A brainteaser...

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Think about this:

Lets assume that the Root biophysical criteria for normalcy is correct...take a foot that meets these criteria, except that it has a true osseous forefoot varus and nothing else is out of alignment and all joint ranges of motions are good. The joint axes are all in what are assumed to be normal positions (ie STJ axis is 16 deg from sag plane and 42 deg from transervse plane).

Assuming all this to be the case, and lets say for the forefoot varus, that the forefoot is inverted relative to the perpendicular posterior bisection of the calcaneus 5 degrees (assume I took an accurate measurement )

Question:
How far does the calcaneu evert in the frontal place as the subtalar joint pronates to compensate for this?

 

A 5 degree forefoot farus can be compensated by the STJ everting 5 degrees. The calcaneal postition will be 5 degress everted.

 

Anyone else? Agree or disagree

 

Blake had some thoughts on a ratio of motion..that 1 deg of frontal plane eversion of the calcaneus was not the same as 1 deg of sub-talar pronation, and that the ratio maybe more like 3:1 or 5:1.
If we just talk calc eversion and that the link between the STJ and the first ray is rigid then the answer would be 5 deg I think...but in reality the foot has other articulations that will/may respond to a dynamic situation....in gait I think the intial 5 deg would be compounded by failure of MTJ lock moving into late stance and further compounded by whatever amount of dorsiflexion of the forefoot results from the lax midfoot...resulting in more STJ and MTJ collapse than you may intially expect. Given that the range of motion is actually available...but be Roots normal I think it would be
Regards Phill Carter

 

Lets assume the MTJ is not involved in the answer and it is functionaling normally about those two fictious axes we used to believe existed.

Any other takers?

 

Craig,

Perhaps the calcaneus everts as much as it possibly can since the foot(STJ)
is pronating throughout contact and propusion.
I have osseous forefoot varus as does my son and one of my grandsons.
I did Root type biomechanical exams for years and , I agree, osseous forefoot varus is rare.

Tony

 

This is a difficult question, Craig. First of all, I don't believe that the Root biophysical criteria for normalcy are correct (regardless of whether this theory is still being taught at most of the podiatry schools around the globe). Second, an inverted forefoot deformity of 5 degrees (i.e. forefoot varus deformity) will tend to cause a STJ pronation moment to be generated but the magnitude of STJ pronation moment will then be dependent on STJ axis spatial location. Third, the dorsiflexion stiffness of the metatarsal rays will also change how the foot reacts to dorsiflexion loads acting on them (i.e. I don't think that there is such a beast as a "true osseous forefoot varus".)

If we ignore all those problems, and if we can assume that the STJ does maximally pronate, then the calcaneus will evert to its maximally pronated position, whether that is inverted, perpendicular or everted relative to the ground.

 

The theoretical correct answer is about 2.5 degrees. The reasoning behind this is the STJ has to pronate to allow the medial forefoot to come down to the ground. If the STJ axis was horizontal, then STJ would pronate 5 degrees and the posterior aspect of the calcaneus would evert 5 degrees in the frontal plane. HOWEVER, the STJ axis is not horizontal and is at 42 degree (lets assume its 45 degrees)...simple trigonometry would say the posterior aspect of the calcaneus moves in the frontal plane half what the STJ moves (given all the assumptions mentioned above).

In reality, because of all the issues Kevin mentioned, this probably does not happen. eg it is possible that the STJ goes to end range of motion when calc goes past vertical.

 

This is very difficult to answser within the paradigm. The classic Root assumption is that you start in neutral position. So, place this foot with a five degree forefoot varus on the ground and start in neutral position. There is force on lateral forefoot and no force on the medial forefoot. Assuming an average location (aka normal) of the STJ axis the center of pressure under the foot will be lateral to the axis and therefore cause a pronation moment. The STJ will pronate until it is stopped by something. As the STJ pronates the range of motion of the MTJ increases and the forefoot to rearfoot angle is more everted when compared with the neutral position. So the theoretical answer does not take this into account. Is this amount significant? I don't know.

This points to one other inconsistancy in the Root et al theory. A flexible forefoot valgus is supposed to cause pronation of the STJ because the long axis of the MTJ is supinated. However, this will be true for all forefoot to rearfoot relationships in which the lateral forefoot hits the ground first in gait. After heel contact the ankle plantar flexes and the lateral forefoot hits the ground. This causes a pronation moment at the STJ and the STJ pronates until something stops it. If there is range of motion available the medial forefoot will stop it. When the medial forefoot hits the ground there will be angular momentum around the STJ and there will need to be significant force applied to the medial forefoot for all foot types that have adequate range of motion to get to this point. This force under the medial forefoot would supinate the long axis of the MTJ in all these feet. Putting a hinge like long axis into the model makes the model impossible to work with.

Now if we think about the location of center of pressure and ignore the MTJ axes....

Cheers,

Eric

 

I agree with Craig’s view on how you would discover the amount of frontal plane motion.

However I think the amount of frontal plane motion would be more like 3.5 degrees as when resolving vectors from a triangle both sin and cos are required to correctly find horizontal and vertical components, not simply halving the total amount.

That is
The amount of frontal plane motion is equal to

COS45 x 5 (assuming the angular inclination of the STJ axis is 42 degrees rounded to 45 degrees.)

.707 x 5

= 3.5 degrees or so.

What is your take on this?

Simon Adam

 

Simon, Kevin, Craig, (and anyone else who did this calculation)

If we are in a world where we believe that we can accurately measure the STJ axis position, and a world where we can measure that it is exactly 5degrees FFT VR etc etc and this fictitious world is one where everything is so accurate and you are going to the trouble of getting out the calculater to use COS and SIN then why in the world dont you just use 42 degrees? why be so careful for so long and then fudge it out of laziness??? honestly? Isnt that the problem with podiatry schools? We are taught to be so very careful for so long and then right at the end (when it counts) someone says, "oh what the hell, just fudge it a little?"

Bec.
(Mere Student)

 

Bec:

I think you are missing the point of the discussion. It is helpful to make measurements, at least it is for me, to compare one patient to another. However, if as a student, you are not taught to do these measurements with fairly good accuracy, then these measurements are useless for comparing one patient to another. What you choose to do once you leave the school is up to you. My suggestion is that you should try and be as accurate as possible during your time at podiatry school so that at least if you want to use these techniques in practice to increase your therapeutic outcomes, you have the ability to do so.

By the way, who ever said we can accurately measure STJ axis position without putting pins into the calcaneus and talus and doing a three dimensional motion analysis of the STJ motion?!!

 

Mr Kirby.

i was only refering to the specific example above where craig said he could accurately measure the position of the STJ axis, etc but then chose not to use it for the maths part of the question. i just thought that was ironic.

I do know that the most accurate measures should be taken at all times, of course, even if the most accurate arent actually accurate (as, as you say that would require pain or surgery in most cases).

But thanks for answering me. I just did a massive assignment on some of your work and it is an honour for a student like me to think a 'name on a paper' is actually taking time to answer.

Bec.

 

Bec:

We all started out like you, being students at one time, and I gratefully appreciated those clinicians that would take the time to thoughtfully respond to my earnest questions (of which there were very many). I'm just returning the same favor to you that was extended to me by some of my professors when I was a much younger man.

The subtalar joint axis is impossible to find accurately clinically, but can be estimated using various methods. The important thing is not to know exactly where the STJ axis is in space, but to have an idea of how its abnormal location affects the mechanics of the foot and the internal forces/moments acting within the foot during weightbearing activities. In this way, the clinician can better design their therapeutic modalities (i.e. orthoses, shoes, strengthening, stretching) around this knowledge of the location of the pathological forces/moments that are causing the patient's painful symptoms.

Hope this helps.

 

Doesn't the same simple trigonometry say that the subtalar joint has to pronate twice as many degrees and the forefoot everts?

Regards,

Stanley

 

I say nil.

The lateral column (assuming it is "normal" and supple) may dorsiflex a bit and the windlass mechanism will plantar flex the first ray thus leveling out the supposed "deformity". Who knows what is happening along the longitudial axis of the transverse tarsal joint- but I would image that it would respond before the STJ.

Thanks Craig, my brain now hurts.:bang:

Nick