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Durometer measurement confidence intervals

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Mart, Jul 31, 2014.

  1. Mart

    Mart Well-Known Member


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    Hi All

    Persistent plantar heel pain can be attributed to periostitis, calcaneal bone edema or pain generated within plantar fibro-fatty pad and this may be associated to the mechanical properties of that individuals plantar fibro-fatty pad. If the plantar fibro-fatty pad is too stiff it may not disperse forces evenly with resulting increase localized pressures at sites such as the medial process of calcaneal tuberosity. Similarly if fat pad tissue characteristics are overly compliant or too thin then the distribution of forces similarly compromised.

    I am considering using a ubiquitous inexpensive digital handheld durometer measurement device to “tune” the mechanical properties of silicone material used to mitigate compression stress at medial process of calcaneal tuberosity in those with poor empiric response to pre-existing foot orthoses design.

    My assumption is that the devise is used by applying a force in the direction of the indentation pin motion until the reference plate touches the surface of the material without deforming it. I can’t find any explicit direction about using the instrument online.

    Does anyone have experience using this type of instrument in this context and better still some confidence limits for multiple measures and associated parameter controls around those values?

    Additionally has anyone considered the merit of trying to determine the predictive value of measuring the weight-bearing indentation properties (Shore value) for an individual’s plantar fibro-fatty pad and trying to determine the durometer and thickness characteristics of an optimally tuned supplementation material embedded within a foot orthoses to normalize or clinically improve plantar fibro-fatty pad behavior?

    Cheers

    Martin

    Foot and Ankle Clinic
    1365 Grant Ave.
    Winnipeg Manitoba R3M 1Z8
    phone [204] 837 FOOT (3668)
    fax [204] 774 9918
    www.winnipegfootclinic.com
     
  2. Keith Rome has done a lot of good research on this same topic.

    In Google Scholar type the following: plantar fat pad stiffness, K Rome

    Happy reading.
     
  3. Mart

    Mart Well-Known Member

    Hi Kevin

    Thanks for your suggestion to look at Keith Rome’s work which I did.

    His work in this area was to validate a custom indention device then use it to look at force/deformation data comparing matched runners with PHP against those without. Essentially, using a non-linear regression equation, he derived 2 different force deformation curve coefficients and examined the difference in mean values between the PHP and none PHP groups finding statistically significant difference using one of the coefficients. BTW his paper (The Foot Volume 8, Issue 4, Pages 179–185, December 1998 Mechanical properties of the heel pad: current theory and review of the literature K Rome) is well worth a read irrespective of interest in durometry.

    Clearly this force/deformation approach is a different than using durometer readings and attempts to measure stiffness as opposed to a measure which for specific materials appears to have a correlation with Young’s Modulus.

    Here’s where my initial idea sits now. Most of the studies I had looked at using durometers used gravity to apply the indention force either by attaching a weigh to the instrument or simply using its serendipitous weight.

    Rex Gauge Co. information states that “One of the most common problems durometer users experience is false readings due to pressing too firmly on the gauge and imbedding the foot of the gauge into the test specimen. To make Rex Durometers a good instrument for testing in hard to reach areas, we designed our gauges with the smallest foot possible which is ½" diameter. This small foot makes it easy for an inexperienced user to imbed the foot of the gauge into the soft specimens thus obtaining false readings.” This company produces a variant with a load a load weight on the gauge, all the user has to do is set the gauge on the test specimen - no further pressure is necessary. Thus the Constant load weight eliminates the human error that would otherwise occur due to users pressing on excessive or inconsistent force. However choosing the correct range will be vital to exploit that idea.

    Ironically one study I looked at measuring plantar heel indentation showed a photograph of the foot of the device compressing the skin around the indentation pin which will clearly give an erroneous Shore value (Proc Inst Mech Eng H. 2012 Apr;226(4):305-11. Association of limited joint mobility and increased plantar hardness in diabetic foot ulceration in north Asian Indian: a preliminary study . Periyasamy R1, Anand S, Ammini AC).

    That is why I was curious to start this thread. Since then I have got a better handle on this.


    “Durometers have successfully been used in skin softness measurements with the foot attachment removed [15, 16]. Falange et al [17] used a durometer to measure the skin softness in patients with scleroderma. Lancy [18] adapted a durometer to assess the skin induration around venous ulcers. The durometer indentation test is thus a common measurement method to make quantitative assessments and comparisons of the material resistance to indentation of elastomeric and elastomeric-like materials. However, the indentation and durometer tests used in the above-mentioned applications did not in general follow the methods described in the relevant ASTM or ISO standards.”
    Measurement Science and Technology Volume 17 Number 7: Winnie Yu et al 2006 Meas. Sci. Technol. 17 1785 doi:10.1088/0957-0233/17/7/017 Softness measurements for open-cell foam materials and human soft tissue Winnie Yu1, Yongbao Li1, Y P Zheng2, N Y Lim1, M H Lu2 and Jintu Fan1

    Winnie Yu et al did use a durometer as it is intended to be used and concluded that the mechanical characteristics of foams (force/deformation approach) can be measured in a repeatable manner by the indentation method (durometer) and that appropriate foams can therefore be selected for use in a soft mannequin which will then accurately simulate the texture and mechanical behaviour of human soft tissue.

    However this falls short of using it on much more complex material such as human plantar fibro-fatty pad.

    I got hold of a cheap digital Shore A instrument to play around with. It has a max sensitivity of 0.5 HA and after a bit of practice I was able to measure homogenous foot orthoses materials such as silicones, PPT and EVA foams with reproducibility of +- 0.5 within range of shore A 5-50.

    What was notable was how much the shore measurement of the silicone sheets I fabricate increased with a very thin embedded fabric surface (which I use to allow adhesion within the foot orthoses).

    On my own foot the reproducibility of the plantar heel fibro-fatty pad was less impressive; the shore values were in range of 1 to 6 according to site and the mean CIs between +- 1-2.

    It would be interesting to use an instrument designed to measure shore 00 which might be more accurate for plantar foot.

    Regardless, it is a bit of a stretch to imagine that the shore value might be sensitive or specific enough to predict risk for plantar heel pain let alone allow a coefficient to emerge which would usefully predict the shore value for a component of foot orthoses design.

    Cheers

    Martin

    Foot and Ankle Clinic
    1365 Grant Ave.
    Winnipeg Manitoba R3M 1Z8
    phone [204] 837 FOOT (3668)
    fax [204] 774 9918
    www.winnipegfootclinic.com
     
  4. Martin:

    If I was going to measure the compression stiffness of the plantar heel fat pad then I would try to simulate the ground by using a device with a flat plate at the compression-application end of the device that was at least 3 cm diameter since this would give a broader surface area of contact and would probably be best at reproducing the foot-ground interface. In addition, it may be helpful to devise a way to simulate the impact of heel strike during walking and/or running on the plantar heel by using a more rapid velocity of the application of compression force to the plantar heel than to use a slower compression rate.

    It is an interesting field of research and glad to see you are getting interested in it.:drinks
     
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