39 results
12 - Commentary on Boyles v. Kerr
- from Part IV - Negligence and Vicarious Liability
- Edited by Martha Chamallas, Ohio State University, Lucinda M. Finley, University at Buffalo, State University of New York
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- Book:
- Feminist Judgments: Rewritten Tort Opinions
- Published online:
- 28 November 2020
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- 10 December 2020, pp 261-290
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Summary
Boyles v. Kerr exemplifies judicial reluctance to permit tort claims for negligently inflicted emotional harm. Severely curtailing prior cases that permitted claims for negligent infliction of emotional distress (NIED), the Texas Supreme Court rejected the claim for a woman whose sexual partner surreptitiously videotaped their intercourse and displayed the tape at college parties. The feminist rewritten opinion reverses this ruling, making the NIED claim fully available to the victim of revenge porn. It demonstrates gender bias in the court’s conflicting precedents, which approved NIED claims for women comporting with traditional notions of femininity, while denying the claim for the sexually liberated plaintiff. It powerfully asserts the value and dignity of all women and the importance of tort law in deterring callous male behavior that objectifies women. The accompanying commentary situates the case in the development of NIED law and highlights how NIED law is permeated with gender stereotypes.
4 - States’ Rights and State Wrongs
- from Part II - States, Federalism, and Antipoverty Efforts
- Edited by Ezra Rosser
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- Book:
- Holes in the Safety Net
- Published online:
- 05 September 2019
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- 01 August 2019, pp 91-109
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Summary
A resurgence in work requirements for safety-net programs, including the Supplemental Nutrition Assistance Program (SNAP), has marked the early years of the Trump administration. Some lawmakers at both the federal and state level have moved to revive and expand SNAP’s work requirements, despite evidence that such work requirements do little to increase self-sufficiency or improve long-term economic outcomes among those living in poverty.This chapter takes up the issue of work requirements in the context of rural communities, where the need for safety-net programs and food system supports is acute. We begin with a brief overview of SNAP and examine the recent push to make SNAP work requirements more strict.We then turn to an overview of the need and current state of use of the social safety net in rural America. If work requirements are to be effective – and, indeed, appropriate – work opportunities must be available.We, therefore, consider employment data and information on safety-net use across the rural-urban axis. Finally, we present a case study about the results of relatively early efforts to impose work requirements on SNAP receipt in Maine.
19 - Planned Parenthood of Southeastern Pennsylvania v. Casey, 505 U.S. 833 (1992)
- from Part II - The feminist judgments
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- By Macarena Sáez, Fellow in International Legal Studies and the Director of the Center for Human Rights and Humanitarian Law at American University Washington College of Law., Lisa R. Pruitt, Professor of Law at the University of California.
- Edited by Kathryn M. Stanchi, Linda L. Berger, Bridget J. Crawford
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- Book:
- Feminist Judgments
- Published online:
- 05 August 2016
- Print publication:
- 02 August 2016, pp 361-383
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Summary
After Roe v. Wade, the polarization of supporters and detractors of the right to abortion resulted in both legislative restrictions on abortion and legal battles to protect Roe. The U.S. Supreme Court had several opportunities to revisit Roe, and each time the Court upheld it, albeit weakening its foundations. Between 1988 and 1989 Pennsylvania's legislature passed five amendments to the Pennsylvania Abortion Control Act of 1982. The provisions required that women must give express consent and receive specific information at least 24 hours prior to the procedure; underage girls must obtain parental consent or judicial authorization; married women must sign a statement indicating that they had notified their husbands; and abortion clinics must comply with a series of reporting requirements. In Planned Parenthood of Southeastern Pennsylvania v. Casey, the U.S. Supreme Court upheld all but the spousal notification requirement. It even reversed sections of prior decisions to uphold the information and waiting-period requirements.
Casey came as a surprise to conservatives and liberals. It reaffirmed Roe, clarifying what the majority viewed as its “essential holding”: (1) recognition of the right to choose an abortion before viability without undue interference from the State; (2) “confirmation of the State's power to restrict abortions after fetal viability,” with limited exceptions; and (3) recognition of a state interest from the outset of the pregnancy in “protecting the health of the woman and the life of the fetus that may become a child.” Casey relaxed Roe's standard of review by moving from strict scrutiny to “undue burden.” Under the new standard, the means chosen by states to further their interest in potential life pre-viability “must be calculated to inform the woman's free choice, not hinder it.” Given that Casey upheld all but the spousal notification, the bar for what constitutes a “substantial obstacle” was left very high. Casey also ended the trimester system, and viability became the dividing line between women's liberty to choose and the right of a state to ban abortions. As a result, the timeframe during which women have access to abortion is more uncertain than Roe envisioned.
Although in practice Casey reduced women's opportunities to access safe abortions by giving more deference to states’ interest in potential life, parts of its reasoning supported women's citizenship and equality, something Roe did not do.
Mechanics of biomaterials: Fundamental principles for implant design
- Lisa A. Pruitt, Ayyana M. Chakravartula
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- Journal:
- MRS Bulletin / Volume 37 / Issue 7 / July 2012
- Published online by Cambridge University Press:
- 12 July 2012, p. 698
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- July 2012
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6 - Elasticity
- from Part II - Mechanics
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp 167-207
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Summary
Inquiry
What modifications could you make to a hip stem to minimize stress shielding of the surrounding bone while still using metals common to orthopedic implants?
An interesting property of human bones is the manner in which they efficiently build up bone density where the bones are under higher loading, and remove bone density where strength is not needed. This property can be seen in the increased bone strength of the dominant arm of tennis players as compared to the non-racquet-holding arm (Ashizawa et al., 1999). Because the dominant arm is constantly subjected to loading as a result of the impact between the ball and racquet, the bone in that arm has greater bone density than the non-dominant arm, which presumably sees only everyday loading.
This property of bones is of importance to designers of hip stems as it has been shown that patients who have hip implants with a high stiffness stem were experiencing noticeable bone loss. This phenomenon, known as stress shielding, was thought to be occurring because the hip implants themselves were taking up so much of the loading that the body removed from the now extraneous surrounding bone. This in turn led to implant problems, as the remaining bone was not strong enough to stabilize the hip stems, as shown in Figure 6.1. Because of this, researchers turned their attention to developing materials and implants that might provide the same strength and durability with decreased stiffness, forcing the bone to take up more of the load. By the end of this chapter, a method for determining the stresses in various configurations of materials and stem cross-sections, in order to reduce stress shielding, will be developed.
3 - Ceramics
- from Part I - Materials
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp 70-91
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Summary
Inquiry
How would you create a bone graft for the supporting structure of a dental implant if the patient has periodontal disease accompanied by bone loss in the jaw and also suffers from osteoporosis (porous bone)?
The above inquiry presents a realistic challenge that one might face in the field of dentistry. A dental implant must be integrated into the underlying bone in order to have the necessary structural support needed for function. A patient who has periodontal disease accompanied by bone loss in the jaw and who also suffers from osteoporosis (porous bone) is unlikely to have bone tissue that can be used as a structural graft. Traditionally bone grafts are obtained from elsewhere in the patient's body such as ribs, pelvis, or skull. If the patient has had severe trauma or other disease such as osteoporosis, there may be insufficient quantity or quality of bone available. In such cases, viable alternatives such as coral may be necessary. The case study presented at the end of this chapter examines coral as a bone substitute.
Historical perspective and overview
Structural materials, such as plaster of Paris (loosely categorized as a resorbable ceramic), were first employed as a bone substitute in the late 1800s (Pelter, 1959). Even today this material is used as a structural support for fractured bones. Plaster of Paris, which is made of calcium sulphate hemihydrate (CaSO4·H2O), is also used in radiotherapy to make immobilization casts for patients and in dentistry for modeling of oral tissues. Only within the last 50 years have medical ceramics been incorporated into load-bearing applications. The use of ceramics in structural applications has been restricted due to their inherent susceptibility to fracture and sensitivity to flaws (Kingery, 1976). For this reason, ceramics have had limited use in medical implants in which any tensile stresses were expected. Modern manufacturing methods, including sintering and high-pressure compaction, have enabled these materials to be produced with fewer defects and better mechanical properties. As a result of such technological development, ceramics are now utilized in a variety of applications including heart valves (More and Silver, 1990), bone substitutes (Bajpai, 1990), dental implants (Hulbert et al., 1987), femoral heads (Oonishi, 1992), middle ear ossicles (Grote, 1987), and bone screws (Zimmermann et al., 1991). Additionally, ceramics can be utilized in inert, active, and resorbable forms.
Index
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
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- 05 June 2012
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- 20 October 2011, pp 645-681
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16 - Soft tissue replacements
- from Part III - Case studies
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
- Print publication:
- 20 October 2011, pp 560-594
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Summary
Inquiry
What methods can be used to keep soft tissue replacements fixed in the body?
A key challenge of developing soft tissue replacements is selecting a method for keeping them in place. Whether the implant is highly load-bearing (such as an artificial ligament) or only minimally (as in artificial cheekbones), implant migration is a serious issue. Because soft tissue replacements are typically made of compliant materials, implant fixation is particularly difficult. Tools such as screws or wires, commonly used in orthopedic implants, will only serve to damage these more delicate materials. Some typical methods for soft tissue replacements include suturing and encouraging tissue ingrowth into porous meshes. Are there ways to incorporate adhesives or less compliant materials for improved fixation?
Historical perspective and overview
Previous chapters in the clinical section of this textbook have described devices used to replace hard tissues (such as bone or teeth) and blood-interfacing implants (including vascular grafts and stents). Hard tissue replacements have their special challenges: for example, the need for a hip implant to be strong enough to withstand loading but have a low stiffness so as to reduce stress shielding. Blood-interfacing devices are required to provide structural support while preventing adhesion of platelets and without damaging blood cells. A third category of implant is soft tissue replacements. These can be further subdivided into mechanical supports (demanding a specific stiffness or strength as in sutures and synthetic ligaments), space fillers (primarily cosmetic implants, but also including artificial skin, which require compliance match and conformability), and highly specialized ophthalmic implants, which have further specifications related to their optical properties.
Prologue
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
- Print publication:
- 20 October 2011, pp xiv-xvi
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Summary
Mechanics of Biomaterials: Fundamental Principles for Implant Design provides the requisite engineering principles needed for the design of load-bearing medical implants with the intention of successfully employing synthetic materials to restore structural function in biological systems. This textbook makes available a collection of relevant case studies in the areas of orthopedics, cardiology, dentistry, and soft tissue reconstruction and elucidates the functional requirements of medical implants in the context of the specific restorative nature of the device. Each chapter opens with an exploratory question related to the chapter content in order to facilitate inquiry-based learning. Subsequently, a general overview, learning objectives, worked examples, clinical case studies, and problems for consideration are provided for each topic.
The organization of the book is designed to be self-contained, such that a student can be trained to competently engineer and design with biomaterials using the book as a standalone text, while practicing engineers or clinicians working with medical devices may use its content frequently in their careers as a guide and reference. The book comprises three basic sections: (i) overview of the materials science of biological materials and their engineered replacements; (ii) mechanical behavior of materials and structural properties requisite for implants; and (iii) clinical aspects of medical device design.
The first section of this book begins with a synopsis of medical devices and the fundamental issues pertaining to biocompatibility, sterilization, and design of medical implants. Engineered biomaterials including metals, ceramics, polymers, and composites are examined. The first segment of the book concludeswith a description of structural tissues. In these chapters, the mechanical properties of common synthetic and natural materials are discussed within the context of their structure-property relationships.
7 - Viscoelasticity
- from Part II - Mechanics
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp 208-240
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Summary
Inquiry
How are the material properties of cartilage and polymer bearing materials affected by the rate and duration of applied loading? How then does one design with biomaterials, accounting for time-dependent material properties?
The majority of important biomaterials, polymers, and tissues, for example, express some of the most complicated mechanical behaviors. When subjected to a constant load these materials will relax and continue to deform potentially indefinitely in a process called creep, making their use in structures problematic. Further, their response to high-frequency loading may differ dramatically from that observed under quasi-static conditions, directly impacting their performance in service. For instance, imagine the dynamic loading experienced by natural cartilage or a total knee replacement during downhill skiing, and how this rapid and severe loading could differ from the carefully controlled quasi-static testing typically performed in a laboratory. Comprehensive and effective modeling techniques are needed to properly account for time-dependent material properties in viscoelastic materials.
Overview
Viscoelastic materials, displaying both solid-like and fluid-like characteristics, are common in biomedical applications, from polymeric structures and coatings, to load-bearing connective tissues. The behavior of these materials cannot be described using only linear elasticity or plasticity theory, as is typical for many engineering materials. Rather, they are best described as having time-dependent material properties, such as an effective elastic modulus that is a function of the duration of applied loading. This time-dependence is usually associated with a characteristic time scale, called the relaxation time, and for time scales much greater or less than the relaxation time the material may not appear viscoelastic at all. Mathematical models of varying complexity can capture this behavior well, many times with only two or three material constants. Energy dissipation from the fluid-like part of the response can be separated from solid-like energy storage using a complex modulus, where they are represented by an imaginary and real part (respectively) of a complex material property (discussed below). In some cases, the material behavior is not simple enough to be represented by linear superposable functions, and nonlinear methods are especially suitable, for example when the unloading response does not match the loading response of the material.
Contents
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp v-x
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11 - Friction, lubrication, and wear
- from Part II - Mechanics
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
- Print publication:
- 20 October 2011, pp 369-394
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Summary
Inquiry
You are designing a total knee replacement – a high-load device for which wear is a major concern. What would your ideal materials be for a low-wear device? Are there other concerns at play here, and how do they affect your material selection?
The inquiry posed above gives an idea of the issues that must be balanced in medical device design. A total knee replacement will see contact stresses of 40 MPa (Bartel et al., 1986) and will undergo 1 million fatigue cycles per year. We have already discussed the dangers of stress shielding which can occur when using high stiffness materials. Selecting a material that can meet these demands and still maintain low amounts of wear is a challenge that must be tackled by a team of engineers and scientists.
Overview
When thinking about designing devices that will have contact with any other materials, it is necessary to consider issues of friction, lubrication, and wear, also known as the field of tribology. These are particularly critical in load-bearing medical device applications such as total joint replacements, where the generation of wear debris is a known issue that begins a cycle of immune response and the eventual need for device replacement. Frictionless contact would be ideal, but does not exist in the real world. It has also been shown that low-friction contact can still lead to appreciable wear (Stachiowak and Batchelor, 2005). In orthodontic implants such as braces, a smooth surface to the arch wires is very important – frictional forces are known to reduce the amount of force applied to the teeth by 50% or more (Bourauel et al., 1998 ). The characterization of the interface between two surfaces, including the composition of the articulating materials, the surface finishes, and the lubricant (if any) will play a large role in determining the friction and wear behavior.
Glossary
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp 620-644
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Frontmatter
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
- Print publication:
- 20 October 2011, pp i-iv
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2 - Metals for medical implants
- from Part I - Materials
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp 26-69
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Summary
Inquiry
All metals, with the exception of high noble metals, are susceptible to corrosion. This problem is exacerbated in the body, where implants must function in aqueous environments under complex loading and often utilize designs that contain inherent stress concentrations and material discontinuities. How would you design a modular hip system composed of metal components that minimizes the likelihood for fretting and crevice corrosion?
The inquiry posed above represents a realistic challenge that one might face in the field of orthopedic biomaterials. At a minimum one would want to minimize the galvanic potential of the alloys involved in the implant design. Fretting is the loss of material owing to the rubbing or contact of components; to minimize this type of surface degradation it is necessary to utilize designs that optimize component tolerances for device fixation. Crevice corrosion occurs in metal components that contain fissures, notches, or other geometries that facilitate local changes in the environment. This type of corrosion is further enabled in the presence of elevated stresses or stress concentrations within the component. To minimize fretting and crevice corrosion in a modular hip it is necessary to diminish geometric gaps at junctions such as the Morse Taper (where the stem and acetabular head join) and to keep the stem and neck at reasonable lengths in order to reduce bending stresses at the stem-neck juncture. The case study presented at the end of this chapter addresses this issue.
Historical perspective and overview
Metals have been utilized in medical implants for several hundred years; in fact gold has been used as dental material from as early as 500 BC and noble metals have been employed in structural dentistry for use in fillings, crowns, bridges, and dentures since the 15th century (Williams, 1990). Noble alloys contain varying percentages of gold, palladium, silver, or platinum. High noble metals are metal systems that contain more than 60% noble metals (gold, palladium, or platinum) with at least 40% of the metal being composed of gold. It is known that gold was used in the 16th century to repair cleft palates; gold, bronze, and iron evolved as suture wires over the 17th–18th centuries; and steel plates were commonly employed as internal fixation devices by the early 19th century (Aramany, 1971; Williams et al., 1973). By the mid-1920s, stainless steels were introduced as implant materials and a decade later cobalt-chromium alloys were utilized in the body to provide enhanced corrosion resistance (Park and Bronzino, 2003). Titanium found its use as a surgical material shortly thereafter (Leventhal, 1951). Much of what motivated the development of metals as implant materials was the need for both strength and corrosion resistance in load-bearing devices.
Epilogue
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp 595-596
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Summary
Mechanics of Biomaterials: Fundamental Principles for Implant Design provides the requisite engineering principles needed for the design of load-bearing medical implants with the intention of successfully employing natural or synthetic materials to restore structural function in biological systems. One challenge in the medical device field is the multifactorial nature of the design process. Numerous elements affect device performance, including clinical variables, structural requirements, implant design, materials selection, manufacturing, and sterilization processes. The crucial requirement of any medical device is that it is biocompatible; the implant must restore function without adverse reaction or chronic inflammatory response in the body. In this respect, choice of materials used in the implant is key to the device integrity.
Moreover, the structural requirements of the implant are determined through an assessment of the expected physiological stresses that vary depending upon the patient's anatomy, weight, and physical activity. Analyses of these stresses are key in making certain that the selected material offers the appropriate mechanical properties such as requisite elastic modulus and yield stress, as well as resistance to creep, fracture, fatigue, and wear. This book addresses the complexities that are encountered in physiological loading such as three-dimensional stress states; the complex interplay of dynamic loading, contact mechanics, viscoelastic deformation, and rupture; as well as the combined effects of environmental degradation of the materials owing to biological attack, aqueous environment, and sterilization method employed.
Part I - Materials
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp 1-2
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8 - Failure theories
- from Part II - Mechanics
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
- Print publication:
- 20 October 2011, pp 241-282
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Summary
Inquiry
How would you safely design a tibial insert of a total knee replacement that is known to experience a complex loading state with a normal stress component that is on the order of the uniaxial strength for this material?
The inquiry posed above represents a realistic design challenge that one might face in the field of orthopedics. Many of the tibial components used in total knee arthroplasty utilize ultra-high molecular weight polyethylene with a uniaxial yield stress on the order of 20 MPa; yet, the contact pressures for many of the clinical designs exceed this value. In order to assess the likelihood for failure owing to yield or plastic deformation, it is important to calculate the effective stress that provides a scalar representation of the multiaxial stress state acting on the implant. It is the effective stress that must be compared to the uniaxial yield strength as an assessment for the factor of safety against failure. Furthermore, localized plastic damage due to the presence of a notch or stress concentration can serve as a nucleation site for cracks if the component undergoes cyclic loading conditions. All of these factors must be considered when designing the implant.
Overview
The process of material failure depends upon the stress state of the system as well as whether its microstructure renders it ductile, brittle, or semi-brittle. In general, ductile materials yield before fracture while brittle materials fracture before yield. A semi-brittle system offers a small amount of plastic or permanent deformation prior to fracture. In the broad spectrum of materials behavior, metals are generally considered strong, tough, and ductile; ceramics are known to be strong in compression but weak in tension, and are notoriously brittle; and polymers are usually compliant, resilient, and highly sensitive to strain rate. Composites and tissues are typically anisotropic and are highly dependent upon the distribution of constituents. The most commonly employed mechanical test for material characterization is the uniaxial tensile test, which provides several important material properties including elastic modulus, yield strength, ultimate tensile strength, fracture stress, energetic toughness, and ductility (as shown in Figure 8.1).
12 - Regulatory affairs and testing
- from Part III - Case studies
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp 397-415
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Summary
A medical device company is developing a material for use in vascular grafts. What should the designers keep in mind from a regulatory standpoint as they move toward developing a prototype implant?
The regulatory aspects of medical device approval can be the least familiar part of the medical device development process, particularly for engineers and scientists. It can be very useful to think ahead to what types of tests will be instrumental in gaining approval from the Food and Drug Administration (FDA). This company will want to decide if it is planning to market the device itself, or merely supply the material to another company that will be the primary distributor of the implant. Second, the company might want to check the FDA database to see what guidance the FDA may provide for these implants. Third, the company will need to determine whether a device made from this material is going to be entirely novel, or whether it can be classified as substantially equivalent to currently approved devices (perhaps the more simple path to market). Further details regarding medical device classification and the approval process will be given in this chapter.
Historical perspective and overview
The Food and Drug Administration (FDA) regulates $1 trillion in products per year, which includes 1.5 million medical devices (Chang, 2003; Swann, 2003). The Center for Devices and Radiological Health (CDRH) within the FDA is responsible for evaluating approximately 4000 new medical device applications each year, as well as monitoring the medical devices already offered on the U.S. market (FDA, 2010). With just over 1200 employees, the CDRH works daily to engage with industry members and physicians in defining the balance between safety, efficacy, and health benefits of medical devices. In addition to the large numbers, the devices approved and monitored by the FDA are also quite diverse, ranging from specialties such as neurology to gastroenterology; from implants as complex as the total artificial heart to items as simple as a bandage.
1 - Biocompatibility, sterilization, and materials selection for implant design
- from Part I - Materials
- Lisa A. Pruitt, University of California, Berkeley, Ayyana M. Chakravartula
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- Book:
- Mechanics of Biomaterials
- Published online:
- 05 June 2012
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- 20 October 2011, pp 3-25
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Summary
Inquiry
All medical implants must be sterilized to ensure no bacterial contamination to the patient. How would you sterilize a total hip replacement comprising a titanium stem, a cobalt-chromium alloy head, and an ultra-high molecular weight polyethylene acetabular shell? Could the same method be employed for all three materials? How do you ensure that there is no degradation to the material or its structural properties? What factors would you need to consider in the optimization of this problem?
The inquiry posed above represents a realistic challenge that one might face in the field of orthopedic biomaterials. At a minimum, one would want to know the sterilization methods available for medical implants and which materials they best serve. For example, steam or autoclaving work well for sterilization of metals and ceramics but are generally unsuitable for polymers due to the lower melting and distortion temperatures of medical plastics. Also, one needs to consider whether there are any changes in the mechanical properties or if any time-dependent changes are expected owing to the sterilization method employed; for example, gamma radiation is known to leave behind free radicals (unpaired electrons) and these free radicals are highly reactive with elements such as oxygen that may be present or may diffuse into the implant material. In certain polymer materials such as ultra-high molecular weight polyethylene, gamma radiation can result in oxidation-induced embrittlement (shelf aging) that can severely degrade its wear and fracture properties. The case study presented at the end of this chapter addresses this issue.
Historical perspective and overview
Designing medical implants is a complex process, and this textbook aims to provide insight into the material, mechanical, and clinical factors that affect implant design and performance. The goal of this book is to integrate all aspects of implant design including clinical issues, structural requirements, materials selection, and processing treatments.