An upper-limb prosthetic is an artificial device designed to replace a missing arm or part of an arm, helping to restore physical function and appearance. These devices are complex tools that act as an extension of the user, but their effectiveness depends entirely on the specific technology selected. The user’s goals, the level of limb loss, and their remaining physical capability all influence which type of prosthetic will provide the most benefit.
Categorization of Prosthetic Arms
The three main types of upper-limb prosthetics offer distinct trade-offs between function, cost, and complexity. Passive or cosmetic prostheses replicate the natural look of the hand and arm, often using lightweight silicone covers. While they lack active movement, they provide a stationary tool for tasks like stabilizing objects. They are generally the lightest and most affordable option.
Body-powered prostheses, also known as mechanical prostheses, are functional devices operated by a simple cable and harness system. Movement of the user’s body, such as shrugging a shoulder or flexing the chest, pulls a cable that opens or closes the terminal device, typically a hook or a simple grasping hand. This mechanical linkage provides the user with direct force feedback, allowing them to feel the resistance of objects they are grasping. They are durable, weatherproof, and do not require a battery, making them reliable for heavy manual labor and rugged environments.
Externally powered, or myoelectric, prostheses represent the most technologically advanced category, relying on a battery and electric motors for movement. These devices use electrodes placed within the custom-fitted socket to detect tiny electrical signals generated by the contraction of residual muscles. These signals are processed by a microprocessor to command the motors, which can actuate a hand, wrist, or elbow joint. Myoelectric arms offer a more natural appearance and can provide greater precision and a range of gripping patterns compared to mechanical devices.
Methods of Control and Operation
The operation of a body-powered prosthetic involves a direct mechanical relationship between the user’s gross body movement and the terminal device. For example, a figure-of-eight harness around the shoulders connects to a cable that runs down the arm, so that shrugging the shoulder opens the hook. This direct control provides immediate and proportional feedback, meaning the user can feel exactly how much force they are exerting on the cable.
Myoelectric control uses electromyography (EMG) signals, which are the electrical impulses generated by muscle fiber contraction. Electrodes embedded in the prosthetic socket sense these faint signals. A control system amplifies and translates these signals into commands that drive the prosthetic’s motors. Users learn to contract specific muscle pairs—like a flexor and an extensor—to produce opposing movements, such as opening and closing the hand.
Advanced control techniques like Targeted Muscle Reinnervation (TMR) improve the function of myoelectric devices. TMR is a surgical procedure where nerves severed during amputation are rerouted to reinnervate a small, non-functional muscle section in the residual limb or chest. This procedure creates new, intuitive muscle sites that produce distinct EMG signals. This allows the user to control multiple joints, like the hand and wrist, with separate, more natural thoughts, enabling simultaneous, complex movements.
Functional Capabilities and Limitations
Modern multi-articulating myoelectric hands achieve a variety of grip patterns, ranging from simple pinch grips to complex cylindrical or spherical grasps. This dexterity is achieved through individually powered fingers and a rotating thumb, enabling users to perform detailed tasks like handling cutlery or typing. The grip force of these electrically powered hands is substantial, often exceeding 20 pounds of force.
These prosthetics face limitations in speed and sensory feedback. The speed of movement is constrained by the motor and battery systems, meaning the hand cannot react as instantaneously as a biological hand. The lack of true sensory feedback, such as proprioception (the sense of limb position) and touch, is a major issue. Without this feedback, users must rely heavily on visual cues to regulate grip force, which increases cognitive demand and can lead to accidental drops or crushing of delicate objects.
Body-powered devices, while less dexterous, are often more reliable and faster for basic tasks. Their cable system provides a form of mechanical feedback that is absent in most myoelectric systems. The high cost and the need for daily battery charging also represent practical limitations for externally powered devices. Research is focused on integrating non-invasive or surgically implanted sensors to provide users with tactile information, improving performance and acceptance.
The Journey to Use (Fitting and Training)
The process of receiving a prosthetic arm begins with a comprehensive assessment where a prosthetist evaluates the residual limb, muscle strength, and the user’s specific lifestyle and functional goals. The most crucial component is the custom-fabricated socket, which must fit the residual limb perfectly to ensure comfort, secure attachment, and effective signal transmission for myoelectric devices.
The prosthetist takes detailed measurements or a digital scan of the limb to create a mold from which the rigid socket is formed. This fitting process is iterative, often requiring several visits to make adjustments as the limb changes shape due to swelling reduction or muscle changes. A comfortable and snug socket is paramount, as a poor fit can cause pain, skin breakdown, and lead to the rejection of the device.
Once the prosthetic is delivered, physical and occupational training is required to master its control system. For myoelectric users, this involves learning to isolate and consistently contract the correct residual muscles to generate EMG signals. This functional training, which can take up to a year, involves practicing basic movements, key grip patterns, and integrating the device into daily tasks, alongside managing maintenance and battery requirements.