5.2. Soft tissue response to NiTi
The general features of the soft tissue response in the studies II and III were similar. The tissue responses to NiTi as well as to the control materials were clearly non-toxic, regardless of the time point. No qualitative differences in histology between the NiTi, Ti-6Al-4V and stainless steel test materials could be seen at the light-microscopic level. Black-brown discolored wear was found in the extracellular space and inside some cells around four Ti-6Al-4V implants and one NiTi implant in study II. The discolored tissue was mainly located at the ends of the implant. There was no necrosis, granulomas or signs of dystrophic soft tissue calcification around any of the test materials.
The inflammatory reaction was most notable in the 2- and 4-week samples. Generally, the inflammatory reaction was mild and restricted to an area very near to the implants regardless of the material. At 4-12 weeks, the encapsulation membrane had two distinct zones: an inflammatory cell layer with a clearly milder inflammatory response than in the 2-week groups, and a distinctive fibrous capsule zone formed by fibroblasts and collagen fibers. In the 26 week group, there was only a thin fibrous layer with practically no macrophages or other inflammatory cells present (Figure 5-7, A1-5, B1-5 and C1-5).
New capillaries were seen between the fibrous capsule layer and muscle tissue. These were more prominent after periosteal than intramuscular implantation, which may be related to different surgical procedures. The number of capillaries diminished over time. An avascular capsule was seen in the 26-week group regardless of the material.
Monocytes and macrophages were the main inflammatory cell types in the studies II and III. They were found very close to the implant-tissue interface. They were most prominent in the short-term implanted samples. After 4 weeks, macrophages began to disappear, and their numbers remained very low afterwards. No differences were seen between the different test materials.
Foreign body giant cells were observed at all time points, except in the 26-week samples. They were equally few in number in the 2- to 12-week samples and with different test materials. A few ring-shaped, small (< 50 µm), low-polarizing, non-metallic material particles were seen near the tissue-material interface in all time groups and materials.
Some polymorphonuclear and round cells (lymphocytes, plasma cells, mast cells) were also present in the 2- and 4-week samples, but cells of these types could only be seen incidentally at the later time points. However, separate mast cells and few eosinophils were seen in the 26-week samples. No signs of lymphocyte accumulation were visible as an indication of immune system activation by the materials, a finding similar in all test implants.
Active fibroblasts were seen in the 2-week samples. After 2 weeks, mature fibroblasts were found to form a distinct capsule between the soft tissue and the test material. The number of fibroblasts increased over time, but the fibroblast density also became higher. At 26 weeks, there were thin, well-defined, dense fibrous capsules between the soft tissue and the implant. The fibroblasts were flattened and elongated with wavy collagen fibers between them. Practically no macrophages or other inflammatory cells were present. Again, no differences were seen between the different test groups.
