The O&P PT’s last entry, Tech Talk, explored recent innovative technology in operative control designs for upper extremity prostheses. Let’s take a deeper dive into these systems…
Upper Limb Prosthetic Control Systems
Actuation of a prosthesis may be classified by the structural design.1 This can be categorized as passive or active; the latter being generated by body forces or external power.1,2
PASSIVE 2
A passive prosthesis indicates there is no active motion at any of the joints. The lack of motors and minimal mechanical systems lends to lighter density of the prosthesis. While often deemed a cosmetic device, the passive prosthesis promotes function by providing a stabilizing assist for the sound upper extremity.
Image by Polina Tankilevitch from Pexels
BODY-POWERED 1,2
As the name denotes, this prosthesis is powered by the user’s body. A harness system is often utilized where scapular abduction controls cables directed to mobilize distal segments and terminal devices. Without the addition of motors, this system remains relatively lightweight. Body-powered prostheses are durable, may be waterproof and provide kinesthetic feedback via control cable tension. This system is appropriate for individuals with sufficient strength & range of motion to manage the harness and operate the cable to activate the prosthesis in various positions. Overhead motions require increased proximal stabilization to support the weight of device.
EXTERNALLY POWERED 1,2
Batteries power this system and can be activated by a variety of inputs. A common externally powered control system is myoelectric. Electromyographic (EMG) signals from two antagonist muscles are utilized to directly control opposing movements. For example, EMG signals from wrist extensor muscles can be used to open the prosthetic hand while those from wrist flexor muscles can control closing. A Certified Prosthetist can set the threshold to allow prosthetic activation with a certain level of muscle contraction as well as ensuring grip force is proportional to speed of device movement. User practice, intuitive technology and options with planes of motion promote more fluid upper extremity movement. While the device may weigh more given the motors and batteries, the intimate fit and additional degrees of freedom can lend to greater energy efficiency.
With any good thing, there are always drawbacks. For myoelectric prostheses, it can be the increased cost, maintenance (e.g., daily charge; more moving parts may lead to greater need for repairs), lack of waterproof options and user activation. The individual’s skin not only must tolerate the close fit of the device, it must be able to conduct signals. Scar tissue and excessive sweating can act as barriers. Additionally, some user’s may find difficulty with sequencing of contractions or use of co-contractions required to activate prosthetic functions. Thankfully, advancements with algorithms, sensors and gesture controls have made for more intuitive triggering of prosthetic movements.
Image by Polina Tankilevitch from Pexels
Overcoming Barriers 1,3
Prosthesis abandonment is a key factor when addressing barriers. Controllability and aesthetics play a large role in user experience. Satisfaction with use, comfort, cosmesis, weight, limited kinematics, associated noises and reduced feelings of security can lead to decreased acceptance of the device.
Research is directed towards finding effective strategies to promote human-device interaction to allow for a more intuitive fit and activation. Two areas of focus are upper limb prosthetic mechatronics (i.e., the combination of the mechanics and electronics required for prosthetic operation) and control strategies (e.g., algorithms, feedback). Interaction of the user and the device must be addressed both at the user-level (e.g., residual limb:socket interface) and device-level (e.g., socket:device connection). Effective communication is necessary to translate to efficient and fluid movement. The intention must be expressed by the user and transmitted through the socket to activate the device. In turn, feedback from both the environment and the prosthesis informs the user of their actions. Together this forms an integral feedback loop for function and requires optimal transmission at the user- and device-level.
Input signals from the user to the device are translated into a motor command to operate the prosthesis. While surface EMG is an oft used form of transmission, other methods have been explored to detect and implement the user’s biosignals [e.g., implantable electrodes, electroencephalography (EEG), surgical technique: targeted muscle reinnervation]. Each comes with its own set of pros (e.g., high signal to noise ratio, eliminates barrier of sweat) and cons (e.g., artifact signals, muscle fatigue, invasive procedures). Additionally, algorithms continue to evolve to better match the biomechanics of the anatomical upper limb allowing for greater prosthetic degrees of freedom and precision of movement (e.g., complex grasp). Use of sensory feedback (e.g., vibration, pressure) is being encoded to improve user awareness and increase the communication and functionality.
Many great advancements on the horizon!
Image by CottonBro Studio from Pexels
References
- Marinelli, Andrea, et al. “Active Upper Limb Prostheses: A Review on Current State and Upcoming Breakthroughs.” Progress in Biomedical Engineering, vol. 5, no. 1, 2023, p.012001 . https://doi.org/10.1088/2516-1091/acac57
- National Academies of Sciences, Engineering and Medicine. The Promise of Assistive Technology to Enhance Activity and Work Participation. Edited by Alan M. Jette et al. Washington, DC: The National Academies Press, 2017. https://www.ncbi.nlm.nih.gov/books/NBK453290/
- Smail, Lauren C., et al. “Comfort and Function Remain Key Factors in Upper Limb Prosthetic Abandonment: Findings of a Scoping Review.” Disability and Rehabilitation: Assistive Technology, vol. 16, no. 8, 2020, pp. 821-30. https://doi.org/10.1080/17483107.2020.1738567