Prosthesis

Parylene s use in the medical field is linked to electronics. Certain pacemaker manufacturers use it as a protective conformal coating on pacemaker circuitry (69). The coated circuitry is sealed in a metal can, so that the parylene coating serves only as a backup should the primary barrier leak. There is also interest in its use as an electrode insulation in the fabrication of miniature electrodes for long-term implantation to record or to stimulate neurons in the central or peripheral nervous system, as the "front end" of experimental neural prostheses (70). One report describes the 3-yr survival of functioning parylene-coated electrodes in the brain of a monkey (71). 

Hea.rt Va.Ives. Since the early 1960s nearly 50 different heart valves have been developed. The most commonly used valves as of the mid-1990s include mechanical prostheses and tissue valves. Nearly 75,000 of these prosthetic valves are implanted aimually worldwide, and about 30,000 in the United States alone. Caged-baH, caged disk, and til ting-disk heart valves are the types most widely used. 

The next advance in total hip arthroplasty came with the development of various porous surface treatments which allow bone tissue to grow into the metal porous coating on the femoral stem of the hip implant and on the acetabular component of the total joint replacement. These developments arose because of patients who were not able to tolerate cemented implants because of allergies to the cement, methylmethacrylate. More youthflil patients are better served by a press-fit implant as well. Figure 12 shows the difference between textured and beaded surface-treated orthopedic prostheses. 

Biomedical. Heart-valve parts are fabricated from pyrolytic carbon, which is compatible with living tissue. Such parts are produced by high temperature pyrolysis of gases such as methane. Other potential biomedical apphcations are dental implants and other prostheses where a seal between the implant and the living biological surface is essential. Plasma and arc-wire sprayed coatings are used on prosthetic devices, eg, hip implants, to achieve better bone/tissue attachments (see Prosthetic and BiOLffiDiCALdevices). 

Table 7. Composition of Gold-Based Alloys Used in Ceramo—Metallic Prostheses, wt %...

Resins are also used for permanent tooth-colored veneers on fixed prostheses, ie, crown and bridges. Compositions for this application include acryflcs, vinyl—acryflcs, and dimethacrylates, as well as silica- or quartz-microfilled composites. The resins are placed on the metallic substrates of the prostheses and cured by heat or light. These resins are inexpensive, easy to fabricate, and can be matched to the color of tooth stmcture. Acrylic facings do not chemically adhere to the metals and are retained only by curing the resin into mechanical undercuts designed into the metal substrate. They have relatively low mechanical strength and color stability, and poor abrasion and strain resistance they also deform more under the stress of mastication than porcelain veneers or facings. 

Maxillofacial polymers include the chlorinated polyethylenes, polyethemrethanes, polysiloxanes (see Elastomers), and conventional acrylic polymers. These are all deficient in a number of critical performance and processing characteristics. It is generally agreed that there is a need for improved maxillofacial polymers that can be conveniently fabricated into a variety of prostheses (218,227,228). 

Allen, K.W., Greenwood, L. and Wake, W.C., The stability of adhesive bonding between silicone mbber and alumina for neural prostheses. J. Adhes., 16(1), 61-76 (1983). 

The highest possible degree of hemocompatibility is essential for certain applications, including small-bore artificial prostheses. 

Samitz, M. H. and Katz, S. A. Nickel Dermatitis Hazards from Prostheses , British Journal of Dermatology, 92, 287-290 (1975)... 

Interest in the T1-6A1-4V alloy and Extra Low Interstitial (ELI) versions of this alloy for total joint prostheses spurted in the United States in the late 1970s. This alloy now finds wide application in orthopaedic surgery ... 

The corrosion fatigue failure of total joint prostheses has been widely studied. It can be concluded from an extensive literature survey carried... 

The successful clinical use of titanium and cobalt-chromium alloy combinations has been reported Lucas etal. also investigated this combination using electrochemical studies based on mixed potential and protection potential theories. Verification of these studies was made by direct coupling experiments. The electrochemical studies predicted coupled corrosion potentials of -0.22 V and low coupled corrosion rates of 0.02 ft A/cm. Direct coupling experiments verified these results. The cobalt-titanium interfaces on the implants were macroscopically examined and no instances of extensive corrosion were found. Overall, the in-vitro corrosion studies and the examination of retrieved prostheses predicted no exaggerated in-vivo corrosion due to the coupling of these cobalt and titanium alloys. 

Intercrystalline corrosion was a serious problem with the austenitic stainless steels early in their development since carbon contents then were relatively high, e.g. En58J contained up to 0.12type stainless steel contained up to 0.08 Vo C. The problem in relation to surgical implants has been reported by Scales eta/. and as a result of this and several other reports the British, American and International Standards specified the use of a 316S12 type austenitic stainless steel which contains 0.03 Vo C max. The use of the lower carbon content stainless steels as specified in the various standards has now eliminated the problem of sensitisation of implants. If manufacturers do use the 0.08% C versions they have to be very careful with the forging temperatures or anneal the prostheses afterwards. 

Rostoker, W., Pretzel, C. W. and Galante, J. O., Couple Corrosion Among Alloys for Skeletal Prostheses , Journal of Biomedical Materials Research, 8, 407-419 (1974)... 

Jackson-Burrows, H., Wilson, J. N. and Scales, J. T., Excision of Tumors of Humerus and Femur with Restoration of Internal Prostheses , Journal of Bone and Joint Surgery, STB, 148 (1975)... 

Coatings for hip j oints, heart valves, and other prostheses DLC is biocompatible and blood compatible,... 

The major biological application of isotropic carbon is in heart valves. The material is performing well and several hundred thousand units have been produced so far. Other applications include dental implants, ear prostheses, and as a coating for in-dwelling catheters. 

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