10.3 Upper Limb Prosthetics — Body-Powered vs Myoelectric

Upper Limb Prosthetics — Body-Powered vs Myoelectric

Upper limb amputation results in significant loss of motor execution, tactile sensation, and body image. In physical medicine and rehabilitation, prescribing an upper extremity prosthesis requires a thorough evaluation of amputation level (transradial, transhumeral, shoulder disarticulation), residual limb characteristics (length, soft tissue coverage, phantom pain), cognitive status, work/recreational goals, and environmental exposure. Upper limb prostheses fall into two major functional categories: Body-Powered Harness Systems and Externally Powered (Myoelectric) Systems.

Body-Powered Prosthetic Harness Systems

Body-powered prostheses utilize a custom fabric harness cable system worn across the shoulders. Muscle movements of the shoulder girdle and intact proximal joints transmit mechanical tension through steel control cables to operate the terminal device (hook or hand) or elbow joint.

Cable System Biomechanics & Motion Inputs

To operate a transradial body-powered prosthesis, two primary body movements create cable tension:

1. Glenohumeral Flexion: Forward elevation of the residual limb pulls the cable attached to the posterior harness ring.
2. Scapular Abduction (Protraction): Spreading the shoulder blades apart pulls the cable across the back.

For transhumeral prostheses, a dual-cable system is required:

• Front Cable (Elbow Lock Cable): Operated by shoulder depression, elevation, and backward extension to lock and unlock the mechanical elbow joint.
• Rear Cable (Main Control Cable): Operated by glenohumeral flexion and scapular protraction. When the elbow is locked, cable tension operates the terminal device; when the elbow is unlocked, cable tension flexes the prosthetic elbow joint.

Terminal Devices: Voluntary Opening vs. Voluntary Closing

The terminal device (TD) serves as the synthetic hand or hook at the distal extremity.

• Voluntary Opening (VO) Terminal Devices:
  • Mechanism: The terminal device is held closed at rest by elastic rubber bands or internal springs. Pulling the control cable actively opens the hook or hand; relaxing tension allows the springs to pull the device closed around an object.
  • Pinch Force: Dictated entirely by the number of rubber bands applied to the hook (each standard rubber band provides approximately 1 to 1.5 lbs of pinch force, typically capped at 6-8 lbs).
  • Advantages & Disadvantages: Extremely low user fatigue when carrying objects, as no continuous cable tension is required to hold items. However, maximum pinch force is limited, and grasping delicate objects can crush them if spring tension cannot be modulated.
• Voluntary Closing (VC) Terminal Devices:
  • Mechanism: The terminal device is open at rest. Pulling the control cable actively closes the fingers/hook around an object; relaxing cable tension allows the device to open.
  • Pinch Force: Proportional to the physical effort exerted by the user (can exceed 50 lbs of high-strength pinch force).
  • Advantages & Disadvantages: Permits precise velocity and graded pressure control, enabling the user to grasp fragile objects softly or heavy objects tightly. The primary drawback is high user fatigue, as continuous muscular exertion and cable tension must be sustained to maintain grip.

Myoelectric Prostheses, Advanced Control & TMR

Myoelectric prostheses eliminate body harnesses by utilizing electric motors powered by internal lithium-ion batteries.

Surface EMG Electrode Control

Myoelectric control relies on surface EMG electrodes embedded flush within the inner socket lining directly over residual muscle bellies.

• Dual-Site Direct Control: The standard control scheme. One surface electrode is positioned over agonist muscles (e.g., wrist flexors or biceps) to trigger hand closing, while a second electrode is placed over antagonist muscles (e.g., wrist extensors or triceps) to trigger hand opening.
• Proportional Control: The velocity and pinch force of the motor are directly proportional to the microvolt amplitude of the muscle contraction detected by the EMG sensors.

Advanced Control: Pattern Recognition

Traditional dual-site control requires isolated, sequential muscle contractions, which becomes challenging for high-level amputations (transhumeral, shoulder disarticulation) where limited muscle sites remain.

• Pattern Recognition Systems: Utilize an array of 8 or more surface EMG electrodes arranged circumferentially around the residual limb. Instead of measuring isolated muscle voltage spikes, a microprocessor analyzes spatial and temporal patterns of muscle activity during complex muscular contractions. Machine learning algorithms compare real-time EMG signals against calibrated movement profiles, permitting intuitive, simultaneous multi-articulating hand movements (e.g., wrist pronation, wrist supination, thumb power grip, precision pinch).

Targeted Muscle Reinnervation (TMR)

Targeted Muscle Reinnervation (TMR) is a revolutionary neurosurgical procedure performed at the time of amputation or as a delayed reconstructive surgery.

• Surgical Technique: Severed major motor nerve trunks (e.g., median, ulnar, radial, and musculocutaneous nerves) are microsurgically transferred and end-to-side anastomosed to denervated motor points of redundant target muscles (such as specific heads of the pectoralis major in transhumeral amputees, or residual flexor/extensor compartments in transradial amputees).
• Biomechanical Impact: The target muscles act as biological signal amplifiers. When the amputee visualizes closing their missing hand, motor impulses travel down the original median nerve into the reinnervated pectoralis muscle segment, generating a robust surface EMG signal directly over that segment.
• Clinical Benefits:
  1. Intuitive Control: Permits simultaneous, multi-joint myoelectric control (e.g., flexing elbow and closing hand simultaneously without switching modes).
  2. Pain Mitigation: Dramatically reduces post-amputation phantom limb pain and symptomatic neuroma formation by providing severed motor axons with target endplates to reinnervate.