Material characterization and biological evaluation of calcium pyrophosphates for use as bone graft substitutes
Bone is the second most frequently transplanted tissue in humans. Autogenous bone, of which a limited amount can be taken from the same patient, is currently the preferred graft material since it is neither immunogenic nor inert. The incidence of complications from this procedure, which may include morbidity, pain, sepsis, fracture, and/or hernia at the donor site, is about 20%. Allogenic bone, although sufficient in quantity, may elicit an immunological response or transmit a disease to the patient. Due to the problems with available bone grafts, this research on synthetic graft substitutes has been conducted. Calcium phosphates have shown superiority over other synthetic graft materials. One type of calcium phosphate, calcium pyrophosphate (Ca$\sb2$P$\sb2$O$\sb7$), has been the subject of discrepancy in the literature. Some have claimed it to be a calcification inhibitor, others an osteoconductive ceramic. To resolve this discrepancy, novel precipitation techniques were used to prepare calcium phosphate powders which were then mixed with sized naphthalene, calcined, and sintered to produce porous discs and granules for implant studies. X-ray diffraction, scanning electron microscopy, and mechanical testing were used to characterize the materials. The biological response was evaluated in two implant studies. In the first, alpha and beta calcium pyrophosphates were implanted in subcutaneous sites of six dogs along with control implants of 60/40 hydroxyapatite/$\beta$-tricalcium phosphate (HA/TCP), Type I collagen, and an HA/TCP with collagen composite. All explants at six months containing calcium phosphate were found to initiate osteogenesis in subcutaneous tissue, a previously unknown phenomenon. The extent of osteogenesis was different for each material. Each had a unique and inherent osteogenic potential. Primary vascular bone of plexiform structure formed by intramembranous endosteal osteogenesis and hematopoietic marrow were observed in the explants. The collagen-only implant was resorbed. In the second study, discs of alpha and beta calcium pyrophosphate, hydroxyapatite, and 60/40 HA/TCP, implanted into the iliac crests of rabbits for three months, demonstrated that calcium pyrophosphates are osteoconductive and biocompatible. These implant series settle the controversy of calcium pyrophosphates by demonstrating their biocompatibility and osteoconductivity, as well as true osteoinductivity in canine subcutaneous tissue.
Jeffrey M. Toth,
"Material characterization and biological evaluation of calcium pyrophosphates for use as bone graft substitutes"
(January 1, 1992).
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