A body-powered prosthesis is an artificial limb designed to replace a missing upper limb. This type of prosthesis functions by harnessing the user’s own body movements to operate its various components. Unlike more complex systems, it relies on a direct, mechanical connection between the user and the device.
Mechanism and Operation
The operation of a body-powered prosthesis begins with a custom-fit socket, which encases the residual limb. This socket serves as the interface, transferring forces from the limb to the prosthetic device. The precise fit of the socket is important for both comfort and effective control, as it prevents slippage and distributes pressure evenly.
Attached to the socket is a harness system, often a figure-eight design, which loops around the user’s shoulders and back. Control cables connect specific points on the harness to the terminal device, such as a hook or hand.
When the user performs specific body movements, such as protracting their shoulder or expanding their chest, tension is created in these control cables. For example, a forward movement of the shoulder might pull a cable that causes a prosthetic hook to open, or a different movement could cause it to close. This direct mechanical linkage provides intuitive control, allowing the user to feel the tension and movement, which aids in proprioceptive feedback.
Common Types and Applications
Body-powered prostheses come in various forms, distinguished by their terminal devices and amputation level. The most common terminal devices are hooks and prosthetic hands. Hooks are often preferred for their durability, lighter weight, and precision in certain tasks, such as grasping small objects or manipulating tools. They are frequently used in industrial or outdoor settings where robust function is a priority.
Prosthetic hands, while offering a more natural appearance, provide basic gripping functions. These hands are often chosen when cosmetic integration is important, or for tasks that require a delicate grip. Both hooks and hands can be designed as voluntary opening (VO) or voluntary closing (VC) devices. Voluntary opening hooks are held closed by rubber band tension and open when the user pulls the cable, while voluntary closing devices require continuous tension from the user to maintain grip force.
These prostheses are adaptable for various levels of upper limb amputation, including transradial and transhumeral levels. For transhumeral amputations, the prosthesis will also include an elbow unit, and for shoulder disarticulation, a shoulder component is added. Body-powered prostheses are suitable for individuals engaged in manual labor or heavy-duty activities due to their ability to withstand challenging environments and provide robust function.
Advantages and Limitations
Body-powered prostheses offer advantages. They are known for their durability and reliability, as they do not rely on an electrical power source or batteries, making them suitable for prolonged use and rugged environments. They are also lighter in weight and less expensive compared to their myoelectric counterparts. The direct mechanical linkage provides users with proprioceptive feedback, meaning they can feel the tension in the cables, which helps in understanding the position and force applied by the terminal device.
Despite these benefits, body-powered prostheses also have limitations. Their cosmetic appearance is a drawback, particularly with hook-style terminal devices. Operating these prostheses requires physical effort, as the user must exert muscle force to tension the cables, which can lead to fatigue over time, especially during prolonged or repetitive tasks. The harness system, while functional, can cause discomfort, skin irritation, or snag clothing. Body-powered devices often have a limited range of motion and grip force compared to more advanced prosthetic systems, making fine motor skills challenging.