Understanding Harness and Control Systems in Prosthetics
Q. Harness & Control Systems Below Elbow Harnessing and Shoulder Amputee Harnessing
Certainly! Below is a 500-word essay on harness and control systems in upper extremity prosthetics, focusing on below-elbow (B.E.) harnessing, the challenges associated with it, and shoulder amputee harnessing, ending with a detailed conclusion.
Harness and Control Systems in Upper Extremity Prosthetics
In upper extremity prosthetics, harness and control systems are essential for the suspension and function of body-powered prostheses. These systems enable the user to operate prosthetic components (such as the terminal device or elbow joint) using residual limb and upper body movements. Though newer technologies like myoelectric systems are increasingly common, harness systems remain a reliable and cost-effective option—especially in low-resource settings or for users who prefer direct feedback.
Below-Elbow Harnessing and Control Systems
For transradial (below-elbow) amputees, the most commonly used harness is the figure-of-eight harness. This harness consists of a series of adjustable straps that go around the shoulders and back, providing both suspension (to keep the prosthesis in place) and control (to operate the terminal device, usually a hook or hand).
Key Components:
- Axilla loop: Anchors the harness under the opposite arm.
- Anterior suspension strap: Helps suspend the prosthesis.
- Control cable: Connects from the harness to the terminal device, usually via a cable housing system.
Control Mechanism:
In body-powered systems, scapular abduction and shoulder flexion generate tension on the cable, causing the terminal device to open or close. The control system offers proprioceptive feedback, allowing users to “feel” the device’s movement through tension.
Common Challenges and Causes:
- Discomfort or chafing: Improper strap positioning or tension can lead to skin irritation or nerve impingement.
- Cable inefficiency: Excess friction or misalignment can reduce the responsiveness of the terminal device.
- Limited range of motion: Tight harnessing or bulky clothing may restrict shoulder movement, especially during activities that require full arm extension.
Shoulder Amputee Harnessing
Harnessing becomes more complex in individuals with shoulder disarticulation or forequarter amputations, where the entire arm and shoulder are missing. In such cases, traditional suspension methods cannot be used due to the absence of a residual limb.
Harness System:
A chest strap harness or shoulder saddle harness is often employed. These systems rely on the thorax and contralateral shoulder for suspension and control. Control cables may operate multiple joints, such as powered shoulder movement or elbow flexion, depending on the prosthesis design.
Control Challenges:
- Multiple control inputs: Shoulder-level amputees may require dual or triple control systems, making coordination more complex.
- Stability: With no residual limb for leverage, maintaining prosthesis alignment can be difficult.
- Comfort and cosmesis: Bulkier harnesses may be more visible and uncomfortable, affecting user acceptance.
Conclusion
Harness and control systems remain foundational components in upper extremity prosthetic design, particularly in body-powered prostheses. For below-elbow amputees, figure-of-eight harness systems provide effective control and suspension, enabling users to regain basic hand functions through body movements. However, challenges such as discomfort, reduced range of motion, and cable inefficiency must be addressed through proper fitting and regular adjustments.
In contrast, shoulder-level amputees require more elaborate harnessing solutions, such as chest-strap or shoulder saddle harnesses, to achieve multi-joint control. While these systems can restore significant functionality, they often involve a higher learning curve and reduced cosmetic appeal.
Ultimately, the success of any harness system depends on individual assessment, appropriate component selection, and comprehensive training.
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