| Biocompatibility evaluation of nickel-titanium shape memory metal alloy: | ||
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Since the first attempts to introduce this material into medical use in the early 1970s certain progress has taken place (Castleman et al. 1976). NiTi superelastic wires were first introduced into orthodontic use (Andreasen et al. 1971). Nowadays, there are some commercial products available worldwide. At the present, the breakthrough of self-expandable stents in gastroenterology, radiology and cardiovascular applications seems convincing. The idea of using NiTi stents was first reported separately by two authors (Cragg et al. 1983, Dotter et al. 1983). By using stents, major surgical operations can be avoided. Sometimes a stent may be the only choice in critically ill patients. Stents have shown NiTi with certain criteria to be a material with huge possibilities.
The first vascular NiTi device was the Simon Nitinol filter (SNF) used to treat pulmonary embolism (Simon et al. 1977). The filter is inserted as a straight thin wire via the small bore catheter used for angiographic diagnosis. Upon reaching the lumen of the inferior vena cava and sensing body temperature, it reverts to its preset complex filter shape and locks into place permanently, trapping any further thromboemboli from the pelvis or the lower limbs. SNF has also been accepted by the U.S. Food and Drug Administration FDA.
The general trend of stenting is towards self-expandable NiTi-based stents. The thin stent is placed in the narrowed artery, where it expands and dilates the artery. The carotid artery stents and the endoluminal polyester-covered NiTi stent-grafts for infrarenal abdominal aortic aneurysms have been proved to be efficient and technically successful, but a careful long-term evaluation is still necessary (Blum et al. 1997, Wholey et al. 1998). Intracoronary (de Jaegere et al. 1996, Oesterle et al. 1998) and peripheral vascular NiTi stenting (Schwarzenberg et al. 1998) also seems to be increasing. Their benefits include good radial expansion capabilities and flexibility. Despite the improvements, restenosis and reocclusion remain a significant problem and the optimal physical and surface properties of an arterial stent have not been defined yet (Schurmann et al. 1995). Endoluminal repair of infrarenal abdominal aortic aneurysms with the use of Dacron-covered NiTi stent-grafts is feasible, safe and clinically effective. The attempts to improve properties with a heparin-coated Dacron cover have shown only a pronounced inflammatory response (Schurmann et al. 1997). As pointed out by the same authors, the need to evaluate the biocompatibility of new vascular devices is evident. Polyurethane stent coating was also associated with an inflammatory tissue response (Rechavia et al. 1998).
A transcatheter approach to the occlusion of atrial septal defects has also been recently reported. The implant consists of two umbrellas placed over a long veno-arterial guide-wire. The system has been described to be technically feasible. It has been used in a few cases of adults and children (Sievert et al. 1995, Hausdorf et al. 1996). However, much further evaluation and long-term data are needed before this technique can be recommended.
Self-expanding stents for esophageal strictures and the palliation of malignomas have been studied by several authors (Cwikiel et al. 1993, May et al. 1995, Acunas et al. 1996). Esophageal NiTi stents are easy to implant, provide effective palliation of malignant esophageal obstructions, and have a low risk of severe complications. The only disadvantage was that incomplete initial stent expansion as well as tumor ingrowth/overgrowth occurred in nearly one third of the patients. Covering the NiTi-based stent with a thin Gore-tex sheath may give a possibility to avoid ingrowth and to use the stent even in the case of fistulas.
Biliary stents are effective in achieving long-term palliation in patients with malignant obstructive jaundice. Recently (1999), FDA released on the market a stent for this purpose. The treatment of benign biliary strictures with metallic stents is associated with a low long-term patency rate (Bezzi et al. 1994, Rossi et al. 1994). The use of stents reestablishes bile flow in the occluded biliary tree. The stent may, however, be technically difficult to insert (Smits et al. 1995).
The insertion of NiTi stents in patients with rectosigmoidal carcinoma is technically feasible and effective in the palliation of malignant rectosigmoid obstruction; they provide an alternative to repeated palliative laser therapy or palliative surgery (Tack et al. 1998).
The use of NiTi prostatic stents has also increased since the first reported experiment by Lopatkin et al. (1989). For high-risk patients with subvesical obstruction caused by prostatic carcinoma, the insertion of a permanent metal stent system offers a useful alternative to transurethral resection (Gottfried et al. 1997).
NiTi stents are also used as an alternative method for the treatment of benign prostatic hyperplasia. Patients were considered suitable for treatment with the stent when they presented with a high operative risk (Gottfried et al. 1995, Gesenberg et al. 1998).
The use of urethral stents was found to considerably decrease the number of repeated dilatations and urethrotomies in recurrent urethral strictures (Yachia 1993). Despite the good biocompatibility of the material in a long-term study on dogs, there were no complete incorporations of the stent by epithelialization, and the authors concluded that clinical application might therefore be problematic (Latal et al. 1994).
According to Castleman et al. (1976) the first thoughts to exploit the potential of NiTi as an implant material were made by Johnson and Alicandri in late 1960s. Since that time, further studies have been carried out into the viability of the alloy for orthopedic operations.
Some of the first in vitro studies were carried out by Baumgart et al. (1978), who examined the NiTi distraction rod in the correction of scoliosis. In China Lu et al. (1986) implanted NiTi rods in 26 patients with scoliosis. Correction was reported to be good and there were no complications. Matsumoto et al. (1993) and Sanders et al. (1993) published further in vivo experimental studies. It seems that the scoliosis-correction system based on NiTi shape memory or superelastic property has quite complicated biomechanical problems related to compression and distraction control. NiTi may not provide any improvements compared to the traditional implant systems.
NiTi compression staples were first introduced in China. According to Dai (1983) a shape memory staple was first used inside the human body in 1981. After that, NiTi staples and clamps have been used in comminuted fractures of the short tubular bone (Yang et al. 1992), for fixation of mandibular fractures (Drugacz et al. 1995), for metatarsal osteotomies (Tang et al. 1996), for anterior cervical decompression and fusion (Mei et al. 1997, Ricart 1997, Silberstein 1997), for fixation of small bone fragments (Musialek et al. 1998), and for several other cursory applications (Iwabuchi et al. 1975, Kuo et al. 1989, Dai et al. 1996). The only NiTi-containing orthopedic implant widely used in the western world is the Mitek G2 suture anchor. It has superelastic NiTi wings which prevent the anchor from pulling out of the bone after insertion and secure the tendons or ligaments to the bone (Barber et al. 1996). Another promising application is a NiTi hook used to restore the dislocated acromio-clavicular joint (Ryhänen et al. 1998).
The problem of most published studies in the orthopedic field is that they rarely satisfy the quality criteria of scientific study. It is not enough to say that a certain NiTi implant can be used without harm. To be considered fully successful, it must be proved to be better than the existing competitors. At the present, there are no comparative clinical studies and the series have generally been small. Randomized prospective studies are needed to apply new NiTi implant devices for constant clinical use in humans.
The indications of stenting increase rapidly. The use of self-expanding NiTi stents to prevent major airway occlusion was first reported by Rauber et al. (1990). According to the early tests, they seemed to be very useful and effective in inoperable tracheal or bronchial stenosis due to intraluminal tumor invasion (Yanagihara et al. 1997, Hauck et al. 1997).
A NiTi-based mesh-expanding prosthesis for laparoscopic hernioplasty significantly shortened the operating time in a study of Himpens (1993).
The good holding and atraumatic characteristics of the detachable clamp have been confirmed by use in laparoscopic and thoracoscopic surgery on the gastrointestinal tract (Frank et al. 1995).
Also, new type of NiTi stapes prosthesis to restore the ossicular fixation after stapedectomy has been introduced (Kasano et al. 1997).