A prosthetic arm is an advanced device designed to restore function and independence following limb loss. These artificial limbs range from simple cosmetic models to sophisticated robotic systems, all aiming to replicate the capabilities of the missing arm. The term “hybrid” in this context refers to a specific design that integrates two distinct methods of control and power within a single prosthetic device. This integration typically combines the user’s physical body movement with an external power source to operate different parts of the arm effectively. The hybrid approach balances mechanical simplicity and powered precision to offer a more versatile solution for complex limb differences.
Defining the Hybrid Arm
The fundamental concept of the hybrid arm is its ability to utilize power from two different sources simultaneously to control separate functions. This design merges the immediate feedback and mechanical reliability of a body-powered system with the enhanced precision and strength of an externally powered system. A common configuration involves the body-powered mechanism controlling the elbow joint, while the terminal device, such as the hand or hook, is operated by an external power source. This division of control is beneficial for individuals with transhumeral, or above-elbow, amputations, who require functional control over multiple joints.
The body-powered component operates through a harness and cable system, leveraging the user’s gross body movements to perform actions like elbow flexion and locking. Conversely, the externally powered component, often myoelectric, uses motors and batteries to provide grip force and fine motor control to the terminal device. By assigning the more power-intensive and precise movements to the myoelectric system, the hybrid arm achieves a functional outcome superior to either pure system. This combination results in a device that is typically lighter and less complex than a fully myoelectric arm, while still offering the benefits of powered grip strength.
The Dual Control Systems
The operation of a hybrid arm relies on the user coordinating two distinct control mechanisms to achieve movement. The body-powered control is initiated by the user’s movement of the shoulder and surrounding muscles, anchored by a harness worn over the shoulders and torso. Tension created by movements like shoulder flexion or chest expansion pulls a cable, often a Bowden cable, which actuates a mechanical component, such as the elbow joint or a mechanical lock. This mechanical link provides the user with an immediate sense of position and force, known as proprioceptive feedback, which is helpful for controlling gross movements.
The externally powered aspect is driven by myoelectricity, which uses electromyography (EMG) sensors embedded within the prosthetic socket. These sensors detect the minute electrical signals generated when the user contracts residual muscles in the limb. The impulses are amplified and sent to a microprocessor, which translates the signal into commands for the motors in the terminal device. Users must learn to isolate and control these muscle contractions, a process requiring focused training to achieve proportional speed and strength control. The simultaneous control of the body-powered elbow and the myoelectric hand allows the user to position the arm with body movement while performing delicate tasks with the powered terminal device.
Components and Structure
The physical design of a hybrid prosthesis must accommodate the hardware for both the mechanical and electronic control systems. The prosthetic socket, which connects the device to the residual limb, is a highly customized component that must securely anchor the entire system. This socket is precisely molded to provide intimate contact for the embedded myoelectric sensors and stable attachment points for the body harness and cable system. A well-fitted socket is necessary for comfort and suspension, and to ensure the myoelectric electrodes maintain consistent contact with the skin to accurately detect muscle signals.
The body-powered structure includes the external harness, often a figure-of-eight design, made of straps that distribute the control forces across the shoulder and back. The cable housing, which contains the flexible control cable, is anchored to the harness proximally and the prosthetic elbow distally, transmitting the tension necessary for joint movement. For the externally powered functions, the structure integrates a battery pack, a microprocessor controller, and small, high-torque motors, often located in the forearm and terminal device. These electronic components power the terminal device, which is typically an advanced electronic hand or hook capable of stronger grip force and more complex movements than its body-powered counterpart.
Application and Candidacy
Hybrid arm prostheses are predominantly prescribed for individuals who have undergone transhumeral amputations, involving limb loss above the elbow joint. At this level, the user needs to control both the elbow and the terminal device, making a single power source inefficient for managing all functions. The hybrid configuration provides a practical solution by allowing the user to operate the proximal joint (elbow) reliably with body power and the distal joint (hand/hook) with the precision of myoelectric power. This dual functionality expands the usable work envelope, enabling a greater range of movement and activity.
The system offers inherent reliability and adaptability to various situations. The body-powered component serves as a mechanical backup, ensuring the user retains function even if the battery powering the myoelectric hand is depleted. This makes the hybrid arm a preferred option for individuals who work in environments where electronic components might be susceptible to damage from moisture, dust, or excessive vibration. The ability to switch between the immediate feedback of the body-powered control for heavy lifting and the fine manipulation of the myoelectric terminal device allows the user to perform a wider array of daily and occupational tasks.