Upper-Limb Prosthesis Fabrication: Clinical and Technical Methods
General Principles of Prosthetic Fabrication
The fabrication of an upper-limb prosthesis is a multidisciplinary process combining clinical assessment, engineering, and customization. The primary goal is to restore function, improve cosmetic appearance, and enhance the patient’s independence and quality of life. The complexity of the device increases significantly with the level of amputation, requiring careful consideration of suspension, control, and weight distribution.
Partial Hand and Digital Prostheses
Partial hand prostheses are designed to restore functional and cosmetic capabilities for individuals with partial hand amputations or congenital absence of digits. The fabrication process is highly patient-specific to meet individualized functional and aesthetic goals.
1. Clinical Evaluation and Assessment
The process begins with a comprehensive patient evaluation, including:
- Detailed history of the amputation or congenital absence (traumatic, surgical, or congenital cause).
- Assessment of the residual limb, including skin condition, length, mobility, and muscle strength.
- Evaluation of the patient’s functional goals (e.g., grasping, cosmetic appearance).
- Psychological and emotional readiness for prosthesis use.
Special attention is given to the shape and size of the residual limb, as this determines the prosthetic fit and suspension method.
2. Impression and Casting
A detailed impression of the residual limb is taken, usually using alginate or plaster of Paris for accurate molding. A negative mold is created and then filled with dental stone or plaster to make a positive model of the limb. This positive model is modified to accommodate pressure-sensitive and pressure-tolerant areas.
3. Prosthetic Design Selection
There are two main types of partial hand prostheses:
- Passive Prosthesis: Mainly cosmetic; may allow limited function such as stabilization or support during tasks.
- Functional Prosthesis: Designed for active use; may be body-powered, externally powered (myoelectric), or mechanical.
The choice depends on the level of amputation, the number of missing digits, and the patient’s activity level and preferences. For congenital absence, the design must be adapted to the anatomical variation present.
4. Material Selection and Fabrication
Modern partial hand prostheses use lightweight, durable materials:
- Silicone elastomers for life-like cosmetic appearance and skin tone matching.
- Thermoplastics or carbon fiber for inner structural strength.
- Stainless steel or titanium for joint components in mechanical or body-powered prostheses.
Fabrication steps include sculpting or 3D printing a socket, creating digits using silicone molds or prefabricated components, and assembling joints. In myoelectric systems, electrodes are embedded to detect muscle signals for finger movement. Finishing involves texturing, coloring, and refining the surface for aesthetics and comfort.
5. Fitting, Training, and Follow-Up
The prosthesis is fitted to ensure comfort, suspension, and functional alignment. Adjustments are made for pressure points or discomfort. Occupational therapy is critical post-fitting to help the patient adapt and learn how to use the prosthesis effectively in daily life. Regular follow-ups monitor the fit, especially in children as they grow, and allow for repairs or component replacements.
Through-Wrist (Wrist Disarticulation) Prostheses
A through-wrist amputation preserves the full length of the forearm. The goal of prosthetic rehabilitation is to restore function, improve appearance, and promote independence. These custom-made devices must accommodate the bulbous end of the forearm caused by the preserved carpal bones.
1. Clinical Assessment and Patient Evaluation
Assessment includes determining the cause of amputation, measuring the length and condition of the residual limb, and evaluating the patient’s needs (cosmetic versus functional use). Since the full length of the forearm is preserved, suspension and rotation control are important design considerations.
2. Impression and Casting
A precise impression is essential for proper socket fitting, typically achieved using the plaster bandage technique or silicone/alginate materials. The positive model is modified to relieve pressure-sensitive areas and ensure optimal contact.
3. Prosthesis Design Considerations
Types of prostheses used:
- Passive (Cosmetic) Prosthesis: For appearance and light support.
- Body-Powered Prosthesis: Uses cables connected to a harness for movement.
- Myoelectric Prosthesis: Uses surface electrodes to detect muscle signals, controlling hand/finger movement.
4. Socket Fabrication and Suspension
The socket is the interface between the prosthesis and the residual limb. Materials commonly used include thermoplastics (e.g., polypropylene) or laminated composite materials. The socket must fit snugly over the bulbous distal end, provide comfort and stability, and offer secure suspension (often using suction, straps, or anatomical suspension). For myoelectric prostheses, electrode placement and wiring are incorporated into the socket.
5. Wrist Unit and Terminal Device Integration
A prosthetic wrist unit may be fixed or allow flexion, extension, and rotation. It connects the socket to the terminal device (hand). Common terminal devices include a cosmetic hand, a functional hook, or a myoelectric hand. Wrist units can be quick-disconnect or locking types to allow interchange of terminal devices.
6. Fitting, Training, and Follow-Up
During fitting, the prosthesis is tested for comfort, alignment, and functional reach. Post-fitting training is essential, especially for body-powered and myoelectric users, to help patients integrate the device into daily tasks. Regular follow-up appointments address wear and tear and reassess fit due to residual limb changes.
Elbow Disarticulation Prostheses
Elbow disarticulation involves amputation at the elbow joint, leaving the humerus intact. The intact humeral condyles provide a broad, bulbous end, offering excellent potential for anatomical suspension but posing challenges for cosmetic alignment.
1. Clinical Evaluation and Assessment
Evaluation focuses on the shape and size of the residual limb, especially the bulbous distal end, skin condition, and range of motion at the shoulder. The length of the residual limb plays a crucial role in the design and function of the prosthesis.
2. Impression and Casting
The casting process involves positioning the residual limb neutrally and using plaster bandages or fiberglass tape to create a negative mold, marking anatomical landmarks (epicondyles, olecranon). The positive model is modified to provide relief at pressure-sensitive areas and ensure optimal fit over the bulbous end.
3. Prosthetic Design Considerations
Main types include cosmetic (passive), body-powered (using a harness and cable system), and myoelectric (controlled by electrical signals from muscles in the upper arm). Due to the residual limb’s length, aligning the prosthetic elbow with the sound side can be a cosmetic challenge.
4. Socket Fabrication and Suspension
The socket must accommodate the wide distal end and ensure even pressure distribution. Secure suspension is typically achieved through anatomical self-suspension around the epicondyles or suction suspension. Materials used include thermoplastics for test sockets and laminated composites (carbon fiber and resin) for definitive sockets.
5. Elbow Joint and Terminal Device Integration
The choice of elbow joint depends on the prosthesis type (manual locking for body-powered systems; powered for myoelectric systems). Terminal devices include mechanical hooks, cosmetic hands, or myoelectric hands. Devices are aligned to ensure symmetrical function and appearance.
6. Fitting, Training, and Follow-Up
Occupational therapy is crucial to train the patient in functional use, especially in tasks requiring bimanual coordination. Regular follow-ups address fit changes due to muscle atrophy or weight change, component wear and tear, and functional upgrades.
Above-Elbow (Transhumeral) Prostheses
A transhumeral amputation involves the loss of the arm above the elbow joint. Fabrication is complex due to the need to restore both elbow and hand functions.
1. Patient Evaluation and Assessment
Key aspects include the length and shape of the residual limb, muscle strength and control of the shoulder, and functional goals. The longer the residual limb, the greater the potential for control and prosthetic suspension.
2. Impression and Casting
An accurate socket begins with capturing the shape of the residual limb using plaster bandages or thermoplastic materials. Anatomical landmarks are marked during casting. The positive mold is modified to relieve pressure-sensitive areas and improve suspension.
3. Prosthesis Design and Suspension
Types of AE prostheses:
- Cosmetic (Passive): Minimal function.
- Body-Powered: Uses a harness and cable system to control elbow and terminal device.
- Myoelectric: Controlled by muscle signals (EMG); offers more natural movements.
Hybrid systems combining body-powered elbow with a myoelectric hand are also common. Suspension methods include harness suspension (figure-8 or chest strap), suction sockets, or liners with pin/vacuum suspension.
4. Socket Fabrication
The socket must fit snugly, distribute pressure evenly, and allow effective transmission of control forces. Materials used are thermoplastics (for diagnostic sockets) and laminated carbon fiber or acrylic resins for definitive sockets.
5. Elbow Joint and Terminal Device Integration
The elbow unit may be manual-locking or electronically powered. The terminal device may include a hook (lightweight, durable), a cosmetic hand, or a myoelectric hand (multi-grip, advanced function). Proper alignment is essential for symmetry, comfort, and function.
6. Fitting, Training, and Follow-Up
Post-fitting occupational therapy focuses on functional training (reaching, grasping, lifting) and ADL practice. Regular follow-ups ensure continued proper fit, component repairs, and adjustments for limb volume changes.
Shoulder Disarticulation Prostheses
Shoulder disarticulation is a high-level amputation where the entire arm is removed at the shoulder joint. This presents unique challenges regarding suspension (due to the absence of a residual limb), control, and weight distribution.
1. Patient Evaluation and Assessment
Evaluation includes the shape and condition of the shoulder region, evaluation of the opposite arm, and determination of patient goals (cosmetic, functional, or both). Given the absence of a residual limb, suspension and control are the major design considerations.
2. Impression and Casting
An impression of the shoulder and torso is taken using plaster bandages or thermoplastic sheets to create a mold of the shoulder, chest, and part of the back. The mold often extends over the contralateral shoulder if harness-based control is planned. The positive model is used for socket fabrication.
3. Prosthetic Design Types
Types of prostheses used:
- Cosmetic Prosthesis: Lightweight, passive device for appearance only.
- Body-Powered Prosthesis: Uses a harness and cable system to control elbow and terminal device.
- Myoelectric/Electronic Prosthesis: Controlled using muscle signals from the torso or opposite side, offering better function.
4. Socket Fabrication and Suspension
The socket design must accommodate the full absence of the arm, typically involving a total contact socket extending over the shoulder and part of the chest/back. Suspension relies on chest wall contour, axilla extension, or a harness system. Lightweight thermoplastics or laminated carbon fiber are used. For myoelectric systems, electrode placement and battery integration are critical.
5. Components: Shoulder, Elbow, and Terminal Device
Components include a shoulder joint (passive, friction-based, or locking), an elbow unit (manually or electronically controlled), and a terminal device (cosmetic hand, mechanical hook, or myoelectric hand). Components are aligned and balanced to avoid strain on the torso and neck.
6. Fitting, Training, and Follow-Up
The socket is checked for comfort, suspension, and stability. The patient is trained in donning/doffing and in using the prosthesis effectively for daily activities. Occupational therapy is essential, especially for complex body-powered and myoelectric systems. Ongoing follow-up is necessary to adjust fit and repair or upgrade components.
Conclusion: Achieving Optimal Outcomes
Fabricating an upper-limb prosthesis, regardless of the amputation level, requires careful planning and customization based on the residual limb’s unique anatomy and the patient’s lifestyle. Successful outcomes depend on proper evaluation, precise socket fabrication, appropriate component selection, and continuous patient support through training and follow-up care. Advances in materials and control systems continue to enhance the function, confidence, and quality of life for amputees.