Effects of different osteopromotive membrane porosities on experimental bone neogenesis in rats

Biologically inert expanded polytetrafluorethylene (e-PTFE) membranes (GORE-TEX) have earlier been shown to improve healing of different types of bone defects, to be able to restore earlier existing bone, and to produce bone neogenesis. This study was performed to investigate the influence of membrane porosity on the osteopromotive efficacy and to determine bone neogenesis times in the rat. Three different e-PTFE membrane qualities with different porosities (internodal distances < 8, 20-25 and 100 microns) and four healing periods (6, 12, 18 weeks and 6 months) were studied. Dome-shaped e-PTFE membranes (5 mm inner diameter) were placed on denuded rat calvaria and covered with the periosteum and skin. After the respective time intervals, specimens were prepared for histology, and the bone obtained within the domes was quantified by an image analysis system. The results confirmed that it is possible to produce new bone by the use of the osteopromotion technique. The rate of bone neogenesis depended on the quality of membrane used; the membranes with the smallest internodal distance were less efficient than the others in that osteogenesis was somewhat delayed. The amount of new bone achievable was essentially already obtained after 6 weeks with the two most porous membranes, whereas the least porous one lagged behind. After 12 weeks there was no difference in the amount of newly formed bone. There was an obvious time sequence, in that the newly formed bone showed an increased maturity with longer observation periods. The material with the smallest internodal distance did not integrate well with the surrounding soft tissue, leading to a lack of stabilization of the membrane and more soft tissue ingrowth from the side. It is concluded that there is a porosity range within which osteogenesis beneath the membranes is optimal, tissue integration for stability is adequate enough, and soft connective tissue ingrowth is avoided.