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Evaluation of the pressure-redistributing properties of prefabricated foot orthoses in older people

Discussion in 'Biomechanics, Sports and Foot orthoses' started by NewsBot, Aug 23, 2011.

  1. NewsBot

    NewsBot The Admin that posts the news.


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    Evaluation of the pressure-redistributing properties of prefabricated foot orthoses in older people after at least 12 months of wear.
    Cronkwright DG, Spink MJ, Landorf KB, Menz HB.
    Gait Posture. 2011 Aug 18. [Epub ahead of print]
  2. DaVinci

    DaVinci Well-Known Member

    Haven't we had a previous discussion (that I can not find) that its invalid to collect plantar pressures on the curved surface of a foot orthoses as the sensor is not perpendicular to the GRF. If so, how can the authors reach the conclusion that they did?
  3. Simon, David and Kevins paper I think you mean. - In-Shoe Pressure Measurement and Foot Orthosis Research A Giant Leap Forward or a Step Too Far?

    Simon linked it here for those that want a read - http://www.podiatry-arena.com/podiatry-forum/showpost.php?p=223213&postcount=3
  4. In this instance it shouldn't be too much of a problem since the same devices were tested- hence the foot-orthosis interface geometry is the same(ish) in the two conditions. So the bending error should be roughly the same(ish) in both conditions too. That is unless the surface topography and/ or the load characteristics of the orthoses were changed through the twelve months of wear. I assume they were to an extent, as this is ultimately what the study was trying to test (along with frictional changes).

    Indeed, the changes of 6%, 5% and 2% are pretty small, I should think that such changes might even be accounted for by between trial variability alone.

    It would have been nice if they could have run the subjects over a force-plate as well though, but I didn't think it was a bad effort. I'd like to see some similar studies showing same topography with different material density of foot orthoses, i.e. Vasyli: low density, medium density and high density= same geometry, different load/ deformation characteristics.
  5. DaVinci

    DaVinci Well-Known Member

    I would have thought that the surface geometry of the device 12 months later would be substantially different and the load characteristics of the material would have changed substantially and this would have distorted the plantar pressure readings? From the abstract it appears they did not assess those changes.
  6. Good job to Karl, Hylton and colleagues for doing this research and getting it published in a major biomechanics journal such as Gait and Posture.:drinks

    I have no problems with using inshoe pressure analysis in a study comparing the plantar pressure in the same insole over time since the absolute pressure pressures aren't as important as are the relative changes over time. As far as I'm concerned, these researchers should be congratulated for doing some badly needed basic research on this important subject for the podiatry profession.
  7. krome

    krome Active Member

    I am in total agreement with Kevin. Studies similar to Hylton and Karl are desperately needed and it illustrates the need to have robust studies that are peer-reviewed. For those, who have not been through the process of developing and implementing a large prospective clinical study then the finite details of bending error is small compared to recruitment, acceptability of the devices from this ‘at-risk’ group and most importantly the wider global understanding of the role of foot orthoses. Congratulations to the authors.
  8. DaVinci

    DaVinci Well-Known Member

    Am I missing something here?

    If the 3d shape or contour of the arch area is different between the new orthotic and the 12 month old one, then the sensors will be at different angle to the ground reaction force and the comparison of the plantar pressures will not be valid due to that error that comes from the measurement. As the authors did not tell us about any 3d shape changes, how can we accept the results?

    Am I not getting something here?
  9. How much change in arch shape would invalidate the results using pressure measurement insoles when comparing the same insoles over a 12 month wear period? I would estimate that the change in sensel angle pre- and post-wear would need to be at least 10 degrees to show signficant changes due to a change in non-normal force on that sensel.

    If I were to review this paper, I would certainly want the authors to mention this potential measurement error within the discussion of the paper, but, in my opinion, do not think that this type of error would invalidate the study or make the study unpublishable in Gait and Posture. (Even though I am not a reviewer for Gait and Posture, I do review manuscripts for the Journal of Biomechanics, JAPMA and the Journal of Foot and Ankle Surgery.)

    There is no perfect research. Therefore, as a reviewer, I am required to weigh whether the research imperfections are worse than the potential benefit of the paper to the medical/scientific literature. My opinion is that as long as the authors describe the potential problems with their research methods and results within the discussion fully, then problems like you mention here of potential change in sensel angle pre- and post-wearing of insole do not outweigh the importance of such research for the medical profession.
  10. Surly it depends upon the initial angulation and the input loading? After all, a 2% change was reported as significant. If I get time later I'll show some working on this. Dave Smith may still have some of the spread-sheet models he did which will save some time- Dave?

    I think this is an interesting problem. What is the hypothesis here? That 12 months wear of the orthoses changes their shape and/ or load/deformation characteristics? Both of these have the potential to change the force recorded by the sensels. Indeed, the sensor array is effectively an indirect way of assessing these changes.

    Lets ignore the load/deformation side for now and focus on the surface angulation. Say the in-shoe sensor is 100% accurate and that there is zero variability in the input loading between trials. If we see a change in the data returned it has to be due to changes in the angulations of the sensels. Nothing else could account for those changes in this situation- right?

    More later, work now.
  11. davidh

    davidh Podiatry Arena Veteran

    "These findings indicate that the prefabricated orthoses evaluated in this study are only slightly less effective at redistributing plantar pressure after at least 12 months of wear."

    No they don't.
    These finding indicate that the prefabricated orthoses evaluated in this study are only slightly less effective at redistributing plantar pressure after at least twelve months of wear provided the subjects have a mean age of 75.4 years, and have the normal reduced activity levels of persons in their middle 70's.
  12. OK quick fag packet calculation: let the surface of the orthotic upon which the sensel sits be initially 45 degrees; let the total load on the sensel be 5 Newtons, then this sensel with give a reading of 3.5N since Force x Cos 45 = 3.5N. Now let the same area of the orthotic be deformed by 3 degrees so that the sensel now sits on a surface angled at 42 degrees, now apply a total load of 5 Newtons, this time the sensel will give a reading of 3.7N, this is about a 5% change (which is commensurate with what was reported in the paper).

    Repeat the same calculation with increasing total load and you'll see the percentage change increases, so for example, with a 30N total load, you get an 8% change with our 3 degree change in surface angulation from 45 to 42 degrees.

    In other words, the change in angulation at the orthosis-sensor interface need only be small to arrive at statistically significant percentage pressure changes.

    Hope this helps.
  13. OOOPs There is an error in my calculation here, it stays at about 5-6% not 8% change regardless of input force. My Bad, but you get the drift.

    Of course another way of looking at this is to assume that despite the 12 months of wear, that hypothetically this could have resulted in zero change in the angulation of the surface that the sensel is sitting upon. So to return a 5% change in the output of a sensel sitting on a 45 degree surface we should need a change in the total load (if the surface angulation and load/deformation is constant between trials). In this example of a sensor on a 45 degree surface, if the initial loading at the sensel was registered as 3.5N we know the total load was 5N so if the sensel output was to be increased by 5% without change in it's angulation the total load applied to it should need to be 5.19 Newtons. So, the (between-trial) error/ variation at the sensel needs to be less than 0.2 Newtons in order for it to not return a (statistically significant) change of 5%- is it? BTW a 2% (which was also reported as being statistically significant within the paper) change should be an increase from the initial loading of 5N to a loading of 5.04N on a 45 degree surface.

    To be fair the authors reported the (statistically significant) 2% change at the forefoot, where the sensor is flat (doesn't mean the loading vector is normal though, but lets pretend it is for a moment) in order to bring a (statistically significant) 2% change in a sensel loaded vertically on a flat surface with 5N force we should need to apply a load of 5.1N. So, is the system accurate to within 0.1 of a Newton? Moreover, was the between trial variability less than this?

    These are the kind of things I hope to see discussed within the paper when I read it in its entirety. Mr Griff, if you'd be so kind.
  14. Any authors who take the germ of an idea through to publication of a scientific study are to be congratulated upon their efforts. However, the whole point of publication is to have your peers read and critique your work. The finite details of bending error might be significant here, whether the researchers have managed to recruit and obtain data from one or one thousand individuals from this "at risk" group becomes superfluous if the data they have collected is inaccurate. Lets be honest, 31 subjects is not that large. We ran many undergraduate studies with larger subject groups. Indeed, as one of my colleagues has already pointed out this "at risk group" is perhaps unrepresentative of population to whom this kind of device is most commonly prescribed. Good effort, but lets not start pretending that it is a study which is above criticism, since it is clearly not. Nor, should I hope, that the authors believe this to be the case. Rather, I should hope that they welcome the feedback, since that is the whole point of publishing the study- isn't it?
  15. Don't want to become Peter the prolific poster on this topic, but it is of interest to me. So lets live in the real world. We know the reaction force vector will rarely be normal to a horizontal surface during gait, so what variation in the angle of the reaction force vector is required to bring about a 2% difference in the output of a force measuring sensel which is placed on a horizontal surface?

    Answer: not a lot.:drinks

    I'd love it if the authors could respond to these comments here, if not I can always send them in writing to the publishing journal.
  16. DaVinci

    DaVinci Well-Known Member

    Thanks Simon, that is what I was getting at. From what I read, the authors were not even aware of this problem.

    If I understand correctly, hypothetically, if the new orthotic has a higher arch profile, there will be greater error as the sensor will be angled at a greater ankle to the GRF and give an artificially lower value. Hypothetically, if the worn out orthotic has a lower arch profile, then the sensor will be angle closer to the perpendicular with the GRF and give a more valid reading of the true pressure.

    Is that understanding correct?

    If it is, then did not the authors actually find the opposite of what they reported? If the pressures were artificially lower in the newer orthotic (higher arch profile), then they actually probably found a difference?

    I only talking about the midfoot here, as this would not affect the forefoot pressures.
  17. Griff

    Griff Moderator

    Leave it with me - Athens playing up tonight

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