1. Principle of NiTi shape memory alloy implants

Nickel-titanium memory alloy (memory alloy for short) is an alloy with shape memory properties. Its main components are nickel and titanium. It was born in the US Navy Instrument Laboratory in 1962 [1]. The advantage is that the material can be shaped at a lower temperature and processed into the desired shape, and when the product is heated to a certain temperature, it can return to the shape before heating. Memory alloys exhibit two completely different crystal structures at different temperatures, namely austenite and martensite. The condition for the appearance of austenite is above the transformation temperature. At this time, the stiffness of the alloy is large and it is not easy to deform; on the contrary, the condition for the appearance of martensite is below the transformation temperature, and the stiffness of the alloy is very small at this time. It is very soft and can be shaped into any shape [1]. At present, implants made of memory alloy materials are widely used in orthopedics [2-4], and their good tissue compatibility and elasticity can meet various clinical needs.

2. Application status of memory alloy implants in orthopedics

2.1 Application of memory alloy embracing device

Memory alloy embracing devices are commonly used in the treatment of multiple rib fractures. In clinical cases, multiple rib fractures belong to multiple rib fractures. This kind of chest injury is more serious and the treatment method is more complicated. The proportion can reach 10% to 26%. Li Jun[5] selected 62 patients with multiple rib fractures as the research objects, 31 cases of the observation group treated with memory alloy embracing device internal fixation surgery and 31 cases of the control group treated with methods other than internal fixation surgery. The total effective rate of treatment was 100% and 74.19% respectively (8 cases died), it can be seen that the observation group has certain advantages in the treatment effect. Zhang Fan et al[6] conducted statistics and observations on 120 patients with traumatic flail chest, of which 60 were treated conservatively, and the other 60 were internally fixed with a memory alloy embracing device. After treatment, it was found that compared with conservative treatment patients, the recovery time of various indicators in patients with memory alloy embracing device was shorter, and the recovery of abnormal heart rate was significantly promoted, and inflammatory factors returned to normal levels faster. Not only that, but the patient's exercise ability is also steadily recovering. It can be seen that the memory alloy embracing device has excellent histocompatibility and is more practical in the treatment of chest trauma.

In addition, the memory alloy embracing device has also exerted its advantages in the treatment of palmometatarsal diaphysis fractures. Mei Hailong et al. [7] analyzed 65 patients with palmometatarsal diaphysis fractures treated with memory alloy embracing device. All fractures healed postoperatively, and no fracture, nonunion, delayed union, malunion, or inflammation were found.

2.2 Application of memory alloy patella concentrator

There are many patellar fractures in clinical fractures, and most of them are caused by violent impact. If they receive timely and effective treatment, they can recover in a short time. However, if the patient does not receive effective treatment in a short period of time, it will cause permanent damage to the knee joint. And it is irreversible, which has a great impact on the life of the patient, and even lifelong disability. Clinically, the shape memory alloy patella concentrator is very effective in the treatment of comminuted fractures of the patella, can significantly shorten the recovery time of patients, and is also extremely beneficial to the recovery of knee joint function [8-10].

Huang Shihao and Song Zihua[11] analyzed the fracture patients who received treatment. 25 patients in the control group were fixed with a tension band wire during the operation, and 25 patients in the observation group were fixed with a memory alloy patella concentrator. The results showed that the patients in the observation group were fixed with a patella The treatment effect was significantly better than that of the control group. Shen et al [8] studied 63 patients with transverse patella fractures. Twenty-nine patients in the patella concentrator group were used to fix the patella with open reduction memory alloy patella concentrator, and the other 34 patients in the conventional group were fixed with an open reduction tension band. Compared with the two groups of patients, the patients who used the patellar concentrator for internal fixation not only had a shorter operation time and higher safety, but also had a shorter and faster recovery time of the knee joint after surgery.

2.3 Application of memory alloy spinal rods

In the treatment of cervical spondylosis, the application of nickel-titanium artificial cervical vertebra joint has epoch-making significance. The memory alloy rod can be pre-bent to correct the diseased spine. In clinical practice, the correction can achieve better results. Xu Hui and Wang Yan [1] confirmed the effectiveness of memory alloy rods in the treatment of scoliosis through various methods.

Compared with traditional materials, the application of memory alloys in spine surgery has better characteristics in mechanical properties and wear resistance. In terms of mechanical properties, its tensile, torsional and impact resistance properties are superior to other materials; it also performs well in terms of wear resistance, which can reduce the damage to bones and soft tissues caused by particles entering the human body. For cases with high difficulty factor in traditional orthopaedic surgery, stronger and more effective internal fixation can be provided, and it is beneficial to reduce the rate of osteoporosis [12].

2.4 Others

In addition to the embracing devices, patellar concentrators, and spinal rods mentioned in the article, the products using memory alloy materials in orthopedic implants also include arch teeth nails [13], skeleton claws, carpal navicular internal fixation forks, and wrist joint fusions. device, etc. The products are mainly used in the treatment of fractures of ribs, metatarsal bones, patella and other parts and the correction of spine. A large number of studies have proved [14] that the treatment effect of memory alloy implants is significantly better than that of traditional treatment methods.

3. Phase transition temperature of memory alloy implants

Since the accuracy of nickel and titanium content measurement of nickel-titanium memory alloys cannot meet the requirements of ensuring shape memory or superelasticity, thermal analysis method or equivalent bending-free recovery method must be used to measure the phase transition temperature of the alloy.

There are many factors that affect the phase transition temperature, one of which is the annealing temperature [15-17], which is positively correlated. Within a certain range, the higher the annealing temperature, the higher the phase transition temperature. During rapid thermal annealing, the change of the alloy precipitation phase and grain size will lead to the change of the phase transition temperature [18-20]. It can be seen that the change of the phase transition temperature is closely related to the deposition temperature and the annealing temperature, and has an inseparable relationship. Another very important factor affecting the phase transition temperature is the composition.

Memory alloys have shape memory effect and superelasticity [21], and the choice of performance depends on the actual use environment. If you want to have superelasticity at room temperature, you need to control the Af point below 22 to 25 °C; if you want to have superelasticity in the human body, you need to control the Af point below 37 °C. Generally, the phase transition temperature of implanted products is lower than 37°C. After entering the human body, austenite will be formed due to the reverse phase transformation of martensite due to heating, so that the product has super elasticity. The DSC curve of the NiTi shape memory alloy is shown in Fig.

How the end temperature of the reverse martensite-austenite transformation changes depends on the annealing temperature and annealing time [22]. The increase of solution temperature and the increase of time will cause the temperature change of Mp, which shows that it decreases and then increases [23]. The cooling rate also affects the transformation temperature. Both Ms and Mf decrease with the decrease of the cooling rate. The decrease in the cooling rate helps to increase the M→A austenite transformation temperature [24] ].

4. Influence of key indicators on memory alloy implants

Memory alloys have good biocompatibility [25-26], and have superelastic properties, coupled with their low magnetic properties, wear resistance, corrosion resistance, and fatigue resistance, their usage rate is increasing. Most of the medical devices used in the market are Class II and Class III medical devices, most of which are implanted in the body and belong to long-term implants. Therefore, its key indicators are very important. The chemical composition is particularly important for the various properties of metal materials. The phase transition temperature is the key factor that determines the function of the product. The mechanical properties are the top priority of the product performance. Any failure to meet the requirements will directly affect the use of the product in the body. Effects, such as causing loosening of the fixator, breakage of the fixator, or loosening of the patellar claw, resulting in nonunion or malunion.

5. The development direction of memory alloy implants

Alloys must have at least 10 basic alloy families if they are to exhibit shape memory capabilities. If all alloying elements are taken into account, there are a hundred or more. The most widely used in the market, the most frequently used Ti-based alloys, Cu-based alloys and Fe-based alloys. Memory alloys are currently the most comprehensively studied materials in shape memory alloys, and their memory properties have significant advantages over other materials [27]. However, due to the existence of Ni element, the biocompatibility of memory alloys is affected to a certain extent. At present, the most used method is to modify the surface of memory alloys by surface coating, thereby improving the biocompatibility of the material.

Kong Xiangque et al. [28] found that the constant voltage DC anodization method can significantly reduce the nickel content on the surface of the material. Zhu Zihong [29] studied the advantages of dealloying technology in surface modification, and the nickel-free layer prepared by it showed good biological activity. In addition, the implanted ion method, electrophoretic deposition method [30], calcium phosphate solution immersion, photoelectric catalytic oxidation method [31], electropolishing technology, micro-arc oxidation [32] and other methods have achieved good results in surface modification. . Surface modification will also change its mechanical properties, mainly manifested as the restoring force of memory alloys, which is also one of the important factors for the performance of memory alloy implants. Wang Eiyuan et al. [33] used titanium-niobium-coated nickel-titanium memory alloy rods and uncoated nickel-titanium memory alloy rods for comparison in the test. The test results showed that for uncoated nickel-titanium memory alloy rods, the temperature increased, The restoring force also increases accordingly; the larger the rod diameter, the greater the restoring force; the greater the pre-bending deflection, the greater the restoring force. In the coated nickel-titanium alloy rods, the mechanical properties of the 6 mm and 6.5 mm rods decreased compared with the previous ones, and the 7 mm rod did not change significantly. [34] found that the internal friction and specific strength of porous NiTi shape memory alloy composites can be enhanced through the synergistic effect of pores and Ti2Ni.

The elastic modulus of memory alloy materials is close to that of human bone [35], and the use of orthopedic implants made of memory alloys is also increasing. Memory alloy implants may induce nickel ion precipitation after implantation, which is toxic. It will be the focus of future development to inhibit the release of nickel ions through surface modification [36] and other methods to improve the biocompatibility of memory alloy products. In addition, with the rapid development of medical technology, if memory alloy can be combined with virtual reality technology and artificial intelligence technology, it will be developed to a greater extent in orthopedics.

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[Author's brief introduction] Li Wenjiao (1988-) female, master, intermediate engineer. Research direction: surgical implants. [Corresponding author] Ma Chunbao (1981-), male, master, deputy senior engineer. Research direction: surgical implants.