IN-VIVO BIOCOMPATIBILITY EVALUATION OF NICKEL-TITANIUM SHAPE-MEMORY METAL ALLOY - MUSCLE AND PERINEURAL TISSUE RESPONSES AND ENCAPSULE MEMBRANE THICKNESS

Nickel-titanium shape memory alloy (Nitinol) has properties that could be very useful in surgical applications. Thermal shape memory, supere lasticity, and high damping properties make such alloys behave differe ntly compared to other implant metals. There has previously been a lac k of sufficient evidence on the biocompatibility of Nitinol. The purpo se of this study was to evaluate general soft tissue response and bioc ompatibility to Nitinol in vivo, and to clarify neural and perineural responses, previously unreported. Seventy-five rats were randomized in to three groups. Test specimens were implanted into paravertebral musc le and near the sciatic nerve. A comparison was made between Nitinol, stainless steel, and Ti-6Al-4V. The animals were euthanized at 2, 4, 8 , 12, and 26 weeks after implantation. General morphologic and histolo gic observations were made under light microscopy Semiautomatic comput erized image analysis was used to measure the encapsule membrane thick ness around the implants. The muscular tissue response to Nitinol was clearly nontoxic, regardless of the time period. The overall inflammat ory response to Nitinol was very similar to that of stainless steel an d Ti-6Al-4V alloy. There were no necroses, granulomas, or signs of dys trophic soft tissue calcification. The immune cell response to Nitinol remained low. Only a few foreign-body giant cells were present. The d etected neural and perineural responses were also clearly nontoxic and nonirritating with Nitinol. No qualitative differences in histology b etween the different test materials could be seen. At 8 weeks, the enc apsule membrane of Nitinol was thicker than that of stainless steel (m ean 62 +/- 25 mu m vs. 41 +/- 8 mu m) At the end of the study, the enc apsule thickness was equal to all the materials tested. We concluded t hat Nitinol had good in vivo biocompatibility after intramuscular and perineural implantation in rats in the 26-week follow-up. Based on the results of the present study, Nitinol appears to have good potential for clinical use. (C) 1998 John Wiley & Sons, Inc.