rabbit posterolateral spine fusion model. The synthetic bone graft comprised of CaP granules with a needle-shaped submicron surface structure (BCP<µm) embedded in a highly porous and fibrillar bovine type I collagen matrix (col-BCP<µm). The fibrillar collagen was introduced as a carrier, designed to facilitate admixing of the graft with biological fluids perioperatively and to minimize graft migration postoperatively. <h3>STUDY DESIGN/SETTING</h3> The novel synthetic bone graft was first mixed in a 1:1 volume ratio with autologous bone (ABG). Subsequently, grafts were implanted in the rabbit posterolateral spine environment for 3 days, 6, 9 and 12 weeks. An autologous bone graft group was used as a positive control in this study. Outcomes for the test article were additionally compared to historical data for the same material (BCP<µm) implanted alone, without the fibrillar collagen carrier. <h3>OUTCOME MEASURES</h3> Spinal fusion was determined by manual palpation (MP), X-ray and µCT imaging (based on the Lenke scale), mechanical testing (range of motion testing at 12 weeks only) and histology (decalcified and undecalcified). <h3>METHODS</h3> Forty skeletally mature New Zealand white rabbits were divided into two groups and underwent single-level bilateral posterolateral intertransverse process spine arthrodesis at L4-5. Animals were euthanized after 3 days, 6, 9 and 12 weeks. <h3>RESULTS</h3> Based on all endpoints, the test group and positive control group showed an equivalent and gradual progression in bone formation and implant resorption over time, leading to mature fusion masses at 12 weeks. No adverse reactions were observed in any of the groups. Bilateral fusion rates by MP were consistent with the literature (0-40% at 6 weeks; 60-70% at 12 weeks) and similar trends were seen by histological and radiographic assessment (both X-ray and µCT). Flexion-extension and lateral bending results at 12 weeks demonstrated fusion of the treated level consistent with the MP results. The results observed for the test group were consistent with those seen historically for the same material implanted without the fibrillar collagen carrier. <h3>CONCLUSIONS</h3> These results show that BCP with needle-shaped submicron-scale surface topography (BCP<µm) combined with a fibrillar collagen matrix (col-BCP<µm) could achieve equivalent fusion rates to ABG when used as a bone graft extender in a clinically relevant single level rabbit intertransverse process posterolateral lumbar fusion model. Over time, a progression in bone healing, graft resorption and remodeling were seen with all experimental endpoints, with equivalency between the groups. The results for col-BCP<µm were consistent with those previously seen for BCP<µm alone, confirming that the fibrillar collagen carrier improved handling properties but was not detrimental to the bone-forming capacity of the BCP<µm. <h3>FDA DEVICE/DRUG STATUS</h3> Col-BCP<µm (Investigational/Not approved)