| Biocompatibility evaluation of nickel-titanium shape memory metal alloy: | ||
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The biocompatibility data from limited studies in which NiTi has been used as a bone implant are slightly contradictory. NiTi bone plates have been found to be well tolerated and comparable to Vitallium in the dog femur (Castleman et al. 1976). Porous NiTi and NiTi clamps have also appeared to be suitable for craniofacial surgery (Simske et al. 1995, Drugacz et al. 1995). Nevertheless, NiTi appears to have less bone contact than pure titanium or other titanium alloys (Takeshita et al. 1997). NiTi showed a slower osteogenesis process and lower activity of osteonectin synthesis compared to Co-Cr alloy, pure titanium and stainless steel in a study by Berger-Gorbet et al. (1996). Unfortunately, the number of animals was insufficient for statistical analysis. The follow-up time was 12 weeks, which can also be considered quite short.
The biologic effects of NiTi on the normal regional acceleratory phenomenon (RAP), new bone and callus formation, (re)modeling and the fracture healing process are mainly unexplained, and they were a focus of research in the present study. Biologic failures in bone healing after surgery include inadequate callus formation or the lack of a normal regional acceleratory phenomenon (RAP), normal (re)modeling, or maldifferentation of the healing tissues, plus combinations. One of the most common failures is the inability to form callus or normal RAP. When such failures co-occur, clinical problems, such as non-union and failed fixation, may ensue. Most biologic failures stem from problems attributable to mitogens, differentiating and priming agents, growth factors, and other labile biochemical and biophysical messengers and signals in the fracture region (Frost 1989). Toxic materials may cause biologic failures by affecting these factors.
The effects of NiTi, Ti-6Al-4V and stainless steel on bone were examined in the RAP model developed for this study. An interesting point suggested by the results of the previous fracture healing studies is that an optimum level of loading and relative micromovement could be beneficial in stimulating bone formation around implants (Goodship et al. 1985, Kenwright et al. 1986, Kenwright et al. 1989). The added influence of trace ions released from the biomaterials and the possible biomaterial surface effects on cell activity must be considered in view of bone formation around implants (Pilliar 1991).
Typical RAP is induced by periosteal stimulation. In the RAP model used here, muscle movements caused intermittent micromotion at the contact surface between the non-fixed implant and the bone cortex. This dynamic loading gives an adaptive (re)modelation stimulus to bone. If the test material itself has toxic or unfavorable biocompatibility properties, it disturbs the normal RAP more than other materials do in the presence of the same mechanical stimulus. This is manifested as decreased new bone formation and increased bone resorption, i.e., a more negative bone balance compared to a more inert material. Thus, if bone formation with NiTi is disturbed by some means (necrosis/ no normal bone formation), or if there is severe disturbance in structure (porosity), maturation (woven to lamellar bone) or modelation (organization of lamellar bone) compared to the controls, we can expect this to be also reflected in the biocompatibility of the material used in similar specific applications.
The differences between the responses to the implants may theoretically be due to two factors other than the implant material itself. The densities of NiTi, Ti-6Al-4V and StSt differ, and the slightly different weights of the test implants identical in size may possibly explain the changes to some extent, but we consider this to be of minor importance. Differences caused by surface roughness are not likely, because the surface was shown by the SEM examination to be similar in all materials. Since the mechanical factors affecting the tested implants were quite similar in all cases, we find it justifiable to suggest that the differences in bone parameters are due to the implant material itself (trace ions, wear particles and other physicochemical factors).
Nickel from NiTi and stainless steel and vanadium and aluminium from Ti-6Al-4V may be deleterious at high concentrations. In vitro nickel appears to be harmful to bone, but less so than cobalt or vanadium, which are also routinely used in implant alloys (Gerber et al. 1980). Aluminium may impair the osteoblastic functions and diminish bone collagen synthesis and mineralization (Rodriguez et al. 1990, Kasai et al. 1991).
The RAP model was used in study III. The subperiosteal resorption under the implants was found to be compensated for by endosteal bone thickening and lateral new bone bracing. This general adaptive bone modeling and normal RAP were seen with all implants (Figure 5-10), but certain differences between the tested materials occurred. New bone formation started sooner in the Ti-6Al-4V group (2 weeks), but also decreased earlier in this group than in the others, which was clearly seen at 8 weeks, when the cortical width of Ti-6Al-4V was thinnest (Figure 5-12D). No significant differences were seen at the later time points. Thus, similar normal RAP was observed with all the tested materials, but the culmination of the modeling process occurred and disappeared earlier in the Ti-6Al-4V group than in the NiTi or StSt groups. Our findings confirm and complement the previous results (Berger-Gorbet et al. 1996). NiTi may have a slower initial osteogenesis process compared to Ti-6Al-4V, but this has no negative effect on the total amount of new bone formation, normal RAP, or normal (re)modeling.
In the present study I, NiTi initially released some nickel in vitro. The amount of released nickel decreased rapidly within a few days. It is assumed that, after an initial release of surface nickel ions, a titanium oxide layer develops, which gives good protection against further corrosion. There is a possibility that the initial nickel ion release from NiTi and stainless steel contributes to the slower osteogenesis process. On the other hand, could the aluminium ions from Ti-6Al-4V have a diminishing impact on the modelation process earlier? Since the different surface treatments have evident influences on metal ion release (Buchanan et al. 1990, Browne et al. 1994), the treatments should be tested in the future to find out the optimum processing for NiTi used in bone surgery. The RAP model described here may be of potential value in this testing.
Based on histomorphometric measurements, the results of the present study show that the bone response to NiTi seems to resemble more the response to StSt than that to Ti-6Al-4V.