Stent Fracture: A Fracture Mechanics Approach - Research Paper Example

of Paper Stents being introduced in human body for treatment of coronary arteries and hence supporting them, have become highly popular in the present times. While the use of stents have proved to be highly beneficial for the purpose of medical treatment; on the other hand, there are threats and concerns associated with the incidences of fractures in the structures, particularly after being implanted into the body of the human. This can not only make the structure ineffective for treatment but also impact the human health. This research has been focused on having an understanding of the stents fracture based on fracture mechanics and hence realizes the different types of fractures in stents, the levels of corrosion and fatigues, and the role of fracture mechanics. Table of Contents Table of Contents 3 1 Introduction 4 2 Literature Review 6 2.1 Stents Fracture and Its Incidence: 6 2.3 Fatigue and Corrosion in Stents: 9 2.4 Stents Made of Steel: Effects of Corrosion on Steel Fatigue: 10 3 Findings and Analysis 11 3.1 Stents Fracture: 11 3.2 Stents Fracture and Fracture Mechanics: Classification: 13 3.3 Detection of Stents Fracture: Corrosion and Fatigue: 14 3.4 Nitinol Alloy in Stents: 15 3.4.1 Stages of Fatigue Crack Propagation in Stents Made from Nitinols: 17 3.5 Fatigue Life Analysis of Stent: 19 4 Discussions: 23 4 Conclusions: 27 References 28 1 Introduction While considering a research on stent fracture, it would be necessary to first understand what a stent is. A stent represents a small mesh tube generally used in patients having weak coronary arteries. Arteries mean the blood vessels that transport blood to and from the heart to other parts of the human body. Angioplasty represents the process that involves placing the stent in the artery [1]. When a stent is introduced within the body, it is in general called stenting. The two most common materials used for construction of stents are metal and plastic mesh-like material. Grafts of stents are made of fabric. These are generally used when there are blocks in the arteries or when the arteries become narrow [2] [3]. Figure 1: Implantation of Stents in Coronary Artery [4]. The above figure presents the process of implantation of stents in the coronary artery. The use of stents has increased owing to the increasing rate of heart diseases, hence requiring the need for angioplasty [5] [6] [7]. A balloon catheter is used by the stent being implanted in the heart. The stent is thin, made of metal, and introduced into the narrowed artery. With inflation of the balloon, the stent gets expanded, and gets permanently fixed in the artery [8]. Stents including within the human body do not create any problem for the patient, unless and until the stent fractures, which is although not very common to happen [9], but there are still records of such incidences, based on which the present research has been concerned. Stents have proved to be highly beneficial to control severity of heart problems [10]. However a major cause of concern is the fracture in stents that can result in disastrous effects in the human body. Such incidences are uncommon and generally occur from some form of complications in the process of implantation [11] [12]. Occurrence of in-stent restenosis (ISR) to neointimal hyperplasia has been attributed to an alternate cause of in-vivo coronary stent fracture. Stent fracture primarily reflects upon an impulsive damage to the structure of stent support exclusive of any simulated process [13] [14]. The most important cause of stent fracture has been associated with excessive mechanical stress on the structure owing to angulations [15]. Figure 2: Stent Fracture [16]. The figure above presents a case where the stent structure has undergone a fracture. It represents a mechanical fault of the structure thereby affecting the durability of the structure [17]. This particular research is based on an understanding of the corrosion and fatigue of stents, focusing on stents analysis and the mechanics of fatigue and fracture in stents structures, based on an extensive literature review of the researches that have been previously conducted on this particular issue and hence overviews the understanding, opinions and views of those researches, to provide for an understanding and analysis for stent fractures. 2 Literature Review 2.1 Stents Fracture and Its Incidence: Although stent fracture is not a very common incidence that occurs, yet there are several reports of the incidence of fracture in the stent structure [18]. Hence not recognizing the level of importance of addressing the issue of fracture in stents cannot be supported, as researchers believe it to be. The use of drug-eluting stents has been made to replace the bare metal stents targeting towards reduction of the rates of restenosis also reducing the requirement of revascularization [19]. According to researchers, there is growing alarm about stent fracture as a prospective reason of stent restenosis and thrombosis, “which can lead to adverse clinical outcomes such as recurrent angina, myocardial infarction (MI), and even sudden death” [20]. The rates of incidence of fractures in stents have been obtained to be different as reported by different researches. This implies that accurate research has not been made on the incidence of stent fracture after its implantation in the human body [21]. The difference rates as could be obtained from different studies can be observed from the following chart. Table 1: Different rates of incidence of stent fractures obtained from different researches [22]. Studies reflect that the incidences of fractures in stents are under recognized [23]. They occur post implantation of the stents owing to severe complications of percutaneous coronary interventions. Earlier it was reported to be 1 percent of the angiograms that followed the process of implantation. However, in the present times, reports of stent fractures of around 7.7 percent to 30 percent are obtained to occur. As researchers obtained, “placement in the right coronary artery, lesion angulation, drug-eluting stents, long stents and longer duration of implantation are all associated with increased prevalence of stent fracture” [24]. Figure 3: Incidence of the Fracture in Implanted Stent [25]. The process of fracture is highly complex and involves “nucleation and growth of micro and macro voids or cracks, mechanisms of dislocations, flip bands, and propagation of microcracks, and the geometry of the material [26] [27]. Fracture mechanics is a field of solid mechanics that is concerned with the cracks and deformations occurring in materials occurring due to stresses and strains [28] [29]. Fracture mechanics is mainly applied for the analysis of cracks and hence trying to solve the problems of cracks in materials, such as stents [30]. In this regard studies have revealed three modes of cracks. These 3 modes of cracks include Opening, Sliding, and Tearing [31]. Cracks might occur due to combined effect of these three modes. If improvement can be achieved in the Mode 1 of the crack, then resistance of the combined action of the modes can be obtained [32]. Figure 4: Three Modes of Cracking in Materials [33]. 2.3 Fatigue and Corrosion in Stents: Corrosion fatigue represents a method in which fractures occur in metals in a premature way where corrosion and cyclic loading occurs simultaneously causing fatigue of the material such as stents [34]. The occurrence of the corrosion fatigue in the material occurs as a result of the development of cracks within the materials that gradually propagates through the material and destroys it. Several factors are involved in causing the stress and corrosion in the materials. These include the interactions of the factors of loading, environment and metallurgy of the materials [35] [36]. Tests of fatigue are usually experiments that are performed for long term periods. Acoustic emissions have been found to have a significant role to play in this regard. Studies reveal that acoustic emissions sensors are highly sensitive in determining initiation of fatigue crack in materials. Movements of dislocation and formation of slip bands can be easily detected by acoustic emissions [37]. Figure 5: Acoustic Emission in Detection of Fatigue in Materials [38]. 2.4 Stents Made of Steel: Effects of Corrosion on Steel Fatigue: When plates of steels undergo corrosion in general, the surface of the material tends to get rough and pitted. An attack is also caused owing to deprivation of oxygen that is named as crevice corrosion. Such attacks are mainly caused in plates of steels that are bolted or butted up against each other. Other factors leading to crevice corrosion include deposition of soil and dust, droppings from birds, or debris that result from manufacture of the steel structures [39]. If the steels are not alloyed, then corrosion is mostly found to occur in the pits of the corrosion, which results in the pits on the surface of the materials. The effect of the corrosion becomes more with the duration of time as propagation of the corrosion gradually gets developed and hence the cracks within the materials keep on spreading causing destruction of the material [40]. Figure 6: Effect of Corrosion in Steels [41]. The micro mechanisms of different fractures in materials represent different variety of physical attributes associated with the material “that affect the properness of a specific fracture toughness parameter to describe that fracture micro-mechanism” [42]. Components that possess high temperature mostly experience multiaxial loading instead of uniaxial loading. The modes of the growth of the cracks in the materials tend to alter from the principle plane of strain to maximum plane of shear as the material experiences increases in temperature [43]. 3 Findings and Analysis 3.1 Stents Fracture: Stents construction is an essential part of medical technologies in the present day with its structure being focused on suitable engineering and construction [44]. Figure 7: A Stent [45]. The materials used for construction of stents have to be such that the structure can be flexible, supportive, expansion being possible, and biocompatible. Since the use of stents are done in medication and the structures are included in the body of patients for purpose of treatment, hence, the structure needs to be comfortable and compatible with the human body, while keeping all factors in focus. In general, stainless steel is used for the construction of the structure. However stainless steel could be found not to be compatible with human body and hence alternatives were researched that includes platforms made of gold, titanium, alloy of cobalt and chromium, tantalum, and nitinol [46] [47]. The use of metals can be traced back to earlier times. It is considered as essential to materials science and engineering. On the other hand, metals are occasionally considered as unsuitable for medical reasons owing to the environmental and health effects that heavy metals tend to have. Sincere emphasis has been placed on the safety of metals when they are used for medical purposes. Substantial attempts are being made towards the enhancement of corrosion resistance and mechanical durability. On another side, the “technological evolution of ceramics and polymers during the last three decades has made it possible to apply these materials to medical devices; as a result, many metal devices have been replaced with those made of ceramics and polymers. In spite of this development, over 80% of implant devices are made of metals because of their strength, toughness and durability [48]. 3.2 Stents Fracture and Fracture Mechanics: Classification: As already mentioned before, the rate of incidences of coronary stents fracture is underestimated. The stent fracture has different aspects that include incidence, classification, predictors, outcome, diagnosis, and management [49]. The classification of stents fracture can be presented through the following figure: Figure 8: Fracture Classification of Stents [50]. As can be obtained from the figure the classification of stents fracture can be done in no fracture, mild fracture, moderate fracture, and severe fracture. There are several factors that lead to the fractures in the stents structures. These include technical factors such as overexpansion, or improper handling, or too many forces on the structure. The type and conformability of the stent structure also determines its chances of being fractured [51]. Fracture and fatigue in stents are obtained in many cases as millions of people are now treated with implantations of stents within their bodies. 3.3 Detection of Stents Fracture: Corrosion and Fatigue: “Corrosion is a surface phenomenon greatly influenced by different media-related factors (chemical, electrochemical and physical) in which the material is placed” [52]. As could be obtained from the previous sections of the report, it is clear that fracture and fatigue of the structure needs to be identified and detected such that propagation of cracks can be prevented. If stents are implanted within the body, it can be understood that such fractures in the structures can affect the human body as well. And hence fracture mechanics has a significant role to play in understanding the nature of fracture and crack that could initiate within the structure and hence make a stress and strain analysis as needed [53] [54]. Figure 9: Stent Fracture [55]. With fracture mechanics involved, the intensities of the stresses and strains can be computed for the structure and hence the cracks and fractures within stents can be detected to either remove it from the body or to repair it at the earliest preventing the further propagation of the crack. Coronary angiography is commonly used for the detection of fracture in stents. Other imaging methods have also been obtained in the present times for the purpose of detection that include plain fluoroscopy without contrast injection. High resolution is used for better imaging and detection of the fractures [56]. Figure 10: Follow-up X-ray showed transverse linear fracture and displacement (type IV stent fracture) [57]. 3.4 Nitinol Alloy in Stents: Nitinol alloy is a material of stent, on which improvement is being focused for designing for successful implantation in the body. The nitinol material is superelastic in nature thus being a suitable material for superelastic stents for medical uses [58]. Nitinol alloys are considered as the most commonly used material for stenting that is said to be self-expanding in nature and is capable of providing increased elasticity and radial force that allow the stent to become accustomed to the shape of the vessel. However, “specific stresses and physiological environments can lead to stent fracture” [59]. Figure 11: Advantages of Biased Stiffness in Nitinol [60]. As a result of such a structure, the stent remains open to a relaxed state of the material. There is an increase in force when the stent gets compressed into the catheter. Again there is a decrease in the force when the stent is organized at the point of constriction. Fracture mechanics is useful for this structure as it helps in providing a design for the suitable construction of the stents. As cracks are always found within materials, hence significant toleration of the components for such cracks is extremely essential [61]. The relevance of the fracture mechanics based on the theories and concepts discussed earlier in the study with stents lies in the way such mechanics leave very less scope for increased propagation of cracks within the materials. Hence the chances of fracture also reduce within the structure making it more useful and dependable for the treatments in cardiovascular diseases. As already discussed, initiation of the crack and the propagation of the crack within the structure are the two main factors of fracture in a material that can be studied by fracture mechanics, which is why it is also used in stents analysis [62]. 3.4.1 Stages of Fatigue Crack Propagation in Stents Made from Nitinols: Figure 12: Stages I and II of Fatigue Crack Propagation [63]. Stage I of the process reflects the initiation of the crack in the material. The propagation of the crack occurs through the planes of high shear stresses. It is the stage of short crack growth propagation. The propagation continues for some time till it starts decelerating by a barrier of microstructure where the initial direction of the growth of the crack cannot be accommodated. Stage II is the stage of the process when the intensity factor of stress, K, increases owing to the growth of the crack or application of excess loads on the material and the crack starts propagating to different planes in the material. Stage III represents the unstable stage of the propagation of the crack within the material. The structure at this stage becomes highly sensitive to the ratio of the load and the microstructure and the state of stress of the material [64] [65]. Figure 13: Micro-mechanism of Fatigue [66]. Although Nitinol is largely used in stents, it has been obtained that such structures of stents have low margin of safety. Calculation of the margin has also not been effectively achieved as of yet. The resistance of fatigue of the material has been found to be capable of being altered through treatments of heat and surface finishing. This is possible as the material is highly thermal-mechanical coupled [67] [68]. 3.5 Fatigue Life Analysis of Stent: “Fatigue is the process of progressive, localized, permanent, structural change occurring in a material subjected to conditions which produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture, after a sufficient number of fluctuations” [69]. The fatigue life of stents can be analyzed with the basics of fracture mechanics. This can be understood from the following two figures – one reflecting the structure of a stent and the second presenting the finite element model of stent [70]. Figure 14: A Stent before its Expansion [71]. Generally such structures are formed of solid tubes of stainless steel. A lattice structure is formed by the walls of the tube of the stent with the use of laser. The surfaces of the materials are then fixed. When the structure is inserted into the body of the patient an internal balloon expands the lattice which is removed from the structure after some time. Studies reveal that failure of the stent could arise from the loading of fatigue that occurs from the beating of the heart of the patient. This has made it necessary to construct highly compatible stents [72]. Figure 15: Finite Element Model of Stent [73]. The finite element model of stent is based on fracture mechanics that studies and analyses the application of radial pressure on the stent structures caused due to heart beat. The forces and loading experienced by the stent are analyzed to determine the ability of the stent to withstand that load such that cracks do not initiate and propagate. The design presented in the above figure has proved to be effective for predicting a level of endurance for the structure to enable understanding of the acceptability of the structure [74]. Figure 16: Detection of Nitinol Stents in Fatigue Testing [75]. Research has found that fractures in stents do occur. These are represented through angiography. Most of the stents occur in any segment of the structure, particularly in the distal segment. Hence the distal region causes most of the concern in relation to stent fracture. Thus challenges are presented by the incidences of such fractures that need prior detection, which is the main cause of fracture mechanics being incorporated in the system [76]. Considering the case of nitinol tubes in stents, in-vivo loadings are experienced by the structure that lead to overload and fatigue of the structure. Such fatigue conditions are presented in the form of fractures causing damage to the stents. The structure thus becomes ineffective in its performance. The following Kitagawa-Tagahashi diagram present the fatigue in the stent structure formed of nitinol and hence the fracture caused from overload [77]. Figure 17: Fatigue and Overload Fracture in Nitinol Stents [78]. For the above figure a generic calculation was obtained for measurement of the intensity of stress on the structure. Thus the geometry factor, Y was set to unity, resulting in the relation: K = Ya [79]. 4 Discussions: Stenting in coronary artery have become a common measure in medical treatment of problems related to the heart. While the use of stents has proved to be effective in treatment of the heart, there are major concerns related to the fractures that occur in stents. Stress and biochemical forces have been found to be main causes causing fractures in stents at different sites of implantation of the stents in the human body [80]. As could be obtained, stent fracture is a complication as considered by medical experts leading to thrombosis and restenosis. 1 to 2 percent of patients who have undergone the implantation of stents in their bodies have been found to have been affected by stent fractures, which could be revealed during follow up of angiography [81]. Although replacement of bare metal stents is being initiated to improve upon the conditions of fractures and reduce effects such as restenosis. However, the concerns have increased over the recent years with studies and research being focused on bringing the issue of stent fracture in front, something that was earlier underestimated. Stent designs are prepared and implanted in the human body depending upon the situation required for the patient. There are mathematical models involved that enable understanding of the behavior of stents in the body, which is also essential to determine the possibilities of fractures in the structure that is implanted [82]. Stents fracture has been associated significantly with the occurrence of corrosion and fatigue in the stent structure, as the implantation of the stents in the human body results in significant amount of external forces. Hence restenosis has been found to be resultant from stent fracture. It is thus a target of the manufacturers of stents to manufacture the structure in a way such that the rates of fractures in the structures are near to zero. Over the recent years, with better understanding and records of occurrence of stents fracture, greater emphasis has been placed on the manufacture of stents trying to reduce its chances of experiencing fractures, particularly after the structure has been implanted in the body. It is the present times that it has been noted and significantly taken into consideration that stents fracture can not only make the structure ineffective for treatment of the heart, but also can negatively impact the human body. Thus while construction of the stents is done, certain factors are necessary to be considered with effect in order to determine that fractures do not occur in the structure. This includes selecting the correct diameter for the structure such that stability of the stent can be achieved. This requires keeping in focus that there would be continuous pressures from the walls of the heart that could lead to excess forces being applied on the structure and hence fracture. The clinical implications of stents fracture have caused the major cause of concern for cardiologists and physicians. It is still a difficult measure to manage the fractures in stents. Prior study of the particular human body and the detection of possible chances of fracture are extremely essential to prevent stents fractures. Relating the issue to fracture mechanics, it can be said that fracture mechanics holds an essential feature of stents fractures that can be used to detect the incidence of fracture in the stents. It can be understood from the findings that once a fracture or crack occurs in the structure, it is bound to propagate and spread across the structure and gradually break the entire structure, which can be fatal to the human body. It has been generally obtained that patients experiencing minor fractures in the implanted stents do not always reflect symptoms [83]. Hence it becomes difficult to realize from outside that the stent has been fractured slightly that would eventually propagate and damage the complete structure. It is due to this fact that detection through imaging is essential. High resolution digital images can be used as obtained from the study, to determine whether the stents are experiencing cracks and fractures. If these can be detected at an early stage, before the fracture damages the structure completely, then repair or removal of the stent from the body can be done such that the patient can remain unaffected. Although the fracture in stents does not lead to loss of human lives, yet such an act is neither desirable in medical sciences nor are accepted by cardiologists and physicians. Fatigue and corrosion in stents have become common owing to the fractures occurring in the structures. As these lead to ineffectiveness of the structure and then the purpose of the structure cannot be achieved, hence in the present times, assessment of stents is currently done on the basis of testing for survival. This implies that before the stents are implanted in the human bodies, they are tested to determine whether the structures are capable of surviving the life cycle that has been planned and designed for the structure. For example, if a stent with a life cycle of 108 is required and planned, then assessment is done to test whether the structure can survive the 108 life cycles [84]. However it can be said that such a method of assessment is not adequate for determining the chances of incidence of fractures in the stents in the future. For instance, when the test is being done, the structure may reflect positive results and performances thereby allowing physicians to be relieved. However, in the near future there might be fractures in the structure, which cannot be realized from this method. Hence, this method cannot be considered to be efficient for prevention of fractures in stents. Instead, fracture mechanics that allows detection of fractures or cracks in the materials can be said to be a better option for this study. Fracture mechanics, involving detection through imaging allows high resolution images being taken of the structure that would clearly enable viewing the incidence of fracture in the structure and its gradual propagation to realize the state of the fracture such that repair or removal may be possible. The factors of stress and intensity on the structures have been associated with the occurrence of fatigue and corrosion in stents. The study of the loads on the structure enable determining the flaws of different lengths in the structure and hence the flow of the propagation of the fracture within the stents. The severity of the flaw in the structure and hence the growth of the crack helps to realize the life time of the stent that is left for the structure to serve the purpose of treatment. If the crack has grown to an extent that the entire structure will be damaged, then it can be realized that the stent would not be effective any longer on the treatment of the heart. It can be obtained and analyzed from the study that the physical and mechanical properties of the stents need to be considered effectively before construction and implantation of the structure in the human body. The materials also need to be chosen effectively. For example, as the study reflects, although the use of stainless steel is made for the construction of stents; yet stainless steel is not considered as suitable for the human body. An in-vivo environment is generally used for the analysis of the stress on the structure. In order to effectively serve the purpose of treatment, the stents need to be stable and consistent and hence fractures are not desired. It is for this reason that importance of fracture mechanics has been realized enabling detection of cracks in the material and prior detection helping in prevention of the spread of the fracture in the structure. Determination of the strength of the structure to endure fatigue and corrosion can be considered as essential for the stents to perform effectively without the fractures affecting the functions of the stents. The use of fracture mechanics and the use of simulation processes can be effectively done towards realizing the state of fracture that has occurred or might occur in the structure and hence take the necessary steps accordingly. 4 Conclusions: The study has been based on having an understanding of the concept of fracture mechanics, and its application on analysis of stents and stents fracture. It could be obtained from the study that stents fracture is a cause of significant concern in medical fields as it is used for coronary artery treatment and support. The causes, types and classifications, and corrosion and fatigue of stents have been discussed in the study. It implies on the necessity to detect fractures in stents such that damage in the stents can be prevented, which is essential to help prevent negative effects on the human health as well as to prevent the ineffectiveness of the structure. Fracture mechanics provide a highly significant method of studying and analyzing initiation and propagation of cracks within materials. It is extremely essential to study the initiation and propagation of cracks in order to prevent damage to the material once it is used for construction and use. Structures like stents that have been primarily focused in this study are used for implantation within the human body for purpose of treatment. Hence it is essential that such structures do not suffer from fractures, which otherwise might prove to be harmful for the body. Corrosion and fatigue of stents formed of steel, or nitinol, as they experience different environments, different compositions of materials, and hence prior analysis is extremely essential. Owing to different stresses and strains on the structure, cracks are formed, which if ignored, can rapidly propagate within the structure destroying its properties and functioning. Prevention of the cracks and prevention of the propagation of the cracks is possible through fracture mechanics as it provides with a detailed analysis of the structure and the possible design that can enable crack prevention in such structures. Fractures in stents are studied and analyzed before implanting the structure in the body and this has been made possible with fracture mechanics. Stents tend to experience loading and pressure from the heart beat, particularly when the implantation of the structure is done in the body. As a result, cracks can be expected to occur which would eventually propagate in the structure destructing its normal functioning. Hence the application of fracture mechanics is essential and is being involved for analysis of the stents to prevent fractures of the structures. As could be obtained from the study, application of stents has become essential for the treatment of heart problems. Hence, learning that the structure can undergo fractures owing to excessive pressure and angulations, it would be necessary to detect such fractures at the earliest such that preventive measures can be taken. Physicians and cardiologists need to be aware and sincere of the fact that a fractured stent within the body of the human can be fatal to the individual. Thus detection through fracture mechanics need to be considered effectively towards prevention, repair, or removal of the stent from the body depending on the occurrence of the fracture. References Read More