What are prostheses? |

Indications and Procedures

Traditionally, prostheses are defined as artificial replacements for missing, malformed, diseased, or damaged human parts. However, partly as a result of biomedical engineering feats, scientists and practitioners disagree as to what defines a prosthesis. Some think that only external devices that are mechanical, such as artificial limbs, are prostheses. Others think that internal devices, such as hip or knee replacements, or internal parts that are biological, such as transplanted organs and parts of organs, are prostheses. Finally, some think biosynthetic materials, such as artificial skin substitutes, qualify. Hence, the broadest definition could include artificial limbs, eyes, and noses; biosynthetic materials; organ transplants; heart valves; penile implants; bladder pumps; wigs; and dental appliances.

Body parts may be ruined or their function impaired as a result of disease, aging, congenital malformation, or trauma. Common diseases that may result in loss of limbs or destruction of tissue are diabetes, cancer, and vascular disease. Osteoarthritis erodes cartilage in the spine and joints and is common among older people. Congenital anomalies arising from exposure to teratogens during embryonic or fetal development may impair function. Trauma from accidents and war is a major cause of damage. Every week, land mines maim approximately twelve hundred people worldwide, many of them children. Generally, a prosthesis is prescribed if normal function or appearance can be partially or completely restored.

The most common prostheses are replacements for amputated limbs or parts of limbs. More than 70 percent of amputations in the United States are caused by peripheral vascular disease
in which blood flow is partially or completely obstructed in blood vessels beyond the chest. Peripheral vascular disease includes arteriosclerosis (hardening of the arteries), thromboembolism (blockage of a blood vessel), and complications from diabetes, a chronic metabolic disease.

Amputations involve the disarticulation of joints (such as through the hips) and the transection of long bones. About two-thirds of lower leg amputations in the United States are at a level below the knee and include transtibial (across the lower leg bone), ankle, foot, and toe amputations. About a third of leg amputations are transfemoral (across the thigh bone). Most amputees are over sixty-five years of age and have peripheral vascular disease; a large proportion of these amputees have diabetes.

A lower extremity prosthesis comprises several components. A socket fits around the residual limb and transmits force for standing and walking. A sheath, sock, and liner serve as an interface between limb and prosthesis, offering protection and comfort, especially for people with peripheral vascular disease or sensitive skin. A suspension system secures the socket to the residual limb for safe ambulation. There may be a knee joint with a single axis and hinge, or a polycentric axis with changing centers of rotation. A pylon or tube connects the socket to the terminal device and may absorb, store, and release energy. The terminal device, such as a foot or ski, can respond to changing terrain if dynamic. For each component, a prosthetist selects the appropriate type after evaluating several factors, including the patient’s upper body strength, trunk control, balance, posture, motivation, vocation, and hobbies and sports, as well as cost and aesthetics. Sometimes, at the patient’s behest, a more lifelike prosthesis is designed instead of one that functions optimally and is easy to
maintain.

About 90 percent of upper extremity amputations are the result of traumatic injury sustained by men between the ages of twenty and forty. Commonly, injury results from accidents with machines (especially farm machines), frostbite, and electrical burns. Prostheses for upper extremities range from being entirely cosmetic to optimally functional. There are two types of functional prostheses: body-powered (involving cable systems) and myoelectric. The latter use surface electrodes to translate energy from muscle contractions to prosthetic elbows, wrists, and hands.

In general, the level of amputation is linked to the degree of function that may be restored by a well-designed prosthesis and appropriate rehabilitation. For example, as the level rises from toes to hips, it becomes increasingly difficult to fully restore mobility and locomotion. Also, it is easier to restore the function of lower limbs than of upper limbs—primarily because of complex hand movements for grasping and manipulating.

Prostheses can replace parts of the face when plastic surgery is not a viable option. Sometimes, neither function nor appearance can be restored by more surgery. The patient may not be able to afford or refuses to endure more plastic surgery and elects instead to wear a prosthesis. Facial prostheses include ears, noses, midfacials, orbitals (eyes and lids), and artificial eyes. An anaplastologist, a member of a craniofacial rehabilitation team, custom designs a prosthesis for the tissue site after considering the symmetry of the face, its proportions, and anatomical landmarks. After sculpting and molding, a prosthesis is made of silicone rubber tinted to resemble the patient’s skin color.

Medical adhesives can keep a facial prosthesis in place. However, a more secure method involves osseointegrated implants of titanium posts. After bone grows around the posts, abutments or extensions are added surgically to support a bar; the prosthesis is clipped to the bar to hold it in place. Sometimes magnets are used. Osseointegrated implants are an important improvement over adhesives in that a person who wears a facial or dental prosthesis is less likely to be embarrassed by losing it in public. Regardless of the method of securing the prosthesis, it must be taken off at night. Both the skin and the prosthesis need to be cleaned daily.

Uses and Complications

People who use prosthetic devices for ambulation expend more energy than do able-bodied people. For example, the energy expended in using a unilateral prosthesis (for one limb or segment) varies directly with the level of amputation. Lower energy costs (related to effort) are associated with longer residual limbs, unilateral rather than bilateral amputation of both limbs or segments, intact knee(s), and loss of limb(s) to trauma rather than vascular disease. Someone with a unilateral transtibial amputation will expend about 9 to 20 percent more energy walking than does an able-bodied person, whereas someone with a bilateral transfemoral amputation may expend almost 300 percent more energy. It is interesting to note that the effort to walk with crutches may be greater than with a prosthesis. Thus, the prognosis for successful ambulation may be better for a nonambulatory patient when fitted with a prosthesis rather than crutches.

Some of the common problems associated with prosthetic devices include initial pain, sensitivity, and edema (swelling) of the residual limb after surgery; discomfort when learning to use a prosthesis; problems with balance and gait (pattern of walking); the breakdown of skin where it comes into contact with an ill-fitting prosthesis; and the effort necessary for donning and doffing (putting on and taking off) a prosthesis. Some types of components are too challenging for the debilitated or the elderly. For example, donning the traditional suction suspension system for holding a transfemoral prosthesis in place requires strength, good balance, and cardiovascular fitness. Some of the dynamically responsive components are bioengineering marvels, but they are heavier and require relatively more strength to use than do traditional counterparts. In general, components suitable for athletes and laborers are not suitable for the weak and sedentary.

People who wear a facial prosthesis depend on its realism and attractiveness for psychological comfort in social settings. The quality of the prosthesis depends a great deal on the artistry of the anaplastologist. When successfully crafted, the prosthesis allows the wearer to feel confident in public whenever ordinary social distances are involved, such as when passing people on the street or making a purchase in a store. However, the prosthesis is detectable close up and may be a source of anxiety for the wearer in more intimate settings.

Although a facial prosthesis may allow a person to “pass” as normal looking, it may not restore the ability to communicate emotion nonverbally or the ability to speak clearly. Even if communication is possible, people may look away, thus disregarding any facial cues. Other difficulties arise when the face has been so damaged that normal eating behavior is impossible or embarrassing. This represents a serious challenge because a great deal of human social interaction involves food; the inability or unwillingness to share food can disrupt families and ruin friendships. The disfigured person may insist on eating alone. The worst consequence of facial disfigurement is social isolation, which leads to depression. However, mutual help groups and nonprofit networks, such as Let’s Face It USA, can offer social support and information.

Typically, a facial prosthesis will last about two years, more with careful care. Over time, the silicone yellows when exposed to ultraviolet light, pollutants, and oil. The condition of both the prosthesis and the wearer’s skin needs to be monitored regularly by a health care specialist.

Perspective and Prospects

Early versions of today’s facial prostheses were crafted by two inspired artists during World War I. Captain Derwent Wood, an English sculptor working out of Britain’s Third London General Hospital, was dismayed by the number of suicides committed by men disfigured in trench warfare. After multiple reconstructive surgeries, the men had healed physically but were ashamed of their faces. In response to their plight, Captain Wood made full or partial masks based on photographs taken before their injuries. He made very thin masks of galvanized copper lined with silver, which he painted with oils to approximate the coloration of their skin. Eyeglasses often held the masks in place.

Anna Coleman Ladd, a Boston sculptor in France with the American Red Cross, created masks using similar methods. The soldiers were particularly appreciative of the eyelashes, mustaches, and whiskers made of fine copper threads. If the mouth had been damaged, Ladd parted the lips of the mask just enough to allow a cigarette holder.

Today, facial prostheses are more lifelike and more securely attached to the face. Although there have been tremendous advances in reconstructive surgery and biosynthetic skin substitutes, facial prostheses are still necessary for some patients disfigured, for example, by cancer treatments or burns.

The earliest prosthetists for limbs were blacksmiths, armor makers, artisans, and amputees themselves. War was often the occasion for ingenuity. For example, during the Civil War thousands of amputees needed limb replacements. Wounded men made their own peg legs or crutches. Replacement limbs and feet from mail-order catalogs were less crude and were made of carved or milled wood covered in rawhide that was glued in place.

Major scientific and technical advances were made in prosthetic design after World War II and the Vietnam War. The National Academy of Sciences organized a conference in 1945 to call attention to the needs of the wounded and to the lack of scientific innovation in the field of prosthetics. An intense research program was sponsored by the Office of Scientific Research and Development, and later by the Veterans Administration. The result was several design breakthroughs, such as the patellar tendon-bearing prosthesis for transtibial amputation and the quadrilateral socket and hydraulic swing-phase knee for transfemoral amputation. After the Vietnam War, myoelectric control systems for upper extremity prostheses and modular endoskeletal lower extremity prostheses were developed. (Endoskeletal refers to a prosthesis in which the pylon is covered with an anatomically shaped, soft foam covering.)

In the early twentieth century, prostheses were made primarily of wood and leather. By the end of the century, major advances in materials science for the aerospace and marine industries yielded stronger and more lightweight materials, many of which were adopted by prosthetists.

Some of the major innovations in prosthetic design depended on the development of thermosetting plastics, which were important for suction socket suspension systems, and transparent plastics, which improved diagnostic fittings. For example, transparent test sockets are used to evaluate potential weight-bearing areas on residual limbs, and transparent face masks are used to evaluate thermal injuries. Other advances in materials engineering, such as the development of carbon composites, contributed to the design of high-performance prosthetic feet for athletes.

Today, leather is still used for suspension straps, belts, and limb cuffs, but there exists an armamentarium of interesting materials useful for prostheses, such as steel, aluminum, titanium, and magnesium alloys; thermoplastics; thermosetting materials; foamed plastics; and viscoelastic polymers.

In the 1960s, computer-aided design and manufacture (CAD/CAM) systems revolutionized the fabrication of prostheses. Using CAD/CAM, surface contours of residual limbs are digitized by reading casts with a probe or scanner. The subsequent numerical models are used for computer-guided fabrication. By the early twenty-first century, medical imaging methods, such as computed tomography (CT) scanning and magnetic resonance imaging (MRI), were used in CAD/CAM systems. This was a major advance because CT and MRI allow the prosthetist to see underlying skeletal and soft tissue structures. Before the incorporation of medical imaging, the prosthetist relied on surface contour maps and palpation (feel) of the residual limb to evaluate anatomical structure. It is likely that advances in medical imaging will contribute further to innovative prosthetic design and fabrication in the future.

Bibliography

Hughes, Michael J. The Social Consequences of Facial Disfigurement. Brookfield, Vt.: Ashgate, 1998.

Lusardi, Michelle M., and Caroline C. Nielsen. Orthotics and Prosthetics in Rehabilitation. 3d ed. Philadelphia; London.: Saunders, 2012.

MedlinePlus. "Artificial Limbs." MedlinePlus, May 2, 2013.

Newell, Robert. Body Image and Disfigurement Care. New York: Routledge, 2000.

Ott, Katherine, David Serlin, and Stephen Mihm, eds. Artificial Parts, Practical Lives: Modern Histories of Prosthetics. New York: New York University Press, 2002.

Schaffer, Erik. "Overview of Limb Prosthetics." The Merck Manual Home Health Handbook, May 2007.

Seymour, Ron. Prosthetics and Orthotics: Lower Limb and Spinal. Philadelphia: Lippincott Williams & Wilkins, 2002.

Vorvick, Linda J. "Prosthesis." MedlinePlus, January 21, 2013.