6.14. Value of NiTi as a biomaterial

NiTi alloy is quite new in medical use. It provides possibilities to make applications that no other implant material has offered before. A few commercial applications have been successfully developed since the 1970s, when Nitinol was first reportedly used for medical purposes. These applications include dental arch wire, vena cava filter and suture anchor for orthopedic surgery (Simon et al. 1977, Pelizzoni et al. 1989, Barber et al. 1993). In the 1990s, further development has been carried out with markedly increasing interest. Urethral, esophageal and intracoronal stents, aneurysm prostheses, and some orthopedic implants seem promising.

The idea behind shape memory applications is to introduce an item with a suitably small cross-section into the body, where its shape changes into a different, much larger structure upon application of heat. This thermal shape change phenomenon can be arranged to have a beneficial impact within the body, such as to restrict, dilate, push apart or pull together body components. Applications based on superelasticity, such as guide wires or laparoscopic tools, are another benefit of this material.

The biocompatibility of NiTi has long been a puzzle. Earlier studies are in agreement with our present findings and support the conclusion that NiTi is a safe material. Multicenter biocompatibility data must be available, including in vitro and in vivo experimental studies with several animal species and in several tissues. Surface treatments, compositional changes in the alloy, and various metallurgical and thermal treatments may all affect notably the biocompatibility of a specific implant application. In orthopedic applications, the amount of material inserted inside the body is usually considerable. The large amount of bulk material and the often larger surface area may adduce some harmful properties of the material due to corrosion. The implantation time is often long, even decades.

Many other problematic issues apart from biocompatibility may also arise when certain applications are manufactured for surgical use. This is especially true of orthopedic implants. The very important and complicated biomechanical considerations of the musculoskeletal system introduce extra puzzles.

The application of NiTi to orthopedic implants has involved problems with such aspects as implant fabrication, design and testing (Brailovski et al. 1996). It is the author’s opinion that some of the prototypes that have been made have been subjected to human tests without due concern. To apply a new material or implant to surgical use, several demanding criteria must be met. In fact, the new implant must be better than the existing ones, or it must solve a clinical problem that the others fail to solve. This means it has to be better for the patient or the surgeon or society. Advanced technical properties, easy use, faster healing, less tissue damage, shorter operation time or lower total costs of operation must be behind every new application.

Some of criteria of a successful NiTi implant can be listed:

1. The design and function of the implant must based on careful biomechanical considerations.

2. The compressive or other effective force caused by the implant must be known in every specific application.

3. The working time must be sufficient for proper implantation.

4. In case the implant is positioned wrongly, it must be able to remove it immediately during the operation or, if necessary, later.

5. There must be high-quality control and standardization of the procedure at every step from raw material to final product.

6. The sterilization method must not affect negatively the implant.

7. The learning of the new operative technique must be considered.

To fulfill these requirements, the new NiTi implants must be developed in close co-operation between surgeons, material scientists and mechanical engineers. The surgeon is rarely able to understand all the limitations of a material, while engineers are unable to evaluate the clinical validity. Computer-aided FEM-based implant design may be of great value for the design of optimal constructions.

Finally, the introduction of NiTi to human trials must fulfill every criterion of the normal protocol of implant testing. The conclusions concerning the superiority of every single NiTi application must be based on large series, multicenter studies and, preferably, randomized trials. The precondition for commercial availability and use is that sufficient background data are available and official national approvals are obtained.

1. The use of NiTi as a biomaterial has several possible advantages. Its shape memory property and superelasticity are unique characteristics and totally new in the medical field. The possibility to make self-locking, self-expanding and self-compressing thermally activated implants is fascinating. As far as special properties and good biocompatibility are concerned, it is evident that NiTi has a potential to be a clinical success in several applications in the future.