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Biomechanics/orthotic question

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Berms, Jul 28, 2008.

  1. Berms

    Berms Active Member

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    I have a question for those who are a little more biomechanically minded than myself... What should my orthotic be doing for the adult patient who pronates excessively causing pathology and pain (often plantar fasciitis) but has significant rearfoot varus. So, in other words they are pronating from an inverted rearfoot position to a more "vertical" calcaneal position as seen in both resting stance and the midstance of gait.

    In the past I have prescribed orthotics to address the excessive pronation using the "old STJ neutral model" and I ahve ended up with devices that can cause lateral instability.... What should my orthotic prescription be in these cases?

    Thanks for any advice.
  2. Craig Payne

    Craig Payne Moderator

    Re: biomechanics question

    Hello from Heathrow (hate these bloody stop overs)

    Think about it intuitively. What is the painful structure? What are the function of the structure? Design an orthotic to reduce the moments that the structure is involved in (ie reduce load in the injured tissue).

    In this case its the plantar fascia; what does the plantar fascia do? .... ie the windlass --- to make the wndlass work easier to reduce stress in the tissue, you need to plantarflex the first ray (first ray cut out; lateral forefoot posting; invert the rerfoot; etc) ... its that simple (see the thread on jacks test - sorry no have time to find link as flight off soon)
  3. pgcarter

    pgcarter Well-Known Member

    Re: biomechanics question

    Have you established that you actually need to take their money for orthoses or will bringing the floor up to the heel varus do all they need? A Heel wedge will stop the eversion to the floor and effectively help plantarflex the first ray. You may just need a varus heel wedge and a lateral forefoot wedge (valgus) to solve their symptoms. Worth finding that out first ?
  4. Berms

    Berms Active Member

    Re: biomechanics question

    Thanks Craig, that makes sense. If I combine rearfoot varus wedging/posting with lateral forefoot wedging, it will reduce the likelyhood of the device creating lateral instability....
  5. Berms

    Berms Active Member

    Re: biomechanics question

    Hi Phill, thanks for the response. Yes, simple wedging/posting may be all this patient needs, and I will certainly trial this.
  6. Re: biomechanics question

    Hey Berms.

    If you have a patient who Pronates excessivly but upon whom you do not wish to use much medial rearfoot wedging you may wish to consider the use of an orthotic with a softer (ie High density EVA) material, partially shank dependant , with a deep heel cup.

    The lateral forefoot wedge is also a very good idea.

    Kind regards
  7. Re: biomechanics question

    This gets into the tissue stress theory that I will be discussing in a few weeks in Sydney and Melbourne. Tissue stress theory states that the orthosis treatment should be directed toward reducing the stress within the injured tissue, not specifically at "putting the STJ in neutral" as was hypothesized in the subtalar joint (STJ) neutral theory promoted by Root et al over 30 years ago.

    In the patient that has significant rearfoot varus, the pronation moment on the STJ is not caused by "the inverted calcaneus", it is rather caused by the relatively large magnitudes of ground reaction force acting on the lateral forefoot when the STJ is in the neutral postiion. This is one of the many issues with STJ neutral theory, that rearfoot varus causes pronation because "the heel must come to vertical". This is hogwash. Pronation in the rearfoot varus foot is not caused by an inverted heel, it is caused by the inverted forefoot while the STJ is in neutral.

    When a foot orthosis is prescribed and is found to cause "lateral instability", the clinician must understand that this means that the orthosis has increased the magnitude of STJ supination moments sufficiently in order for the patient to feel as if they are close to suffering an inversion ankle sprain. The suggestions of adding a lateral forefoot wedge to the orthosis is a method of orthosis adjustment that I have used for over 20 years to treat lateral instability. In addition, other methods of reducing the magnitude of STJ supination moment is to grind the medial forefoot and rearfoot of the orthosis to evert it and/or lower the medial longitudinal arch of the orthosis and/or raise the lateral longitudinal arch of the orthosis. Any of these methods will reduce the feeling of "lateral instability" from the orthosis and each method has its benefits and weaknesses.

    Hope this helps.
  8. Re: biomechanics question


    Interesting one.

    I wonder what specifically causes such a feeling. I see several possibilities.

    Is the sensation caused by movement of the COM closer to the STA, a kinematic shift detected in the change in muscle lengths?

    Is it caused by relative increase / decrease in tensile stress in those same muscles (kinetic change rather than kinematic?).

    Is it caused by a delay in the adaptation of tibialis posterior to the altered demands placed upon it in a new anatomical position (less GRF to balance + same tib post pull = lateral instability?)

    Is it caused by sensory change in residual moments in the end range structures (deltoid ligaments / Sinus tarsi compression / ST bony end range)?

    Is it (dare i say) caused by changes to the exteroceptive sensation on the planter structures of the foot?

    Obviously all of these will be created by an orthotic which generates sufficient supination moments. However the point in space at which pronation is controlled / arrested and the manner of such arrest is variable depending on the nature of the prescription and orthotic and understanding what exactly causes the SENSATION of ankle stability might be useful. I emphasise sensation because i have had patients who feel laterally unstable if taken off of end range pronation by even a few degrees.

    Would value your thoughts.


    PS (nice to have you back)
  9. efuller

    efuller MVP

    Re: biomechanics question

    Hi Berms,

    Check the location of the STJ axis in the transverse plane. A foot with a laterally positioned STJ axis will tend to sit with an inverted heel in stance and hence appear to be a rearfoot varus foot. However, the classic rearfoot (partly compensated or uncopmensated [I hate the old terminology]) will not evert further becuase there is no more STJ range of motion available. If you see the foot evert farther, at the STJ, something is preventing the STJ from pronating to its end of range of motion. That could be because of a laterally positioned STJ axis where there is no net moment from the ground in the position that it is in. These kinds of feet, during gait will get a supination moment from tension in the Achilles tendon and this would cause supination (sprained ankle) if the peroneals did not increase their activity to create a net pronation moment at the joint. This is why I hypothesize that you will see late stance phase pronation in feet with a laterally deviated STJ axis. The ground is not causing the pronation, the muscles are. Consequently a rearfoot varus wedge, in this foot type, will make things worse. I like the idea of the lateral forefoot valgus wedge. So, check the axis location.

    When Kevin and I were lecturing in Oxford we were talking on prescription we would write for members of the audience. One of the volunteers had pronation related problems even though she had a laterally positioned STJ axis and a very easy to supinate foot. There is more than one source of pronation moment.


    Eric Fuller
  10. Craig Payne

    Craig Payne Moderator

    Re: biomechanics question

    Here it is: Hubscher manoeuvre
  11. Re: biomechanics question


    Sorry for not replying sooner. My wife and I are touring in a campervan around the Northen Territory of Australia and I finally have an hour of internet access to check my e-mails. Tomorrow is a boat ride up the Katherine Gorge. Next week are the lectures in Sydney and Melbourne. It is a tough life....isn't it!;)

    You have some very good questions and I can only speculate as to the answers. My current feeling on this subject is that the "feeling of lateral instability" is actually the center of pressure (CoP) becoming more medially located on the plantar foot so that there is either less STJ pronation moment or more STJ supination moment occurring during gait.

    During late midstance, the CoP will invariably be located lateral to the STJ axis so that there is an external STJ pronation moment occurring. However, also during late midstance, there will normally also be a internal STJ supination moment occuring from the tensile force within the Achilles tendon acting medial to the STJ axis.

    I believe that the individual can sense, with their peripheral and central nervous systems, very small changes in STJ rotational position and detect changes in tensile forces in the ligaments and tendons surrounding the STJ so that if there is, for example, an orthosis added under the foot that increases the external STJ supination moments too much, that the individual may start having a sensation of supination instability. When this supination instability occurs, their central nervous system will subconsciously resist the tendency of the STJ to supinate excessively during gait by either adding extra internal STJ pronation moment (e.g. increased contractile activity of the peroneals) or lessening the internal STJ supination moment (e.g. decreased contractile activity of the gastrocnemius-soleus complex). This makes sense especially considering the shortened stride length that often occurs with increased varus correction being added to feet with foot orthoses.

    Hope this makes sense and I'll probably be able to comment more in few days once we fly into Sydney.

    By the way, just found out last week that I will be able to publish my third book of Precision Intricast Newsletters by the end of this year. Lots of good information in this third book!!

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