33. Fluorapatite Scaffolds With Gyroid Architecture: Advancing Synthetic Bioceramics for Bone Defect Repair in Cranioplasty

PURPOSE: Synthetic bioceramics are increasingly being explored as artificial grafts for cranioplasty and reconstructive surgeries, where autografts are the standard but have limitations such as high failure rates and donor site morbidity. Results from our previous studies suggested that fluorapatite (FA), a substituted form of hydroxyapatite (HA), enhances in vivo bone formation compared to HA. Additionally, literature data demonstrated that scaffolds with gyroid architectures sintered at high temperatures support mechanical stability and enhanced bone deposition in vitro. The purpose of this study was to evaluate whether FA scaffolds with optimized gyroid architectures could achieve bone regeneration comparable to autografts in critical-size bone defects in sheep ilium. METHODS:Synthesis and Fabrication: Using a previously published method, FA powder was synthesized in-house; calcined at 900°C; and characterized. Porous scaffolds were fabricated by gel-casting aqueous FA suspensions into 3D-printed wax-resin templates (negatives). This was followed by polymerization, drying, and sintering at 1150°C for 2 hours. The scaffolds were analyzed for porosity and structural parameters using micro-CT. Surgical Procedure: Six 18-month-old Rambouillet sheep received full-thickness bicortical defects in the ilium with a trapezoidal shape (2 cm height × 3 cm top width × 2 cm bottom width × 1 cm depth). The defects were treated with either an autograft (positive control), left untreated (negative control), or filled with an FA scaffold. Autografts and FA scaffolds were secured using Kirschner wire (K-wire) fixation. Animals received pain management pre- and postoperatively and were monitored for 12 weeks. At necropsy, the scaffolds and surrounding tissues were harvested for histological and radiological analysis. RESULTS: The characterization data confirmed that the sintered scaffoldings comprised fluorapatite, with no indication of secondary phases. These scaffolds had a volumetric porosity ranging from 58% to 62%. The mean pore and strut diameters were 478 ± 59 µm and 525 ± 84 µm, respectively. Postoperatively, the animals did not exhibit any gait abnormalities or behavioral changes. Preliminary micro-CT analysis at 12 weeks indicated incomplete regeneration in the defect-only group. The implanted autograft bone was mostly integrated with the surrounding tissue in the autograft group. Still, large areas remained devoid of mineralized tissue at the center of the defects, and the cortical shell was not regenerated in the defect region. In the FA scaffold group, bone formation was most prominent near the periphery of the defect, with mineralizing bone ingrowth throughout the pores. CONCLUSION: Ongoing histological and microCT analyses aim to quantitatively assess ossification within the defects for both the autograft and FA scaffold groups. Early micro-CT results suggest that FA scaffolds with gyroid architectures promote bone formation, predominantly in peripheral regions, with some ingrowth extending into the inner scaffold pores. These findings highlight the potential use of porous FA scaffolds in bone regenerative applications, including cranioplasty and cosmetic bone reconstruction.