Like worn-out shock absorbers on a car, parts of the human body wear out from age or overuse — often both. Now it's time for baby boomers, the 78 million-member generation born between 1946 and 1964, to cope with the inevitable deterioration that accompanies aging.

Recent advances in repairing worn body parts — shoulders, knees, toes and eyes — are providing options that would have been unimaginable in the 1950s, when the artificial hip was pioneered.

As the tail end of the enormous baby boom generation enters middle age, Americans are living longer and expecting to enjoy better fitness and health than previous generations.

The human body can't necessarily do at 50 what it did at 25, but when a part wears out, older Americans increasingly expect that it can be fixed or replaced.

Consider this:

— 71 million Americans 65 and older will account for roughly 20 percent of the population by 2030.

— 80 percent of older Americans are currently living with at least one chronic condition.

— 25 million Americans have some type of medical implant, such as artificial knees, replacement lenses in the eyes or pacemakers.

How does medicine try to keep pace with all these aging bodies?

Harvesting tissue from one location and transplanting it into another part of the same patient.

Example: For a coronary bypass, veins are removed from the patient's leg, then transplanted to the heart to allow blood to flow around blocked arteries.

Risks: Potential infection and pain at the harvesting site.

Harvesting tissue or organs from a donor and transplanting it to the patient.

Examples: Whole organ transplants, such as lung or kidney. Or cadaver parts, sometimes used in partial knee replacements.

Risks: Organ rejection and infection, despite sophisticated anti-rejection drugs. Donor shortages.

Transfer of cells, tissues or whole organs from one species to another.

Examples: Pig valves, horse tissue.

Risks: Organ rejection and infection, ethical concerns.

Replicating, augmenting or extending functions performed by biological systems.

Examples: Artificial heart valve, prosthetic hip.

Risks: Do not behave physiologically as true organs or tissues. Parts are subject to fatigue, fracture, toxicity, inflammation and wear over time.

Refurbishing diseased or damaged tissue or organs using the body's own healthy cells.

Example: Bladders.

Risks: A developing science.

A look at some replaceable parts. The list focuses on age-related dysfunction and less on parts replaced for cosmetic reasons or to repair trauma:

— An implantable hearing aid can improve deterioration in the inner ear brought on by age-related hearing loss.

— Tooth implants embedded into the jaw can replace individual teeth or bridgework. A basic implant involving a single tooth costs between $1,500 and $3,000.

— Gum recession may lead to gum grafts, including tissue taken from the underside of the palate at the roof of the mouth to cover exposed roots of the teeth.

— In the past few years, doctors performing shoulder replacement surgery have used tissue from horses' hearts as a patch to fix a worn rotator cuff.

— Elbows may succumb to severe arthritis and need to be replaced.

— Artificial disks that fit into the vertebrae replace disks in the lower back and neck that have worn out.

— Wrists, fingers, ankles and toes are among the newer candidates for joint replacement, often to combat severe arthritis.

— The number of total hip replacements grew from 117,000 in 1991 to 234,000 in 2004. Patients as young as 50 undergo hip replacements.

— Retinas damaged by age-related macular degeneration can be replaced by a microchip implanted under the retina, which allows retinal cells to send visual signals to the brain. The chip is 25 microns thick, thinner than a human hair.

— An artificial cornea helps those whose corneas cannot be fixed with a transplant. The device, shaped like a shirt collar button, is made of clear plastic. Approved by the FDA in 1992, it has been implanted more than 1,200 times. A soft contact lens is placed on top.

— Cataract surgery to replace a cloudy lens with a clear artificial one is among the most commonly performed surgical procedures in the United States. Last year more than 2.7 million Americans underwent the outpatient operation.

— The artificial heart is still in experimental stages. Artificial heart valves, made from metal, plastic or pig tissue, are in wider use. Mechanical valves can last 20 years or longer.

— In the United States, more than 300,000 people get their knees replaced each year, and nearly two-thirds are women. At least one company is pursuing knee-replacement implants that are gender-specific — tailored for the anatomical differences between men and women.