A citric acid-based hydroxyapatite composite for orthopedic implants

We describe a novel approach to process bioceramic microparticles and poly(diol citrates) into bioceramic-elastomer composites for potential use in orthopedic surgery. The composite consists of the biodegradable elastomer poly(1,8-octanediol-citrate) (POC) and the bioceramic hydroxyapatite (HA). The objective of this work was to characterize POC-HA composites and assess the feasibility of fabricating tissue fixation devices using machining and molding techniques. The mechanical properties of POC-HA composites with HA (40, 50, 60, 65 wt.%) were within the range of values reported for tissue fixation devices (for POC-HA 65 wt.%, S_b=41.4±3.1, E_b=501.7±40.3, S_c=74.6±9.0, E_c=448.8±27.0, S_t=9.7±2.3, E_t=334.8±73.5, S_s=27.7±2.4, T_s=27.3±4.9, all values in MPa). At 20 weeks, the weight loss of POC-HA composites ranged between 8 and 12 wt.%, with 65 wt.% HA composites degrading the slowest. Exposure of POC-HA to simulated body fluid resulted in extensive mineralization in the form of calcium phosphate with Ca/P of 1.5-1.7 similar to bone. POC-HA supported osteoblast adhesion in vitro and histology results from POC-HA samples that were implanted in rabbit knees for 6 weeks suggest that the composite is biocompatible. Synthesis of POC-HA is easy and inexpensive, does not involve harsh solvents or initiators, and the mechanical properties of POC-HA with 65 wt.% HA are suitable for the fabrication of potentially osteoconductive bone screws.