Printable Prosthetics R&D Q&A FAQ: Part 2 – The Wondering

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In order for me to design an OpenSCAD parametric model that can be adjusted to work for more people, I need to get a better handle on the necessary measurements and how they effect the final design.  Below is my understanding of the necessary measurements and how those measurements necessitate changes in the final prosthetic.

  1. What are the design ideals, besides functionality?
    1. While answering a different question, Marc Petrykowski suggests, “My goal as the designer and printer is to make the hand as near perfect as the other hand so it feels the same to the body and brain, thus they will respond with the effected hand like it was their real non effected hand. Also as stated above, the degrees of flexion and extension and the size/length of the fingers are all incorporated into the final design before the printing the hand.”
    2. Thus, all other things being equal, he tries to craft a hand that is as similar as possible to the non-effected hand.
  2. What are the main parts of the Cyborg Beast?
    1. I’ve drawn a picture with the main features of the Cyborg Beast printable prosthetic.  There are really just a few parts – the palm, the “gauntlet,” fingers, and thumb.  The gauntlet fits over and is secured to the forearm and is connected to the palm by two hinges.  The palm goes over the user’s effected hand and is connected to the fingers and thumb.
  3. What are the necessary measurements?
    1. Marc Petrykowski has provided a set of photos to demonstrate the various measurements.  They appear to all be in millimeters.  Please forgive my layman’s description of these various measurements.  Measurements are taken of the effected and non-effected sides so that a prosthetic can be made that will fit the effected side, but have similar characteristics to the non-effected side.
    2. Flexion angle.  This would be the maximum angle of movement from holding your hand out and then bending the hand at the wrist towards the inside of the wrist.  An example is pictured above as “Figure 1.”
    3. Extension angle.  This would be the maximum angle of movement from holding your hand out and then bending the hand at the wrist away from the inside of the wrist.  An example is pictured above as “Figure 2.”
    4. Knuckle width.  This is the width of the hand at the knuckles.  In Figure 3, you’ll see this as “H1″ and “h1.”
    5. Wrist width.  This is the width of the hand at the wrist.  In Figure 3, you’ll see this as “W” and “w.”
    6. Hand measurements.  I’ve identified these as “H1 – H3″ and “h1 – h4″ in Figure 3 above.
    7. Forearm width measurements.  I’ve identified these as “F1 – F3″ and “f1 – f4″ in Figure 3 above.
  4. How does each measurement inform the design?
    1. Again, this is merely my guess, impression, or understanding of how each measurement results in a design change.  For the purposes of these diagrams, I’ve assigned each measurement a letter or letter/number combination.  When applicable, I’ve differentiated between the effected (lower case) and non-effected (upper case) hands.
    2. Hand Measurements (Figure 1,blue and green)
      1. Knuckle width, non-effected hand, “H1″.  This is necessary to creating a prosthetic of the size that will match the non-effected hand.
      2. Knuckle width, effected hand, “h1″.  This is necessary to creating a prosthetic of the size that will fit the effected hand inside the palm.
      3. Wrist to pinky knuckle, “H2″ and “h2,” the purpose of which is to ensure a prosthetic that will fit the effected hand inside the palm.
      4. Wrist to middle finger tip, “H3″ is the overall length of the uneffected hand.  The purpose of this is to create a prosthetic of roughly the same size as the uneffected hand.
      5. Wrist to index finger knuckle, “h3″ is for making sure the prosthetic palm will fit around the effected hand.
      6. Wrist to middle3 finger, “h4″ is for making sure the effected hand will fit inside the prosthetic palm.
    3. Wrist Measurements (Figure 1, orange)
      1. Wrist width, “W” for the non-effected hand and “w” for the effected hand.  The purpose of the effected hand measurement is to ensure a good fit between the prosthetic palm and the effected hand and the purpose of the non-effected hand measurement is to allow the prosthetic palm to match the non-effected hand more closely.
    4. Forearm Measurements (Figures 1, purple and red)
      1. Various measurements from “F1″ (and “f1) just below the wrist to “F4″ (and “f4″) which is the width of the elbow. As best as I can tell, these measurements are to ensure a good fit of the “gauntlet” on the effected forearm.
      2. Elbow to wrist, “F5″ on the uneffected arm and “f5″ on the effected arm.  I’m not sure what the purpose of this measurement is, but perhaps it is to ensure the effected arm with prosthetic is roughly the same length as the unaffected arm.
    5. Angle Measurements (Figures 2, 3)
      1. Somehow the flexion and extension are incorporated into the design.  I do not know how these settings inform the design.
  5. How accurate do these measurements need to be?
    1. Within 1mm, rounded up would be best.  Thanks to Peregrine Hawthrone and David Orgeman for the input.
  6. Questions begetting questions
    1. If you’ve ever made one of these prosthetics, please let me know if there’s anything I’ve gotten wrong.
    2. It appears the measurements effect the design as follows:
      1. Measurements “h1, h2, h3, h4 and w” dictate the size of the palm.  The ratio of the increase/decrease is then applied to all the finger bits.  The measurement “H3″ is used to adjust the size of the palm and fingers on the effected arm.
      2. Measurements “f1, f2, and w” dictate the size of the gauntlet.
      3. The additional measurements on the corresponding uneffected arm could be used to make the prosthetic over the effected arm appear more like the uneffected arm.
      4. I’m guessing the other unused measurements (“f3, f4, f5″) are used as part of the Creighton University research study, to measure the physical changes in the extremities before, during, and after use of these prosthetics.
    3. How does the flexion and extension change the design?
    4. Have you printed the Cyborg Beast designs I’ve uploaded?  What are your thoughts?

Thanks for reading and helping!  Comments appreciated!

4 Responses to “Printable Prosthetics R&D Q&A FAQ: Part 2 – The Wondering”

  1. Marc Petrykowski says:

    Questions #1,2 are good.
    #3 the measurements are in CM, the rest of the question is accurate.
    #4 each measurement for my cases are required. That is because every hand is somewhat different and I manually measure the hands on the computer before I start the print. This ensures the hand will be the best fitting for the individual which is very very very crucial because then they will keep wearing it.
    #4.5 (angle measurements) are used for the tightness of the strings. They only take a few more minutes to take and include the measurements, but it is nice to have to understand the height of the hand (even though it is not necessary, I just like having it in case) and then you can see how much flexion and extension of the wrist/hand they have compared to their other hand. Then you can modify the hand to look similar when being flexed or extended based on the dial settings on the gauntlet.
    #5 when measuring, it is always good to round up to the nearest CM or so. Also remember that you are adding foam to some parts of the device, so you will need to compensate for that.
    #6 when you make a piece of the device at a certain size/scale, then that should be the same exact scale for ALL of the other components.
    #6.3 stated above is the flexion and extension answer.
    +Maker Block Let me know if this helps!

  2. Jascha Wilcox says:

    Okay to get things back on topic, getting these standardized measurements sorted out is extremely important. +Maker Block did a great job of illustrating the state of the art, and makes it fairly apparent that the current measurement definitions are quite arbitrary and need some work.

    Some specific feedback I have in an attempt to better define and streamline things:
    – Why are flexion and extention necessary measurements?
    – Specific anatomical “landmarks” need to be included in the definitions, otherwise repeatability is difficult to achieve. I would propose defining all forearm measurements based off common bony protrusions such as the styloid process (bump on wrist), elbow (olecranon), and knuckles.
    – Are so many transverse forearm dimensions (f1-f4) necessary? I would also propose that those measurements be taken as a circumference as opposed to a width and height.

    I would also like to push to make ball bearings standard components. This would involve slight modification to existing designs (ie. parametric cyborg beast), but is quite feasible. +Nick Parker 

  3. Marc Petrykowski says:

    Sure thing +Jascha Wilcox
    Specific land marks are hard to determine for the average person. It is easier for a person to just divide the forearm up into the elbow and wrist and then the parts in between those two. For the forearm, we don’t need specific precise spots because it is a rough estimate for the gauntlet. It is more of a guide then an exact number.
    For the circumference, it is not needed. The length and the width is all we need because the gauntlet and the palm of the hand have an “open bottom” thus creating the “Z” axis not super important. The X and Y axis are the important ones, which is taken with photo description I have been talking about. I also shared my guide as I just finished it. Hopefully that helps

  4. MakerBlock says:

    @Jascha Wilcox: I’ll try to address your questions the best I can.
    1) From my discussions with Jorge, the flexion and extension are not necessary measurements to create a printable prosthetic – they are collected because they are useful to the research study.
    2) I agree – using anatomical landmarks is a great idea.
    3) None of the measurements of the affected arm are required – just F5 (elbow to wrist), F2 (width of forearm 1/2 of the way from the elbow to the wrist). As Marc mentions, since the prosthetic uses velcro to tighten to the appendage, there is no need for a “Z” or height measurement. This also dispenses with the need for the circumference measurement. The gauntlet should fit over the forearm, then gets fastened in place with the velcro. If it rides a little high, that shouldn’t be a problem. (As far as I understand how these things work).
    Once I’m done translating the Cyborg Beast into a fully parametric design, I’ll work on incorporating some of the latest developments (bearings, springs, etc).

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