20071011

Artificial Body Parts

Many people lead active, enriched lives thanks to artificial body parts—devices that either temporarily or permanently replace a missing or faulty part of the body. Artificial body parts are also known as prostheses. Some, such as artificial limbs and kidneys, are located outside the body. Others, such as artificial hips and heart valves, are implanted in the body.

A Young Technology

The earliest known artificial body part was a leg made about 300 B.C. of metal plates surrounding a wooden core. Over the following centuries, wooden pegs took the place of legs, hooks replaced lost hands, wooden or ivory teeth replaced decayed teeth. Compared with the prostheses of today, these parts were very crude.

Today the study and development of artificial body parts constitute a discipline within the science of biomedical engineering, or bioengineering. Another term for this relatively young science is bionics, a term made popular by such television heroes as the Six Million Dollar Man and the Bionic Woman.

One major problem facing bionics researchers is finding materials that are biocompatible—that is, materials that will not be rejected by the body's immune system as foreign invaders. There are three basic groups of biomaterials: metals, ceramics, and polymers.

Metals that have proven to be biocompatible include titanium, stainless steel, platinum, and cobalt-chromium alloys. Because they are strong, metals are particularly useful in artificial limbs and joint replacements. They also are used in heart valves, dental implants, and electronic devices used to regulate various artificial organs.

Ceramics can be created with a broad range of properties. Some are very hard. Others make good contact surfaces. Still others are flexible and can be fashioned into artificial tendons and ligaments.

Polymers are large, chainlike molecules that can be custom-made to exhibit a wide range of properties. Two broad types are used in making artificial organs: elastomers and plastics. Elastomers, or rubbers, are very flexible; if stretched, they will return to their original shapes. Plastics are rigid, although they can be combined with other materials to make flexible parts. A type of silicone rubber is used for artificial finger joints: It can be bent 90 million times without breaking! Other polymers commonly used in implants include acrylics, which are used to make artificial eyes and lens implants; Teflon, used for artificial blood vessels; and polyethylene, used on the surface of artificial joints.

After a person receives a prosthesis, it may take awhile for him or her to learn how to use it and become accustomed to it. This process is called rehabilitation. Medical specialists assist people in learning how best to function with a prosthesis. For example, a physical therapist teaches a person with an artificial leg how to balance on the leg and how to move it; what exercises to do to strengthen the remaining leg muscles and the back muscles; how to stand up and sit down; and how to climb stairs and walk on slopes.

Orthopedic Devices

The medical specialty that concentrates on the care of the skeletal and muscular systems is orthopedics. Artificial parts are widely used by orthopedic surgeons to replace lost or severely damaged limbs, joints, and other body parts.

Following the removal or amputation of part or all of an arm or leg, the patient is fitted with a prosthesis. To attach the prosthesis, a piece of metal is usually cemented to the bone in the limb's stump; a hard carbon button passes from the metal through the skin of the stump to hold the artificial limb.

Artificial legs are made to look like natural legs. In fact, if an amputee has one natural leg, a mold can be made of the leg and then used to create an artificial leg that looks exactly like the person's real leg. The amputee controls movement of the prosthesis by moving his or her thigh.

Artificial arms also look natural. Often, however, an artificial arm ends in a hook rather than a natural-looking hand. The hook provides more dexterity than do currently available natural-looking artificial hands. An amputee may use a hook for everyday use, then substitute a natural-looking prosthesis for social occasions.

An artificial arm is operated by a system of cables and pulleys. The amputee uses various movements of the shoulder and other body parts to move the arm or lock it in place. The process requires a lot of concentration, and relatively few people are able to use these arms successfully. Even people who become adept at using the arms are frustrated by their limitations.

Most artificial hands also are operated by movements of the voluntary muscles. For example, a hand's thumb and fingers may be connected to a strap leading to the shoulder. Movements of the shoulder cause the thumb to move toward or away from the fingers.

No currently available artificial limbs are able to mimic the wide variety of movements that can be made by a natural limb. Within a natural limb, movement is controlled by muscles composed of millions of fibers, each of which is controlled by a nerve fiber. Movement occurs when the brain sends commands—in the form of electrical impulses—via the nerve fibers to the muscle fibers. In other words, to move a part of your body, all you need do is think about it, either consciously or subconsciously. Most amazing of all, the process takes only a small fraction of a second.

Researchers are trying to build prostheses that duplicate these processes—prostheses that will move when wearers think, "I want to move my arm in such and such a way, or do such and such." This type of device is called a myoelectric prosthesis because it depends on the electrical impulses sent to the missing limb's muscles (myo is a Latin root for "muscle"). Even though a person has lost a limb, the brain still sends out electrical impulses. However, they travel only as far as the stump of the amputated limb. A small portion of these impulses reach the stump's skin, where they can be used by electrodes to power the prosthesis.

The most successful myoelectric arm was introduced in 1981 by Stephen C. Jacobsen, director of the Center for Biomedical Design at the University of Utah. It is called the Utah arm. Its electrodes can pick up signals from the shoulder or the upper arm. The signals are transmitted to a microcomputer in the plastic elbow, which tells the arm's plastic "muscles" to respond to the brain's commands.

Another electrically powered prosthesis is the Otto Bock hand. It picks up myoelectrical signals via electrodes placed on the forearm stump. It can open and close at different speeds, pick up objects, and even shake hands with the natural hand of another person. And in 1998, researchers at Rutgers University in New Brunswick, New Jersey, built a prosthetic hand that can move its fingers independently.

Work on a myoelectric leg is occurring, though progress is much slower than with myoelectric arms. Measuring myoelectric signals from a leg stump is more difficult than from an arm stump. Perspiration and the weight put on an artificial leg affect the quality of the readings from the electrodes. Meanwhile, new materials have led to more-successful knee replacements. The replacement's upper surface is made of oxidized zirconium, which reduces wear by 85 percent and thereby extends the knee's life.

In addition to external parts, orthopedists also replace some internal parts. The junction between two bones is called a joint. Most joints allow motion to occur between the two bones. But injuries and diseases such as arthritis can cause deterioration of the ends of the bones—stiffening the joints, limiting movement, and causing pain or discomfort. Each year, thousands of people who suffer from such problems have their afflicted hips, knees, shoulders, elbows, fingers, knuckles, ankles, and toes replaced by artificial implants.

The oldest and most common joint replacement is the artificial hip. Although experiments with artificial hips date back to the 19th century, modern methods were pioneered by Sir John Charnley of England in the early 1960s. The artificial hip has two moving parts, a cup and a shaft. The cup commonly is made of ceramics; the shaft is made of a metal alloy. The cup fits into the socket of the hipbone. One end of the shaft fits into the hollow of the cup, and the other end is implanted into the femur (thigh-bone). At one time, the shaft was cemented to the femur, but the cement dried and cracked, loosening the shaft. Today, computers can be used to design implants that fit perfectly within the hollowed femur. Some implants have porous surfaces, allowing tissue to grow into the implant, forming a bond that becomes stronger with time.

Tendons and ligaments are bands of tough, fibrous tissue. Tendons connect muscles to bone. Ligaments hold bones together to form joints. Injuries may cause torn tendons or ligaments, which surgeons sometimes repair with implants made of flexible Dacron, polyester, and other materials.

The Circulatory System

Many things can go wrong with parts of the circulatory system. For example, disease can cause heart failure; fatty deposits can block blood vessels; and wounds can result in massive blood loss. Several types of implants enable physicians to save many patients who would previously have died because of such problems.

Open-heart surgery, heart transplants, and the implantation of artificial-heart devices are possible only if the heart stops its natural pumping action. But a person cannot live if blood does not constantly circulate through the arteries and veins. The solution to this dilemma is the heart-lung machine, which performs the functions of the heart and lungs while those organs are temporarily stopped. One tube carries blood from the heart to the machine—blood that would ordinarily travel to the lungs. A second tube carries blood from the machine to the aorta, the large artery that carries blood from the heart to the rest of the body.

In 1969, Dr. Denton Cooley placed an artificial heart into a dying man until a human heart became available three days later. The next day, the patient died of pneumonia and kidney failure. Another patient, William Schroeder, received a Jarvik-7 heart in 1984 and lived 620 days. A handful of other patients received artificial hearts, with only limited success. In 1990, the use of artificial hearts was suspended by the FDA.

Nevertheless, research continues; significant advances have been made, including the development of a type of artificial heart called a left-ventricular assist device (LVAD), which boosts the performance of the left ventricle, which does most of the heart's work. LVADs, which include a device called the Jarvic 2000, are most often used in patients awaiting heart transplants (and the FDA approved the device for this use in the late 1990s), but in a few cases they have been placed in people who were too ill to qualify for a conventional transplant. In July 2001, in Louisville, Kentucky, a self-contained mechanical device, the AbioCor, a kind of artificial heart, was implanted in a male patient in his 50s. By late 2005, 14 AbioCor hearts had been implanted, and the device awaits FDA approval.

Sometimes it is not necessary to replace an entire heart. The four valves in the heart keep blood flowing in one direction. If a valve does not close tightly, blood is allowed to flow backward, decreasing the heart's efficiency. If a valve becomes stiff, the heart must work harder.

Today there are three types of artificial valves used for implantation. The ball-in-a-cage valve looks like a hollow ball inside a cage. The bi-leaflet valve has two flaps that open and close, rather like swinging doors. The tilting-disk valve has one or two flat or slightly curved disks.

Some people suffer from arrhythmia, or irregular heartbeat. Nerves in the heart emit a stream of weak electrical impulses that control heartbeat. The impulses stimulate the muscles that contract and relax the heart's chambers, thereby indirectly controlling the opening and closing of the heart valves. To correct irregular heartbeat, a pacemaker may be implanted under the skin of the chest. Today, an estimated 1 million Americans have pacemakers.

A pacemaker is sealed in a titanium case. It has three main parts: a long-life battery, a pulse generator, and an insulated wire lead with a tiny electrode at its tip. The wire lead is passed through a vein until it reaches the inner wall of the right ventricle of the heart, where it is attached by means of small hooks on the electrode. The wire carries a series of electrical signals from the pulse generator to the heart muscle, replacing or supplementing the signal of the heart's own timing system.

Today's pacemakers also contain a microprocessor. They can be programmed to send out signals only when needed, or they can be reprogrammed from outside the body. This does away with the need for an operation if signal changes are necessary. The microprocessor enables a physician to monitor the pacemaker via telephone.

Since 1955, surgeons have used artificial tubes to replace clotted and damaged blood vessels. The tubes are usually made of a Dacron or Teflon fabric that is porous and elastic. They function much like real vessels, dilating and contracting in response to changing blood flow.

A pacemaker-like device known as an implantable cardioverter defibrillator (ICD) made headlines in mid-2001 when one was implanted in U.S. Vice President Dick Cheney. The ICD can differentiate between cardiac arrest and cardiovascular stress. It not only stimulates the wearer's heart rate electrically if it slows, but also responds at once if the heart begins to beat rapidly.

Because natural blood is so complex, it is difficult to create a synthetic substitute. Red-blood-cell substitutes deliver oxygen and remove carbon dioxide. But they do not perform other vital functions, such as forming clots and fighting infections. Red-blood-cell substitutes include perfluorocarbon-based solutions, bovine-derived hemoglobin solutions, and actual hemoglobin-containing artificial cells. The solution-based substitutes may escape from crevices in cells, and blood vessels may contract, leading to increased blood pressure. Artificial cells, which are lipid sacs, may lack the flexibility to negotiate the thinnest blood vessels. The search for better red-blood-cell substitutes that address these issues continues. Studies are being conducted on recombinant plasma proteins, transgenic therapeutic proteins, and artificial platelets.

Artificial Kidneys

The kidneys remove wastes from the blood and control the amount of water in the body. If the kidneys cease to function, a person will soon die unless proper treatment is received. In many cases, drugs cure the problem. In other situations, transplants are used. But in some cases, an artificial kidney is the only possible solution.

An artificial kidney, or dialyzer, is a machine located outside the body and temporarily connected to the patient. Its principal part is a tank containing small fibers made of a semipermeable plastic membrane. The space around the fibers is filled with a solution called a dialysate. This solution is similar to human blood plasma. Plastic tubes connect the patient to the artificial kidney. One tube is attached to an artery in the person's arm or leg. A second tube is connected to a vein. Blood is pumped from the artery into the tiny fibers in the machine. Waste products and excess water diffuse out of the blood into the dialysate. Afterward, the newly cleansed blood is pumped back into the body via the vein.

This process usually takes four to six hours and must be repeated several times a week. Most patients undergo the process at a hospital or special dialysis center, although some are treated at home.

A portable artificial kidney is now available, offering increased freedom to dialysis patients. It has three principal parts: the kidney itself, a 7-pound (3-kilogram) device that is connected by short tubes to the patient's bloodstream; a mixer that mixes dialysate concentrate with water; and a tank that holds the mixed dialysate. During use, the kidney is connected to the mixer or the tank via tubing some 25 feet (7.6 meters) long. Although tethered, the patient is able to move around while undergoing dialysis. Because the machine is portable, users of this technology can travel farther than a day's journey from their dialysis center.

Breast Implants

Many women opt for breast implants for breast augmentation or reconstruction after mastectomy. The two most common types of breast implants are silicone-gel and saline implants. The silicone-gel implant consists of an elastic bag filled with jellylike silicone. Because of silicone's consistency, the augmented breast feels much like a normal human breast. The saline implant is a silicone bag filled with a salt-water solution.

Various problems have been associated with breast implants. Silicone-gel implants impede mammograms—X rays of the breast used to detect tumors—preventing them from revealing tissue behind the implants. The silicone-gel bag has been known to leak, releasing silicone into the body. Silicone is an immune-system irritant, and fears were raised in the early 1990s that it could cause connective-tissue disorders. These fears, coupled with inadequate safety data, led the FDA to ban the use of silicone-gel implants. But in June 1999, the Institute of Medicine concluded that although the implants can leak and cause infections and scarring, there was no evidence that they cause connective-tissue or autoimmune diseases. In 2005, an FDA advisory committee recommended that silicone implants be allowed, as long as patients were warned of the potential dangers.

Saline implants, while they do not entirely obstruct X rays, can prevent detection of certain small tumors. A tiny percentage deflate and require replacement.

Synthetic Skin

One of the skin's major functions is to protect the body against invasion by bacteria, viruses, and other organisms that might cause fatal infections. Thus, if people who have experienced severe wounds or burns are to survive, it is essential that their lost skin be replaced as quickly as possible.

Artificial skin should have several basic characteristics. It should prevent infection, promote rapid healing, and minimize the amount of scar tissue that forms. It must prevent moisture and blood from seeping out of the body, but allow oxygen to permeate into the underlying tissues. And it should conform to the contours of the body and not limit a person's ability to move.

Artificial skins of today are temporary implants. One such skin is made from plastic, collagen (a connective-tissue protein), and a polysaccharide (a sugar). Another type of artificial skin is made from keratin (a component of hair and nails) and chitosan (a starchy substance found in insect skeletons and crab shells). Recently introduced products include TransCyte, intended for use as a temporary wound cover for burns; Dermagraft, for the treatment of diabetic foot ulcers; and Integra, which is used to replace the dermal skin layer. Integra and TransCyte have been used together successfully to treat patients who have extensive burns over large portions of their bodies.

Sensory Implants

People who have lost an eye or an ear, or who have a sightless and disfigured eye, can be fitted with natural-looking artificial organs. The artificial outer ear is functional as well as cosmetic, since it funnels sound waves to the eardrum. The artificial eye is cosmetic. However, it is attached to the eye muscles; thus, if the person has one natural and one artificial eye, the two eyes will move in unison.

Eyeglasses and contact lenses can be considered to be prosthetic devices, since they correct problems caused by a faulty lens. Similarly, hearing aids are prosthetic devices, since they amplify sound to compensate for hearing loss.

The most dramatic developments in sensory prostheses in recent years have been in the introduction of implants.

Many people suffer from cataracts, a condition in which the lens of the eye, normally transparent, becomes cloudy, impeding or preventing the passage of light. When the lens is surgically removed, some sight can be restored with corrective glasses or contact lenses. Today, however, the preferred method of treatment is to implant a lens made of a soft, transparent plastic or silicone rubber. The lens is attached to adjoining parts of the eye by metal or plastic clips or thread. The patient regains near-normal eyesight.

Deafness, or loss of hearing, can result from damage to the ear or brain. Researchers have not yet developed prostheses to help people whose deafness results from nerve damage. But prostheses are available for people who are deaf because of structural damage to the ear.

The middle ear contains three small bones that conduct sound to the inner ear. These bones are the hammer, anvil, and stirrup (so named because of their shapes). In a disease called otosclerosis, spongy bone forms in the middle ear, causing the stirrup to become rigid and unable to transmit sound. Surgeons can remove the stirrup and replace it with a metal or plastic implant. Prostheses are also available for hammer and anvil replacement.

The inner ear has a shell-shaped structure called the cochlea that is filled with fluid and lined with thousands of tiny hair cells. Movements of the stirrup against the membrane between the middle and inner ear cause the cochlear fluid to vibrate. These vibrations trigger the hair cells to generate electrical signals that are picked up by nerve fibers and carried to the brain. In many deaf people, the hair cells are damaged, and thus are unable to convert sound waves into electrical impulses.

Several types of cochlear implants are being marketed. All of them work on the same principle: A microphone attached to the person's shirt or eyeglass frame picks up a sound and sends it to a speech processor, which converts the sound into an electrical signal. A detachable cable carries the signal from the processor to a coil behind the ear. The coil is held by magnetism to a receiver implanted under the skin and embedded in the skull. The coil sends the signal to the receiver, which is connected to one or more electrodes implanted in the cochlea. The signal electronically stimulates the nerve fibers, which carry the sound message to the brain.

Thanks to cochlear implants, people who once were totally deaf can now hear voices and other sounds. Although hearing through an implant may sound peculiar, sophisticated speech processing allows many people to fully participate in oral communication with others.

Tooth Implants

Dentures are sets of artificial teeth that replace all or some of a person's natural teeth. The teeth are made of metal covered with porcelain; they are set in plastic gums that sit atop a person's natural gums. Although today's dentures are highly efficient and appear lifelike, they are not problem-free. Fitting problems can develop, especially as a person ages. Also, dentures can harm the jawbone by not distributing the stress of chewing evenly across the bone.

In the 1960s a procedure called osseointegration was developed in Sweden. In this procedure, holes are drilled in the jaw and hollow, threaded metal cylinders are implanted in the holes. Over a period of several months, the bony tissue of the jaw "integrates with," or grows around, the cylinders. In the next step of the process, a metal post is screwed into each cylinder. These posts extend slightly above the surface of the gum. Then an impression of the jaw is made. This impression becomes the basis for creating the patient's new artificial teeth. The teeth are attached to the posts with tiny screws.

Like natural teeth, those attached to the embedded posts do not move when the person chews, and they do not need to be removed for cleaning. They also distribute the stress of chewing more naturally within the jaw than do dentures.

The information provided should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies.

Copyright Information: Public domain information with acknowledgement given to the U.S. National Library of Medicine.

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