Implants replacement

Valve Problems. The primary solution to valve problems has been implantable replacement valves. The introduction of these devices necessitates open-heart surgery. There are two types of valves available tissue (porcine and bovine) and mechanical. The disadvantage of tissue valves is that these have a limited life of about seven years before they calcify, stiffen, and have to be replaced. The mechanical valves can last a lifetime, but require anticoagulant therapy. In some patients, anticoagulants may not be feasible or may be contraindicated. Of the valves which require replacement, 99% are mitral and aortic valves. The valves on the left side of the heart are under much greater pressure because the left ventricle is pumping blood out to the entire body, instead of only to the lungs. Occasionally, two valves are replaced in the same procedure. 

Biomaterials Carbon has high biocompatibility and can be used for dental implants, replacement of broken bones, and heart valves. 

Metallic materials such as stainless steels, Ti and Co-Cr based alloys are currently used as permanent implant materials. These alloys are also used for temporary implant applications in the form of plates, screws and pins, for the repair of bone fracture. However, these metal implants have to be removed by a second surgical proceditre after the tissues have healed sufficiently, since they pose a risk of releasing toxic metalhc ions and/or particles through corrosion or wear processes. The second surgical procedure adds to the cost and bmden to the patients. Identification of implants to support tissue regeneration and healing in specific applications by material corrosion and simultaneous implant replacement by the surroimding tissues is an important area of research. 

Biomaterials for Cardiovascular Devices. Perhaps the most advanced field of biomaterials is that for cardiovascular devices. For several decades bodily parts have been replaced or repaired by direct substitution using natural tissue or selected synthetic materials. The development of implantable-grade synthetic polymers, such as siHcones and polyurethanes, has made possible the development of advanced cardiac assist devices (see... 

Implantable valves, particularly mechanical valves which continue to encroach on tissue valves, are unique. Methods such as valvuloplasty, mitral valve repair, or use of ultrasound are unlikely to reduce the number of valve replacements into the twenty-first century. Valve selection remains in the hands of the surgeon because of the critical nature of the procedure. If anything goes wrong, the result can be catastrophic to the patient. Cost of a valve, from 3000— 4000, is a relatively small part of the cost of open-heart surgery which can mn as high as 30,000. Growth of the cardiovascular valve market has slowed in the United States with the decline of the threat of rheumatic fever. 

Vascular grafts are tubular devices implanted throughout the body to replace blood vessels which have become obstmcted by plaque, atherosclerosis, or otherwise weakened by an aneurysm. Grafts are used most often in peripheral bypass surgery to restore arterial blood flow in the legs. 

Fig. 11. Three views of a hip implant for joint replacement (a) insertion of the implant into the femur (b) implant in place and (c) femur and implant...

The materials used in a total joint replacement ate designed to enable the joint to function normally. The artificial components ate generally composed of a metal piece that fits closely into bone tissue. The metals ate varied and include stainless steel or alloys of cobalt, chrome, and titanium. The plastic material used in implants is a polyethylene that is extremely durable and wear-resistant. Also, a bone cement, a methacrylate, is often used to anchor the artificial joint materials into the bone. Cementiess joint replacements have mote tecentiy been developed. In these replacements, the prosthesis and the bone ate made to fit together without the need for bone cement. The implants ate press-fit into the bone. 

Breakage of an implanted joint is rare. Breakage occurs when the bone flexes and the metal implant does not flex as much, thereby exceeding its mechanical fatigue point causing the implant to break or crack. A revision joint replacement operation is necessary if breakage occurs. 

Nerves are rarely damaged during the total joint replacement surgery. However, nerve damage can occur if considerable joint deformity must be corrected in order to implant the prosthesis. With time these nerves sometimes return to normal function. 

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. 

At the in vivo assay level, the classic ip-ip (iateraperitoneal) in vivo model has been replaced as a selection criteria for advancement of new dmg candidates to clinical trial. More stringent alternative models iaclude subcutaneous or subrenal capsule implantation of tumor followed by iatravenous dmg dosiag (7) and the human tumor xenograft models ia aude mice (8). 

Transosteal and Staple Implants. This implant goes through the bone and is for the purpose of attaching dentures. The mandibular bone staple plate has replaced the transosteal pin. Since 1975 the five-pin staple bone plate has been used predominantiy, and in 1985 showed a success rate of 93% after five years (317). 

Figure 1.6 shows the implant for a replacement hip joint. In the operation, the head of the femur is cut off and the soft marrow is taken out to make a hole down the centre of the bone. Into the hole is glued a long metal shank which carries the artificial head. 

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