Metallic biomaterials of knee and hip--a review.
Nasab, Marjan Bahrami
Hassan, Mohd Roshdi
Sahari, Barkawi Bin
|Publication:||Name: Trends in Biomaterials and Artificial Organs Publisher: Society for Biomaterials and Artificial Organs Audience: Academic Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2010 Society for Biomaterials and Artificial Organs ISSN: 0971-1198|
|Issue:||Date: August, 2010 Source Volume: 24 Source Issue: 2|
One of the most significant current discussions in orthopedic is the total joint replacements especially hip and knee and the increasing trend to replace degraded and destroyed biological materials by artificial organs. It is estimated that approximately 1 million hip replacements and 250,000 knee replacements are carried out per year . This number is expected to double between 1999 and 2025 as a result of aging populations worldwide and growing demand for a higher quality of life . Another statistical data estimated that by the end of 2030, the number of total hip replacements will increase by 174% and total knee arthoplasties is predicted to grow by 673% from the present rate . An increasing trend of the number of knee replacements in different countries over the last 10-15 years is shown in fig 1.
Yet-increasing demand for implants makes it crucial to accelerate efforts on biomaterials.
Unfortunately, the currently used materials have been found to have tendencies to fail after long-term usage due to not fulfilling some vital requirements such as modulus close to that of bone, high wear and corrosion resistance and good biocompatibility. Rimnac et al. investigated the failure of orthopedic implants in three case studies (hip and knee) and illustrated that both material and design deficiencies contribute to failure of total joint replacements. Failure of current biomaterials imposes pain for patient and after some time revision surgery should be performed.
The purpose of this paper is to review some recent researches about presently used metallic biomaterials and discuss on great potential of NiTi and porous NiTi shape memory alloys (SMA) for orthopedic implant. Meanwhile this study seeks to address the following questions:
1) When a material is going to be used in the human body, what kinds of requirements should be fulfilled by that material to be considered as a successful biomaterial?
2) What kind of problems can occur if these requirements are not satisfied by the material?
3) Which of the requirements present in the currently used materials and which cannot be fulfilled?
4) What solutions are available for improving the properties which are not completely satisfied by the biomaterials?
5) Do the NiTi and porous NiTi shape memory alloys have necessary requirements to be utilized as a metallic biomaterial for orthopedic implants especially hip and knee?
[FIGURE 1 OMITTED]
Requirements of Biomaterials: general issues and concerns
An implant should possess some important properties in order to long-term usage in the body without rejection. The design and selection of biomaterials depend on their mechanical and non-mechanical characteristics.
The mechanical properties such as hardness, tensile strength, modulus, elongation (strain), fracture resistance and fatigue strength or life play an important role in material selection for application in the human body.
Long fatigue life
The fatigue strength is related to the response of the material to the repeated cyclic loads. Teoh  pointed out in his paper that fatigue fracture leads some of major problems associated with implant loosening, stress-shielding and ultimate implant failure and it is frequently reported for hip prostheses. Fatigue characteristics are strongly depends on the microstructures. The microstructures of metallic biomaterials alter according to the processing and heat treatment employed .
Strength of materials from which the implants are fabricated has influence the fracture of artificial organ. Alvarado et al.  and Geetha et al.  explained that inadequate strength can cause to fracture the implant. When the bone-implant interface starts to fail, developing a soft fibrous tissue at the interface can make more relative motion between the implant and the bone under loading. This fact causes pain to the patient and after a certain period, the pain becomes unbearable and the implant must be replaced, by a revision procedure.
Modulus equivalent to that of bone
For major applications such as total joint replacement, higher yield strength is basically coupled with the requirement of a lower modulus close to that of human bones [9, 10]. The magnitude of bone modulus varies from 4 to 30 Gpa depending on the type of the bone and the measurement direction . Au et al.  and Geetha et al.  emphasized about the modulus and described that the large difference in the Young's modulus between implant material and the surrounding bone can contribute to generation of severe stress concentration, namely load shielding from natural bone, that may weaken the bone and deteriorate the implant/bone interface, loosening and consequently failure of implant. The modulus is considered as a main factor for selection of total knee replacement (TKR) materials.
[FIGURE 2 OMITTED]
In addition to the above mentioned mechanical properties, some non-mechanical requirements which have significant role in performance of the material in the human body are describe here.
High corrosion resistance
Singh & Dahotre  researched on corrosion resistance as an important issue in selection of metallic biomaterials because the corrosion of metallic implants due to the corrosive body fluid is unavoidable. The implants release undesirable metal ions which are non-biocompatible. Corrosion can reduce the life of implant device and consequently may impose revision surgery. In addition the human life may be decreased by the corrosion phenomenon. Okazaki & Gotoh  expressed the fact that dissolved metal ions (corrosion product) either can accumulate in tissues, near the implant or they may be transported to other parts of the body. They revealed for example, replacement of 20 stainless steel Charnley hip arthroplasties in the human body after 10-13 years showed a considerably higher metallic concentration in body fluid in comparison with that without implant. This included Ni concentration in blood of ~0.51 [micro]g/L, in plasma of ~0.26 [micro]g/L and in urine of ~2.24 [micro]g/L, and Cr level in plasma of ~0.19 [micro]g/L. Alike, the Ti concentration of ~135.57[micro]g/L was found in the serum of patients with failed Ti-6Al--4V total knee replacements after 57 months which was greatly more than in the control.
High wear resistance
The low wear resistance or high coefficient of friction results in implant loosening [7, 15]. Wear debris are found to be biologically active and make a severe inflammatory response that lead to the destruction of the healthy bone which supports the actual implant. Corrosion caused by friction is a big concern since it releases non compatible metallic ions. It should be pointed out that mechanical loading also can result in corrosion fatigue and accelerated wear processes.
One of the most important non-mechanical requirements of orthopedic biomaterials is the biocompatibility. Biocompatibility is the ability to exist in contact with tissues of the human body without causing an unacceptable degree of harm to the body. It is not only associated to toxicity, but to all the adverse effects of a material in an biological system [8, 16]. Navarro et al.  supported the study of Smallman and Bishop  and with retrospect to the last 60 years, categorized three generations for evolution of biomaterials: bioinert materials, bioactive and biodegradable materials and materials designed to stimulate specific cellular responses at the molecular level. Bioinert is related to reduce the body reaction to the implant to a minimum. Bioactivity defined as the ability of the material to interact with the biological environment to enhance the biological response. The third generation refers to the capability of the material to stimulate specific cellular responses at the molecular level. Williams  defined the biomaterial requirements of total joint replacements in terms of biocompatibility as, optimizing the rate and quality of bone apposition to the material, minimizing the release rate of corrosion and the tissue response to the released particles, minimizing the release rate of wear debris and the tissue reaction to this debris and optimizing the biomechanical environment in order to minimize disturbance to homeostasis in the bone and surrounding soft tissue.
Osseointegration is fundamental in orthopedic. Several literatures explained about the integration of the implant with adjacent bone and tissue [7, 8, 19]. Osseointegration defined as the process of formation of new bone and bone healing. The incapability of an implant surface to join with the adjacent bone and other tissues due to micromotions, results in formation of a fibrous tissue around the implant and promote loosening of the prostheses. Thus, materials with a proper surface are extremely essential for the implant to integrate well with the surrounding bone. Surface chemistry, roughness and topography are all parameters that influence both the osseointegration and biocompatibility . It should be considered that in addition to properties of the implanted biomaterial, the characteristics and regenerative capability of the host bone affect the osseointegration of biomaterial . All the above required properties for biomaterials are summarized in table1.
Implants are fabricated from a wide variety of materials, including metals, polymers, ceramics and their composites. Among these materials, metals are an important group, for instance knee implant has some metallic parts. Current total knee replacement mainly has three components: femur, tibia (includes tibial tray and tibial insert) and patella or kneecap. The tibial insert and the patellar components are usually made of plastics such as ultra-high molecular weight polyethylene (UHMWPE) or cross-linkedpolyethylene. The femoral component and tibial tray are metallic parts and tend to be made of titanium alloys, stainless steel or cobalt chromium with small amount of molybdenum (Co-Cr-Mo) [22, 23]. Another application of metals is in design of hip joint implant which includes an alloy femoral stem (Ti alloy) with a metallic or ceramic femoral head moving in an acetabular cup that is normally made of UHMWPE . The components of knee and hip implants shown in fig 2. In the following, metallic biomaterials are divided to current and promising materials and described respectively.
Current Metallic Biomaterials
Stainless steel, Co-Cr alloys and Ti alloys are the current metals used in orthopedics application such as knee and hip implant. This section provides information about these materials and answers research question 3 and 4.
Stainless steel is the generic name for a number of different steels used primarily because of their resistance to a wide range of corrosive agents due to their high Cr content. The Cr in the stainless steel has a great affinity for oxygen, which allows to the formation of a film of chromium oxide on the surface of the steel at a molecular level which is passive, adhesive, tenacious and self-healing [7, 16]. In spite of this fact, Singh & Dahotre  indicated that stainless steel implants are often degraded due to pitting, crevice, corrosion fatigue, fretting corrosion, stress corrosion cracking, and galvanic corrosion in the body. Their corrosion resistance can be modified by lowering the nickel content and alloying them with Mn or N. The wear resistance of austenitic stainless steel is relatively poor. So rapid loosening is generated by the large number of wear debris. Worse corrosion resistance as well as the danger of allergic reaction which appears in a big number of patients [16, 24] restricts their application in orthopedic joint prosthesis. Moreover the modulus of stainless steel is about 200 GPa which is much higher than that of bone.
Stainless steel has several types and the most mainly used for manufacturing implants is austenitic stainless steel. Stainless steel 316L is the type widely used in traumatological temporary devices such as fracture plates, screws and hip nails. Stainless steel has been used for wide range of application due to easy availability, lower cost, excellent fabrication properties, accepted biocompatibility and great strength.
Cobalt chromium alloys can be basically categorized into two types; one is the Co-Cr-Mo alloy (which is usually used to cast a product) and the second one is Co-Ni-Cr-Mo alloy, (which is usually wrought by hot forging). The castable Co-Cr-Mo alloy has been used in dentistry for long time and recently in making artificial joints. The wrought Co-Ni-Cr-Mo alloy is a relative newcomer material which is now used for making the stems of prosthesis of heavily loaded joints such as the knee and hip .
Cobalt-based alloys are highly resistant to corrosion even in chloride environment due to spontaneous formation of passive oxide layer within the human body environment [7, 15, 16, 25, 26]. These materials have superior mechanical properties such as high resistance to fatigue and cracking caused by corrosion with a good wear resistance. Also they are not brittle because they have a minimum of 8% elongation. These materials have a high elastic modulus (220-230 GPa) similar to that of stainless steel (approx. 200 GPa) which is higher than that of cortical bone (20-30 GPa)[7, 15]. Elements such as Ni, Cr and Co are indicated to be released from the stainless steel and cobalt chromium alloys due to the corrosion in the human body . It has been found that Ni, Cr and Co are the most toxic ions. The corrosion products of Co-Cr-Mo are more toxic than those of stainless steel 316L. The thermal treatments used to Co-Cr-Mo alloys modifies the microstructure of the alloy and alters the electrochemical and mechanical properties of the biomaterial .
Ti and Ti Alloys
Manufacturing of titanium implants date back to the late 1930s. There are three structural types of titanium alloys: Alpha ([alpha]), Alpha-Beta ([alpha]-[beta]) or metastable [beta] and Beta ([beta]).The [beta] phase in Ti alloys tends to exhibit a much lower modulus than [alpha] phase, and also it satisfies most of the other necessities or requirements for orthopedic application [27, 28].
Shenhar et al,. , Alvarado et al. , Budzynski et al. , Geetha et al.  and Navarro et al.  described the Titanium-based materials properties. Ti alloys due to the combination of its excellent characteristics such as high strength, low density (approx. 4700 [Kgm.sup.-3]), high specific strength, good resistance to corrosion (due to the formation of an adhesive TiO2 oxide layer at the surface), complete inertness to body environment, enhanced biocompatibility, moderate elastic modulus of approximately 110 GPa are a suitable choice for implantation. Ti and its alloy also have this ability to become tightly integrated into bone. This high capacity to join with bone and other tissues considerably improves the long-term behavior of the implanted devices, decreasing the risks of loosening and failure. Typically good clinical outcome from rough surfaces of Ti and its alloy is resulted when compared to smooth-surfaced implants due to the good osseointegration between the bone and the implant . Commercially pure Ti (CP Ti) and Ti-6Al-4V ELI (Ti64, Extra Low interstitial) are most commonly used titanium materials for implant applications. Ti-6Al-V4 is slowly replacing CP Ti due to the greater mechanical strength . According to Navarro et al.  and Geetha et al., long-term performance of titanium and its alloys mainly Ti64 has raised some concerns because of releasing aluminum and vanadium. Both Al and V ions are associated with long-term health problems, like Alzheimer disease and neuropathy. Furthermore when titanium is rubbed between itself or between other metals, it suffers from severe wear . High friction coefficient and a rather high propensity to seizure are attributed to this alloy . Therefore their application is limited to the locations on the implant surface where wear resistance is not of vital importance . For example, the poor wear resistance of titanium alloys avoids their use for femoral head applications in typical hip implant although the femoral stem is often made of these alloys. Rostoker & Galante  measured wear rates by in vitro methods on UHMW-PE when rubbed by a oppose face of the Ti-6% AI-4% V, ELI grade (ASTM F-136) with various prepared surfaces and compared the result with stainless steel or cast Co-Cr-Mo alloy.
[FIGURE 3 OMITTED]
Simply polished titanium alloy demonstrated an unusual wear rate which was one order of magnitude larger than polished AISI 316 stainless steel or cast Co-Cr-Mo alloy. Wear-corrosion also can occur for Ti, but Ti-6Al-4V can be modified by replacing V with Nb, Zr or Ta in order to make it more biocompatible and corrosion resistant .. Semlitsch  developed a hot forged and surface treated Ti-6AL-7Nb and observed the same [alpha]/[beta] structure as Ti-6AI-4V and equally good mechanical properties. Moreover Nb solved the problem of releasing vanadium. The authors concluded that the alloy is a real alternative to the wrought Ti-6Al-4V alloy manufacturing of endoprosthetic components. A variety of surface treatment methods, such as ion implantation, Titanium nitride (TiN) coating, and thermal oxidation, have been proposed to enhance the wear resistance [29, 30] and osseointegration [20, 31] by altering the nature of the surface. Processing of Ti and its alloy include of machining, forging or heat treating is not easy [8, 16]. Two recently developed promising biomedical alloys, Ti-35Nb-7Zr-5Ta (TNZT)  and Ti-29Mo-13Ta-4.6Zr (TNTZ) , show significant improvement, in the aspect of accompanying the high yield strength and low modules, compared to previous generation alloys such as Ti-6Al-4V, stainless-steel and cobalt-chromium-based alloys .
The applications of Ti and its alloys include dental implants and parts for orthodontic surgery, joint replacement components such as knee and hip, bone fixation devices like nails, screws and plates, artificial heart valves and surgical instruments.
Shape memory alloys (SMA) provide new insights for the design of biomaterials for artificial organs and advanced surgical instruments, since they have unique characteristics and superior properties [37, 38]. Some unusual properties of these materials are: one-way and two-way shape memory effects, superelastic effect, high damping property and rubber-like effect. Some literatures about NiTi SMA and porous NiTi SMA are overviewed in this section and important aspects about these materials (RQ5) are illustrated.
Dense NiTi shape memory alloy
Among several tens of shape memory alloys, NiTi alloy is considered to be best due to its excellent properties.
Phase Transformation and properties
NiTi can have three different forms; martensite or the low-temperature phase, stress-induced martensite or superelastic and austenite or the high temperature phase. The reference temperatures indicated in figure 3 show the start and finish temperatures for the forward transformation to martensite and the reverse transformation to austenite, respectively. The martensitic transformation can occur when the alloy in the austenitic phase is cooled through [M.sub.s]--->[M.sub.f], producing martensite. The reverse transformation occurs when the alloy is heated through [A.sub.s]--->[A.sub.f] and the material structure returns to that of austenite. The transformation from austenite to martensite accompanied by a large recoverable strain. The martensitic transformation also can be induced by stress in a shape memory alloy at temperatures above [M.sub.s]. An increase in the applied stress provides an effect analogous to a decrease in temperature.
The main properties of the SMA are generally determined by the phase transitions from austenite to martensite and vice versa. When the material is in its martensite form, it is soft and ductile and can be easily deformed. Superelastic NiTi is highly elastic (rubber-like) while austenitic NiTi is quite strong and hard (similar to titanium). Their specific expression depending on the temperature in which they are used. Alvarado et al.  explained the properties of NiTi material. NiTi alloys have this ability to combine high recovery strain, high strength as well as a relatively low Young's modulus. The low elastic modulus of NiTi which is much closer to that of bone than any other implant metal might provide benefits in specific applications. NiTi has unique high fatigue resistance and ductile properties, which are also related to its martensitic transformation. High dampening capacity could be useful in some cases. These properties are typically favorable in orthopedic implantation applications. Also, high wear resistance has been reported compared to the Co-Cr-Mo alloy. In addition NiTi is a non-magnetic alloy, so magnetic resonance imaging (MRI) is possible.
Concerning Issues and Biocompatibility
Ni release from the surface of NiTi implants which is the concerning issue of NiTi alloys have been discussed by Kapanen et al. , Mantovani , Machado & Savi  and Geetha et al.. It has been found that although, Nickel is a necessary element for life and it is able to stimulate the immune system , but it can be severely poisonous when the high nickel content of NiTi are generated due to the dissolution of nickel ions or wear particles from the alloy. Releasing of this element above certain concentrations brings some allergic reaction and biocompatibility problems such as pneumonia, chronic sinusitis and rhinitis, nostril and lung cancer for patients.
Researches on the biocompatibility of shape memory alloy started from 1976. Castleman et al. investigated on the biocompatibility of nitinol alloy through in vivo studies on femurs of beagles using Cr-Co reference controls (sham). Their results demonstrated no evidence of either localized or general corrosion on the surfaces of the bone plates and screws, no signs of adverse tissue reactions resulting from the implants and no metallic contamination in the organs due to the implants. Shabalovskaya  reviewed biocompatibility of NiTi and performed X-ray surface investigation. The author drew conclusions include good biological response in vivo and tendency of Nitinol surfaces to form TiO2 oxides with only a minor amount of nickel. He pointed out that a certain toxicity which usually observed in vitro studies, probably resulted from the higher in vitro Ni concentrations that are impossible to attain in vivo. At the end it was concluded that biocompatibility of Ni-Ti alloys is similar to that of titanium, Co-Cr and stainless steel alloys. Berger-Gorbet et al.  evaluated the biocompatibility of Nitinol screws by comparison with vitallium, CP titanium, duplex austenitic-ferritic stainless steel and stainless steel 316L and observed a slower osteogenesis process with no close contact between implant and bone in NiTi screws compared with the others. Ryhaenen et al. studied soft tissue response and biocompatibility of Nitinol in vivo and performed a comparison between Nitinol, stainless steel and Ti-6Al-4V. The authors observed clearly nontoxic response of muscular tissues to NiTi regardless of the time period, similarity of overall inflammatory response to Nitinol, stainless steel and Ti-6Al-4V alloy, no necroses, granulomas, or signs of dystrophic soft tissue calcification, low response of immune cell to Nitinol and no qualitative differences in histology between the different tested materials. In addition the encapsulated thickness was equal to all the materials examined. Based on the results of their study, Nitinol has good potential for clinical use. Ryhanen  reviewed researches on biocompatibility of Nitinol and considered fundamental aspects of biological responses to Nitinol. It was indicated that most studies supported the good biocompatibility of Nitinol but it was not well demonstrated the long-term in vivo performance of Nitinol and the host-Nitinol interactions at cell and molecular level. Finally it was concluded that Nitinol is a safe biomaterial, at least as good as stainless steel or titanium alloys. Kapanen et al. determined the biocompatibility of NiTi alloy on bone formation in vivo study. They compared NiTi with stainless steel and Ti-6Al-4V. The researchers concluded good biocompatibility as its effects on ectopic bone formation are similar to those of stainless steel and the amount of nickel released from NiTi implants was lower than the concentration required inducing toxic reactions. Balakrishnan et al.  tested the biocompatibility of Nitinol, a nickel titanium alloy, and stainless steel as bladder implant materials. Similar tissue effects in all groups with small or no inflammation was observed and it was designated that Nitinol may be more inert than stainless steel. Es-Souni  reviewed papers published on the biocompatibility of NiTi alloys and indicated that NiTi SMA are usually characterized by good corrosion properties, in most cases better than those of conventional stainless steel or Co-Cr-Mo based biomaterials. The majority of biocompatibility studies demonstrated good biocompatibility. It is also obtained that smooth surfaces with well controlled structures and chemistries of the outermost protective TiO2 layer results in to negligible amount of Ni leaching, with concentrations below the normal human daily intake.
It is obvious that although, there are few reports on release of Ni from NiTi implants, but most of in vivo studies and in vitro experiments demonstrate excellent biocompatibility of this material.
[FIGURE 4 OMITTED]
Surface Modification (enhanced biocompatibility)
In addition to the above studies which expressed good biocompatibility of NiTi, other scholars investigated on surface modification and coating to achieve high connection between bones and implant, high corrosion resistance and good biocompatibility. Firstov et al. performed surface oxidation in air on mechanically polished NiTi alloy in temperature range 300-800 [degrees]C and observed that oxidation treatment at temperatures close to 500 [degrees]C generates a smooth protective nickel-free oxide layer that is in favour of good biocompatibility of NiTi implants. Chen et al. produced a thin apatite layer on NiTi alloy implants in situ. The results illustrated that large amount of new bone was directly in contact with the host bone but the uncoated implant/bone interface has gaps and fibrous layer was generated. Chan et al. worked on oxygen and sodium plasma on NiTi SMA. The method enhanced the bioactivity and corrosion resistance of NiTi but the treatment changed the bulk mechanical property and phase transformation temperature. Chu et al. described the recent applications of plasma immersion ion implantation (PIII) to the surface modification of NiTi orthopedic materials. It was found that the method produced an effective surface barrier to diminish Ni out-diffusion and the PIII treated NiTi rods maintained the shape recovery properties by using the proper conditions. Michiardi et al.  compared the electrochemical behavior of NiTi surfaces oxidized by a new oxidation treatment with untreated NiTi surfaces and concluded that the new oxidation treatment was capable of protecting NiTi surfaces from electrochemical degradation and, thus, NiTi can be a superb candidate for biomedical applications. Ng et al. modified surface of NiTi by laser surface modification methods. It was demonstrated that the modified layers which were free from micro-cracks and porosity acted as barrier to Ni release and improved bulk properties, such as hardness, wear and corrosion resistance. Shabalovskaya et al.  reviewed different studies on surface modifications of NiTi SMA and discussed about bare Nitinol surfaces, mechanical, chemical and electrochemical techniques and heat treatments for their modifications. Meanwhile the authors investigated about the biological response to bare Nitinol surfaces. Surface modifications with ion and energy sources includes conventional and plasma immersion ion implantation and laser surface melting, sol-gel and hydrogen peroxide, bioactive surfaces and Nitinol surface under strain all are overviewed in their work. Although most of literatures on surface modification of NiTi have provided good results in favor of Ni release as well as the biocompatibility, but it should be considered that these modifications may affect mechanical properties. There have been a few studies which examined the mechanical properties after surface treatment. However Chu et al. developed an electropolishing (EP) and photoelectrocatalytically oxidation (PEO) surface on biomedical nickel titanium shape memory alloy and investigated the microstructure, nickel suppression and mechanical characteristics. They indicated that PEO resulted in formation of a sturdy titania film on the EP NiTi substrate. A Ni-free layer near the top surface and a graded interface between the titania layer and NiTi substrate were observed which was greatly efficient for both biocompatibility and mechanical stability. Moreover, Ni ion release from the NiTi substrate was suppressed (10-week immersion test). The modulus and hardness of the modified NiTi surface was greater than before, with larger indentation depths which were slightly higher than those of the NiTi substrate but a lot lower than those of a dense amorphous titania film. The authors compared mechanical properties of NiTi with after undergoing only EP. Their results demonstrated that surface modification by dual EP and PEO can remarkably decrease Ni ion release and improve the biocompatibility of NiTi without the degradation of the surface mechanical properties. It makes the treated materials suitable for hard tissue replacements.
[FIGURE 5 OMITTED]
It can be concluded that NiTi shape memory alloy has good biocompatibility and can be improved to a higher level of compatibility with biological system by the help of surface modification. Meanwhile the recent work emphasize that mechanical stability can also be obtained after the treatment.
Porous NiTi has been considered as one of the promising biomaterials in surgical implants which have been used in medical fields in Russia and some other countries. The porous materials have many applications, ranging from spinal fixation to acetabular hip prostheses, dental implants, permanent osteosynthesis plates, etc . Porous biomaterials are divided into two categories: solid substrate with porous coating and integral porous body. Study of integral porous body has attracted more attention due to some problems such as developing only at the interface and not easy machining due to brittleness in the first group .
Properties and privileges
Some literatures demonstrated that interconnected open pores and large surface area induce transport of body fluids (helps to accelerate the healing process) and in-growth of bone tissues in this special material. Porous nature of NiTi biomaterial enables tissue and bone cells to penetrate and integrate with the implant and provides strong anchor between surrounding bone and tissue with the implant. This promotes long-term fixation without the need for bone grafting and prevents the loosening of implants [8, 57, 59-61]. Super-elastic behavior also remains after tissue in-growth . Zhang et al,. observed that the porous Ti-50.8 at. % Ni SMA which was fabricated by capsule free hot isostatic pressing process had a recoverable strain as high as 4% in terms of a linear superelasticity. They indicated that under a high-cyclic strain level, the degradation of superelastic effect only was in the first fatigue cycle and thereafter the good linear superelasticity was maintained. Studies on the superelastic behavior, different pore size and various heat treatment conditions of NiTi produced by gas expansion method revealed that the NiTi with 16% porosity exhibited excellent combination of mechanical properties such as high strength (1000 MPa), low young modulus (15 GPa), large compressive ductility (>7%), large recoverable strains (>6%) and high-energy absorption (>30 MJ/[m.sup.3]) . Bansiddhi et al,. introduced some of the important properties of porous NiTi. These porous materials provide a combination of high strength, high toughness and relatively low stiffness. High strength is an important parameter for preventing deformation or fracture, high toughness is essential to avoid brittle failure and low stiffness or low modulus is useful to minimize stress shielding effects [8, 61, 65]. Shape-recovery behavior can make good mechanical stability within the host tissue . Meanwhile it has been obtained that an appropriate range of pore sizes and interconnectivity enable a morphology similar to that of bone .Generally porous SMAs have the ability to carry significant loads. These materials offer the possibility of higher specific damping capacity under dynamic loading conditions in comparison with dense SMA materials . It has been demonstrated that a considerable part of the impact energy is absorbed .
Biocompatibility and surface treatment
Porous NiTi has good biocompatibility, comparable to conventional porous stainless steel and titanium implant materials . No adverse tissue response due to the implant occurs and no fibrous tissue is formed at the interface of new bone/ implants for the porous NiTi alloy. Porous NiTi alloy shows better osteoconductivity and osteointegration than bulk one . Porous NiTi SMA is less corrosion resistant than the solid one  because the Ni release is unavoidable due to the large exposed surface area which directly contacts with adjacent bone and tissue. In order to prepare porous NiTi surfaces with minimal or negligible Ni release and corrosion rates, several surface treatments have been used to create uniform, homogeneous, and thick TiO2 on NiTi surfaces. Some surface modification can reduce Ni release rate by a factor of 3-24 to levels below the normal daily Ni intake . The amount of nickel that is normally present in food with the dietary intake of nickel estimated to be in the range of 300-600 [micro]g per day . Current surface treatments applied on porous NiTi consist of thermal annealing, oxygen plasma immersion ion implantation, pre-soaking in SBF solution, HA coatings, TiN and TiO2-PVD coatings, chemical treatment and combinations thereof. Jiang & Rong  produced hydroxyapatite coating on porous NiTi shape memory alloy. The results illustrated greatly decrease in the amount of nickel release from the porous NiTi SMA after formation of a uniform hydroxyapatite layer. The amount of Ni release after 50 days for untreated was 6.7 ppm and for treated surface was 0.48 ppm. Wu et al.  performed thermal annealing for porous NiTi. The optimized annealing temperature was found to be 450 [degrees]C. In lower temperatures (300-450 [degrees]C) Ni leaching level of treated NiTi decreased by a factor of two as compared to untreated NiTi (from 0.45 ppm to 0.2 ppm). Ho et al.  investigated oxygen plasma immersion ion implantation (PIII) method for porous NiTi. Their observation showed that Ni release was considerably reduced after oxygen PIII and excellent durability of the layer in a biological medium was approved. The authors concluded that depletion of Ni from the near surface region after oxygen PIII and the high corrosion resistance of the oxygen rich surface layer can provide better properties. All the literatures confirm that the amount of Ni release can considerably decrease after surface modification, but in their study the effect of surface treatment on superelasticity and other mechanical properties is not investigated. Practical use of porous NiTi SMA requires a complete research on surface modification accompanying with examining mechanical properties and biological response to the treated material. Figure 4 shows failure of different metallic biomaterials for long term use in body. Since NiTi SMAs have enough potential, so their superelastic behavior and damping properties are explained in the next section.
Advantages of the superior properties of dense and porous NiTi shape memory alloy
Shape memory alloy in addition to fulfilling the requirements have some especial characteristic such as superelastic and damping properties which are efficient to be utilized in orthopedic application. So these two important parameters are explained here.
The superelastic behavior is similar to that of elasticity but in a higher and complex order of magnitude. Super-elastic materials (SEM) return to their original shape upon unloading after a substantial deformation . Since the mechanism is not conventional, this effect is also termed as "pseudoelasticity" or "transformational superelasticity". It is caused by stress-induced martensitic transformation . If a stress is applied above [M.sub.s], the martensite can be stress-induced. The transformation from austenite to martensite is accompanied by large recoverable strain and the material returns to the original shape when the stress is no longer applied because the martensite is completely unstable without stress assistance . A schematic superelastic stress-strain curve is shown in figure 5 while [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] are the loading stress, unloading stress, the total strain and unrecoverable strain respectively.
Fully austenitic NiTi material usually is suitable for surgical implantation . So this property probably can be useful for joint replacement where the load is frequently applied and removed during daily activities from the aspect of unrecoverable or residual strain.
The high-damping effect is the ability of a material to transform mechanical energy which is provided for example by an applied force into thermal energy. This irreversible energy transformation helps the material to resist shocks and absorb vibrations . When a shape memory material is cyclically loaded further than a critical value, the transformation from austenitic phase to martensitic one takes place which represents a significant hysteresis loop whose area demonstrates the amount of energy dissipation . In NiTi shape memory alloy high dampening capacity could be useful in, for example, dampening the peak stress between the bone and the articular prosthesis . This property cause that the material also can absorb the impact energy when a sudden load affects the joints, but in conventional material it may cause hard damage to prostheses.
Biomaterial selection is one of the most challenging issues due to crucial requirements and biocompatibility, so it has been of major interest to material designers in recent years. The present study reviewed the currently used metallic biomaterials in hip and knee; stainless steel, chromium cobalt alloys and titanium alloys. Meanwhile NiTi and porous NiTi shape memory alloy as promising materials were explained in detail. It has been indicated that in spite of that all the current metals have some capabilities, there are some concerning issues about them, for instance low wear resistance in case of Ti-6Al-4V, high Yang's modulus about Co-Cr alloy. Returning to the last question posed at the beginning of this study, it is now possible to state that NiTi shape memory alloy and its porous form, can be considered as high potential biomaterials to be used for orthopedic application such as knee and hip implant. These two materials satisfy most of the requirements and additionally have superior characteristics which help the long term use of material in the body. Practical use of NiTi SMA particularly the porous NiTi requires a complete research on surface modification accompanying with examining mechanical properties and biological response to the treated material. The evidence from this study suggests more investigation on mechanical properties of NiTi after treatment. This review should be of value to researchers who are interested in the state of the art of metallic biomaterial evaluation and selection of knee and hip prostheses.
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Marjan Bahrami Nasab, Mohd Roshdi Hassan and Barkawi Bin Sahari
Department of Mechanical and Manufacturing Engineering, Engineering Faculty University Putra Malaysia
Received 1 July 2009, Accepted 5 September 2009, Published online 27 January 2010.
Table 1: Requirements of biomaterials and problems derived from inadequate requirements Significant requirements Consequences of not fulfilling the requirements Long fatigue life Implant mechanical failure and revision surgery Adequate strength Implant failure, pain to patient and revision surgery Modulus equivalent Stress shielding effect, loosening, to that of bone failure, revision surgery High wear resistance Implant loosening, severe inflammatory response, destruction of the healthy bone Producing wear debris which can go to blood High corrosion resistance Releasing non compatible metallic ions and allergic reactions Biocompatibility Body reaction and adverse effects in the organic system Osseointegration Fibrous tissue between the bone and the implant, not well integration of the bone and implant and finally implant loosening
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