Nitinol Stents – Nitinol used in Vascular Surgery The shape-memory alloy, nitinol, discovered only 37 years ago, has brought about many new surgical possibilities, and is gradually being incorporated into products available in the market place. Vascular surgery has adopted nitinol for various devices during he rapid evolution of endovascular techniques. Great potential still exists for further application of in surgery and other fields, given their biocompatibility and unique physical properties. Commercial and sci- entific groups will undoubtedly exploit these op- portunities. As such, a thorough understanding of these alloys by surgeons should facilitate development of exciting new applications.
Thermal shape memory
Some nitinol stents or stent/grafts currently used to treaarterial aneurysms or vascular occlusive disease rely on the thermal shape-memory effect. They exploit the ability of the alloy to recover its trained shape above Af. For thermal shape memory devices, nitinol is manufactured with the Af near to the working temperature. Cooling allows them to be delivered in an easily deformable martensite form. Characteristically, the TTR is set at 27 °C for change due to heating. The TTR is some 25–50 °C lower for change due to cooling, known as trans formation temperature hysteresis. Thus, the device may require cooling to near 0 °C to fully retransform to martensite. Flushing the sheath with ice-cold water prior to deployment allows the prosthesis to move more easily within the sheath in its pliable martensite form. This allows the sheath diameter to be reduced. Exposure to body temperature in the circulation after ejection from the sheath then allows it to expand to the strong austenite phase to assume its pre-set shape.
Inferior vena cava filters to prevent pulmonary embolism have been made from nitinol.This was the first vascular implant to utilise thermal shape memory for deployment. Again, the major advantage is that they can be stored in the martensite phase in a small delivery system to allow access from the external jugular or antecubital veins to be released as the austenite form into the inferior vena cava.
Superelasticity and damping
Self-expanding nitinol coronary and peripheral arterial and venous stents have advantages over stents made from other materials in certain situations. Under appropriate conditions, the austenite phase can be trans formed into martensite by applying stress, and can then withstand considerable deformation up to 10 times that of the best stainless steel. The undeformed shape in the austenite phase rapidly recovers when the stress is released. This confers nitinol with incredible flexibility and kink resistance over a temperature range of some 50 °C above Af. Some commercial groups refer to them as ‘‘Muscle Wires’’. For these devices, nitinol is manufactured with the Af well below body temperature (Af 0–20 °C). Within strain limits, these stents are not permanently deformed by external forces and the force hysteresis curve of nitinol makes the metal resistant to radial compression.
Nitinol can be used as very fine or heavy duty vascular guidewires and snares to provide high flexibility and kink-resistance.Nitinol discs are used for transcatheter closure of atrial septal defects and patent ductus arteriosus.Nitinol stents can be used for benign and malignant strictures of hollow tubes such as the ureter, prostate, urethra, tracheobronchial tree, oesophagus, rectum or bile ducts. Nitinol forms the framework for a patch used for laparoscopic inguinal hernioplasty and as clips for intracranial aneurysms. Nitinol has been used to construct surgical instruments that can be worked into a shape that is suited for each surgeon or for a particular procedure.
A long-standing major application for nitinol is for archwires of orthodontic braces, A nitinol wire is connected across each tooth with a brace and pulled tight. The strain in the alloy thus formed induces a phase transition to martensite and the force created by the alloy’s attempt to return to its austenite form slowly pulls the teeth back into position, with a constant force that does not diminish as the teeth are gradually approximated.
A nitinol suture anchor has had a major impact in orthopaedic surgery by providing a secure attachment site for tendons, ligaments and other soft tissues to bone. Nitinol staples are used for internal fixation of fractures and nitinol Harrington rods have been investigated for potential use to treat scoliosis.
Biocompatibility and Biological Safety
Nickel is a normal component of the diet, but exposure to excess levels can lead to severe respiratory disorders, local and systemic allergic reactions and inhibited cellular reproduction. It may be the most common contact allergen in developed countries. In contrast,titanium appears to be entirely inert. Nickel and titanium form an extremely strong intermetallic bond, so that nitinol is resistant to oxidation of nickel, that would otherwise liberate nickel ions and raise the risk of adverse reaction. The components could be released by corrosion but experiments with nitinol exposed to saliva or placed in animals’ major veins show that this results in blood levels that are far below those from normal dietary intake. In addition, long-term implantation into sheep showed that the principal product released by surface pitting is TiO2, which is then promptly taken up by phagocytes. In vitro and in vivo investigations show that biocompatibility for nitinol is very good, even in subjects with nickel sensitivity. There are studies in animals and humans to assess its effects on cell reproduction and neointimal hyperplasia, but further long-term studies appear to be required to assess thrombogenicity, sensitisation and carcinogenicity.
Cellular reactions
An in vitro study to compare the effect of nitinol with that of nickel and titanium separately on mitoses in human foreskin fibroblasts grown in cell culture showed that nickel alone caused detrimental effects on mitotic activity but that nitinol closely ap- proximated the inert nature of titanium. A similar study that compared the effects of nitinol, stainless steel and titanium on human osteoblasts and fibro- blasts in cell cultures showed that none of these metals caused significant toxic effects or inhibition of cell growth or proliferation.
An in vivo study involved fixing nitinol plates to beagles’ femurs compared to control groups using Co–Cr alloy implants and ‘‘sham’’ operations. There was no significant corrosion, histological changes in tissues adjacent to the plates, nor incorporation of metal into adjacent bone, liver, lung, or brain at up to 18 months. A further study compared the histological response to nitinol, stainless steel and titanium im- plants placed in paravertebral muscles adjacent to the sciatic nerve in rats. All produced minimal reaction at up to 26 weeks. Another study described nitinol as ‘‘indistinguishable from stainless steel at similar time periods’’, for tissue responses in rats In humans, MRI has shown intense but transient perivascular inflammation to nitinol stents covered with Dacron which was not observed in uncovered nitinol stents, and the reason for the difference is still not clear.Implantation of nitinol stents into the animal or humanoesophagus produces a severe inflammatory reaction, muscle degeneration and fibrosis
Neointimal hyperplasia
Most experiments with nitinol stents placed in dog and sheep arteries show that a relatively thin intimal lining develops with little evidence of thrombosis or inflammation at 1–2 years. The cellular proliferative response reaches its maximum by 3–6 months without further progression. It is accelerated and increased if the surface is grooved.60 A study with stents placed in sheep iliac arteries showed little difference in neo- intimal thickness over or between the stent struts for nitinol stents, Palmaz stents or wallstents. However another study placed various stents across the orifice of renal arteries in pigs and found that there was a disorganised acellular collagen matrix on some nitinol stents that partly obstructed the arteries. Intra- vascular ultrasound performed at 6–18 months after implanting nitinol stents into human iliac arteries showed approximately 20% restenosis over ap- proximately 10% surface area due to soft hypo- echogenic plaques.