A prosthesis is an artificial device that replaces a missing or nonfunctional body part. While most people picture a prosthetic arm or leg, prostheses also include dental implants, artificial eyes, heart valves, and joint replacements. The field that designs and fits these devices is called prosthetics, and the specialists who build them are prosthetists.
Types of Prostheses
Limb prostheses are the most visible category, but they represent only a fraction of what the term covers. A prosthesis can replace nearly any structure in the body. Artificial hips and knees are prostheses implanted during joint replacement surgery. Dental prostheses include crowns, bridges, and full dentures. Cochlear implants restore hearing by converting sound into electrical signals sent to the auditory nerve. Ocular prostheses (artificial eyes) restore the appearance of an eye socket after removal of a damaged eye. Even breast prostheses worn after mastectomy fall under this umbrella.
For limb prostheses specifically, the two broad categories are upper-limb (arms, hands, fingers) and lower-limb (legs, feet, toes). Within each category, the design depends on where the amputation occurred. A below-knee prosthesis, for instance, is mechanically simpler than an above-knee one because the person still has their own knee joint to control movement.
How a Limb Prosthesis Is Built
A typical limb prosthesis has three core components: the socket, the pylon, and the terminal device.
- Socket: The custom-molded piece that fits over the residual limb. It’s the most critical part of the prosthesis because a poor fit causes pain, skin breakdown, and reduced use. Sockets are typically made from rigid plastic or carbon fiber composites, often with a soft liner against the skin.
- Pylon: The structural shaft connecting the socket to the terminal device. Traditional pylons are rigid tubes, usually made of aluminum, titanium, or carbon fiber. Some newer designs use shock-absorbing pylons that compress slightly with each step to cushion impact forces, similar to how a natural ankle absorbs shock.
- Terminal device: The functional end piece. For a leg, this is the prosthetic foot. For an arm, it could be a hand, a hook, or a specialized tool designed for specific tasks like sports or manual work.
A suspension system keeps the whole assembly attached to the body. This might be a suction seal, a pin-lock mechanism, or a harness system, depending on the type of prosthesis and the user’s anatomy.
Three Ways Prosthetic Limbs Are Controlled
Not all prostheses move. Passive (cosmetic) prostheses restore the appearance of a missing limb without active grip or motion. They’re lighter and require less maintenance, which makes them a practical choice for people who prioritize comfort and appearance over mechanical function.
Body-powered prostheses use a cable-and-harness system that translates shoulder or trunk movement into motion at the terminal device. Shrugging a shoulder, for example, pulls a cable that opens or closes a prosthetic hand. One advantage of this design is that users get direct physical feedback through the cable tension. They can feel how much force the device is exerting and sense its position, much like the feedback loop in a natural limb.
Myoelectric prostheses take a different approach. Sensors on the skin of the residual limb detect electrical signals from the underlying muscles. When the user contracts specific muscles, those signals are amplified and translated into motorized movement of the prosthetic hand or wrist. These devices look more natural and don’t require a harness, but they rely heavily on vision for feedback. Users tend to watch their prosthetic hand closely during tasks because they can’t feel what it’s doing the way a body-powered user can through cable tension.
Materials and Durability
Early prostheses were made from wood and leather. Modern devices use engineered materials chosen for their strength-to-weight ratio. Titanium and aluminum provide structural strength in pylons and joint mechanisms. Carbon fiber composites are increasingly common because they’re both lightweight and extremely strong, which is why the curved “blade” feet used in competitive running are made from layered carbon fiber.
Plastics and silicone play important roles too. Silicone liners cushion the residual limb inside the socket, while thermoplastics can be molded into complex shapes for socket fabrication. Prosthetic feet and hands often combine rigid internal frames with flexible outer coverings that mimic the appearance and give of natural tissue.
Most prosthetic limbs need replacement or significant refurbishment every three to five years, though this varies with activity level and the complexity of the device. Sockets may need adjustment or replacement more frequently, especially if the residual limb changes shape due to weight fluctuation or muscle atrophy. Mechanical components like cable systems and joints wear down with daily use and require periodic maintenance.
What Rehabilitation Looks Like
Getting a prosthesis isn’t a one-day event. The rehabilitation process for a limb prosthesis typically spans several months. Before a prosthesis is even fitted, physical therapy focuses on strengthening the residual limb, maintaining range of motion, and managing swelling. Studies of lower-limb rehabilitation programs report average durations of roughly 78 days, though individual timelines range from about 40 to 126 days depending on factors like overall health, amputation level, and motivation.
Once the prosthesis is ready, gait training begins. For a leg prosthesis, this starts with standing balance, progresses to walking on flat surfaces, and eventually includes stairs, uneven terrain, and community walking. Training sessions are typically 30 minutes, two to three times per week, continuing for several weeks. Upper-limb rehabilitation follows a similar pattern, starting with basic grip-and-release exercises before moving to complex daily tasks like cooking, dressing, or typing.
The first prosthesis a person receives is rarely their last. As strength and skill improve, users often graduate to more advanced devices. And because the residual limb continues to change shape over the first year or two after amputation, socket refitting is a normal part of the process.
Osseointegration: A Different Way to Attach
Conventional prostheses sit on top of the residual limb inside a socket. Osseointegration offers an alternative: a metal implant is surgically anchored directly into the bone, and the prosthesis clicks onto a post that extends through the skin. This eliminates the socket entirely, which can be life-changing for people who experience chronic socket discomfort, skin irritation, or an unstable fit.
The procedure is best studied for below-knee amputations. For above-knee cases, adequate bone length is needed, and some patients require additional internal reinforcement. Candidates are generally between 22 and 65 years old (per FDA guidelines in the U.S.), and people with conditions that impair bone healing, such as osteoporosis, uncontrolled diabetes, or a history of radiation therapy, are typically excluded. The recovery demands commitment: patients need to be psychologically prepared and willing to follow a structured post-operative rehabilitation program to allow the bone to fuse securely with the implant.
Cost and Accessibility
The price of a prosthesis depends enormously on its technology level. A basic mechanical prosthetic arm operated by cables and a harness runs between $5,000 and $10,000. A myoelectric arm with motorized fingers and wrist rotation costs $20,000 to $50,000. Microprocessor-controlled legs, which use onboard sensors to adjust automatically to walking speed and terrain, range from $30,000 to $60,000. Specialized devices designed for athletics or demanding occupations can exceed $60,000.
Insurance coverage varies widely. In the U.S., Medicare and most private plans cover prosthetic devices, but they may limit which technology tier is approved. Many people end up paying significant out-of-pocket costs for higher-function devices, and replacement cycles every few years compound the financial burden over a lifetime. Globally, access is far more uneven. In lower-income countries, even basic prostheses can be out of reach, which has driven efforts by nonprofits and researchers to develop low-cost designs using locally available materials and 3D printing.
Sensory Feedback Technology
One of the biggest limitations of any prosthesis is the loss of sensation. You can’t feel the temperature of a coffee cup, the firmness of a handshake, or the texture of fabric through a conventional prosthetic hand. Researchers have been working on this problem for decades, using approaches that include vibrating motors on the skin, small electrical pulses, pressure cuffs, and more recently, electrodes implanted directly into peripheral nerves.
Surgically implanted systems have shown the most promise for restoring something close to natural touch. In laboratory settings, users with these systems can distinguish between objects of different sizes and stiffnesses and apply more appropriate grip force without having to watch their hand constantly. Non-invasive approaches, like vibration patterns mapped to specific fingers, are simpler and don’t require surgery, but the sensory information they provide is cruder and takes longer to learn to interpret. Both approaches remain largely in the research phase, though the pace of development has accelerated significantly in recent years.