Developing porosity of poly(propylene glycol-co-fumaric acid) bone graft substitutes and the effect on osteointegration: A preliminary histology study in rats

Bioresorbable bone graft substitutes could eliminate disadvantages associat ed with the use of autografts, allografts: and other synthetic materials. W e investigated a bioresorbable bone graft substitute made from the unsatura ted polyester poly(propylene fumarate) which is crosslinked in the presence of soluble and insoluble calcium filler salts. This compact bone graft sub stitute material develops porosity in vivo by leaching of the soluble fille r salts. In attempt to develop materials whose in live porosity can be desi gned such that implant degradation would occur at a rate that remains suppo rtive of the overall structural integrity of the repairing defect site, we studied the early tissue response upon implantation in a bony defect. Three grout formulations of varying solubilities using slightly soluble hydroxya patite (HA) and soluble calcium acetate (CA) were evaluated in 3 mm holes m ade in the anteromedial tibial metaphysis of 200 g Sprague Dawley rats (n = 16 per formulation for a total of 48 animals). Grout formulations cured in situ. Animals from each formulation were sacrificed in groups of 8 at 4 da ys and 3 weeks postoperatively. Histologic analysis of the healing process revealed improved in vivo osteointegration of bone graft substitutes when a higher loading of calcium acetate was employed. All formulations maintaine d implant integrity and did not provoke sustained inflammatory responses. T his study suggested that the presence of a soluble salt permits in vivo dev elopment of porosity of a poly(propylene fumarate) based bone graft substit ute material.