Shoulder harnesses for body powered arms are used to pull open the terminal device (hand, hook). Using rubbers, springs or other elastics, the terminal device then closes. That is the most common way these prosthetic arms work. Terminal devices with that mechanism are also termed VO or “voluntary opening”.

The force of the cable pulls on the strap, day in, day out. And it works well. Mostly.

1. It works well. Too well : )

After modifying and tweaking all parts of my prosthetic arm to complete super function, I am wearing it for over 14 hours a day these days. Mostly. And I am really using it for just about anything I do. Of course – skin issues are under control, socket fits neatly, prosthetic length is correct, the quick release wrist is reliably locking and unlocking at all times, my Becker hands and various hooks are optimized for grips or looks, the cable setup was seriously tweaked and so nothing to stop me from using it.

2. Double crush injury and dual nerve injury –

My body works like a precision machine. Of course it doesn’t – but I am doing so much physical stuff that even slight problems start to get my attention, and I catch up with things relatively fast.

It started with my (left) hand falling asleep or getting numb around the thumb, index and middle finger after about an hour of extremely long stretched crawl swimming. Nothing wrong with good old DPS (distance per stroke) swimming – except this happened after a while.

My doctor got an MRI, ultrasound and a neurologist to check things up. The MRI showed some anatomical variation of my left shoulder bones but no clear problem. Definitely, there was no cervical spine or visible compression there.

With a change of swimming routine from over an hour of DPS to some more interesting workouts, the numbness was gone also. My hand then was alright for a year until it became numb in the mornings. Not always but sometimes.

I quickly figured out that after not wearing the body powered arm for even one day, the problem was gone.

The ultrasound showed an ever so slightly compressed median nerve as found with carpal tunnel syndrome. Now, I am using my left hand for lots of stuff including computer work – but it is not possible to blame the carpal tunnel syndrome on other daily activities when there is a clear presence / absence pattern that correlates with my usage of the body powered arm.

What became clear after a visit to the neurologist was that this all does fit a pattern. I do have a reduced nerve conduction velocity of both ulnar and median nerve. With compression of both at the level of the brachial plexus, the neurologist explained to me that congestion of the median nerve at that level also explains carpal tunnel syndrome in what is termed “double crush injury”.

Copyright (C) Lippincott Williams & Wilkins [1].

This illustration in that article that describes very similar injuries in backpackers [1] makes this very clear – the anterior and lower region of the axillary fold contains branches for both median and ulnar nerve.

Some people already suffer such issues at light loads or weights. Harness redesign would be called for according to the article [1] – but reducing load generally and by means that are sustainable and long lasting is a far better solution in my eyes.

3. Option – harness redesign

Designing a new type of shoulder anchor sounds like an easy task. Just do it, they say.

However, the rounded and flexible shoulder, breast and back are not meant to be used as leverage for a cable control. Of course there are some things we can build on – previously learned things, the ‘military principle’ (if you dodge a bullet even by a few inches you still dodge it), and that none of the new clowns ever built something really new – but instead copied and reused other people’s old ideas. And if everyone else can do that so can we.

But now, that is not enough. Any setup that provides some anchor point will invariably do one of the following two things:

• if a flexible option is used, the harness will end up compressing the brachial plexus no matter what; try it and you will see;
• if a stiff option is used, the function of the (sound) shoulder is so seriously restricted that one is better off without the prosthesis.

Both seem to seriously motivate me to give up wearing shoulder harnesses completely. Grip as well as freedom of motion are seriously limited. Also, harness redesign to achieve a true shoulder anchor solution that work beautifully would have been done already in the last 20 years if it was easy.

The new stuff that is promoted appears to fail. We already tried that. Using 3D capturing and casting stiff material, one may establish that the shoulder of the remaining (less disabled) arm is meant to be flexible and not restrained. Not a new thing to learn but interesting to find out once more.

Using a wide silicone strap, invariably and after just about three or four strong pulls, the anterior axillary fold is compressed yet again. So quite possibly that was tried in the past and abandoned for good reason.

As anyone should realize the actual performance of various options (below) is not that widely differing. I will get my garbage bags out no matter what. I will get the house cleaned no matter what.

Question is where to target center of general life balance and then spend enough time trying to get there.

4. Option – minimizing forces

Cable forces can be minimized by improving these two functions of the prosthetic arm:

• Grip. If grip works well at low closing forces, then opening the gripper using cable pull can be performed with minimal force. Work in progress.
• Cable friction. If cable friction is minimized, less force is necessary to operate the cable. Done that.

5. Option – myoelectric prosthesis

Accepting that functionality is lost anyway, one may turn to myoelectric prostheses. Not in my instance due to issues around these arms – but in other instances.

6. Option – cosmetic arm

My cosmetic arm is very light and very pretty. It is also relatively functional – not as functional as the body powered arm, but still.

7. Option – no prosthesis

As the stump skin deforms and has good friction for gripping, it performs very well in comparison to prosthetic devices inasmuch as activities of daily living are concerned. Particularly compared to one of the more restricting harnesses. This option does not require a lot of research. I just think it is off in terms of looks. And over the months and years, asymmetry may end up feeling bad.

8. Krukenberg forearm split

With all its drawbacks – the Krukenberg forearm split reprents a rather sensible option for balanced, nerve and joint saving option to implement grip function to the stump. While closing force in weight is not that high, skin surface is the ideal grippers’ cover. It also heals and replaces itself once damaged.

There are some more intricate issues regarding this that would go too far right here. But this is definitely an option that requires more research.

Actual solution

About two months after I first blogged about this, we came up with a shoulder anchor that effectively remedied all issues.

The shoulder anchor really shifts the weight off the strap type pressure location to the humerus bone’s head.

References

[1] Mäkelä, JP, Ramstad, R, Mattila, V, Pihlajamäki, H (2006). Brachial plexus lesions after backpack carriage in young adults. Clin. Orthop. Relat. Res., 452:205-9.

@Article{pmid16906084,

   Author="Makela, J. P.  and Ramstad, R.  and Mattila, V.  and Pihlajamaki, H. ",
   Title="{{B}rachial plexus lesions after backpack carriage in young adults}",
   Journal="Clin. Orthop. Relat. Res.",
   Year="2006",
   Volume="452",
   Pages="205--209",
   Month="Nov"
}