• Category Archives: Hosmer Dorrance

Body powered arms [overview]

Body powered arms are not the same. Despite everyone saying they understand what these are, these arms are not the same.

My setup explained, a generic setup explained. To show similarities and differences. For those in need to learn about this, technical differences to myoelectric arms are explained.

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Approximating the Carroll Quantitative Test for Upper Extremity Function comparing hooks [V2P, Dorrance, Otto Bock] and hands [Otto Bock, Becker Lock Grip]

I am not saying that the Carroll quantitative test for upper extremity function is necessarily the test most related to my own Activities of Daily Living (ADL). It is not.

But manufacturers are going above and beyond their call of duty to build “bionic” prostheses that seem to excite their engineers, the media and that by and large miss the point of functional prosthetic support while public money for research is wasted away on similarly useful gadgetry. Can you believe it? With a BeBionic hand you can even grab an apple? How amazing! With the iLimb, you can hold a water bottle! Wild! And the Otto Bock Michelangelo hand can also grab an apple. Oh, well.

So again amputees are mostly on their own as far as the real works are concerned.

On my way trying to get an understanding of the interplay between grip angles and usefulness I am trying to work towards better defining a modern test for relevant dexterity. And for that, it helps to play with a previously established test and then discuss maybe what it does well, what it does not show, and what it can be used for.

Furthermore, current advertising for “bionic” prostheses [Michelangelo, BeBionic, iLimb] usually show activities that any prosthesis can achieve and as such do not prove a particular point.

After reading through Carroll’s paper, I decided to implement the following tasks:

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Dorrance 555 aluminum hook with nitrile covers

I also use a Dorrance 555 aluminum hook with nitrile covers. It is light, and it is extremely powerful and sturdy. Compared to the Otto Bock Movohook and right after wearing a few days, I would rate it as more precise, more sturdy, lighter and more versatile.

The rubbers were a feature that some may look at as outdated – but I would not go that far. After wearing the Otto Bock Movohook 2Grip for over a year now and after working with it a lot, I have to say that it is heavy (the Dorrance 555 is light), even freshly after a service/overhaul the Otto Bock hook’s tips are not precise and neither is the Otto Bock hook joint (the Dorrance hook has a ball bearing and is rather precise), the Otto Bock hook has two spring settings (but I often only use one spring tension setting and that is the strong action one; if I need two spring settings these are ‘strong’ and ‘ridiculously strong’), the Otto Bock spring has a slightly stronger initial resistance to pull (whereas the haptics of rubber tension is – hard to describe – pleasant to my shoulder). And the Otto Bock spring accidentally falls off, as they don’t seem to fix the spring too well.

First I cut myself an adapter piece (instead of buying yet another Otto Bock bolt) and placed a washer under the ball of my cable so it would fit and stay in place.

Then I increased the hook’s strength by buying some castration bands [see Open Prosthetics on that issue] and now adding one. I don’t have this tool they use to put on rubber bands, so I just rolled it over the claws and pulled it into position. No problem.

Very nice. I do like the vertical claw position, also I find it better for typing.

Some more about highway robbery on a great Open Prosthetics blog page: I use castration rubber bands instead of the official rubbers that sell 40 pieces for 37 US$ or so at amputeesupply.com. Alternatively they sell castration bands for 17$ a pack (without shipping) at amputeesupply.com.

Now, I got 100 pieces for 15 CHF – one for 15 cents (that includes shipping and handling; price without shipping is ~ 2.70 CHF). That’s a fair price for standard rubber rings that I can live with.

Get your supplies at proper stores. Not at amputeesupply.com.

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