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medial arch apex in CAD design

Discussion in 'Biomechanics, Sports and Foot orthoses' started by vivo, Sep 4, 2019.

  1. vivo

    vivo Welcome New Poster


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    hello everybody,

    recently i started using insole design software . i'm trying to understand what reference point should i use as medial arch apex. right now i'm measuring the distance to TNJ using navicular tuberosity (about 33% of foot length) as reference point , and designing the medial arch to reach the apex gently and down again.
    i have seen that some CAD designers use the mid point of the visual arch apex as reference point. the problem with that to my opinion is, it's not the true apex and also the arch comes out too high with the insole .

    how do i determine the medial arch apex ?

    i'll be happy to get some experienced insight on that issue.

    thank you
     
  2. Konrad Job

    Konrad Job Welcome New Poster

    Hi Vivo,

    Craig has asked me to chime in on this one. I might not have the perfect answer, however I can give you a perspective of someone that has worked with several design packages. There are some things you can do to decrease the subjectivity as an orthosis designer.

    Coming from a CAD design perspective I have found that having a clinician mark the location of the peak of the MLA on the foot prior to 3D scan capture is optimal. You can then capture the foot in a 3D colour scan format (VRML, PLY, OBJ) and this marking will be available to you during modelling. This can help to minimise some of the subjectivity of the modelling. Clinicians tend to be in favour of being in control of a more objective process.

    Here’s an example of the basic process in CAD for your reference – this is oversimplified to just cover the arch component.

    First align the cast according to a frontal plane reference (e.g. – forefoot varus, forefoot valgus, heel bisection angle). This is critical as it will effect arch height, etc. You can see below that the blue line is in line with the bisection. As the clinician would mark this on the foot directly I have immediately decreased CAD subjectivity.

    upload_2019-9-6_15-26-59.png

    I then place spheres to help generate an orthosis model with the required dimensions. Note that the peak of the MLA has been marked with an orange sphere. This is based on the clinicians marking on the cast (usually it’s a foot scan, but I have a cast in front of me to demo for you).

    upload_2019-9-6_15-28-43.png

    As you can see the arch peak is then positioned according to the clinicians marking on the foot.

    upload_2019-9-6_15-29-30.png

    The amount of fill in the arch from this point is different from lab to lab. When designing I use percentages as they are more relative to foot morphology than mm values. As long as you are consistent in your approach and communicate this effectively then this should present a good prescription framework for a clinician.

    upload_2019-9-6_15-30-41.png

    You can see above I measure from mid arch point to ground and you can then assess a percentage of fill from this point. In the image above the fill is 10% (20mm height at the midpoint and a 2mm reduction from this point). A clinician could request any percentage they like.

    This example does not come from a clinical perspective (even though I am a Podiatrist), but more from a perspective of how to translate prescription variables into consistent design features, which is a challenge that faces the industry.

    I also though it worth mentioning that colour allows for more information than just peak arch location. You can get creative. For example, many clinicians will mark a plantar representation of the subtalar joint axis and ask me to apply a skive medial to the axis (example below).

    upload_2019-9-6_15-32-39.png

    upload_2019-9-6_15-32-46.png

    upload_2019-9-6_15-32-52.png

    I hope that you found this interesting/helpful.
     
  3. vivo

    vivo Welcome New Poster

    Hi Konard,
    that's helps a lot.

    thank you
     
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