EFFECTS OF HYDROXYAPATITE IMPLANTS ON PRIMARY MINERALIZATION DURING RAT TIBIAL HEALING - BIOCHEMICAL AND MORPHOMETRIC ANALYSES

The effect of 40- to 60-mesh hydroxyapatite (HA) granules (Calcitek, I nc., Carlsbad, CA) on the process of primary mineralization during bon e healing was examined following insertion of the HA granules into rat tibial bone after marrow ablation. Response to HA was assessed by mon itoring morphometric and biochemical changes in matrix vesicles, which are extracellular organelles associated with initial calcification. F ollowing insertion of HA, matrix vesicle-enriched membranes (MVEMs) we re isolated from the tissue adjacent to the implant and from the endos teum of the contralateral limb at 3,6,14, and 21 days and from a nonim planted control group (t = 0). MVEM alkaline phosphatase- and phosphol ipase A2-specific activities were increased on days 6 (peak) and 14; p hosphatidylserine content was also elevated on days 6 and 14 (peak). C omparable changes were seen in the contralateral limb but at lesser ma gnitudes. Morphological changes were observed as well. The number of m atrix vesicles/mum2 matrix increased on days 6 (peak) and 14. The mean diameter of the matrix vesicles was elevated on days 6 (peak), 14, an d 21. Mean distance from the calcifying front increased on day 6 but w as decreased on days 14 and 21. These results indicated that HA behave s like bone-bonding implants in that there is a stimulation of matrix vesicle enzymes, increased phosphatidylserine content, and increase nu mbers of matrix vesicles. However, the increases occur only after 6 da ys postimplantation, indicating a delay in response when compared to n ormal healing. This delay is confirmed by the morphometric measurement s. HA causes a reduction in the response associated with marrow ablati on. In addition, the effects of HA are comparable locally and systemic ally but with different intensity. These observations suggest that ost eogenic cells are able to compensate for the inhibitory effects of HA and primary calcification involves normal matrix vesicle production an d maturation, if somewhat delayed and reduced in magnitude. The abilit y to support primary mineral formation may contribute to the successfu l bonding of HA with surrounding osseous tissue. (C) 1993 John Wiley & Sons, Inc.