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A little advice please - forefoot valgus confusion

Discussion in 'Biomechanics, Sports and Foot orthoses' started by AdamB, Apr 3, 2014.

  1. AdamB

    AdamB Active Member


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    Apologies for the probably very novice question...

    How do we manage the forefoot valgus component in a foot that also has a moderately high supination resistance and a slightly medially deviated STJ axis and is in a valgus position in resting stance?

    In order to reduce stress on the injured structures (and reduce the degree the foot is in a valgus position) my thoughts are to use a slightly inverted pour and a 4mm deep 10* medial heel skive...

    The problem is that when I lay the casts on the table, the forefoot valgus that I have captured causes the bisection line on the casts to be inverted approx. 5*.... So if I prescribe an inverted pour of 5* then I am effectively not correcting the forefoot valgus?
    Conversely, if I pour at vertical to address the FF valgus, I am then not addressing/correcting the valgus nature of the rearfoot??

    I hope this makes some sense to someone.

    Thanks for any advice.
     
  2. Adam:

    These are all good questions and are actually many of the same questions I asked my Biomechanics Professors (and asked myself) at CCPM during my years as a podiatry student and Biomechanics Fellow.

    If the patient that you are treating has symptoms which are related to excessive pronation moments (e.g. posterior tibial tendinitis), then I would tend to balance the cast slightly inverted (2-3 degrees) and use a 2-3 mm medial heel skive. Balancing the cast inverted will reduce the amount of intrinsic forefoot valgus correction that is present in the resulting positive cast and foot orthosis.

    However, for example, if your patient suffered from a supination-related pathology such as peroneal tendinopathy, then I would like balance the heel of the cast vertical which would give the positive cast the full amount of intrinsic forefoot valgus correction that was present within the negative cast. Also a slight lateral heel skive (2 mm) may help shift the orthosis reaction force (ORF) more laterally if a supination-related pathology was being treated.

    When I see a patient like you describe aboe, that has a high degree of forefoot valgus but is maximally pronated and everted at the calcaneus during stance (John Weed and Mert Root would have called this foot a "flexible forefoot valgus"), then another option is to reduce the forefoot valgus in the cast by dorsiflexing the medial column during negative casting to the approximate degree that you plan on inverting the heel of the positive cast. This will still produce an inverted heel in the orthosis but without the intrinsic forefoot valgus correction.

    For example, let's say that the above patient has a 7 degree forefoot valgus but you plan on inverting the heel of the positive cast 3 degrees when the orthoses are being manufactured. Then I would push up on the medial column during casting to reduce the forefoot valgus in the resultant negative cast to only being 3 degrees of forefoot valgus. Now, when the orthosis is made around this 3 degree inverted positive cast, there will be no intrinsic forefoot valgus or varus correction in the orthosis and the heel will be slightly inverted from the 3 degree inverted balancing position. Further varus wedging in the heel can be added with a medial heel skive correction of your choice (Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992).

    In this way, you can manipulate each area of the orthosis to suit your patients biomechanical needs and optimize their symptom resolution and gait function.

    Hope this helps.:drinks
     
  3. AdamB

    AdamB Active Member

    Thanks so much for your detailed explanation Kevin, it all makes sense now!


    In this way, you can manipulate each area of the orthosis to suit your patients biomechanical needs and optimize their symptom resolution and gait function.


    This is great advice. Thanks for taking the time to reply. :drinks

    Adam
     
  4. fabio.alberzoni

    fabio.alberzoni Active Member

    @kevin:
    where do you push to dorsiflex the first ray? on the navicular plantar aspect?

    Another question: how does MPJ hypothetically work in a foot like your last example?with the orthosis?
    During mid-stance the first ray dorsiflex so much to let pronate STJ?..so...

    During the gait analysis of 2 feet which evert at rearfoot of the same amount where could I find differences between a valgus FF and a varus one ?

    thank for any thought.
    fabio
     
  5. I push on the plantar aspect of the first metatarsal head during casting while simultaneous pushing on the plantar aspect of the hallux to, effectively, dorsiflex the whole medial column and "pre-load" the plantar fascia. This negative casting modification will elongate and flatten the medial longitudinal arch and decrease the forefoot valgus deformity in the negative cast by a few degrees.

    Fabio, you must remember that just because you are modifying the negative cast shape, this does not necessarily mean the foot kinematics will follow the changes in foot shape caused by this negative casting modification.

    What do I mean by this? If for example, I dorsiflex the medial column by 5 degrees during negative casting, this does not mean that the foot, when standing or walking or running on the resultant orthosis made over the 5 degree dorsiflexed medial column position, will also have a medial column or first ray that is 5 degrees more dorsiflexed than what it would have when no orthosis was inside their shoes.

    In other words, negative casting modifications (and positive cast modifications or orthosis modifications, for that matter) don't necessarily cause changes in foot position or foot motion (i.e. foot kinematics) but should cause changes in the magnitudes, temporal patterns and plantar locations of ground reaction forces acting on the plantar foot and, therefore, will cause changes in foot and lower extremity kinetics.

    Hope this helps.:drinks
     
  6. efuller

    efuller MVP

    There are many different ways to get to the same orthotic. As Kevin pointed out, what you do will depend on the structure that you are trying to reduce stress on.

    There is another important measure and that is the maximum eversion height test. Google Podiatry arena:maximum eversion height to see the thread. If you make an orthotic with an intrinsic forefoot valgus post of say 5 degrees that will tend to create a wedge with a height of around 4mm. If the eversion range of motion is less than that you can overload the lateral column or create sinus tarsi pain.

    The other important measure is STJ axis position. The high resistance to supination implies a medially deviated STJ axis. (Check for peroneal activity when you do the supination resistance test). So, if your patient had eversion range of motion that is equal to the height of the intrinsic post that you create, then you could balance to vertical, correcting the valgus, and then add a medial heel skive. The real advantage of using the medial heel skive is that you are not confined by the shape of the cast that you take. The feet that have a lot of eversion range of motion, and have a medially deviated STJ axis, will tend to have high load on the medial forefoot. The valgus wedge will tend to increase the load on the lateral forefoot, which will decrease the load on the medial forefoot. So, even though there is a large valgus component in the orthotic it will still be decreasing stress on the medial forefoot. The medial skive will help.

    In your post you mentioned a 10 with the medial heel skive. I believe there are diminishing returns to a skive above 6mm. You are carving away too much plaster and you can actually be lowering the arch height if you make your skive that deep. Especially in smaller feet. I think Kevin said once that the maximum that he uses is 6 even though the original paper goes to 8mm. I also like to use a modification that I call the plantar lateral expansion. The medial heel skive removes plaster from the medial side of the heel. You can achieve a similar wedge effect in the shape of the orthotic heel cup by adding plaster to the lateral plantar side of the heel. You can also do this without loosing arch height.

    Eric
     
  7. fabio.alberzoni

    fabio.alberzoni Active Member

    hi eric!

    1. If we are talking about a medially deviated STJA foot type a valgus wedge on the forefoot would have a very long arm on STJA. I'm always scared doing such modifications that a valgus wedge could increase pronation of STJ during the Heel Off..
    Anyway we got the medial skive helping, from the HC to the MS, to mantain a less everted position...but later? the same matter for me happened when I use inverted morton extension..by now I'm solving this increasing the skive....


    2. Plantar Laterl Expansion: which is your way to do this?
    In which cases you prefer to have a higher arch?

    thanks, fabio
     
  8. fabio.alberzoni

    fabio.alberzoni Active Member

    @ kevin.

    "In other words, negative casting modifications (and positive cast modifications or orthosis modifications, for that matter) don't necessarily cause changes in foot position or foot motion(i.e. foot kinematics) but should cause changes in the magnitudes, temporal patterns and plantar locations of ground reaction forces acting on the plantar foot and, therefore, will cause changes in foot and lower extremity kinetics"

    I understand that the goal is to "cause changes in the magnitudes, temporal patterns and plantar locations of ground reaction forces acting on the plantar foot and, therefore, will cause changes in foot and lower extremity kinetics" and not to change the position of the foot and kinematics.
    But to do this I think that minimal positional differences must appear cause to elastic deformation of ligaments and bones.

    Anyway, my problem is that to try to "think like an engineer" I had to figurate models in my mind...figurate an ideal functionality of how better could work the foot of the patient (even to understand where the stress is located).
    Even if I read a lot about "tissues stress theory" I can imagine to let the foot in the same position lowering the stress on the structures...

    should I change my way to approach to the patient?how?

    thanks fabio
     
  9. efuller

    efuller MVP

    True about the long lever arm of a forefoot valgus post. However, if the problem is high load on the first then I will still add a forefoot valgus extension. My foot has a very medially deviated STJ axis and I prefer a device with a small forefoot valgus post. (When I was doing the biomechanics fellowship I experimented quite a bit on my own feet. My maximum eversion height is about 2mm. If I go any higher than that I get some pretty nasty sinus tarsi pain.)

    A varus forefoot extension will shift the center of pressure more medially. However, it will also increase force under the first met head. Often high loads on the first is the problem and the problem is not STJ pronation. Yes, supination of the STJ will tend to decrease load on the first, but not if the supination moment comes from increasing load on the first met head.



    Eric
     
  10. Fabio:

    Indeed, sometimes the changes in apparent foot position are very small with foot orthoses. However, even though the change in foot position is small, the change in plantar foot forces are quite large and this large change in plantar foot forces and plantar locations of those forces may cause significant changes in internal joint, ligament, cartilage, tendon and muscle forces. Remember, foot orthoses show significant changes in kinetics, but show small to nearly non-measurable changes in kinematics.

    You must create a mental model of the forces present within the foot in order to understand how foot orthoses may change those forces from a pathological level to a physiological level. Eric Fuller and I spend a lot of months writing a chapter on this very subject, which was finally published last year, which goes through this Tissue Stress Approach in great detail (Fuller EA, Kirby KA: Subtalar joint equilibrium and tissue stress approach to biomechanical therapy of the foot and lower extremity. In Albert SF, Curran SA (eds): Biomechanics of the Lower Extremity: Theory and Practice, Volume 1. Bipedmed, LLC, Denver, 2013, pp. 205-264).

    Here is one of the pages from our chapter showing how modelling may be used to better understand the biomechanical effects of medial and lateral heel skives in foot orthoses. Hope this helps.:drinks
     
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