Properties of coralline hydroxyapatite and expanded polytetrafluoroethylene membrane in the immature craniofacial skeleton

Although extensive research regarding the treatment of calvarial defects ha s been done in adult models, little is known about the response in the matu ring skeleton. The role of coralline hydroxyapatite and expanded polytetraf luoroethylene membrane in augmenting bone growth and repair of calvarial de fects in a neonatal model is explored. Utilizing a 3-week-old neonatal swine model, bone growth into 28 calvarial defects was measured. After exposure of the calvaria in seven animals, four defects of 10 mm in diameter were created. In each animal, one defect was treated with a 10-mm disc of porous hydroxyapatite alone (Interpore 500, In terpore International), and a second defect tvas covered with an expanded p olytetrafluoroethylene membrane (Gore-Tex OV-6) secured by four 3-mm micros crews (Luhr Microsystem, Howe-Medica Inc.). The third defect combined all i mplanted hydroxy-apatite disc covered hy an expanded polytetrafluoroethylen e membrane, whereas the fourth defect served as an untreated control. Histo logy and histomorphometry were performed on undecalcified specimens harvest ed at G weeks after surgery. In both hydroxyapatite groups, the bone growth into the inorganic matrix pr ovided complete osseous union in all specimens, and the amount of fibrosis was significantly lower (p < 0.02) in comparison with the control. Unexpect edly, there was significant osteoclastic resorption of the hydroxyapatite m atrix (35.1 percent decrease) with simultaneous bone deposition and remodel ing. The addition of an expanded polytetrafluoroethylene membrane covering the hydroxyapatite implant provided an insignificant advantage in bone grow th (27.3 percent versus 28.3 percent, respectively). Finally, the expanded poly tetrafluoroethylene membrane alone afforded no qualitative advantage s econdary to intrusion of brain and dura into the defect as well as displace ment of the membrane inward during appositional growth, leading to incomple te healing of the defect with thinning of the sill-rounding cranial bone. Unique in this maturing model was morphologic evidence of complete union at the calvaria-hydroxyapatite interface in all specimens as well as active r emodeling of the hydroxyapatite matrix. The results of this study suggest t hat porous hydroxyapatite may be a suitable bone substitute in maturing cal varial bone defects, achieving superior osseous integration and volumetric bone gain while undergoing concurrent resorption and remodeling.