Fibre reinforced bioresorbable composites for spinal surgery

Composites containing different amounts of beta-tricalcium phosphate (beta-TCP) embedded in a poly-lactide (PLA70) matrix with and without poly-lactide (PLA96) fibre reinforcement were studied and the feasibility of using these composites in spinal fusion implants was examined. Compressive yield strength was measured in two directions: parallel to (83-97 MPa) and perpendicular to (108-123 MPa) the laminated structure of the composites. In the parallel direction, the addition of beta-TCP decreased compressive yield strength while in the perpendicular direction this was increased when compared to plain specimens (p<0.05). Fibre reinforcement had no significant effect on compressive yield strength (p<0.05), but did increase impact strength by 127-216% for notched specimens (parallel direction) and by about 65% for un-notched specimens (perpendicular direction) (p<0.05). A 24 week in vitro analysis of implant prototypes in simulated body fluid revealed a decrease in compressive yield strength, which was greater for the samples containing 50 wt.% beta-TCP than for those containing 25 wt.% beta-TCP. After 12 weeks incubation the composites retained 66-99% of their initial compressive strength, depending on composition. After 24 weeks incubation the lowest compressive strength was 51% (56 MPa: 50/50) and the highest was 94% (90 MPa: 75/25) of the initial value. Calcium phosphate precipitation on the surfaces of the materials in vitro was also observed. The initial compressive strengths of the studied composites were comparable to materials used in spinal fusion applications, but adequate strength retention behaviour needs to be confirmed before undertaking clinical experiments.