Histomorphometric study on high-strength hydroxyapatite/poly(L-lactide) composite rods for internal fixation of bone fractures

The purpose Of this study wits to investigate the bone-implant interface of high-strength hydroxyapatite (HA)/poly(L-lactide) (PLLA) composite rods. A s reinforcing particles, two types of HA particles-calcined HA (c-HA) and u ncalcined HA (u-HA)-were applied to allow comparison of their suitability a s bioactive fillers. Four types of composites (c-HA30, c-HA40, u-HA30, and u-HA40), which contained 30 or 40% by weight of each HA particle, were used . Unfilled PLLA rods were used as controls. A hole was drilled in the dista l femora of 50 rabbits, and a composite or unfilled PLLA rod was implanted in a press-fit manner. Two, 4, 8, and 25 weeks after implantation, the samp les were examined histologically by light microscopy, scanning electron mic roscopy (SEM), and transmission electron microscopy (TEM). An image analyze r was used for histomorphometric analysis of the bone-implant interface. An affinity index was calculated for each material; this was the length of bo ne directly apposed to the rods expressed as a percentage of the total leng th of the rod surface. In all the composites, histologic examination showed new bone formation at 2 weeks after implantation. The bone gradually grew along the composite surface. SEM showed direct bone contact with the compos ites without intervening fibrous tissue. During follow-up, the affinity ind ices of all the composite rods were significantly higher than those of the unfilled PLLA rods (p < 0.01; two-way ANOVA). The maximum affinity index (4 1%) was attained at 4 weeks in c-HA40 rods. In contrast, little bone contac t was seen in unfilled PLLA rods. The only significant difference in affini ty indices among the composites was that c-HA40 had a higher affinity index than u-HA40 (p < 0.05 at 4 weeks). No disintegration of rods or polymer de bris, which could elicit inflammatory tissue reactions, was observed even a t 25 weeks. Our results indicate that osteoconductive bone formation on com posites could enhance the stability between hone and implant in fracture re pair. (C) 2000 John Wiley & Sons, Inc.