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List of possible prosthetic arm failure points [prosthetic reliability]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - List of possible prosthetic arm failure points [prosthetic reliability]; published October 31, 2018, 19:55; URL: https://www.swisswuff.ch/tech/?p=8792.

BibTeX: @MISC{schweitzer_wolf_1571678676, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - List of possible prosthetic arm failure points [prosthetic reliability]}}, month = {October},year = {2018}, url = {https://www.swisswuff.ch/tech/?p=8792}}


Here is a list of my experience in prosthetic part degradation and failure intervals.

Prosthetic part Type of problem Occurs first when Typical solution Cost / consequences
Otto Bock Movowrist (body powered) Irreversible jam of joint mechanism.

This is not just functionally disturbing but there is probably no worse brand experience than to have a part irreversibly die, totally, within seconds of first use.

Within seconds of first use. Replacement: new / different wrist connector. Warranty replacement
Otto Bock metal hook (body powered) Temperature based mechanism jam due to lack of metal compability and joint mechanism tolerances.

This is not a very serious problem but it makes you wonder what design thinking was applied.

With warm temperatures. Replacement: different / hook, or, only use for cold ambient and gripped object temperatures. Warranty replacement (design flaw)
Otto Bock Movohook 2Grip (body powered) Hook joint wiggles due to a simple plastic disc being ground down by dust, dirt and possibly metal particles between the two hook claws in absence of an actual ball joint mechanism.

This might clearly be a CE conformance issue in that the product documentation states service free function, and the joint is technically fragile, built weakly and will require cleaning if not user repair (brass inlay) for any extended real use.

Within a few weeks of usage. User repair with brass inlays. Repair ~50 USD
Otto Bock Movohook 2Grip (body powered) Hook stops fully touching claws in high spring setting, so no paper, thread or other very small or thin object can be grasped due to the tolerances of metal lever and spring length being ill designed. Occurs once the metal lever indents the (softer) hook body thus functionally shortening the spring in high spring tension setting.

This is just an irritating issue as result of a technical design that with little effort could be avoided altogether. For a one handed user this is not trivial to fix however.

Within a few months of usage. User repair by shortening the spring a little bit. Repair ~50 USD
Otto Bock metal control cable mount (body powered) The Otto Bock parts to mount metal cables typically wear them down fast. Both clamps and cable sheath are not at all constructed for longevity of metal cables.

The technical design of Otto Bock metal cable mounts were never state of the art from view point of correctly mounting metal cables.

This might be a CE conformance issue in that body powered arms under the Otto Bock label are advertised as "whether your life involves social activities, sports, intricate tasks, or heavy-duty labor, our range of body-powered prosthetic components provides solid and reliable options to help you meet your needs and regain your independence", when really the product documentation and then their technical performance are neither solid or reliable under any real world constraints.

Metal cable regularly rips within 4-10 days of use. Replacement; typically, the cable is replaced.

While that has to be repeated every 4-10 days which is extremely frustrating, we built our own cable mount and sheath system (patented) - that runs service free under full load for at least 9-10 months.

The strategic goal of a body powered part that incapacitates the prosthesis every 4-10 days is to get the user to demand a myoelectric prosthesis which generates significantly more revenue for the manufacturer.

Significant cumulative time and financial effort due to high frequency, or abandonment or reduction of use of prosthesis.
Our own cable mount and Bowden type sheath mount (body powered) With a revised cable mount, fatal cable tears occur far less often.

Mount points and cable sheath interacted problematically at first, so sheath ends were damaged after 9-10 months. With dedicated attention to correct cable end cutting and mounting, one will expect around 1 year of unserviced full load operation.

 

Within 9-10 months. Replace cable sheath and cable.

We now revised the cable end mount points so the problem is delayed or does not occur as fast. It will occur eventually as that is the consequence of wearing longitudinally embedded metal wires in PVC plastic sheaths - the same way, bike cable sheaths (which these are) may degrade after 1-2 years of use.

Reduced efforts for repair. A set of cable sheath and cable may cost 4-20 USD.
Conventional figure 9 harness (Otto Bock) (body powered) Sweat smell of plastic sheath of harness fabric if worn directly on skin.

Otto Bock claims that "their body-powered solutions often provide the right balance of performance and convenience". I am not sure whether CE conformance provides for extremely smelly components under the aspect that they are explicitly sold for "convenience".

Within 1-2 days. User repair.

The solution is to use entirely different materials. Starting with bicycle tubes as protective cover (Latex, Butyl), which hardly ever attract sweat stink as a soft deformable plastic material.

The Otto Bock figure 9 harness cannot be freed of sweat smell. The plastic chemically interacts with sweat in a really uncomfortable way. It is not possible, also with the recommended detergent or soap, to remove that sweaty stink.

The strategic goal of a body powered part that stinks is to get the user to demand a myoelectric prosthesis which generates significantly more revenue for the manufacturer.

A bike tube may cost a few USD. Otherwise abandonment or reduction of use of prosthesis.
Conventional figure 9 harness (Otto Bock) (body powered) Brachial plexus compression with consequential neurological complications such as carpal tunnel syndrom (double crush injury).

Otto Bock claims that "their body-powered solutions often provide the right balance of performance and convenience". I am not sure whether CE conformance provides for design inherent brachial plexus lesions under the aspect that they are explicitly sold for "convenience".

Possibly some time after 6-24 months. Replacement: the sustainable real solution is to get the prosthetist to build an anatomically fitted shoulder anchor with a carbon bridge that does not compress the brachial plexus.

The strategic goal of a body powered part that compresses the brachial plexus and causes difficult neurological problems is to get the user to demand a myoelectric prosthesis which generates significantly more revenue for the manufacturer.

Shoulder anchor ~3000 USD. Otherwise abandonment or reduction of use of prosthesis.
Conventional Ossur Iceross silicone liner (body powered) Friction rash. After 2h to a few days, typically. Stop usage for a while: take a break from wearing the prosthesis for 1-5 days mostly resolves the problem, and the rash heals. Mostly no cost, what may help is creme with zinc.

However, frequent rashes are a reason for abandonment or reduction of use of prosthesis.

Conventional Ossur Iceross silicone liner (body powered) Congestion eczema due to an overly cylindric rather than widely conical shape thus not properly fitting the stump. Possibly some time after 6-24 months. Replacement: switch to a different liner, possibly requires a new socket. A new socket may be around ~1000 to ~5000 USD.

Otherwise, congestion eczema is a reason for abandonment or reduction of use of prosthesis.

Conventional Ohio Willowwood Alpha gel liner (body powered) Friction rash. After 2h to a few days, typically. Stop usage for a while: take a break from wearing the prosthesis for 1-5 days mostly resolves the problem, and the rash heals.

A gel liner however also allows to wear a cotton tubular gauze fabric underneath, extending friction rash issue problems to only occur after 1-2 weeks of continuous heavy use.

Tubular gauze is not very expensive.

Frequent friction rashes otherwise may lead to abandonment or reduction of use of prosthesis.

Conventional Otto Bock wrist (body powered) Wiggle.

This might be a CE conformance issue in that body powered arms under the Otto Bock label are advertised as "whether your life involves social activities, sports, intricate tasks, or heavy-duty labor, our range of body-powered prosthetic components provides solid and reliable options to help you meet your needs and regain your independence", when really the product documentation and then their technical performance are neither solid or reliable under any real world constraints.

Wiggles are a really irritating problem because there is about 2-4 months time between start of wiggling and swap of wrist, until you get your technician and insurance to make sure you have a new wrist. So you have to suffer irritating but not fatal design failure. Which it so clearly is: our own wrist is designed differently and cannot wiggle in that way.

3-4 months. Replacement of wrist connector. Replacement with conventional wrist unit ~600 USD.
Myoelectric socket (general) Lack of fit: too wide. Daily variation of stump size causes this problem to occur erratically. Stop usage for a while: there is no solution.

As a socket that is too wide does not reliably place the electrodes on the skin, the prosthesis thus has to be removed and one has to wait.

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Myoelectric socket (general) Lack of fit: too narrow. Daily variation of stump size causes this problem to occur erratically. Stop usage for a while: there is no actual solution.

A slightly more narrow socket will press electrode and socket rim structures harder against the stump, causing blisters or friction rashes faster, causing subsequent recovery days to let the stump heal.

Frequent occurences are a reason for abandonment or reduction of use of prosthesis.
Myoelectric socket (general) Lack of robust fit to withstand pull forces: inability to lift daily activity related weights When lifting over ~4-6 kg. Restrict usage: there is no solution really.

A Munster type socket will be relatively easy to slide in and out. That causes any other weight that pulls on the arm to pull it off at least to a degree where electrodes lose contact.

Frequent rashes are a reason for abandonment or reduction of use of prosthesis.
Myoelectric control (general) Body position or limb position effect: unintended open or close function is triggered by lifting, flexing or extending arm at shoulder and / or elbow.

The consequences also may possibly contain somewhat embarassing object drops.

During use, frequent occurence. Restrict usage: stop using the myoelectric arm if avoiding limb position is critical. There is no real solution.

One may try to separate body motions to submit the myoelectric control to signals that are only intended but given any actual situation, that can be impossible.

Frequent embarassing situations or object damage are a reason for abandonment or reduction of use of prosthesis.
"Bionic" hand glove or cover Any "bionic" hand is a mechanism with a weight vs. motor strength trade-off: either the hand is heavy and the motor delivers good grip strength, or, the hand is light but grip strength is insufficient. The consequence of a lighter hand is that the grip strength is so low that only very thin gloves or covers can be used. These, however, perforate or rip very easily.

Touchbionics advertises "As a leading manufacturer of upper limb prosthetic solutions, Touch Bionics understands the needs of patients with upper limb loss and deficiency. Touch Bionics is one of the few companies worldwide that solely manufacturers upper limb prosthetic devices and is committed to providing the most innovative technologies". With that, they so far never replied to my questions regarding more durable gloves. Ever. I had mailed them several times. I am not sure that the absence of commitment in light of advertised claims are in keeping with their claimed CE conformance.

Such a glove may perforate or rip within ~10 minutes of use, or, even fall apart unused in the cupboard. Restriction of usage - there is no solution. Do not use the prosthesis if the glove might get damaged, which, given the thinness, may be very often.

One can use thicker gloves but these risk to impede the prosthetic "bionic" hand function.

One can use different manufacturer derived products. I had great experiences using a BeBionic cosmetic glove on an iLimb.

Frequent glove damage and ensuing cost and efforts for replacement are a reason for abandonment or reduction of use of prosthesis.
"Bionic" hand precision grip Unstable grip mechanics - the swiveling thumb and variably flexible index finger may create a type of geometric constellation where plane of contact is entirely unforeseeable by the user. The thumb and index finger also may entirely miss each other.

This creates potentially embarassing situations.

During use, frequent occurence. User trick that may cost a bit of time: the best solution is to reset the hand grip before every single new attempt at performing a precision grip.

A sustainable fluent user solution is to use prosthetic hooks or a Becker Mechanical hand.

Frequent absence of required grip performance are a reason for abandonment or reduction of use of prosthesis.
Myoelectric control (general) Lack of function for no particular reason.

These systems may not perform at any moment. There is no solution. Frequent lack of function is a reason for abandonment or reduction of use of prosthesis.
Prosthetic hand or hook (generally) Contamination: any gripper or hand mechanism may be contaminated, from a profuse splattering with pizza parts to biologically hazardous or chemically problematic substances. Newspaper print may darken any light skin colored prosthesis rapidly and irreversibly. At any point in time. Adapt user choice of terminal device: the best solution is to use a gripper or hand cover that is easy to clean.

Other use may cause irreversible discoloration, staining, damage or perforation, or other unwanted consequences.

The actual consequence is that one does not just perform any grip activity without proper planning.

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Prosthesis looks funny (general issue) Prosthetic arms may be perceived to look funny, strange or weird in the eyes of some people. At any point in time. Difficult: improving the look of a prosthetic arm may be possible or not, depending on specifics. A strange, funny or weird look of a prosthetic arm may cause abandonment or reduction of use of prosthesis.
Terminal device damaged or defunct (general issue) Reduced or absent terminal device function is a serious issue. Increasingly with increased age or use of terminal device. Repair or replacement which, however, depending on specifics, may be impossible (customized or bespoke devices) or extremely costly (particularly for myoelectric devices). Reduced or absent terminal device function will cause abandonment or reduction of use of prosthesis.
Myoelectric control (general) Electrodes may cause burn-like skin changes that look like eczema but may heal only very slowly. Increasingly with use. Possibly different electrode metal / material. Repeated occurrence of slow healing skin injuries may cause abandonment or reduction of use of prosthesis.
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