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How do you decide on how much 'arch fill' to prescribe

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Craig Payne, Oct 16, 2010.

  1. Craig Payne

    Craig Payne Moderator


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    This ones been bugging me for a while ...

    How do you decide how much plaster to prescribe in the arch? What is the criteria that you use to decide between minimal, moderate or maximum arch full? (or arch release if CAD/CAM used).

    I ask many people this question and it is difficult to get any sort of consensus or anything that could be tested and developed into guidelines.

    Most people say its based on 'tolerance'; ie those who are less likely to tolerate a foot orthotic are given more arch fill .... how do actually you determine 'tolerance' before giving someone and orthotic?

    We playing with a few clinical tests that may prove useful.... but would really like to know how you decide.

    This is also based on the premise that the lab actually does what you ask (its been my impression that labs that tend to get a lot more badly taken casts, that they use more arch fill, so the orthotics are more likely to be comfortable).

    Does more arch fill = less effective foot orthotic?

    This was partialaly touched on in this thread: Does plantar contact necessarily = control?
  2. I don't. I've never grasped the concept that if you want a lower arch, you cast a higher arch then make it lower! So for an insole on which I want a lower arch than neutral, I cast the foot more pronated than neutral.
  3. Craig:

    This is a fundamental question regarding the prescription and manufacture of custom foot orthoses. As you know, there are many factors, other than the thickness of the medial expansion plaster on the positive cast, that go into determining the transfer of orthosis reaction force (ORF) from the medial longitudinal arch (MLA) of the orthosis to the MLA of the plantar foot. Factors such as rotational position of the joints of the foot during negative casting, frontal plane balancing position of the positive cast, stiffness of the MLA of the orthosis shell, plantar MLA filler/shank dependent materal, presence/absence of rearfoot posts, and fit of orthosis inside the shoe, to name a few, will also significantly affect the transfer of ORF from the MLA of the orthosis to the plantar foot.

    Here is my theoretical and clinical rationale for use of minimal, normal and extra thickness medial expansion on the positive cast.

    The only instances where I use extra thickness medial expansion on the positive cast is when I have a patient with a laterally deviated STJ axis and I don't want the orthosis to add any extra external subtalar joint (STJ) supination moment to the foot. For example, in many of my patients with peroneal tendinopathy and chronic inversion ankle sprains due to laterally deviated STJ axes, I will use extra thickness medial expansion plaster on the positive cast.

    I use minimal thickness medial expansion plaster on the positive cast in the following instances:

    1. In those feet where I want to have the tightest MLA conformity from the orthosis and want to generate the maximum external STJ supination moment.

    2. In feet where I will also use the medial heel skive in order to optimize generation of external STJ supination moment from the orthosis by shifting ORF as medial as possible in both the rearfoot and midfoot (see #4 for exceptions to this rule).

    3. When I have a patient with a prominent plantar fascia and I will be using a plantar fascial accommodation on the positive cast to avoid plantar fascial irritation from the orthosis.

    4. In children and teenagers with excessive medial deviation of the STJ axis and with symptoms due to excessive STJ pronation moments. However, in some older adults, if I feel orthosis irritation may be likely in the MLA of the foot due to bony MLA prominences and poor muscle strength in the STJ supinators, I may use a normal thickness medial expansion along with a medial heel skive in order to improve foot orthosis tolerability.

    For most other patients, I will use normal medial expansion thickness for their orthoses.:drinks
  4. What's that in mm? Say in a size 9 foot.
  5. Craig Payne

    Craig Payne Moderator

    That is part of the problem. 'Minimal' arch fill to one lab is 'normal' or 'moderate' to another. Consistency is a big problem.

    With the CAD systems, you can specify mm, but then you get issues with the desire/wish to get into 'mirroring' (I will start a seperate thread on that soon).
  6. RobinP

    RobinP Well-Known Member

    I know of a lab who, when sent foam impression negatives(which probably constitutes 90% of its throughput), lower/infill the arch by "rubbing" the foam away in the arch. The spec sheets they use have no arch infill measure, they just do it as standard to "improve compliance."

    The bit that takes the biscuit is that it is done by a technician with no real anatomical knowledge and it is then scanned and modified by CADCAM. Surely this is not good practice. What is the point of the CAD part?

    I am no stranger to modifying a foam box to achieve the desired orthotic reaction force (ORF) but at least I am doing it with the patient in front of me. I agree with Robert, minimal infill for a size 12 foot could be a lot more aggressive than for a size 2 foot if the measure is rigidly 3mm.

    My own method is a little artisan in that I am looking at the foot and lowering the arch of the negative mould to an amount I perceive to be correct based on experience of patient tolerance versus my biomechanical goals. On the occasions when I take POP casts, I guess the amount that seems right and I am probably more conservative than I need to be on most occasions.

    Some kind of calibration formula would be handy to make it a little more objective. Something like:

    In weight bearing with a normal base of stance

    Minimal arch infill = 1.5% of distance(in mm) from posterior heel to 1st met head(apex)

    Normal arch infill = 3 % of distance (in mm) from posterior heel to 1st met head(apex)

    Deep Arch infill = 5.5% of distance (in mm) from posterior heel to 1st met head(apex)

    The reason that the deep infill carries a greater percentage is that the most likely reason for a deep infill is for a foot with a laterally deviated STJ axis/cavovarus foot type and as the distance from the heel to the 1st met apex from the heel is likely to be shortened then perhaps a higher percentage infill is required?

    Obviously there will be cases where there is a shortened first ray due to surgery etc so perhaps a different marker would be more appropriate but there are not many as easy as the 1st MPJ. Better than guessing? Perhaps not.

    Good discussion. Look forward to the mirroring thread

  7. Like robin I will sometimes modify the foam with the patient in front of me. However I dislike the terminology "to improve compliance". To me, this infers that the ideal would be an unmodified, and the infill / modification is a compromise. I don't beleive this is the case.

    I beleive it comes back to the old debate about the "ideal" position. To me, the foot is just something I use when arriving at the shape I decide I want the insole to be. This may involve shaping it in such a way I can't shape the foot.

    I believe that philosophically, this approach stems from the fact that I make my own devices, 50 - 100 per month. If you send them off to a lab it's is natural that you focus more attention on the foot position element, as that is the bit you control. Making the devices gives you more control of the manufacture process so instead of thinking in terms of casting position, one thinks in terms of orthoses shape. Arch fill is a part of this.

    My 0.02$ anyway.
  8. RobinP

    RobinP Well-Known Member

    That was a direct quote from the particular lab in question which I inherantly disagree with. The point i wanted to make about it was that, despite using CADCAM where they could be consistent about the arch infill, they chose to eyeball it and lower it on the foam box having no information from the clinician - i find this a little suspect.

    On a side note, I think that this smiley :butcher: is manufacturing his very own specialised "axethotic"........just grinding in a an extra little bit of edge....for compliance!
  9. efuller

    efuller MVP

    It has been bothering me for a while too.

    I have the patient stand and, with my finger, press gently up into the arch and then measure the height of my finger off of the ground. When I make my positive cast, or send it to the lab, I ask for that height in mm. Some labs have asked to mark on the cast where I want that height to be achieved. I will mark the navicular tuberosity. I've seen other people/labs prefer the high point of their orhtotic's arch to be around the base of the 1st ray or 1st cuneiform. I will push up less hard in feet that need less push up in the arch (lateral instability, peroneal tendonitis, laterally deviated STJ axis.)

    I may be treating my own feet, more than the patient's foot when I do this. In my very medially deviated STJ axis foot, excessive pressure in the arch really hurts. First, do no harm. Feet with more average axes may actually get some supination moment from pressure in the arch. In my foot, not so much. There are a lot of people out there who are recommending higher arched devices and they claim that they work. I'm not convinced that it is needed.

    Here's another experience that has contributed to my thought process. When I was a student, I got my first pair of orthotics and they hurt my very promenent plantar fascia. A plantar fascial groove was added and I was able to tolerate and enjoy them. Later, I decided to take that cast with hump to create the groove and fill in the arch of the cast to cover up the hump. The devices pressed from this modification were equally as comfortable. What was interesting about these orthoses was that when I stood on them, they did not touch my arch. However, when I ran with them the edge of the orthotic left a red line in my foot. So, when I was running on them, the arch of my foot did touch the orthotic. This was before I added the medial heel skive. There are many shapes of orthotics that can make a patient more comfortable. All of those shapes were better than walking without an orthotic. (Well, except for the high arch ones without the plantar fascial groove.)

    The nice thing about asking for an arch height in mm is that if the orthotic comes back with a different height, I can send it back and say that they did not make it to my specifications. You would have a harder time doing that with a minimal/maximal fill request.

    I'm still wondering if my method is the best. I'm getting fewer opportunities to make multiple orthotics for patients to see which ones they like best.

    Another thought, there is variation in the amount of change in arch height from neutral position to standing arch height. Taking that fact into account should be important if you are going to choose max or min fill.

  10. Craig Payne

    Craig Payne Moderator

    A test described to me by Geza Kogler was to get patient standing --- have them rotate the pelvis as far a possible left and then right. While doing that observe how far the arch comes up ... if it does not come up very far --> prescribe more arch fill; if it comes up a lot --> prescribe less arch fill.
  11. DanthePod

    DanthePod Member

    Arch fill would obiously vary according to the design of orthotic you prescribe. Most inverted style devices have compleatly different arch fill parameters than contouring devices. The amount of medial skive can also effect the amount of arch fill that may be required for functional effectiveness. Also people with tight plantar fascial chord would be far happier with more arch fill and a fascial accomidation. Myself for a device where i am prescribing an essentially contouring the device with average amounts of skive I would use around 10% arch fill. :dizzy:
  12. N.Smith

    N.Smith Active Member

    Originally Posted by Craig Payne View Post
    This ones been bugging me for a while ...

    How do you decide how much plaster to prescribe in the arch? What is the criteria that you use to decide between minimal, moderate or maximum arch full? (or arch release if CAD/CAM used).

    If you could apply an amount of force to raise the MLA to whatever height you wanted or the patient could tolerate when weight bearing, wouldn't that take out any need for guessing what height the arch should be in the prescription?

  13. Yes.

    Assuming the patient's weight bearing tolerance / supination resistance when static is the same as when dynamic.
  14. Which is one of the reasons I developed a jig to measure supination stiffness.
  15. Can the jig look at supination resistance when a patient is say running?

    if so no wonder you did not want to give the design to Robin for a beer, very impressive.

    If not, how does a static supination resistance measurment relate to dynamic running supination resistance ? or the higher the supination resistance in stance indicates higher resistance to re-supination in general - Which may not always be so- due the different forces invovles in running .

    Doing a bit of a review of my orthotic process at the moment and the why, why not of measurments and biomechancial assessment verse measurment required to produce a device.
  16. I'm sorry to disappoint you, it's static only. It remains to be seen how it relates to dynamic function, you could extrapolate the data such that you make certain assumption eg. 2.5 x body weight during running.
  17. Phil Wells

    Phil Wells Active Member

    Couple of things that help me decide as to how much arch fill is required (Clinician perspective rather than lab worker)
    1. How high is the arch of the cast relative to the assumed pathological foot position
    2. How much transverse movement/planal dominance is seen in the MTjt.
    3. Activity level - if the orthoses is aiming to apply its ORF in stance/walking then little arch fill (see point 1). If the ORF are needed in sport, I assume a certain amount of fatigue related function e.g. tight gastocs, weak Tib ant, peroneals etc. This is subjective and in some cases I assess the patient pre and post fatigue. In these cases it may be necessary to lower the arch to help minimise arch irritation.
    4. How much ORF needs to be applied in the arch to help relieve symptoms?

  18. Craig Payne

    Craig Payne Moderator

    We got data on static supination resistance force correlated to the dynamic peak pressures between the foot and orthotic where the two fingers (or the strap of pur jig) goes -- the r was 0.54 ... which I thought was a pretty awesome correlation.
  19. Yeah, but that gives an r square of only 0.29 meaning that 70% of the variance was unaccounted for by this variable. Did you attempt any scale transformations? What was the model that you fitted?
  20. Craig Payne

    Craig Payne Moderator

    The supination resistance test is static standing. Dynamically, the muscles are firing, the windlass is supinating the foot; the tibia is externally rotating; etc, so to get and r of 0.54 (or r squared of .29) is an awesome result. How many other static tests have any correlation to dynamic function? (we use a simple pearsons).
  21. N.Smith

    N.Smith Active Member

    Sure. It would also depend on what the patient wanted the orthotics for and if they were running, walking, standing in them, (to determine high or low MLA) but it gotta be more accurate than non or semi-WB. So...if you could determine the height of the MLA and feel resistance to make it higher or lower before the prescription was written and not leave it up to lab to decide, it should be more accurate. Shouldn't it?

    In any case, there are a lot of variables in orthotic prescriptions but I figure the less steps (modifications in this case) in the process, the better the outcomes, if the force, pressure, shape was achieved and you were satisfied with it, before manufacture.

  22. DanthePod

    DanthePod Member

    You would think that dynamic supination resistance would be influenced by sagittal plane restriction occuring at any key sagittal plane pivot site. Simply measuring sup resistance staticly would not necessarily take this into account. A sagittal plane blockade would cause a spike in supination resistance as dynamic function is impeeded. You would expect that this spike would be far greater than a static measure of sup resistance. What do you think Craig et al??
  23. Are these occurring when peak pressure occurs at the foot-orthosis interface in the region of the navicular? I should hypothesise that peak pressure between the foot in this area and the foot orthoses occurs just before the point of equilibrium between the foot and orthotic shell in this region, this should be prior to heel lift and resupination.

    Coming back on topic, the amount of arch fill prescribed needs to be taken into consideration with the load/ deformation characteristics of the device prescribed. How much dynamic navicular drop do you wish to allow before equilibrium is reached and how rapidly do you want to arrest talo-navicular motion? I could put the same amount of arch fill onto the cast, but make two different devices: one of a relatively compliant material, one relatively much stiffer material... they will function very differently despite having the same geometry. Arresting the motion at different rates and positions (i.e. reaching equilibrium at different points during the gait cycle and with different kinematic/ kinetic patterns occurring prior to equilbrium).
  24. Craig Payne

    Craig Payne Moderator

    Probably not, but my point is that there is a lot going on dynamically, so to get a weak correlation with a static test is good!
  25. I'm not trying to take that away from you, Craig. But we both know that a predictive model is more useful than a straight Pearson's correlation, we also know that when it comes to model fitting not all relationships are linear functions. So if you had attempted to model different relationship between the data points, or used scale transforms we may have found even stronger relationships between the variables.

    What kind of devices did you use? n=?
  26. Craig Payne

    Craig Payne Moderator

    It was a Blake inverted type custom device, so it conformed to the area where we put the strap of the supination resistance device. It was done a while ago and I will have to go and look up the n, but it was >20
  27. RobinP

    RobinP Well-Known Member

    It depends....:wacko:

    Simon's point seems correct really, the amount of arch fill is largely irrelevant if you are using different fabrication materials. I presume then(and this has been touched on in other threads) that the top cover material will influence the transfer of ORF to the foot and therefore could feasibly affect the degree of arch fill required? So, even more "it depends"


    If this is a question that requires a massive answer then please feel free to say so but what does this mean? What is r?


  28. RobinP

    RobinP Well-Known Member

    Thanks Simon

    OK - I think I sort of grasp the correlation of static to dynamic as a Pearsons Linear correlation. So why is r squared? I don't want you to do my work for me but I read through some of the links from the Wikipedia ref you gave and couldn't see anything - I could have totally missed it though

  29. Actually I'd give you that. Whilst there will still be a lot of Swaging in terms of predicting interface variation, dynamic variation, internal kinetics and suchlike, I would agree that the less steps between cast and orthotic, the less swags are needed and the better one should be able to predict the effect of the device. Hypothetically.

    Neil, You and I both know the system we're talking about but a lot of the folk following along are probably wondering what the hell we're referring too. I think a breif description and perhaps a picture of the FAS system and why it involves fewer steps might be appropriate on this thread
  30. http://en.wikipedia.org/wiki/R-squared
  31. RobinP

    RobinP Well-Known Member

  32. N.Smith

    N.Smith Active Member

    No problem but before I get burned for "having a different method" I'd like to explain what the "Vertical Foot Alignment System" isn't and can't do!

    *It isn't a device that will pop an orthotic or prefab out.
    *It won't tell you what the perfect position of the foot is.
    *It can't predict outcomes.
    *It can't tell you what or how to prescribe.
    *It will only be as good as the person using it.

    What it can do:

    *Place vertical, translational, and rotational forces on the R/F, Mid/F and F/F individually or all at once, to supinate or pronate these segments as the patient is standing and capture this shape in the negative cast.
    *Deflect or offload pressure to any part of the foot on Weight bearing and capture this in a corrected (or whatever position you want) WB cast.
    *Allow you to look at the feet and lower limbs and what's happening, as you cast.
    *Raise and tilt the rearfoot platforms of the device to compensate for foot deformities, heel height and pitch and leg length discrepencies.
    *Feel supination and pronation resistence as you cast and have patient interaction as you do so
    *Look at the kinematic effect of the anterior aspect of the lower limbs as you adjust the feet and legs.

    Basically... it's up to the SKILL OF THE PRACTITIONER to decide what they want and how much force they want to apply to the L or R, R/F, Mid/F and F/F as the patient stands.

    Sure it's not dynamic but from what I've seen, there dosen't seem to be anything that can alter foot function with forces and capture shape like this device.

    I'm at home at the moment and I don't have the newest photos of the latest prototype (that I'll be taking to the US in Nov.) but I'll show those who are interested next week.

  33. Lab Guy

    Lab Guy Well-Known Member

    To brush up on your math, statistics, physics, et. there is a great website that won the top 2million prize from Google. Khanacademy.org. Its a Free classroom with 1800 short videos that give great explanations of subjects we took but did not completely comprehend (at least for me). The site is also great for kids to get a leg up in school.


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