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Tuesday afternoon I had the good fortune to talk to Professor Jorge Zuniga of Creighton University regarding his insights on printable prosthetics, measurements of uneffected/effected hands, and various important design considerations.  Getting to talk to him really helped crystallize my understanding of the various measurements and the way in which the parts of the printable prosthetic1

1. Design Ideals
   1. One of the design ideals of the Cyborg Beast prosthetic is to fashion a device that strives for symmetry with the unaffected hand.  Thus, all of the necessary measurements are taken from the unaffected extremity.  This serves two purposes.  First, it allows for the prosthetic to be similar in scale to the unaffected hand.  Secondly, the unaffected extremity tends to be, in most cases2 , slightly larger than the affected extremity.  The size difference may be due to the unaffected extremity being used more, and thus having more muscle mass, or due to the loss of muscle tone and muscle atrophy in the affected extremity.  Either way, a prosthetic designed using the measurements from the unaffected extremity should generally fit the affected extremity.  Since this particular prosthetic design uses velcro straps to fasten to the affected forearm, a prosthetic that is slightly too large can easily be adjusted to fit well by tightening the straps.
   2. Another design ideal is to create a core prosthetic design which works for the vast majority of persons.
2. Critical Printable Components
   1. A rough sketch of the various parts of the Cyborg Beast prosthetic appear above as “Figure 2.”
   2. Palm.  This is the part that fits over the hand.
   3. Gauntlet.  This is the part that fits over the forearm, between the wrist and elbow.
   4. Four fingers, each comprised of two pieces.  The above simplified sketch only shows the fingers as a single piece.  Do not let my sophisticated drawings fool you.
   5. One thumb, comprised of two pieces.  Like the fingers, the thumb is comprised of two plastic pieces.
3. Critical Measurements
   1. These measurements refer to the lines labeled in “Figure 1.”  All measurements relate to the unaffected extremity.
   2. F5.  This is the length of the forearm, from the interior of the elbow to the wrist.  While this could be measured along the side of the forearm, it very likely doesn’t matter.
   3. F2 (measured at 1/2 F5).  At a location along the forearm, half way long F5, the width of the forearm.
   4. H1.  This is the distance across the knuckles, from the pinky to the forefinger.
      1. When I lay my own hand flat on a table top, I perceive that an imaginary line drawn through my pinky and forefinger knuckles would end up being not exactly perpendicular to an imaginary line drawn from my elbow to my wrist.  More on this below.
      2. All of that is another way to say that I suspect H1 is not perpendicular to F5.
   5. W.  This is the width of the wrist.  Rather than being strictly measured from either side of the wrist, this measurement appears to best made using the endpoints of the H2 and H3 lines closest to the wrist.
   6. H2 and H3.  H2 is the length from the wrist to the pinky knuckle and H3 is the length from the wrist to the forefinger knuckle.
   7. All other measurements indicated might possibly be useful for refining the design, but they are primarily important for the Creighton University research study purposes.
4. How Each Critical Measurement Informs Design
   1. F5.  Gauntlet length is not longer than 1/2 F5 and not shorter than 1/4 F5.
   2. F2.  Gauntlet forearm width is F2.
   3. W.  Gauntlet wrist width is W.  Theoretically, if the prosthetic’s palm is scaled up to accommodate the wrist width (W), the affected hand  should fit under and inside the prosthetic palm.
   4. H3 can be used to inform the relative lengths of the fingers to match the overall length of the unaffected hand.   This isn’t strictly required for a functional prosthetic.  As designed, the Cyborg Beast appears to use fingers of equal length.  However, the fingers could be scaled up or down along with the rest of the prosthetic hand.  Alternatively, and as will be discussed below, its possible that the fingers could be designed to be of different lengths.  Prosthetics for young children should contemplate fingers based upon slightly larger, 1-2cm, measurements.  The reason being that they quickly outgrow existing parts.
5. Functional Design Considerations
   1. Thickness of parts is 3mm – 5mm, 20% fill.
   2. The wrist hinges should line up as exactly as possible with where the user’s wrist bends.  Additionally, the wrist hinge should be perpendicular to the line of the forearm/gauntlet.
   3. There should be about 1 – 2 mm of space between the hinge part on the palm and the hinge part on the gauntlet.  This allows a washer to be inserted for more fluid movement.
   4. Eliminate square corners when possible, as sharp edges can contribute to possbile injury.
6. Cosmetic Design Considerations
   1. Using the unaffected hand for measurements also allows us to seek symmetry between the hands.
7. Advanced Considerations
   1. Degree tilt to H1.  As mentioned above, it seems like the “H1” line is not perfectly perpendicular to an imaginary line drawn from my elbow to my wrist.  An educated guesstimate would be that there is a 9 degree tilt to this line.  While existing Cyborg Beast designs do not include this knuckle “tilt,” including this feature in future designs may allow the prosthetic to appear and function more naturally.  However, I don’t know if there’s any real ergonomic benefit to using incorporating this knuckle tilt.
   2. Different knuckle positions for fingers.  The Cyborg Beast has a knuckle “block” that positions the attachment points for all fingers in a straight line.  The reason for this is simple – it’s a lot easier to put one long screw through the entire knuckle block to secure and strengthen all four fingers at once.  At a recent e-NABLE meeting I had the chance to inspect a 3D printed prosthetic which used different knuckle positions for each finger.  Rather than all of the knuckles in a straight line, this model featured each knuckle at a different, and more natural seeming, position.  While this can appear more natural, I’m not sure there’s an ergonomic or aesthetic benefit.
   3. Different finger lengths.  Fingers are different lengths.  The Cyborg Beast, with all fingers having the same relative knuckle positions and same finger sizes, has a more mechanical look than might otherwise be possible.  I don’t know if there’s an ergonomic benefit to using different finger lengths, but this is certainly something to explore.

Based on the above, I think I’m ready to dive back into the OpenSCAD code and work out a parametric gauntlet, fingers, and thumb.  Stay tuned!

1. I’m basing my own designs off of his Cyborg Beast designs [↩]
2. Let’s just choose the large and arbitrary percentage of 95% [↩]