Not so handy

The aphorism that you’re best of with an old hand (a synonym for veteran) can’t ever have been more spot on. Tests show that the best performing body-powered hand on the market today is the so-called Sierra hand prostheses, which dates back to 1945 and has hardly changed since.

This is one of the conclusions of a team of researchers from TU Delft and the University of Groningen. They published their article (‘Efficiency of voluntary opening hand and hook prosthetic devices: 24 years of development?’) earlier this month the Journal of Rehabilitation Research and Development.

The researchers tested nine body-powered prostheses – prosthetic devices you control by moving your shoulder – and then compared the pinch forces and the required activation forces with results from similar tests done in 1987 by researchers from the Hospital for Crippled Children in Texas.

“We found that the hand models that are most commonly used haven’t improved for at least two decades,” says biomechanical engineer, Gerwin Smit (MSc), the first author of the article.

Tom Chau, an upper limb prosthesis expert at the Holland Bloorview Kids Rehabilitation Hospital in Canada, who investigated the use and abandonment of upper limb prosthesis over a period of 25 years, says he’s not surprised. “There is not that much research being done in this field. There are good prostheses but they are either exorbitant in price or still just prototypes.”

“Our findings show that there is room for improvement”, says Smit. Improving the hands is something the Delft biomechanical engineers would like to take on themselves. They are old hands in this field (pardon the pun). The group has been working on body-powered prosthetic hands for children since 1967.

However, making prostheses for children and for adults are different things, Smit explains. “The different parts of the prostheses do not scale up the same way.”

“We want to investigate how we can make the mechanisms within the prostheses for adults more efficient – we might for instance reconsider the pivoting points.”

These last decades millions have been invested, according to Smit, in the amelioration of electrical prostheses. “These prostheses are commercially much more interesting. Yet despite these investments the electrical hands are slower and heavier than body powered prostheses.”