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6mm ff varus wedge prescrption from Dr.

Discussion in 'Biomechanics, Sports and Foot orthoses' started by B. Englund, Aug 19, 2008.

  1. B. Englund

    B. Englund Active Member


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    Today I was visiting a orthotic technician for a couple of hours just to have a look on how they work. One patient, female 70+ yo, complained of forefoot pain. The patient had recently had HAV corrective surgery on one foot and was waiting for surgery on the other. She showed moderate STJ pronation and on NWB examination she showed signs of forefoot supinatus. The orthopedic doctor had prescribed an orthotic with a met dome and a 6mm ff varus wedge. wouldn’t this stop the 1:st ray from plantarflexing and create a FHL? according to the technician The doctor wanted to create a more even pressure distribution but when I think about it wouldn’t it be better to plantarflex the first met when casting and promote 1:st ray plantarflexing. Or what was the doctors intention?

    Björn Englund student
    Soon to begin 3:rd year
    Karolinska Institute
    Sweden
     
  2. Hey Bjorn. Welcome to the arena. :drinks

    It really all depends on the morphology of the foot and the ROMs available.

    If the 1st met has been shortened by the surgery that would give the planter apeuronosis / intrinsics a bit more range making fnHL less likely, and if the forefoot is fixed in supinatus then it makes little difference if you create an orthotic with a planterflexed first ray positon. If the foot can't make it into that position it ain't going to work! Also significant is the degree of Structural hallux limitus which exists post surgery.

    On the other hand if the first ray still has movement available and the planter structures are tight then a forefoot varus extension may indeed do more harm than good.

    A jacks test may be a useful assessment to carry out here.

    In my experiance orthotists tend to work on more of a "fill in the gaps" basis than us. Perhaps it is more to do with the demographic they tend to work with.


    Regards
    Robert
     
  3. B. Englund

    B. Englund Active Member

    HI
    thanks for the welcome, first Swedish member I think :)

    the foot was very flexible.

    would it be better to cast post surgery on the other foot.
     
  4. If it was flexible, ie not supinatus, then forefoot varus extension may indeed not be the way to go.

    And i would certainly consider examining the foot post op to see what (if any) type of orthotics are needed. HAV surgery generally shortens the first met which can reduce the buttressing effect of the 1st met to limit pronation.

    If you look at Kevins Thought experiments (#2 for eg) and consider what would happen if the 1st met is removed from the equation and the second met becomes the medial - most WB structure on the forefoot you will get an idea what i mean.

    Regards
    Robert
     
  5. efuller

    efuller MVP


    Hi Bjorn,

    There are times when it is appropriate to add a 6mm varus forefoot wedge, but not that many. I think creating a more even pressure distribution is an admirable goal, but a generic prescription without knowing the foot is not a way to achieve this. I will add forefoot varus wedging when in my examination with the patient standing in relaxed stance, I can easily slide my fingers under the first metatarsal head. This assumes the patient is not actively supinating their foot. When you see this finding you can be sure that the STJ is pronated and the patient is likely to have very high load on the lateral column and little load on the medial column.

    I realize that you were probably taught that plantar flexing the first ray when casting will plantar flex the first ray when standing on an orthotic. This is an assumption you should question. A piece of plastic cannot reach up an pull down a metatarsal. A classic functional orthotic can only push upward. It can push upward less in some areas, but it cannot pull downward.

    Plantar flexing the first ray when casting will increase the medial arch height of the cast and may increase the forefoot valgus measurement of the cast. A lot of the time those are good things. You can go too far with each of those corrections.

    If you have your patient stand and place your fingers under the lateral forefoot and your fingers get crushed and you can slide them under the 1st met head then you should not increase the forefoot valgus intrinsic post in the cast/ orthotic. It is important that you know how cast corrections put an intrinsic post on an orthotic.

    Have fun questioning your instructors,

    Eric
     
  6. B. Englund

    B. Englund Active Member

    hello again
    thanks for your answers

    I do agree on that the orthotic not can grap on to the 1:st ray and pull it down, but if you increase the material under that area wouldnt this stop the 1:st ray from plantarflexing in a dynamic motion? you may create something, like better pressure distibution, but create problem elsewhere, like in the MTPJ?

    regards
    Björn
     
  7. Björn:

    Welcome to Podiatry Arena. I believe your question is a very good one as it raises valid mechanical concerns as to whether varus forefoot wedging is beneficial or harmful to patients.

    Will a varus forefoot wedge "stop the first ray from plantarflexing and cause a functional hallux limitus (FnHL)?" Not in all cases, but probably in some cases.

    FnHL is most often caused by two factors:

    1. Medially deviated subtalar joint (STJ) axis.
    2. Lower medial longitudinal arch height.

    The increase in STJ pronation moments that result from a medially deviated STJ axis will resist the tendency for any additional STJ supination moments acting on the foot to cause STJ supination motion. Therefore, when propulsion is initiated and ground reaction force (GRF) is attempting to dorsiflex the hallux, plantarflex the first ray, and supinate the STJ, if there is a signficantly medially deviated STJ axis, then the STJ pronation moments will be so large that the foot will resist STJ supination, first ray plantarflexion and hallux dorsiflexion.

    Lower medial longitudinal arch (MLA) height will increase the resting tension within the central component of the plantar aponeurosis for a given load on the forefoot. A higher MLA will decrease the resting tension within the plantar aponeurosis. Increased tensile force within the plantar aponeurosis will increase the hallux plantarflexion moment which will cause decreased tendency for hallux dorsiflexion to occur during propulsion and an increased tendency for FnHL to occur.

    Now, if the patient has a MLA that is stiff, and resists medial column dorsiflexion well, and the foot has a relatively normal to increased in arch height, the varus forefoot wedge is more likely to cause an increase in STJ supination moments, STJ supination instability, shorter stride length and possibly peroneal muscle fatigue/pain. However, if the patient has a MLA that is relatively compliant, easily allows medial column dorsiflexion to occur, and the foot has a relatively flat MLA height, the varus forefoot wedge is more likely to cause dorsiflexion of the medial column than STJ supination motion and will likely cause an increase in tendency toward FnHL to occur.

    The mechanical analysis becomes more complicated as one considers adding a foot orthosis along with this varus forefoot wedge since the foot orthosis may be adding extra STJ moments or midtarsal/first ray moments into the equation. But, if one considers a varus forefoot wedge by itself, the analysis is fairly uncomplicated and can be best appreciated by considering the dorsiflexion stiffness of the first ray and medial forefoot and how feet with different dorsiflexion stiffnesses of the first ray/medial forefoot will alter how the rest of the foot will respond to an increase in dorsiflexion loading forces on the medial metatarsal rays.
     
  8. Björn:

    Check out my Thought Experiment #1 to learn more about how medial longitudinal arch height may affect resting tension within the plantar aponeurosis.
     
  9. efuller

    efuller MVP

    Hi Bjorn,

    Again you have to understand how, or if, your modification in the negative cast is going to affect the finished product. Most labs will add a medial expansion plaster that will lower the arch height of the finished device so that plantar flexing the metatarsal when takingthe cast would not be seen in the finished product other than the increased forefoot valgus, or decreased forefoot varus, measurement seen in the cast.

    I can tell you from experience, in at least my foot, some feet will not tolerate minimal arch fill in the cast. For other feet it would be critical for the orthosis to be effective. I would bet that one predictor of this would be the magnitude of difference in neutral position arch versus relaxed weight bearing arch height.

    Cheers,

    Eric
     
  10. B. Englund

    B. Englund Active Member

    thank you all for your answers and your time.

    I now understand there are many things to conisder before prescribing this solution, things no one seemed to analyse at the clinic I attended. sadly I wont be around to see how it works out for the patient.

    thanks Dr. Kirby for your, as always, descriptive answer.

    regards

    Björn Englund
    student
    Karolinska Institute
     
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