Materials Science And Engineering Engineering Essay

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In the area of materials science and engineering, corrosion of biomaterials is of paramount importance as biomaterials are required for the survival of the human beings suffering from acute arthritis, osteoporosis and other joint complications. This term paper discusses various issues associated with biological corrosion of different kinds of implants used as orthopaedic and dental implants. Since, the materials used for these implants are manifold starting from metallic materials such as stainless steel (SS), cobalt chromium, titanium and its alloys, polymers etc. are in constant contact with the aggressive body fluid, they often fail and finally fracture due to corrosion. The corrosion behaviour of various implants and the role of the surface oxide film and the corrosion products on the failure of implants are discussed. Surface modification of implants, which is considered to be the best solution to combat corrosion and to enhance the life span of the implants and longevity of the human beings, is dealt.

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INTRODUCTION

Objectives

Failure is one of the most important aspects of implant materials behaviour. It directly influences the choice of materials and production methods in manufacturing. Analysing implant failures is intricate due to the involvement of many criteria. Despite the best efforts taken during the design, material selection, installation, operation, and machining, components fail. Even though just a minor portion of implants fail, presence of large number of implants in daily life makes failure analysis an extremely important topic. Finding the major causes of failure will help in gathering information for improving design and the operating procedures, which will indeed facilitate a better usage of components. Failure of implant components can be caused by several factors such as: implantation errors; design errors, including wrong selection of materials; corrosion; fatigue; and wear [1]. Among these causes, the risk of materials failing due to corrosion is higher because of the unfavourable setting within the human body [2]. This paper analysis the various orthopaedic and dental implant failures caused by corrosion of biomaterials and offers solutions for preventing it.

Background

With the accessibility of better technology coupled with advancements in the knowledge on materials as well as on surgical procedures has increased the usage of biomedical implants in the field of dentistry, orthopaedics, cardiovascular surgery, plastic and reconstructive surgery, neurosurgery, ophthalmology etc. [3]. The different applications of implantology possible today are shown in Figure 1.

Figure 1: Biomaterials for human application.

(Adapted from Biomedical Implants: Corrosion and its Prevention - A Review by Geetha, M., Durgalakshmi, D., & Asokamani, R. (2010) [3])

Various classes of materials such as metals, alloys, polymers ceramics and composites have been widely used to fabricate the bio-implants [3]. For the material to be biocompatible, they must satisfy the following condition: first and foremost, they must not damage the body and must sustain minimal damage from the body, this is a biological criterion. Then they must satisfy the required physical, chemical and mechanical properties such as providing the required strength, especially high fatigue strength and toughness, for joints and other applications. Moreover, the cost factor should also be taken care of. Thus, the heart of a biomaterial is an economic implant with the mechanical, physical, and chemical properties that provide structural capability without deleterious effects on the body [1].

Among the various biomaterials, two polymers which are extensively used in orthopaedic implants are: ultra-high molecular weight polyethylene (UHMWPE) and polymethyl methacrylate (PMMA). UHMWPE is used for bearings and PMMA is used as a grouting to 'cement' implants into bones [4]. The other class of material, ceramics also are used for bearing surfaces and as coatings to invoke amalgamation with host tissues [5]. The most widely used class of materials in biomedical implants are the metals. Among them, the most commonly used metal and metal alloys are chromium cobalt alloys, stainless steel (316L), and titanium and its alloys. The α+β-type titanium alloy, such as Ti6Al4V, has been most widely used as an implant material for artificial hip joints and dental implants due to its high strength and excellent corrosion resistance [1; 6].

Corrosion

According to ISO 10271:2001, the definition of corrosion is 'physicochemical interaction between a metal or an alloy and its environment that results in a partial or total destruction of the material or in a change of its properties' [7]. Corrosion changes the chemical environment around the implant, inducing an acidic pH and thus increasing the likelihood of corrosion [8].

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Although most of the carefully chosen biomaterials which satisfy the criteria of biocompatibility are highly corrosion resistant, large number of chemical reactions taking place within the human body produce abundance of oxidizing agents [9]. It includes blood and other body fluid constituents such as water, Na, Cl, amino acids in saliva [10]. This creates an unfriendly environment for metals and other materials, which can make even the most corrosion-resistant material to undergo corrosion up to a certain level [11].

FACTORS DETERMINING CORROSION AND CORROSION SUSCEPTIBILITY OF MATERIALS

Factors determining corrosion

The two physical characteristics which determine implant corrosion are: the thermodynamic forces which correspond to the energy required or released during a reaction [12]. It causes corrosion either by oxidation or reduction reaction. The second factor is the kinetic barrier which is the well known process of formation of the surface oxide film on a metal surface, which in turn impedes further corrosion reactions [13]. However, only those metals, which have the capacity to form a protective oxide layer against corrosion, can be used in orthopaedic implants. In order to limit oxidation, passive films must have certain characteristics. They must be non - porous and must fully cover the metal surface; they must have an atomic structure that limits the migration of ions and electrons across the metal oxide - solution interface; and they must be able to remain on the surface of the material even with mechanical stressing or abrasion, which can be expected in association with orthopaedic devices [12].

Corrosion susceptibility of Materials

Materials can be judged as whether corrosion resistant or not based on 4 criteria. They are:

Ease to be oxidized, strong adherence of formed oxide to the substrate, dense of formed oxide, and protectiveness of formed oxide.

The Pilling -Bedworth (P-B) ratio is the very simple indication to judge whether the formed oxide is protective or not [14]. If P-B ratio is less than 1, since oxide occupies small volume than the metal, so that formed oxide is porous and non-protective. If it is larger than 2, since oxide occupies a large volume and may flake from the surface, exposing fresh substrate surface and again exhibits non-protectiveness. If P-B ration is between 1 and 2, the volume of oxide is similar to that of metal, so that the formed oxide is adherent to substrate, nonporous, and protective.

The most common materials used in orthopaedic and dental implants are Stainless steel, Cobalt chromium alloys and Titanium and its alloys. Hence, in order to do proper material selection, it is important to know about their vulnerability to corrosion. A brief account of the vulnerability of these materials to corrosion is given below:

Stainless Steel:

Stainless Steel contains sufficient amount of chromium to impede corrosion by passivity. The passive layer (chromium oxide) is not as robust as in the case of titanium or the cobalt - chromium alloys. The relatively resistant varieties of stainless steel are the austenitic types 316, 316L and 317, which contain Molybdenum (2.5 - 3.5%). However, these types of stainless steels are vulnerable to pitting and to crevice corrosion around screws, under certain circumstances such as in a highly stressed and oxygen - depleted region [15].

Cobalt chromium alloy:

Both the cast and wrought varieties of Cobalt chromium alloy are passive in the human body and do not exhibit pitting, though they are moderately susceptibility to crevice corrosion [16].

Titanium:

Titanium is a base metal, in context of the electrochemical series. Hence, it should be easily corroded. Titanium is a highly reactive metal and will react within microseconds to form an oxide layer when exposed to the atmosphere [17]. Hence, it forms an adherent porous layer (Tio2) and it was calculated that P-B ratio for TiO2 formation is 1.76, indicating that the formed TiO2 is protective and remains passive under physiological conditions [17]. Titanium implants remain virtually unchanged in appearance and offer superior corrosion resistance [15].

RESULTS

Discussion on the corrosion of Dental and Orthopaedic implant

Corrosion of Orthopaedic Implants

Orthopaedic implants include both temporary implants as well as permanent implants. Temporary implants are plates and screws and permanent implants are used to replace hip, spinal, toe etc. The most common forms of corrosion that occur in orthopaedic implants are uniform corrosion, intergranular, galvanic and stress corrosion cracking, pitting and fatigue corrosion [18].

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The common corrosion mechanisms that occur in temporary implants are crevice corrosion at shielded sites in screw interface and beneath the heads of fixing screws [19]. Crevice corrosion is a form of local corrosion which arises on account of the differences in oxygen concentration of electrolytes. In other words, it is due to the change in pH in a confined space, such as in the crevices between a screw and a plate [20]. The narrower and deeper the crack is, the more likely crevice corrosion is to start [16].

The other corrosion mechanism which occurs in temporary implants is pitting corrosion of the implants made of Stainless Steel [19]. Figure 2 below, shows a SEM observation of a typical pitting-type failure occurred due to corrosive attack, which originated the fracture from inside the hollow rod. The rod was removed from another patient [1].

Figure 2: Pitting-type failure occurred due to corrosive attack [1]

Other common corrosion mechanism in orthopaedic implants is stress corrosion. Stress corrosion is a phenomenon in which a metal in a certain environment, especially those rich in chlorides, is subjected to stress and falls at a much lower level than usual as a result of corrosion [21]. Some of the factors that causes stress corrosion are misalloying, aggressive body environment, and residual stresses. Figure 3, below, shows the fracture surfaces of a vertebral implant made of 316L, where stress corrosion caused brittleness and resulted in brittle fracture. Stress corrosion was caused by residual stresses in this failure [1].

Figure 3: Scanning electron micrograph of stress corrosion [1]

In addition, the corrosion of orthopaedic implants are accelerated due to wear. Hence, Cobalt chromium alloy, ceramics which is highly resistant to wear are often preferred to fabricate orthopaedic implants. Titanium alloys are used in hip implants, but they are used only to make the femoral component. The ball which again needs to sustain wear is either made of hard ceramics or Chromium-cobalt alloy. The femoral components are sometimes coated with cement to have good fixation. Willer et al. have observed crevice corrosion in femoral components made of Ti-6Al-4V and Ti- 6Al-7Nb when they were implanted with bone cement [22].

The accelerated corrosion test performed by Khan et al. On Cp Ti, Ti-Nb-Zr and Ti-Mo alloys in in vitro conditions demonstrated Ti-6Al-7Nb and Ti-6Al-4V possessed best combination of corrosion and wear [23]. However, the nature and distribution of corrosion products released into the body from these orthopaedic implants remains, still as an important issue. Hence, currently several researchers are working on the enhancement on the improvement of surface properties of titanium based alloys [11].

Corrosion of Dental Implants

According to Dr. A. P. Abraham (Prosthodontist and Implantologist; Associate Professor, SRM University, Chennai) dental implants can be broadly classified into two categories: endosseous and subperiosteal implants [24]. Endosseous implant is an implant that is inserted into the alveolar and/or basal bone and protrudes through the mucoperiosteum [25] and in the subperiosteal implant, a dental metal appliance is made to conform to the shape of a bone and is placed on its surface beneath the periosteum [25]. These implants face very aggressive environment in the mouth, the pH of saliva varies from 5.2 to 7.8. Thus the major reasons for corrosion of metallic implants and fillings are temperature, quantity and quality of saliva, plaque, pH, protein, and the physical and chemical properties of food and liquids as well as oral health conditions. As two metallic components are used together in making dental implants, galvanic corrosion occurs very frequently in dental implants [3]. This occurs most commonly between the pair of metallic implants such as Co-Cr alloys, Ni-Cr, silverpalladium, gold and Ternary Ti dental implants. Pitting corrosion of cobalt based alloys leads to the release of carcinogens into the body [3]. On the other hand titanium and its alloys are highly resistant to pitting corrosion in different in vivo conditions encountered; however they undergo corrosion in high fluoride solutions in dental cleaning procedures [3] . The corrosion products cause discoloration of the adjacent soft tissue, allergic reactions and rashes in some patients. The wound healing process is also found to be modulated by the metal ions released by corrosion. The corrosion of the implants is further accelerated in the absence of poor osseointegration. Table 1 summarizes some of the common types of corrosion in the conventional materials used for biomaterial implants.

Table 1: Types of Corrosion in the Conventional Materials Used for Biomaterial Implants

Type of Corrosion

Material

Implant Location

Shape of the Implant

Pitting 304 SS

Cobalt based alloy Orthopedic/ Dental alloy

Pitting 304 SS

Crevice

316 L stainless steel

Bone plates and screws

Corrosion fatigue

316 SS, CoCrNiFe

Bone cement

Fretting

Ti6Al4V, CoCrSS

Ball Joints

Galvanic

304SS/316SS, CoCr+Ti6Al4V,

316SS/Ti6Al4V Or CoCrMo

Oral Implants

Skrews and nuts

Selective Leaching

Mercury from gold

Oral implants

[Ref: Blackwood DJ. Biomaterials: past successes and future problems. Corrosion Rev 2003; 21(2-3): pp. 97-124.] [26]

Measures to be taken to minimize and prevent corrosion

Corrosion of material will lead to material loss, which in turn will weaken the implant. Moreover, the corrosion products might also escape into the tissue and can result in undesirable consequences [27]. Hence, it is important to establish measures to prevent and minimize corrosion in biological implants.

Surface modification is one way to combat corrosion. Ti dental implants are often surface modified to reduce corrosion and also this improves osseointegration and increases the biocompatibility. Surface modification involves surface machining, sandblasting, acid etching, electropolishing, anodic oxidation, plasma-spraying and biocompatible/biodegradable coatings are performed to improve the quality and quantity of the bone-implant interface of titanium-based implants [28] .Unlike the above treatments, laser-etching technique was introduced in material engineering originally which resulted in unique microstructures with greatly enhanced hardness, corrosion resistance, or other useful surface properties [28] . Laser processing also is now being used in implant applications to produce a high degree of purity with enough roughness for good osseointegration [3].

With regard to orthopaedic implants also, different surface modification methods have been adopted to improve their corrosion resistance [3]. Thair et al. Studied the corrosion behavior of nitrogen ion implanted Ti-6Al-7Nb alloy by varying the dose of the nitrogen ions using an accelerator [3]. They observed that the passive current density and area of the repassivation loop were decreased as the dose values increased. Similarly the work carried out by Kamachi et al. on nitrogen alloying on the cold worked 316L austenitic stainless steel showed a substantial improvement in the pitting corrosion resistance [3].

CONCLUSION

The field of corrosion with respect to dental and orthopaedic implants faces lots of challenges. Present research is focussed on developing composite materials since our bone, dentin etc are natural composites. However, more studies are to be made to understand the behaviour of composite materials to understand their bio-fluid absorbing behaviour, interfacial bonding between the matrix.

Ceramics are another class of materials which have high biocompatibility and enhanced corrosion resistance. They are widely used today for total hip replacement, heart valves, dental implants and restorations, bone fillers and scaffolds for tissue engineering. But one problem with ceramics is that they are brittle, have high elastic modulus and can fracture as they posses low plasticity. Corrosion resistance is often improved by doing Surface modifications. Also the wear resistance, surface texture and biocompatibility are achieved through this [3].

In a nutshell, the field of corrosion in biological systems is young and fertile as man knows only little about his physiology and its interactions with the foreign body is much more complicated and hence the mission will continue.

APPENDIX

Glossary

Endosseous implant

:

An implant that is inserted into the alveolar and/or basal bone and protrudes through the mucoperiosteum.

Mucoperiosteum

:

Mucous membrane and periosteum so intimately united as to form practically a single membrane, as that covering the hard palate

Osseointegration

:

 It refers to the direct structural and functional connection between living bone and the surface of a load-bearing artificial implant.

Periosteum

:

The thick, fibrous membrane covering the entire surface of a bone except its articular cartilage and the areas where it attaches to tendons and ligaments.

Subperiosteal implant

:

An artificial dental metal appliance made to conform to the shape of a bone and placed on its surface beneath the periosteum.