The in vitro responses of human fibroblasts and osteoblasts to Nitinol in an environment simulating physiological conditions were evaluated in cell cultures. No responses of human osteoblasts to NiTi have been published before.
The toxic effects of different metals can be well quantified in vitro, although cell culture studies cannot directly mimic the cellular response and environment in vivo. Generally, the cell culture method is considered as a very sensitive method of toxic screening (Rae 1986). This method also provides a possibility for direct observation of living cells.
The main question was whether the amount of nickel dissolved from NiTi is high enough to affect cell proliferation in a cell culture and what the dissolution rate is compared to another nickel-containing implant alloy, stainless steel. At high concentrations, nickel itself is known to have toxic effects in cell cultures and in tissues, but less so than cobalt or vanadium, which are also routinely used in implant alloys (Gerber et al. 1980).
The volume of media and cells were quite small compared to the volume and surface area of the tested metal disks. Thus, the corrosion effect on cells is expected to be much more prominent than in vivo. Also, the normal clearance of tissue fluids is missing, and the cultured cells are isolated from the normal detoxification pathways of the body. These factors enhance the effect of corrosion products on cells. For the above reasons, cell cultures can be considered a good basic screening method before animal tests.
Human mesenchymal cells were used in the cultures, because they are the cells present on the site of implantation. Osteoblasts were used as an in vitro model in order to determine the cytotoxicity of NiTi as an orthopedic material. The interesting point was if there is a difference in behavior between fibro- and osteoblasts in the presence of the test materials. The responses of both cells were similar, however, confirming the validity of the selected method.
Titanium was chosen as one of the experimental materials for two reasons: it has been widely used in traumatology and orthopedics and it is the other main component of NiTi. It is known to be one of the best tolerated metal biomaterials (Albrektsson et al. 1981, Serre et al. 1994).
Stainless steel is the most commonly used implant material in surgery. It is very well tolerated by human tissues. It contains 10-14% nickel. Composite material (Silux Plus®) is used as a tooth filling material in dentistry. It is known to have shown some toxicity in vitro (Peltola et al. 1992). That was the reason for choosing it as a toxic positive control for the other tested materials. White soft paraffin was used as a fixative in testing discs. Its effect was also analyzed separately.
Fibroblasts appear to be more sensitive to the growth-inhibiting effect of composite material and white soft paraffin or the growth stimulation of titanium than osteoblasts. The responses of both cells were similar, however. Parallel findings were made by Morrison et al. (1995). The proliferation of cells was undisturbed and similar in NiTi, stainless steel and the control group (Figure 5-6).
The osteoblast proliferation with titanium was not more abundant than that with NiTi, stainless steel or control, but it gave some proliferatory stimulus to fibroblasts. Enhanced fibroblast proliferation is not always a good thing. It may contribute to the fibrous membrane, which may act as a conduit for the transport of polymeric debris produced at the articulating surfaces of hip prostheses and contribute to loosening or osteolysis (Maloney et al. 1993). In present study, NiTi caused neither inhibitory nor stimulatory effects with fibroblasts.
The nickel concentration was higher in the NiTi media than in the stainless steel media at the beginning of the test. However, this had no effect on the proliferation and the total amount of fibroblasts and osteoblasts. There are two possible explanations for this: the maximum concentration was well below the toxic level, or the high concentration was present for only a short time. Cells formed in quite close contact with the NiTi test discs (Figure 5-3), which is also a sign of good biocompatibility. A clearly different reaction was seen in the case of composite material and white soft paraffin. The observed toxic influence of white soft paraffin on cultured cells was rather surprising.