Functional Upper Extremity Prosthetics: Types, Training, and Tech
Harness and Control Systems in Prosthetics
Harness systems are an essential part of body-powered prostheses. They serve two main purposes: suspension, which holds the prosthesis securely on the residual limb, and control, which transmits body movements (usually shoulder girdle motions) to operate terminal devices, joints, or locks.
Harnesses are usually made of leather, Dacron, or webbing straps with metal or plastic control cables.
Below-Elbow (Transradial) Harnessing
Aims
- Provide suspension for the prosthesis.
- Enable control of the terminal device (hook/hand) and, in the case of long stumps, wrist units.
Components
- Figure-of-8 or Figure-of-9 Harness: The Figure-of-8 provides both suspension and control, while the Figure-of-9 offers mainly control with less suspension.
- Axilla Loop: Positioned around the opposite shoulder, it serves as an anchor point.
- Control Cable & Housing: Connects the harness to the terminal device.
- Anterior Support Strap (Optional): Provides better suspension.
Mechanism of Control
Biscapular abduction (scapulae moving apart) and shoulder flexion pull on the control cable. This force is transmitted to the terminal device, causing the hook or hand to open. Relaxation allows a spring force to close the device.
Indications for Use
- Transradial (below-elbow) amputation.
- Cases where the prosthesis must be light, simple, and reliable (e.g., for children or in rural settings).
- Situations where cosmetic appeal is less important than function.
Shoulder Amputee Harnessing
This includes shoulder disarticulation and forequarter amputations, where suspension and control are much more challenging.
Aims
- Securely suspend a heavy prosthesis.
- Provide control of the terminal device, elbow, and shoulder joints in a mechanical prosthesis.
- Allow stability during limb movements.
Harnessing Systems
- Chest Strap & Axilla Loop Harness: Encircles the chest and opposite axilla, providing anchorage for the control cable.
- Modified Figure-of-8 Harness: Adapted to cover the chest.
- Shoulder Cap or Saddle: Distributes the weight of the prosthesis over the shoulder and chest.
Mechanism of Control
Scapular abduction, chest expansion, and shoulder elevation generate cable movement. Control forces are weaker compared to below-elbow cases; hence, there is more reliance on mechanical joints with locks (elbow lock, shoulder lock). These systems are often combined with switches, ratchets, or harness pads to improve efficiency.
Indications for Use
- Shoulder disarticulation or forequarter amputations.
- Cases where a body-powered prosthesis is chosen over a myoelectric one.
- When high suspension stability is needed due to the lack of a residual limb.
Comparison: Below-Elbow vs. Shoulder Harnessing
Below-Elbow Harness
- Suspension: Easy, as the stump provides leverage.
- Control Force: Strong (utilizing scapular abduction and shoulder flexion).
- Harness Type: Figure-of-8 or Figure-of-9.
- Indication: Transradial amputation.
- Cable System: Single cable for simple control.
Shoulder Amputee Harness
- Suspension: Difficult, requires chest/shoulder support.
- Control Force: Weak (relying on chest expansion and elevation).
- Harness Type: Chest strap, shoulder cap, or saddle.
- Indication: Shoulder disarticulation or forequarter amputation.
- Cable System: Dual cable or auxiliary control system.
Summary
Harnesses in prosthetics provide both suspension and control. Below-elbow harnessing, typically a Figure-of-8 or Figure-of-9, uses shoulder motion to control terminal devices. In contrast, shoulder amputee harnessing is more complex, needing chest straps and shoulder caps to compensate for the loss of limb leverage.
Clinical Aspects of Upper Extremity Prostheses
An upper extremity (UE) prosthesis is an artificial substitute that replaces part of an amputated upper limb. Unlike lower-limb prostheses, the primary role of UE prostheses is not only locomotion but also function, cosmesis, and psychological rehabilitation. The clinical approach to prescribing and fitting a UE prosthesis involves understanding the amputation level, the patient’s functional goals, their medical condition, and lifestyle requirements.
Levels of Amputation and Clinical Implications
The clinical challenges and prosthetic designs vary depending on the level of amputation:
- Partial Hand Amputation: Involves the loss of digits or part of the hand. The prosthesis may be cosmetic silicone fingers, a functional partial hand prosthesis, or an activity-specific device. The main clinical concern is maintaining grip strength and dexterity.
- Wrist Disarticulation: Amputation through the wrist joint. The prosthesis provides good leverage and suspension due to the long lever arm. The challenge is the limited space for prosthetic wrist units.
- Transradial (Below-Elbow): The most common UE amputation. The prosthesis can be body-powered, myoelectric, or hybrid. The clinical concern is ensuring adequate pronation-supination and socket comfort.
- Elbow Disarticulation: Amputation through the elbow joint. The prosthesis offers a long lever arm and allows for strong suspension. Limitations include cosmesis and a bulky elbow joint.
- Transhumeral (Above-Elbow): Involves the loss of the forearm and elbow. The prosthesis is more complex, requiring an elbow joint and a control system. The clinical concern is training the patient in dual control (elbow and terminal device).
- Shoulder Disarticulation / Forequarter: The most disabling, involving the loss of the entire arm or the arm with the scapula/clavicle. The prosthesis is heavy with complex harnessing and is often more cosmetic than functional. Clinical concerns include suspension and psychological adjustment.
Types of Upper Extremity Prostheses
Clinically, prostheses are classified as:
- Cosmetic (Passive) Prostheses: Made of silicone or PVC for a lifelike appearance. They provide balance and cosmesis with limited function. They are used for patients prioritizing appearance over function.
- Body-Powered Prostheses: Controlled by a harness and cable system. Terminal devices (hook, hand) are operated via shoulder or scapular movements. They are durable, inexpensive, and suitable for children and laborers.
- Myoelectric Prostheses: Controlled by electrical signals from residual muscles (EMG). They offer a strong grip, better cosmesis, and less dependence on a harness. They are used for patients needing fine motor function and better cosmetic appeal.
- Hybrid Prostheses: A combination of body-powered (for elbow lock) and myoelectric (for terminal device) systems. They are often used for transhumeral amputees requiring dual control.
- Activity-Specific Prostheses: Designed for special tasks like sports, driving, or using work tools. They improve a patient’s participation in hobbies and work.
Clinical Aspects in Prescription
Before prescribing, clinicians assess:
- Medical Status: Residual limb condition, skin integrity, pain, and range of motion (ROM).
- Level of Amputation: Determines the complexity of the prosthesis.
- Patient Factors: Age, occupation, lifestyle, and psychological readiness.
- Financial and Environmental Factors: Cost, availability, and maintenance needs.
Socket Design and Clinical Relevance
The socket is the most important part of the prosthesis and must provide total contact to distribute pressure evenly. For transradial amputations, self-suspending sockets (Muenster, Northwestern) are common. For transhumeral amputations, a split-socket design aids suspension and control. The primary clinical challenge is preventing skin breakdown and ensuring comfort.
Suspension and Control Systems
- Suspension: Options include a Figure-of-8 harness, shoulder caps, suction sockets, or liners.
- Control: Body-powered systems use a cable and harness. Myoelectric systems use EMG sensors. Hybrid systems use a mix of both.
Clinical Aspect: Suspension must be secure but comfortable, and the control system should match the patient’s abilities.
Training and Rehabilitation
Successful UE prosthetic use requires structured rehabilitation:
- Pre-prosthetic Phase: Wound healing, edema control, desensitization, and strengthening exercises for the residual and contralateral limbs.
- Prosthetic Fitting Phase: Socket fitting, alignment, and trial use for comfort and adjustments.
- Prosthetic Training Phase: Donning/doffing training, control training (harness/cables or EMG electrodes), and functional training for Activities of Daily Living (ADLs) like eating, grooming, and dressing. Advanced training covers vocational and recreational activities.
Clinical Complications
- Skin Problems: Pressure sores, dermatitis, and sweating.
- Phantom Limb Pain: Requires desensitization, mirror therapy, and medications.
- Mechanical Issues: Cable breakage or electrode malfunction.
- Psychological Issues: Depression, prosthesis rejection, and poor body image.
Outcome Assessment
Clinicians evaluate prosthetic success using:
- Functional Outcome Measures: Ability to perform ADLs.
- Cosmetic Satisfaction: Appearance and social acceptance.
- Patient-Reported Outcomes: Comfort, confidence, and quality of life.
- Prosthesis Wearing Time: An indicator of acceptance.
Clinical Decision-Making
- Children: Often prefer body-powered prostheses due to durability and cost, or cosmetic options in early years.
- Adults: May choose myoelectric/hybrid options for improved function and cosmesis.
- Elderly/Low-Resource Patients: Body-powered or cosmetic prostheses are chosen for their simplicity.
Future Clinical Trends
- Osseointegration: Direct skeletal attachment, eliminating the need for a harness.
- Advanced Myoelectric Control: Pattern recognition and AI-driven prostheses.
- Sensory Feedback Systems: Restoring the sensation of touch.
- 3D Printing: Creating affordable, customized prosthetics.
Conclusion
The clinical aspects of upper extremity prostheses revolve around a careful patient evaluation, choosing the appropriate type of prosthesis, ensuring proper socket fit and suspension, and training the patient in functional use. Each level of amputation presents unique challenges, and the prosthesis must balance function, comfort, cosmesis, and cost. Ultimately, successful clinical management depends not just on the prosthesis itself but on a comprehensive rehabilitation program, patient motivation, and ongoing follow-up to maximize independence and quality of life.
Training for Upper Extremity Prosthesis Use
Introduction
The fitting of an upper extremity prosthesis is only the beginning of rehabilitation. True success depends on the patient’s ability to learn, control, and integrate the prosthesis into daily life. Training ensures that the patient gains functional independence, confidence, and long-term acceptance of the device. This training must be systematic, starting with pre-prosthetic preparation, followed by basic control training, functional skill training, and finally integration into activities of daily living (ADLs), as well as vocational and recreational activities.
Pre-Prosthetic Training
Before introducing the prosthesis, the patient undergoes preparation to optimize outcomes:
- Residual Limb Conditioning: Includes desensitization (massage, tapping, vibration), edema control (compression bandaging, shrinkers), and skin care (hygiene, moisturization, scar management).
- Range of Motion (ROM) Exercises: Active and passive exercises to prevent contractures in the shoulder, elbow, or wrist.
- Muscle Strengthening: For muscles required for harness control (scapular muscles, shoulders) and strengthening of the contralateral limb for bilateral activities.
- Psychological Preparation: Counseling for body image issues, building motivation, and setting realistic expectations.
Initial Prosthetic Orientation
Once the prosthesis is fitted, the patient is introduced to its parts and functions:
- Identification of Components: Socket, suspension, harness, control cable, terminal device (hand/hook), wrist unit, and elbow joint.
- Donning and Doffing Training.
- Wearing Schedule: Start with short periods (30–60 minutes) and gradually increase.
- Skin Inspection: Teaching the patient to recognize redness, irritation, or pressure spots after wearing.
Control Training
This phase focuses on learning to operate the prosthesis.
Body-Powered Prosthesis Training
Patients are taught harness-controlled motions, such as using scapular abduction and shoulder flexion to open the terminal device and relaxation to close it. This involves repetition drills and cable control exercises against resistance.
Myoelectric Prosthesis Training
This involves electrode site training, where the patient learns to contract residual limb muscles (e.g., wrist flexors/extensors, biceps/triceps). Visual or auditory feedback is used initially. Patients practice graded contractions, where a gentle contraction leads to slow hand closure and a strong contraction results in a fast, powerful grip.
Hybrid Prosthesis Training
Typically for above-elbow amputees, this training teaches dual control. For example, a shoulder motion (cable) controls the elbow lock/unlock, while an EMG contraction controls the hand opening/closing. This requires sequencing practice.
Functional Training
This phase involves learning to use the prosthesis in purposeful activities.
- Gross Motor Activities: Reaching, picking up large objects, and stabilizing objects.
- Fine Motor Activities: Picking up small items, buttoning shirts, and writing.
- Bimanual Coordination: Using the prosthesis and the sound limb together, such as cutting food or opening a jar.
- ADL Training: Eating, grooming, bathing, dressing, cooking, and light household tasks.
- Vocational and Recreational Training: Tailored to the patient’s job, hobbies, or sports.
Prosthetic Skill Progression
Training typically follows a step-wise progression:
- Basic Control: Open/close the terminal device without objects.
- Controlled Grasp/Release: Practice with objects of different sizes and weights.
- Bimanual Tasks: Integrate the prosthesis with the sound limb.
- Complex Tasks: ADLs, vocational, and recreational activities.
- Speed & Accuracy Training: Timed activities to improve efficiency.
Problem-Solving Training
Patients are taught to recognize and solve common problems, such as adjusting harness tightness, identifying socket pressure points, replacing cables, recharging batteries, and cleaning the prosthesis.
Psychological and Social Reintegration
Training must include confidence-building for public use. Counseling for anxiety or self-consciousness is often provided, along with access to support groups and peer mentoring.
Common Challenges During Training
- Phantom limb pain interfering with practice.
- Skin irritation from prolonged wear.
- Poor motivation or unrealistic expectations.
- Difficulty mastering dual control in above-elbow prostheses.
- Rejection of the prosthesis if training is incomplete.
Role of the Multidisciplinary Team
- Physiatrist/Rehabilitation Physician: Provides overall medical supervision.
- Prosthetist: Handles fitting, alignment, and mechanical adjustments.
- Occupational Therapist: The primary trainer for functional use and ADLs.
- Physiotherapist: Manages pre-prosthetic exercises and strengthening.
- Psychologist: Addresses motivation, coping, and adjustment.
- Social Worker/Vocational Counselor: Assists with reintegration into work and society.
Outcome Assessment
The success of training is measured by wearing time per day, ability to perform ADLs independently, efficiency of control, patient satisfaction, and return to work, school, and social activities.
Conclusion
Training in the use of an upper extremity prosthesis is a systematic, multi-stage process. The key to success lies in a well-fitting prosthesis, patient motivation, skilled rehabilitation training, and strong follow-up support. When properly trained, patients can achieve remarkable levels of independence and quality of life.
Externally Powered Prosthetic Systems
Introduction
Traditional upper limb prostheses are body-powered, controlled by harnesses and cables. While reliable and durable, they are limited in grip strength, cosmetic appearance, and fine motor control. Externally powered prostheses were developed to overcome these limitations. These devices rely on external power sources (electrical motors, batteries) to move joints and operate terminal devices. Among them, the myoelectric prosthesis is the most widely used and clinically accepted today.
General Features of Externally Powered Prostheses
- Power Source: Batteries (usually rechargeable lithium-ion).
- Control System: Electrical signals, switches, or sensors.
- Actuators: Electric motors or servomechanisms to move prosthetic joints.
- Functional Parts: Terminal devices (hands, hooks), wrist units, and elbow joints.
- Cosmetic Design: Often covered with a silicone glove for a natural appearance.
Electro-Mechanical Prostheses
These are prostheses where electric motors and mechanical linkages drive prosthetic movements. They are powered by rechargeable batteries, and movement is controlled by switches, buttons, or sensors rather than muscle signals. They are useful for patients who cannot generate reliable muscle signals but are less intuitive and have limited fine motor control.
Myoelectric Prostheses
Myoelectric prostheses are the most advanced externally powered prostheses. They use electrical signals generated by muscles (Electromyography – EMG) to control motors that move the prosthesis.
Principle
When a muscle contracts, it produces tiny electrical signals. Surface electrodes on the skin over the residual limb detect these signals, which are then amplified and processed by microprocessors. These processed signals drive electric motors to open/close the hand or move joints.
Components
- Power Source: Rechargeable battery.
- Electrodes: Detect EMG signals from residual muscles.
- Amplifier & Controller: Convert raw EMG into commands for motors.
- Motors & Gears: Drive the hand, wrist, or elbow.
- Terminal Device: Myoelectric hand or hook with multiple grip patterns.
- Cosmetic Glove: Silicone glove for a lifelike appearance.
Control Strategies
- Single-Site Control: One electrode site, toggling between functions.
- Dual-Site Control: Two electrode sites (e.g., flexors for closing, extensors for opening).
- Proportional Control: The strength of muscle contraction determines grip strength or speed.
- Pattern Recognition (Advanced): Multiple EMG signals are interpreted by AI software to control several movements.
Functions
Myoelectric prostheses can perform hand functions (power grip, precision grip), wrist functions (rotation, flexion/extension), and elbow functions. Advanced multi-articulating hands offer multiple pre-programmed grip patterns.
Other Externally Powered Prostheses
- Pneumatic/Hydraulic Prostheses: Use compressed gas or fluid pressure. These are rarely used in upper extremities today due to bulk and complexity.
- Hybrid Prostheses: Combine body-powered (e.g., for elbow lock) and myoelectric (e.g., for hand function) control. This balances function, weight, and reliability, especially in transhumeral amputations.
- Externally Powered Partial Hand Devices: Special designs for finger amputations that use myoelectric control to restore individual finger movement.
Clinical Aspects
Indications
Myoelectric prostheses are suitable for patients with sufficient muscle signals who require fine motor skills and prioritize cosmetic appearance.
Advantages
- Strong, consistent grip force.
- Natural, lifelike appearance.
- Reduced harnessing and increased comfort.
- Multiple grip patterns for daily tasks.
Limitations
- High cost.
- Requires training to master control.
- Needs battery charging and maintenance.
- More fragile and heavier than body-powered prostheses.
Training for an Externally Powered Prosthesis
Training involves learning to selectively contract residual muscles, practicing graded force control, and integrating the device into ADLs, bimanual tasks, and vocational or recreational use.
Complications and Challenges
Potential issues include skin irritation at electrode sites, signal interference from sweat, battery failure, mechanical breakdown, and prosthesis rejection due to weight or complexity.
Future Advances
- Pattern Recognition Myoelectrics: For more natural control.
- Targeted Muscle Reinnervation (TMR): Surgical rerouting of nerves to produce stronger EMG signals.
- Osseointegration with Electrodes: Direct skeletal attachment with implanted sensors.
- Sensory Feedback Systems: Restoring touch and proprioception through haptic feedback.
- Bionic Hands with AI: Self-learning prostheses that adapt to a user’s activities.
Conclusion
Electro-mechanical, myoelectric, and other externally powered prostheses have revolutionized upper limb rehabilitation. While body-powered systems remain popular in certain settings, myoelectric prostheses are the gold standard for patients seeking fine motor function, cosmetic appeal, and greater independence.