Arm prostheses require too much force

PhD candidate Mona Hichert says we should get rid of current body-powered prostheses because they require operational forces that are too high.

Picking up an empty mug is easy. A pen is a little harder, and to write with it is nearly impossible. My right hand is locked inside a socket at the end of which there is a gripper that I can activate by moving my shoulders. With this device, people like me can experience what it’s like to have to live without hands. Now, onto a plastic water bottle. A little force to get a grip on it and, whoosh!, water gushes over the table. Nicely done. Once the device was removed, I felt a tremendous relief to be able to use my hand again.

High force needed

Former top handball player and biomechanical engineer Mona Hichert has studied why almost half of arm prosthesis users eventually do not use their artificial arm and hand. Rejection rates reportedly vary between 26 and 45%. The main reason is that the high force needed to operate the hand causes fatigue, irritation or pain.

Another reason may be the aesthetics. People may not want to be seen with a hook instead of a hand. Unfortunately, the most hand-like prostheses require larger operation forces than hooks.

Large variety

Hichert studied body-powered arm prosthesis. There is a large variety in forms between hook and hand. The different forms can be divided into two categories: active opening (like a clothes pin) or active closing (like tweezers). A harness around the shoulder is attached to a steel cable, which activates the artificial hand. A spring returns the ‘prehensor’ (hand or hook) in its initial state.

Hichert mentions the more futuristic electrically powered prosthesis, but she hasn’t included these in her study. Electrical prostheses respond to electrical signals coming from the remaining muscle tissue. The trouble with these myoelectric devices is that they’re expensive ($25,000 – $75,000), they’re heavy and users don’t get feedback of the pinch forces exerted on an object.

The much cheaper body-powered prostheses ($4,000 – $10,000) can function as an extension of the body much like a tennis racket. Once you get used to a body-powered prosthesis, you can control it like it’s a part of you. Unfortunately, however, the large operational force needed prevent people from getting comfortable with most of these devices.

Individual capacity

Tests with a representative group of 23 prosthesis users showed that men can deliver more force than women, but also that the spread is quite large. The average force for men is 332 ± 117 newtons and for women 188 ± 87 newtons. ‘The wide range of measured forces indicates that clinicians need to evaluate each individual’s capacity to operate an available body-powered prosthesis based on his strength,’ Hichert concludes.

Hichert compared the force that users can provide routinely with forces needed to handle various prostheses. Guidelines say that the routine force is 20% of the maximum. The results are shocking, as they show that, depending on the device, 26 to 100% of the users cannot use active closing devices without fatigue. Actively opening prehensors score even worse with 52-91% of the users experiencing fatigue or irritation, depending on the device.

Engineers should do a better job in designing prostheses that require less force to operate, says Hichert. Equally, clinicians should recommend only devices that their patients can routinely use, based on individual strength measurements.

User designed techology

At the 2016 Cybathlon in Zürich, the Delft Institute of Prosthetics and Orthotics (DIPO) together with Bob Radocy, founder, and owner of TRS Prosthetics, finished first. Radocy lost his hand and wrist in a car accident in 1971. He was so disappointed with available devices that he has set out to develop something better. He did so partly in collaboration with TU Delft. Their success showed that technology designed with the end-user firmly in mind can make a large difference.

* Mona Hichert, ‘User Capacities and Operational Forces, Requirements for Body-Powered Upper-Limb Prostheses’, PhD thesis supervisors Prof. DirkJan Veeger, Dr. Dick Plettenburg and Dr. David Abbink (3mE Faculty), February 24, 2017.

* Watch the TV program Buitendienst with Mona Hichert between 03:50 and 08:50 for a demo with prostheses.

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