6.3. Encapsule membrane thickness around NiTi

Measurement of the encapsule membrane thickness around the implant is widely used for estimating biocompatibility. There are many variations in the measuring techniques used by different investigators (Taylor et al. 1983, Ellies et al. 1988, Christel et al. 1989, Limberger et al. 1991, Benghuzzi 1996). The semiautomatic image analysis technique used here was a modification of the normal histomorphometric point calculation method developed for this study. Utilization of the thickness of the scar capsule around the implant alone is problematic, because there are other factors than the material itself that can affect capsular thickness. These include movement, location, implant surface texture and morphology, some animal-dependent factors and the surgical procedure used (Taylor et al. 1983, Ansbacher et al. 1988, Barone et al. 1992). The effect of surgical trauma was minimized using a cannulated needle. The implantation was done far from the skin incision, and no sutures were made. The overall capsule thickness was found to be time-dependent, and it generally decreased over time (Figure 5-7). There was, however, an increase in thickness with Nitinol at 8 weeks and with Ti-6Al-4V at 12 weeks compared to stainless steel. The stainless steel capsule was thickest at 2 weeks. These differences were not found to be statistically different. The morphology of the capsule was equal with all materials. There was no accumulation of special cell types, for example, to explain the change. In optimal situations, the inert biomaterial forms a thin, relatively avascular and acellular fibrous scar capsule at the interface (Williams 1986). A capsule of this kind was present with the NiTi implant capsules, demonstrating good acceptance (Figure 5-6, A5).