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Unformatted text preview: Biomaterials: Properties, Types and applications MECH 634- SPRING SEMESTER 2008 Prepared by: Nasser-Eddin M., Ph.D 1 OUTLINES OUTLINES Biomaterials: Properties, Types, and Applications 3.1 Mechanical Properties and Mechanical Testing 3.2 Metals 2 MECHANICAL MECHANICAL TESTING • The most common way to determine mechanical properties is to pull a specimen apart and measure the force and deformation. •Materials are also tested by crushing them in compression or by bending them. •Standardized test protocols have been developed to facilitate comparison of data generated from different laboratories. The vast majority of those used in the biomaterials field are from the American Society for Testing and Materials (ASTM). •For example, tensile testing of metals can be done according to ASTM E8, ASTM D412 is for rubber materials, and ASTM D638 is for tensile testing of rigid plastics. These methods describe specimen shapes and dimensions, conditions for testing, and methods for calculating and reporting the results. 3 MECHANICAL MECHANICAL TESTING EXAMPLE: ASTM E8 • Test done with a ‘‘dog bone’’ shaped specimen that has its large ends held in some sort of a grip while its narrow midsection is the ‘‘test’’ section. • The mid portion is marked as the ‘‘gage length’’ where deformation is measured. • A mechanical test machine uses rotating screws or hydraulics to stretch the specimen. Force is measured in Newtons (N), and how much the specimen stretches— deformation—is measured in millimeters. 4 StressStress-Strain Testing • Typical tensile test machine • Typical tensile specimen extensometer specimen Adapted from Fig. 6.2, Callister 7e. gauge length Adapted from Fig. 6.3, Callister 7e. (Fig. 6.3 is taken from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, p. 2, John Wiley and Sons, New York, 1965.) 55 ASTM ASTM E8 (Cont.) Typical stress–strain curve for a metal that stretches and deforms (yields) before breaking. Stress is measured in N/m2 (Pa) while strain is measured as a percentage of the original length. The minimum stress that results in permanent deformation of the material is called the yield strength (YS). The ultimate strength (UTS) is the maximum stress that is tolerated by the material before rupturing. The stress at which failure occurs is called the failure strength (FS). Region A represents the elastic region since the strain increases in direct proportion to the applied stress. If a small stress is applied (e.g., to point 1), the material will return to its original length when the stress is removed. Region B represents the plastic region in which changes in strain are no longer proportional to changes in stress. Stresses in this region result in permanent deformation of the material. If a stress is applied that results in the strain at point (2), the material will follow the dotted line back to the baseline when the stress is removed and will be permanently deformed by the amount indicated by the offset. 6 3.2 3.2 Metals Metals used as biomaterials have high strength and resistance to fracture and are designed to resist corrosion Applications: see table 1-Case Study 1 Many orthopedic devices are made of metal, such as hip and knee joint replacements The implants provide relief from pain and restore function to joints in which the natural cartilage has been worn down or damaged. 7 Metallic Metallic Biomaterials B. Amsden 9 Materials and Their Medical Uses-I Mechanical Properties of Materials with Literature Values or Minimum Values from Standards 10 Metals (cont.) Plates and screws that hold fractured bone together during healing also are made of metal (a) Metal plates and screws are used to hold fractured bone segments together during healing. Depending on the extent of injury, the plates and screws or rods may be removed when the bone is fully repaired. (Photograph of the HALLU1-FIX MTP Fusion System (registered mark of NEWDEAL) is courtesy of Wright Medical Technology, Inc.) (b) Through the use of x-rays an implanted metal plate with screws can be visualized in this patient’s foot and hand. (X-ray courtesy of Wright Medical Technology, Inc.) 11 Metals (cont.)-Case Study involving metallic application (cont.)A typical total hip joint replacement is made primarily of metal. The ball of the femoral hip stem fits into a pelvic acetabular cup that is lined with ultra high molecular weight polyethylene (UHMWPE) for friction-free motion. (Photograph of the PROFEMUR 1 Z minimally invasive hip stemwith modular necks courtesy of Wright Medical Technology, Inc.) 12 Case Study-Hip Implants involving metals + other materials General General Anatomical Overview The The hip is one of your body's largest weightweightbearing joints. Consists Consists of two main parts: a ball (femoral head) that fits into a rounded ball (femoral socket (acetabulum) socket (acetabulum) in your pelvis. Ligaments Ligaments connect the ball to the socket and provide stability to the joint The The bone surfaces of your ball and socket have a smooth durable cover of articular cartilage that articular cushions the ends of the bones and enables them to move easily. Case Study-Hip Implants Hip Hip Anatomy Case Study-Hip Implants More… More… All All remaining surfaces of the hip joint are covered by a thin, smooth tissue called synovial synovial membrane. In a healthy hip, this membrane makes makes a small amount of fluid that lubricates and almost almost eliminates friction in your hip joint. Normally, Normally, all of these parts of your hip work in harmony, allowing you to move easily and without pain. Case Study-Hip Implants Total Total Hip Replacement A prosthetic hip that is implanted in a similar prosthetic fashion as is done in people. It replaces the painful arthritic joint. The The modular prosthetic hip replacement system used used today has three components – the femoral stem, stem, the femoral head, and the acetabulum. Each component has multiple sizes which allow for a custom fit. The The components are made of cobalt chrome stainless steel and ultra high molecular weight polyethylene. Cementless and cemented Cementless cemented prosthesis systems are available. Case Study-Hip Implants Bone Bone replacement criteria include the following: 1. 1. Appropriate tissue-material interface tissue2. Non2. Non-toxic 3. 3. Non-corrosive 4. 4. Adequate fatigue life 5. 5. Proper design 6. 6. Proper density 7. 7. Relatively inexpensive 8. 8. Elastic and mechanical properties comparable to those of bone Case Study-Hip Implants Early Early history Case Study-Hip Implants Joseph Joseph Lister /misc/evprev/fig23.jpg Case Study-Hip Implants John John Charnley Case Study-Hip Implants Charnley Charnley Prosthesis Case Study-Hip Implants Statistical Statistical Overview First First performed in 1960. Since Since then, improvements in joint replacement surgical techniques and technology technology have greatly increased the effectiveness effectiveness of this surgery. Number of Total Hip and Total Knee Replacement Procedures Performed in Canada, 1994–1995 to 2001–2002 Case Study-Hip Implants Case Study-Hip Implants Number and Distribution of Total Hip Replacement Procedures by Age Group and Sex, Canada, 2001–2002 Compared to 1994–1995 Males Age group <45 years 45-54 years 55-64 years 65-74 years 75-84 years 85+ years Total 19941995 489 716 716 1,609 2,475 1,470 194 6,953 20012002 553 1,055 1,753 2,798 1,976 315 8,450 7-year % chang e 13.1% 47.3% 8.9% 13.1% 34.4% 62.4% 21.5% 19941995 475 630 1,659 3,746 2,798 526 9,834 Females 20012002 484 943 1,966 3,748 3,547 839 11,527 7-year % chang e 1.9% 49.7% 18.5% 0.1% 26.8% 59.5% 17.2% Source: Hospital Morbidity Database, CIHI Common Common Causes of Hip Pain and Loss Loss of Hip Mobility Osteoarthritis Usually Usually occurs after age 50 and often in an individual with a family history of arthritis. In this this form of the disease, the the articular cartilage cushioning the bones of the hip wears away. The bones then rub against each other, causing hip pain and stiffness. Causes Causes (cont’d) Rheumatoid Arthritis a disease in which the disease synovial membrane becomes inflamed, produces excessive synovial fluid, and damages damages the articular cartilage, cartilage, leading to pain and stiffness. Causes Causes (cont’d) Traumatic Arthritis Can Can leads to a serious hip injury or fracture. A hip fracture can cause a condition condition known as avascular avascular necrosis. The articular cartilage becomes damaged and, over time, causes hip pain and stiffness. Osteoarthritis Fracture Operation Operation Removing the Femoral Head Once Once the hip joint is entered, the femoral head is dislocated from from the acetabulum. Then Then the femoral head is removed by cutting through the femoral neck with a power saw. Reaming Reaming the Acetabulum After After the femoral head is removed, the cartilage is removed from the acetabulum using using a power drill and a special reamer. special The The reamer forms the bone in a hemispherical shape to exactly fit the metal shell of the acetabular component. Inserting Inserting the Acetabular Component A trial component, which is trial an exact duplicate of your hip prosthesis, is used to ensure that the joint will be the the right size and fit for the client. client. Once Once the right size and shape is determined for the acetabulum, the acetabular component is inserted into place. Preparing Preparing the Femoral Canal To To begin replacing the femoral head, special rasps are used to shape and scrape out femur to the exact shape of the metal stem stem of the femoral component. component. Once Once again, a trial component is used to ensure the correct size and shape. The surgeon will also test the movement of the hip joint. Inserting Inserting Femoral Stem Once Once the size and shape of the canal exactly fit the femoral femoral component, component, the stem is inserted into the femoral canal. Attaching Attaching the Femoral Head The The metal ball that replaces the femoral head is attached attached to the femoral femoral stem. The The Completed Hip Replacement • • Client now has a new weight bearing surface to replace the affected hip. Before the incision is closed, an x-ray is made xto ensure new prosthesis is in the correct position. See video(s) 36 37 37 38 38 39 39 40 40 41 41 42 42 43 43 44 44 45 45 46 46 Metals (cont.) Metallic devices are also used to fuse segments of the spine together when the disk has degenerated and as dental root prosthetic implants Metallic devices are used to fuse segments of the spine together when vertebral bones are fractured due to osteoporosis or back injury. The metal cage can accommodate the patient’s own bone particles to assist with new bone formation which will eventually span and fuse the adjacent vertebral bones. (Photograph of the VERTESPAN1 spinal fusion cage courtesy of Medtronic Sofamor Danek.) As an alternative to dentures, patients can have metallic dental root prosthetics implanted to replace each missing tooth. The implant is then topped with a porcelain crown. One advantage of dental implants over dentures is that the implant transmits mechanical forces into the jaw bone and stimulates it, resulting in less bone recession over time. (Photograph courtesy of Dr. Martin Freilich of the University of Connecticut 47 Health Center.) Metals choiceMetals choice-I • The selection depends on a number of factors, including the mechanical loading requirements, chemical and structural properties of the material itself, and the biological requirements. • The longstanding use of metals for knee and hip joints, bone plates, and spinal fusion devices is due to the high mechanical strength requirements of these applications and proven biocompatibility in these settings. • The advantages of metals over other materials such as ceramics and polymers are that they are strong, tough, and ductile (or deformable, particularly as compared to ceramics). • Disadvantages include susceptibility to corrosion due to the nature of the metallic bond (free electrons). 48 Metals choiceMetals choice-II The steels that were used in the early 1900s for hip implants corroded rapidly in the body and caused adverse effects on the healing process. Today: preferred selection of alloys of titanium or cobalt-chrome for hip, knee, and dental implants. Certain metals known as shape memory alloys (e.g., nitinol) can be bent or deformed and still return to their original shape when the stress is released. These metals have found application in eye glasses and coronary artery stents that can be inserted through a catheter while collapsed and then spring into a cylindrical shape once they are pushed beyond the confines of the catheter. 49 Metals choice-III choice- Metallic devices are typically made by investment casting, computer-aided design and machining (CAD/CAM), grinding, or powder metallurgy techniques. The specific steps involved in the fabrication of a medical device will depend on factors such as final geometry of the implant, the forming and machining properties of the metal, and the costs of alternative fabrication methods. 50 Metallic Metallic Biomaterials There There are 3 main groups of metals used as biomaterials: stainless stainless steels Co Co-based alloys titaniumtitanium-based alloys 52 52 53 53 54 54 HOMEWORK –Question 1 55 Metals Metals Are Crystalline Body-Centered Cubic The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley & Sons, 1976. Metal Metal Bonding The The electrons in metals are mobile and surround a core of cations. This gives rise to their high electrical conductivity. Product Product Manufacture There There are different methods of metal product manufacture: machining machining melt melt casting Forging Forging Influence Influence of Manufacturing Process Casting Defect Polished-etched view of a cast ASTM F75 femoral hip stem. Note dendrites and large grains In vivo fracture initiated from an inclusion formed during the casting process From : H. Alexander et al., Chapter 2, Biomaterials Science, BD Ratner et al., Academic Press, 1996. Design Design Considerations typically typically want to match mechanical properties of tissue with mechanical properties of metal have have to consider how the metal may fail in vivo • corrosion • wear • fatigue need need to consider cost Corrosion Corrosion The extraThe extra-cellular environment is a chemically aggressive space. Metallic biomaterials are good conductors in an electrolyte solution, leading to galvanic corrosion. Corrosion Corrosion Wear Wear The The effects of wear are most predominant in joint prostheses. There There are two types of wear : Interfacial Interfacial Wear Fatigue Fatigue Wear Fatigue Fatigue Recall Recall that fatigue is progressive failure of a material due to the application of cyclical stresses below the ultimate stress of the material material causing crack propagation. Crack Crack usually starts at a stress concentrator or stress riser. Methods Methods for reducing fatigue failure : Fatigue Fatigue ...
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