Engineering design GE
Table of contents
1.0. Abstract…………………………………………………………………………………4
1.2. Introduction……………………………………………………………………………..4
1.2.0. Valvular disease………………………………………………………………4
1.2.1. Causes of valvular diseases…………………………………………………..5
1.3. The design intent of the Bjork-Shiley Heart Valve (what need or function did it satisfy, what was its design purpose?)……………………………………………………………………6
1.3.0. Mechanical design……………………………………………………………6
1.3.1. Design purpose or function it was intended to satisfy……………………….8
1.3.2. Mechanical heart valves that were present in the market, the competition they imposed and how Bjork-Shiley Heart Valve differs with them…………….8
1.4. Problem with the Bjork Shiley heart valve…………………………………………….9
1.4.0. Nature of the problem and what it meant…………………………………….9
1.4.1. Testing procedure used to understand the problem………………………….10
1.4.2. Concluding remarks of the company………………………………………..14
1.4.3. How they arrived at these conclusions………………………………………16
1.4.4. Ethical issues…………………………………………………………………17
1.5. References…………………………………………………………………………….18
Abstract
The Bjork Shiley heart valve was founded and clinically utilized in late 1969. Between the years 1969-1981, at least 24,000 Bjork Shiley heart valves were implanted in patients with heart problems. Despite that, it was realized that these mechanical valves ended up providing
quiet and low profile prosthesis with exceptional hemodynamics. Because of the fracturing of inlet struts, there have been only two reported cases regarding the mechanical failure of this device. According to the recent medical reports concerning the wear of the BS valve discs, a scientific advisory panel had been established to review and validate the general status of the Bjork Shiley Derin (BSD) tilting heart valve. In return, such a strategy is perceived to be the one that could enable physicians to make sound recommendations regarding the effectiveness of BSD heart valves implanted to patients. Therefore, the purpose of this paper will entail presenting the details collected regarding the scientific effectiveness of BSD heart valves and those that were initially recommended by the panel. On the other hand, explanted Bjork Shiley Derin (BSD) heart valves that could have been returned to Shiley, experimental publications of the Bjork Shiley Derin (BSD) heart valves, BSD heart valves that were subjected to hasten wear tests, test reports, unimplanted BSD heart valves stored in Shiley inventory, and so on are some of the materials that will be used to validate the effectiveness of this device to patients.
Introduction
Valvular disease
Usually, clinical researches regarding the development of human valvular diseases involve two main conditions, especially valvular insufficiency and valvular stenosis. Valvular insufficiency also termed as incompetence or regurgitations evolves whenever an individual's valves do not close normally. When the valve does not seal properly, there is the leakage of blood across the heart valve. As the leak continues to worsen, the heart is forced to work harder to counter the effect of the blood that is lost as a resulting of the backward leaking of blood (Rajamannan, 2014 p.21). In return, little blood is supplied to the body. Taking into consideration the valve that is affected, such a condition is termed as aorta regurgitation, pulmonary regurgitation, tricuspid regurgitation, or mitral regurgitation. On the other hand, valvular stenosis evolves whenever the patient's opening valves become relatively smaller than normal because of the fusing or stiffness of the leaflets. Such a scenario is what results in heart failure. The hardening or restriction of blood flow through the valves is termed as aortic stenosis, pulmonary stenosis, tricuspid stenosis, or mitral stenosis (Wang & Bashore, 2009 p.1). As a result of the narrowing of the opening valves, the heart works harder so as to pump blood through them.
Causes of valvular diseases
Human valvular diseases have been realized to commence developing before birth. A person can also acquire it sometime during a lifetime. Although medical research stipulates that the main cause of its development is not yet known, congenital or birth heart valve diseases have the propensity of affecting the pulmonary or aortic valve. At times, the heart valves might have malformed leaflets, be of the abnormal size, or having leaflets not correctly attached to the annulus. The failure to treat bacterial infections is what results in rheumatic fever. The use of antibiotics to treat bacterial infections is what is perceived to have the likelihood of reducing it. Although initial bacterial infections mainly occur in children, the heart problems that are linked with it can only be diagnosed when an individual is aged between 20-40 years (Newton et al., 2011 p.43). At this age, his or her heart valves could have been inflamed, thus making leaflets to fuse and become shortened, rigid, thickened, and scarred. Such a condition results in mitral regurgitation.
Additionally, endocarditis evolves in an individual when germs, practically bacteria, enters his or her bloodstream and affects the normal functioning of the heart valves. The alteration of the normal functioning of the heart valves is what causes the growth of holes in them. The entering of germs into an individual bloodstream has been realized to be caused by IV drug use, surgery, dental procedures, or because of severe infections. Individuals having heart valve ailments are perceived to be at a higher risk of developing life-threatening mitral heart valve infections (Tisdale, 2010 p.554). As a result of that, the dilation of the annulus, stretching or tearing of the papillary muscles, or stiffness of the valve leaflets can occur. Human valvular diseases, especially MVP (mitral valve prolapsed) is one of the common condition that affects 1 to 3 percent of the entire population. During heart contraction, MVP has been realized because mitral heart valve leaflets to collapse back to the left atrium. The alteration of the normal functioning, stretching, and the leaking of the left atrium valves is as a result of MVP (Runge et al., 2010 p.493). Connective tissue ailments, syphilis, cardiomyopathy, aortic aneurysms, hypertension, heart attack, and coronary heart diseases are some of the most commonly known causes of heart valve disease.
The design intent of the Bjork-Shiley Heart Valve (what need or function did it satisfy, what was its design purpose?)
Mechanical design
The Bjork Siley heart valve is perceived to be one of the mechanical human heart valve prostheses. From the year 1971, this device was developed for the purpose of replacing mitral or aortic valves. It marked one of the first examples of clinical devices that could be successfully utilized for tilting disc heart valves. The Bjork-Shiley heart valve comprises of a carbon-coated disc that is enclosed by tantalum housing. An outflow, inflow, and two metals struts are also used to hold the disc firmly. Ideally, the standard design of this device makes use of the materials that are long-lasting (Anderson et al., 2012 p.1073). Chromium cobalt alloy also termed as Haynes 25 is the main housing material used to manufacture them.
Figure 1: The Bjork-Siley heart valve
Due to the fact that the use of the plano-convex design was noticed to be ultimately susceptible to thrombosis, such a device was improved with the use of the convexo-concave (CC) design. The reason for considering the CC design is because it was proved to have the propensity to speed up its manufacture as well as reduce the formation of thrombus. Within the CC metal housing, such a device comprised of a tilting disc coated with carbon and held in place firmly by an inflow, outflow, and two metal struts. Although the outlet strut of the CC device is welded separately, the inlet strut is flushed with metal flange (Ratner, 2013 p.1376). The size of the metal flange ranges from 21-33mm with an opening angle ranging from 60-70 degrees. The suture ring of the CC device is made of Teflon. The need to sew of the suture ring to the cardiac heart tissues is what assists in ensuring that the valves are held firmly in place.
Design purpose or function it was intended to satisfy
The Bjork-Shiley Heart valve was manufactured to replace heart valves that could have been damaged. Traditionally, artificial heart valves were extensively used for the purpose of replacing diseased or defective heart valves. Although from time to time numerous heart valves have been manufactured they differed greatly in terms of the number of leaflets, materials used, and valve geometry. In the modern medical world, the implantations of artificial heart valves in patients have declined because they induce numerous health complications, such as hemolysis (Anderson et al., 2012 p.1073). For the case of mechanical heart valves, such complications are only believed to be linked with non-physiological flow of blood. Nevertheless, the evolution of mechanical heart valves, especially the Bjork-Shiley Heart valve is perceived to provide superior hemodynamics with relatively lower aerodynamic resistance (Baura, 2006 p.100). Ideally, the development of the Bjork-Shiley Heart valve was anticipated to have the propensity of providing effective and quick relief with least complications to patients with ailing native heart valves.
Mechanical heart valves that were present in the market, the competition they imposed and how Bjork-Shiley Heart Valve differs with them
Initially, there were various mechanical heart valves that were used to replace damaged heart valves. They include the Starr-Edward ball valve, Carpentier-Edwards, Hancock porcine prosthesis, and Bovine pericardium. As compared to these mechanical valves, the use of the Bjork-Shiley heart valve was found to reduce the survival rates of patients because of the failure of such valves. One of the factors that made them be preferable is that the use of the Bjork-Shiley Heart Valve increased the episodes of bleeding once the defective heart valves were replaced with it.
On the other hand, the use of mechanical heart valves was associated with the effectiveness of reducing embolism. In this case, minor embolism is regarded to be episodes of brief neurological deficiency or any other medical events that affect the normal functioning of the heart valves. Major embolism also occurs as a result of a residual neurological deficiency or limb ischemia that requires surgery. The Bjork-Shiley prosthesis differs from the other mechanical heart valves because it consists of a graphite disc that is insulated with pyrolite carbon. This mechanism makes it tilt easily between the two struts contained in its housing (Baura, 2006 p.100). Furthermore, the use of titanium or stainless steel to manufacture the housing makes it to be long-lasting. The modification of the original Bjork-Shiley heart valve was to enable it to increase its opening angle as well as change graphite disc into a CC model or a convexo-concave shape.
Problem with the Bjork Shiley heart valve
Nature of the problem and what it meant
Outlet strut fractures (OSF) is one of the main problems that made the use of Bjork-Shiley heart valves to diminish. Once the deceased heart valves have been replaced with this device, after few months, t was found out that one end of the strut could fracture which is later preceded by the fracturing of the other end. The fracturing occurs as a result of brief and sudden impacts on the outlet strut connections. The fracturing of the outlet connections also occurs when the closing disc over-rotates for at least ten times so as to force the disc to open. This in return creates bending stress beyond the capacity that strut's survival limit (Topol & Califf, 2007 p.396). After multiple occurrences of the outlet strut tip overloading, it causes fatigue-induced fractures. The failure of the valve makes the disc to be disconnected from it hence resulting in the unregulated flow of blood. In case that malfunctioning is not detected immediately, it leads to cardiac death.
Although the majority of these fractures have been realized to occur during premarket tests, one of the assumptions that were given was that it could have been brought by strut welding. Conversely, according to the views of Shiley Company, those failures were accidental. Because of that lower risks of thrombus in the newly designed heart valves were much more effective as compared to the small chances of the Outlet strut fractures (OSF). Even though the actual failure mechanisms were not yet known, the use of the Bjork-Shiley heart valves was approved by the FDA (Food and drug administration). The company was mandated by the FDA to provide a detailed report regarding any problem that could later arise in the process of using these valves to repair diseased or ailing valves (Aura, 2006 p.105). Regardless of those directives, it was discovered that patients were implanted with faulty Bjork-Siley heart valves.
Testing procedure used to understand the problem
During the testing and validation of the problems associated with the Bjork-Shiley heart valve, 533 patients who had previously undergone heart valve replacement and who were qualified to receive warfarin were sampled randomly. Those who were sampled were the ones who were to be implanted with a mechanical Bjork-Shiley disc valve that tilts at 600 or 700. The Bjork-Shiley spherical disc heart valve advanced the CC (convexo-concave) model which was first introduced in the year 1979. The reason for that is because the CC model had subsequently proven to have a relatively strut fracturing rate. Out of the 533 patients randomly selected, 261 of them were subjected to mitral valve replacement while 211 of them were treated with aortic valve replacement. Despite that, both valves were replaced in 61 patients while 8 patients who had could have undergone extra tricuspid heart valve replacement were eliminated from further examination or analysis. Out of the total number of patients who were studied, 267 of them received the Bjork-Shiley prosthetic implants.
Figure 1: Number of outlet strut fractures (OSF) against years of outcome
Y-axis
80
70
60
50
Frequency
40
30
20
10
X-axis
1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002
Year
The patients’ age and clinical characteristics were recorded. During heart valve replacement, the patients’ mean SD (standard deviation) age was 54.3 (10.3) years and 53.8 (10.5) years respectively. The two treatment groups selected for analysis were comparable with other pre-treatment variables. Overall 56 % (295) of the study patients were female and 7.4% (39) of them have previously received heart valve replacement. At least 8% of them have reported suffering from ischaemic heart complications.
Table 1: Heart valve-related risk factors for outlet strut fractures (OSF)
|
Risk factors (RFs) |
Approximated risk rate (RR) of outlet strut fractures (OSF) |
Approximated number (percentage of Bjork-Shiley heart valves with attribute) |
Approximated number (percentage of OSF heart valves with attribute |
|
Angle |
|
|
|
|
700 |
5.1 |
3900 (5.1) |
153(23) |
|
600 |
1.1 |
81750 (94) |
479(75) |
|
Size in mm |
|
|
|
|
32 |
9.2 |
1650(2) |
57(9) |
|
30 |
5.4 |
10250(11) |
204(32) |
|
28 |
4.0 |
14850(17) |
180(27) |
|
26 |
2.7 |
32150(37) |
154(24) |
|
24 |
1.1 |
26450(30) |
33(4) |
|
Weld date |
|
|
|
|
<1980 |
1.1 |
7550(8) |
34(4) |
|
1980 |
0.4 |
18350(21) |
44(6) |
|
1981-1982 |
0.5 |
33100(38) |
462(74) |
|
1982-1985 |
1.5 |
18450(21) |
88(13) |
|
>1985 |
0.1 |
7500(8) |
0(0) |
|
Shop order |
|
|
|
|
<1.1% |
1.0 |
69750(80) |
230(35) |
|
1-6% |
1.8 |
12050(13) |
246(38) |
|
>6% |
2.3 |
3750(3) |
154(23) |
|
Welder group |
|
|
|
|
A or B |
1.1 |
70750(81) |
390(62) |
|
C |
1.6 |
15150(19) |
241(39) |
|
Position |
|
|
|
|
Aorta |
1.1 |
47150(54) |
137(21) |
|
Mitral |
2.4 |
38150(44) |
495(78) |
|
Rework status |
|
|
|
|
No rework or strut cracks |
1.1 |
78150(90) |
537(84) |
|
Rework or strut cracks missing |
1.5 |
7650(8) |
94(14) |
Nonetheless, from the testing conducted, it was found out that 50% of all the patients who were undergoing mitral heart valve replacement had initially suffered from mitral valvotomy. The age and the clinical conditions of surviving patients are also described in table 2 below. From the microscopic examination conducted, it was found out that there were no signs of structural defects of the explanted prosthetics. Only shallow abrasions were the ones that were noted on the heart's ventricular surface. From the data collected regarding the re-operative procedures recommended, it is found out that the mortality rate fluctuates or varies with age with younger patients experiencing lower mortality rates as compared to older patients.
Table 2: Risk rates (RR) of outlet strut fracture (OSFs) incidence against patient age at clinical follow-up
|
Age |
Number of outlet strut fractures (OSFs) |
% incidence rate per year |
95% confidence interval (CI) |
|
>45 |
31 |
0.27 |
0.18-0.38 |
|
45-53 |
22 |
0.14 |
0.08-0.20 |
|
54-63 |
40 |
0.14 |
0.09-0.18 |
|
64-73 |
12 |
0.03 |
0.01-0.06 |
|
<73 |
2 |
0.02 |
0.00-0.03 |
Concluding remarks of the company
From the test conducted, it was realized there was the possibility of outlet strut fractures (OSF) occurring the patient's diseased heart valve are replaced with the BSCC valve. Therefore, the company recommended that any patient with more than one of these valves ought to familiarize themselves with the symptoms associated with heart valve or valves that are not functioning normally. In case the outlet could have fractured, the normal clicking sound that is produced whenever the disc closes or opens ceases (Nelson et al., 2009 p.251). Other symptoms that are associated with these conditions include loss of consciousness, shortness of breath, rapid or irregular heartbeat, sharp chest pains, and so on.
In case a patient experiences any of these symptoms, it is recommended for him or her to consult a doctor immediately. His or her condition can be examined using chest X-rays so as to view and determine the state of his or her heart valve. Likewise, for the patients who could have received the Bjork-Shiley heart valve and experienced new fractures, it is vital for them to make regular consultations with their physician (SYMPOSIUM-FATIGUE AND FRACTURE OF MEDICAL METALLIC MATERIALS AND DEVICES et al., 2007 p.77). The annual fracture rates of patients should also be approximated based on the position of valve implant, welder identity, weld date, valve size, accurate age of the patient, and so on.
How they arrived at these conclusions
The testing conducted made FDA (food and drug administration) to be aware of the multiples incidences of fracturing of the outlet strut of the BSCC heart valve. As a result of that, directives were given to Shiley Company to ensure that they regularly notify patients who were legible to receive CC valves about such a problem. Shiley Company decided not to recommend patients to undergo surgery as a therapeutic means for replacing the defective valve or valves. The reason underlying such an option is because of the anticipated re-operative risks associated with the replacement of those valves were relatively higher as compared to that of the fractured intact valve (Topol & Califf, 2007 p.396). Again, the company was required to ensure that they have notified all patients about the increased health risks of outlet strut fractures (OSF) depending on valve specifications and patient condition.
Furthermore, the report published after testing the validity of the Bjork-Shiley heart valve mandated the company to ensure that they have advised all patients about the importance of consulting their doctors about the newly discovered fractured figures and the challenges associated with valve replacement. In return, the company accepted the implantation of faulty valves in patients globally (Nelson et al., 2009 p.251). Although such recommendations were not clinically effective, the company accepted to pay $10 million more to cater for the medical expenses the government could have initially incurred or is anticipated to incur in the future because of the replacement of defective Bjork-Shiley heart valve in patients.
Ethical issues
One of the ethical issues surrounding the use of the Bjork-Shiley heart valve is disc abrasion. Disc abrasion is perceived to be the initial step towards prosthesis heart valve dysfunction. Although it recommended that this condition be subjected to echocardiographic diagnosis, the whole process is challenging. The relatively high flow of blood across the prosthetics accompanied by shear-stress forces is the ones that have also been realized to make the causes of disc abrasion to be unclear. Despite that, the disc connection with the strut is another issue that ought to be taken into consideration (Topol & Califf, 2007 p.396). The reason for that is because the Bjork-Shiley heart valve has been realized to be prone to strut fracture which makes such a device not to function as anticipated.
Furthermore, for patients who could have received the Bjork-Shiley prosthetic heart implants, bleeding occurs more often. Regular bleeding episodes frequently occur in patients receiving either mitral valve or aortic valve groups. Despite that, there exist no considerable differences in patients receiving both the aortic valve and the mitral valve. When both minor and major bleeding episodes occur, the risks of bleeding after the patient's damaged heart valves have been replaced with Bjork-Shiley heart valves are relatively higher (Ratner, 2013 p.1376). Such a higher risk of bleeding has been realized to have a likelihood of occurring in a patient with deceased or ailing aortic valve as compared to those undergoing mitral heart valve replacements.
Conclusion
The information presented in this essay suggests that medical regurgitations arising from wear of the BSD disc might arise in some heart valves after several years of implantation. From the engineering perspective, it means that there exists no solid reason to deduce an increasing rate of failure of this device because of fatigue or fracture. At least all of the information collected suggests that Bjork-Shiley tilting heart valve has the propensity of continuing to provide the required medical aid to patients. In case it occurs, at the slowest rate, BS disc wear offers enough time for not only diagnosis but also for non-emergency therapy. The same information stipulates the fact that it will not be logical to remove the Bjork-Shiley tilting heart valve prophylactically. As a result of that, it is important for clinicians to ensure that the treatment of patients should be based on individual requirements as well as according to the functionality of their heart valves.
References
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