Design of Solid Scaffolds with Time-Released Porosity for Endogenous Bone Engineering
Alexander C. Allori, M.D., M.P.H.1, Alexander M. Sailon, B.A.1, Clarence D. Lin, M.A.1, Elizabeth Clark, M.S.2, John L. Ricci, Ph.D.2, Cornelia Cretiu-Vasiliu, M.S.3, James Smay, Ph.D.3, Phuong Nguyen, M.D.1, Stephen M. Warren, M.D.1, Robert J. Allen, Jr., M.D.1, Xiaoxia Wang, D.D.S.1.
1New York University, New York, NY, USA, 2New York University College of Dentistry, New York, NY, USA, 3University of Oklahoma, Stillwater, OK, USA.
BACKGROUND: While scaffolds employed in skeletal repair today have excellent osteoconductivity, their porous structure permits soft-tissue ingrowth, and bacterial colonization. To address these problems, a novel hydroxyapatite (HA) and tri-calcium phosphate (TCP) scaffold was designed with filled internal porosity for in vitro and in vivo testing.
METHODS: A 3D printing process was used to create microstructural lattices (11mm diameter, 3mm thickness; 250um struts; pore size 100-750um; HA:TCP ratio 60/40 or 15/85; left porous or filled with calcium sulfate/chitosan). To assess for scaffold dissolution over time, porous and solid scaffolds were placed in DMEM+10%FBS and weighed daily for 64 days. Porous scaffolds were seeded with MC3T3 cells and cultured for 2, 4, and 8 days, and scanning electron microscopy (SEM) was used to assess cellular adherence and density. In vivo, porous and solid scafffolds were implanted in a critical-size (11x3 mm) calvarial defect model in rabbits. Samples were harvested at 8 and 16 weeks and analyzed by micro-computed tomography, histology, and SEM.
RESULTS: All empty porous scaffolds retained 97±2% of their weight at 64 days. When filled by calcium sulfate, relative mass decreased linearly through 64 days (0.44%/day, R2=0.92), which correlated with gradual centripetal exposure of the internal porosity at ~1mm/wk. In contrast, chitosan filling of the scaffolds was irregular in its dissolution. SEM showed strong cellular adherence to exposed struts (density of 0.14 ± 0.09 cells/um2). In vivo, HA scaffolds conducted bone across critical defects by 8 weeks (85±18%) compared to non-healed controls (p<0.05). Bony ingrowth corresponded to the degradation rate of the calcium sulfate. Scaffold filling also prevented ingrowth of soft tissue. Sixteen week samples are being processed.
CONCLUSIONS: Filling with calcium sulfate successfully limited bloody infiltration and soft tissue ingrowth but retained osteoconductivity. Present work is focused on improving scaffold osteoinductivity by using these fillers to encase bioactive molecules for rate-controlled release as the filler degrades.