Biodegradation behavior of ultra-high-strength hydroxyapatite/poly (L-lactide) composite rods for internal fixation of bone fractures

The purpose of this study was to investigate the biodegradation behavior of the ultra-high-strength hydroxyapatite/poly(L-lactide) (HA/PLLA) composite rods for fracture repair. Two kinds of composite materials were used in th is study: u-HA/PLLA, which contained 30% by weight of uncalcined HA as rein forcing particles, and c-HA/PLLA, which contained 30% by weight of calcined HA as reinforcing particles. These composite rods were implanted in the su bcutis and in the medullary cavities of rabbits. The specimens were removed at specific intervals between 2 and 52 weeks and the mechanical strength w as measured for the rods in the subcutis, and the molecular weight and crys tallinity were measured for the rods in both the subcutis and medullary cav ities. The rod surfaces were examined using a scanning electron microscope (SEM). The specimens were examined histologically by light microscopy. The bending strength of the composites implanted in the subcutis was maintained at more than 200 MPa at 25 weeks and at 150 MPa at 52 weeks. The molecular weight dropped to 45% of the initial values at 8 weeks and to approximatel y 10% at 52 weeks. Significant differences in the molecular weight were see n between c-HA/PLLA and u-HA/PLLA, with u-HA/PLLA showing a faster rate of decrease than c-HA/PLLA after 8 weeks. SEM demonstrated that HA particles d isappeared increasingly from the rod surfaces over time and that the spaces left by these HA particles formed many pores in the composite surfaces at 52 weeks. Histologically, a fibrous tissue layer was formed around the comp osite rod from 4 weeks in the subcutis and in the diaphyseal area of the me dullary canal. This became more mature over time. Bony tissue contact to th e composites without fibrous tissue lavers was seen in the metaphyseal area of the medullary canal. During the experimental period, there were no infl ammatory cells such as mono- or multi-nuclear phagocytes. Although further long-term studies for degradation are needed, the composites have promising mechanical strength and no adverse tissue reaction for use as fracture-fix ation devices during the experimental periods. (C) 2000 Elsevier Science Lt d. All rights reserved.