DISSOLUTION OF PARTICULATE HYDROXYAPATITE IN A MACROPHAGE ORGANELLE MODEL

It is controversial as to whether debris from hydroxyapatite (HA)-coat ed implants jeopardizes the longterm success of total joint replacemen ts. It has been hypothesized that Liberated HA particles are engulfed by macrophages and through normal cellular digestion prevent osteolysi s and third-body wear. HA particulates, however, have been observed at the interface and on polyethylene articulating surfaces. There is lim ited data demonstrating the ability of HA to dissolve at the acidity l evels associated with macrophage organelle digestion. The objective of this study was to determine if particulate HA could dissolve at the p H levels found in macrophage organelles. Characterized HA particles we re placed into buffered solutions corresponding to phagosomal organell e pH levels: cytoplasmic (pH 7), phagosomal (pH 6), and lysosomal (pH 5). Flasks were under continuous agitation in a shaker chamber at 37 d egrees C. Calcium and phosphate ions were measured beyond the maximum life span of an activated macrophage. The data showed that calcium ion s rose within the first 24 h and then remained constant throughout the experiment for all pH groups. Phosphate ion concentration showed a si milar pattern at the lysosomal pH but remained undetected at the other organelle pH levels. The saturation point was highest at the lysosoma l pH level and lowest at the cytoplasmic pH level. The results of this experiment leave the potential for HA particles to dissolve following macrophage digestion. However, caution must be exercised when interpr eting the macrophage organelle digestion hypothesis; the size of the H A particle, the length of time required to completely dissolve the par ticle, and potential cellular toxicity all are factors that have yet t o be determined before this hypothesis can be validated. (C) 1998 John Wiley & Sons, Inc.