Re examining and Defining terms for STJ axial position

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Dear All and particularly Kevin Kirby

When locating and marking the position of the STJ axis it is common to find that it is medially rotated. However it is also usual, for me at least and I think you can see this in Kevin and Simons paper on determining dynamic STJ axis location (J Am Podiatr Med Assoc 96(3): 212-219, 2006 page 218 fig 7 B) that the posterior exit point is lateral to the calcaneal bisection. Therefore although it is medialy rotated it is also lateraly translated relative to the longitudinal axis of the foot.
This means that although there is less ability for GRF acting on the forefoot to supinate the STJ at the same time there is increased ability for the rearfoot to increase STJ supination at heel strike and in particular if the foot strikes with an internally rotated position.

This query was promted by a query made on another site whcih remained unanswered and also by the fact that I had a rugby player who had an extremely internally rotated and laterally deviated STJ axis and (really noticeable with slo mo video) internally rotated his hip and foot just before heel strike resulting in frequent inversion ankle sprains while playing rugby and always just when stopping and changing direction.

How should we describe this STJ axis position, just medial rotation or medial rotation in dgs and lateral translation in mm perhaps. Or is medial rotation sufficient and it is taken that since the centre of rotation is in the centre of the rearfoot somewhere, that this will automatically mean that the exit point of the STJ axis will always be lateral to the calc bisection when it is medially rotated? What do you think?

Cheers Dave

 

Dave:

Very good observation. Soon after I wrote my initial paper on subtalar joint (STJ) axis from over 20 years ago, I realized that simply saying that the STJ axis was "medially deviated" and "laterally deviated" would not be sufficient to precisely describe the possible permutations of interindividual STJ axial variation in the human foot. Like you have also observed, Dave, there are some individuals where the STJ axis is both medially translated and internally rotated but others that are only internally rotated and not medially translated. There are also individuals with laterally translated STJ axes which pass posteriorly more out of the lateral border of the posterior calcaneus but which are neither more internally rotated or externally rotated than normal. There are numerous possiblilities of STJ spatial location, all of them having their own unique mechanical effect on the human foot.

My first attempt to mathematically quantify STJ axis position came in a paper where I colloborated with the department of Mechanical Engineering in 1991 at my alma mater, the University of California at Davis. This was a bicycle study where Maury Hull, PhD, and Trish Ruby, master's degree candidate, had asked me to provide them with measurement parameters of the foot and lower extremity that may explain variations in knee loading forces in cyclists, with pedal loading forces measured by specially designed pedal that measured three-dimensional forces and center of pressure. Our paper was eventually published in the Journal of Biomechanics in 1992 and was the first biomechanics paper to ever include measurement of STJ axial location, where I used a method that measured both the angle of the STJ axis within the transverse plane and the intersection point of the STJ axis with a longitudinal bisection of the foot. The paper that we published 16 years ago is attached below (Ruby P, Hull ML, Kirby KA, Jenkins DW: The effect of lower-limb anatomy on knee loads during seated cycling. J Biomech, 25 (10): 1195-1207, 1992).

For clinical usefulness, however, I prefer to use anatomical landmarks to discuss the STJ axis location such as, for example, saying the STJ axis exits anteriorly above the first metatarsal-cuneiform joint and exits posteriorly at the lateral border of the posterior-superior calcaneus. Even though angular values will likely be more preferable for research, knowing the anterior and posterior anatomical exit points seem to me much more valuable from a clinical standpoint.

It is critical to understand the very significant effects that Achilles tendon tensile force has on foot function, especially in regards to STJ axis location. A STJ axis that passes posteriorly through the more lateral aspect of the calcaneus and, thus, gives the Achilles tendon a larger supination moment arm will increase the internal STJ supination moment from Achilles tendon tensile force especially during the latter half of stance phase. A more medial posterior exit point for the STJ axis will greatly diminish the internal STJ supination moment from the Achilles tendon during the latter half of stance phase and, in extreme cases of medial deviation, may even cause the Achilles tendon to cause an internal STJ pronation moment.

I suppose that I should have been writing more on this subject for publication in JAPMA or some other peer-reviewed journal over the past 7-8 years. However, considering that my 2001 paper (Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001) almost never got published because one of the reviewers from JAPMA (who will remain anonymous to all of you) thought it was too long and not respectful enough of the contributions of Mert Root, even though the paper was later awarded the "Journal of American Podiatric Medical Association Scholl Award for Most Outstanding Paper Published in 2001", I lost some of my enthusiasm for "going to battle" again with reviewers that simply don't understand, or want to understand, much of what I am write and lecture on. Therefore, I now publish most of my worthwhile ideas and theories in my Precision Intricast Newsletters and books (Kirby KA: Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997; Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002.), and also write many of my new ideas here on Podiatry Arena where I seem to have an audience that has an interest in trying to understand the mechanical nature of the human foot and lower extremity.

Dave, you are one of those individuals that make me want to continue to contribute to venues such as Podiatry Arena.:drinks

 

Kevin

I guess then that this is a good example showing that thorough examination of the individual and consideration of their unique mechanical anatomy will give better outcomes than just remembering paradigms eg mediall rotated STJ axis = reduced ability to supinate, whereas sometimes it can be the opposite.

I guess the best way to record the position on a form would be to use a diagram and draw it on, ie no ambiguity.

I admit I often forget to consider the effect of achilles tendon force on STJ moments during stance phase and in some cases this would be very significant,--- mental note to myself -----

Thanks and you are one of those who inspire me.


Cheers Dave