28. Evaluation of three commercially available synthetic bone grafts in a clinically relevant ovine model of instrumented lumbar posterolateral fusion

Abstract

BACKGROUND CONTEXT Synthetic bone grafts are important tools for spinal surgeons who wish to avoid the invasive harvesting of autologous bone graft for spinal fusions. Various synthetic bone grafts are available on the market, including materials based on bioglass, calcium phosphate or a combination of both. Side-by-side comparisons of synthetic bone grafts in relevant preclinical study models can provide valuable insights that can aid in the selection of treatment options. PURPOSE The current study compared three commercially available synthetic bone grafts as stand-alone grafts in a clinically relevant ovine model of instrumented posterolateral fusion (PLF). STUDY DESIGN/SETTING The three different synthetic bone grafts evaluated are: (1) putty formulation of biphasic calcium phosphate with submicron topography that was previously shown to have equivalent performance to autograft in this model (BCP; MagnetOs Putty, Kuros Biosciences BV), (2) putty formulation of 45S5 bioglass (BG; Novabone Putty, Novabone Products LLC), (3) composite of beta-tri-calcium phosphate, 45S5 bioglass and bovine type I collagen (TCP+BG; Vitoss BA2X Foam pack, Orthovita, Inc). Autograft bone (AB) was used as positive control. Twelve weeks after implantation, the treatments were evaluated. OUTCOME MEASURES Main study end points include fusion assessment by manual palpation, X-ray imaging and micro-CT scans. Range of motion (ROM) of spine levels was assessed in lateral bending (LB) and flexion-extension (FE). Quantification of fusion mass volume was performed using micro-CT. Histology was performed to evaluate bone formation and tissue responses with the different graft materials. METHODS Following ethical approval, 9 sheep underwent instrumented PLF surgery at L2-3 and L4-5 levels. Autograft was harvested from the Os Ilium and distal femur. After decortication of the transverse processes (TPs), 10 cc of each graft material (AB; n=6, BCP; n=6, BG; n=3, TCP+BG; n=3) was implanted bilaterally in the posterolateral regions, bridging the inter-transverse process space. After sacrifice, specimens were assessed by manual palpation, X-ray imaging, Micro-CT and ROM testing at 7.5 Nm after removing instrumentation. Subsequently, treated spine levels were processed for histology in polymethylmethacrylate to visualize bone tissue in the fusion mass. RESULTS Fusion assessment by manual palpation revealed a 6/6 fusion rate for AB and BCP, while for both BG and TCP+BG, a 1/3 fusion rate was determined. Fusion assessment of X-ray and Micro-CT were consistent and determined an either unilateral or bilateral robust bone bridge in 6/6 levels for both AB and BCP, while The BG group exhibited unilateral bridging in 2/3 levels and the TCP+BG group did not include a robust bone bridge in any of the levels. ROM testing revealed equivalent ROMs in LB and FE for AB and BCP, being substantially and significantly lower than those of BG and TCP+BG groups. Fusion mass volume quantification determined fusion mass volumes of 5.7 ± 1.6 cm3 for AB, 9.6 ± 0.4 cm3 for BCP, 3.0 ± 1.1 cm3 for BG and 2.4 ± 1.0 cm3 for TCP+BG. Correspondingly, histology revealed significant graft resorption for BG and TCP+BG groups, while abundant bone tissue in the inter-TP space was observed for AB and BCP. A continuous bony bridge in between TPs was frequently observed in the BCP group on histological sections. CONCLUSIONS Calcium phosphate with submicron topography was the only synthetic bone graft to perform equally well as autograft as a stand-alone graft in this PLF model. Both bioglass-based bone grafts significantly underperformed as compared to autograft and BCP on every single assessment method. The bioglass-based synthetics were observed to undergo significant resorption and were not able to induce bone formation in the soft tissue region in between the TPs. Calcium phosphate with submicron topography may be the better treatment option than bioglass-based bone grafts as a synthetic bone graft substitute. FDA DEVICE/DRUG STATUS MagnetOs Granules (Investigational/Not approved), Novabone Putty (Approved for this indication), Vitoss BA2X (Approved for this indication).