Effects of Subtalar Joint Axial Location on Midtarsal Joint Dorsiflexion Stiffness

Here's another thought: Let us assume that we can model each of the midtarsal joints as springs of stiffness Ktnj and Kccj respectively. In a pronated foot these two springs are basically in parallel, thus the equivalent stiffness (Kmtj) should be something like Kmtj = Ktnj + Kccj, whereas in supinated foot the springs are pretty much in series, thus the equivalent stiffness should be more like 1/Kmtj = 1/Ktnj + 1/Kccj.

Your thoughts...

 

I don't understand your thought process here, Dr. Spooner.

 

I can see the parallel, but I don't see the series. A wider beam will be stiffer than a narrower beam of the same thickness. I'm not sure that the MTJ is going to behave nicely when we try to apply formulas.

One problem we have is that there are different zones of stiffness. Take a forefoot and plantar flex it as far as you can. Now assess dorsiflexion stiffness. An applied force will dorsiflex the joint quite easily. Keep dorsiflexing until resistance increases. At this point value of stiffness has changed. If only one of the two joints (cc or tn) has tight ligaments then as further dorsiflexion is attempted a point might be reached where the second joint's ligaments become tight and you will get a third value for stiffness. So, the starting position of the joint will be very critical for determining stiffness. The concept of range of motion of the joint is still an important one. I like getting theoretical as much as the next guy, but we still have to look at the anatomy to make sure our models are valid.

Eric

 

Rob, I agree that we need to explain why there is an increase in range of motion of the MTJ with STJ becoming more pronated. You are correct, that observation won't go away. Elftman made a good observation and then added a very poor explanation of that observation.

I don't think the change in range of motion observation will be explained by talar head torsion. The talar head is not a perfect ball. I have taken a cadaver's navicular and compared the joint surface with the navicular 90 degrees to its original orientatation and there is a difference in radius of curvature of the joint at a 90 degree shift. My sense is that limitation of motion by the ligaments will occur before the shape of the articular surface matters.

Eric

 

To Eric, and anyone else interested. My interest is not with the midtarsal joint restraining mechanism per se, it is with human evolution in the most general sense. I happen to have focused upon the foot, perhaps because I was once a podiatrist. Please view the attached slides. First lets look at the talar neck angle. We all know from our ontogenetic lectures as students that it decreases, but did we see it in the context of this? We all know that humans changes from ~30-~18 degrees (Slide 3). An adult ape value is the same as the infantile (newborn) human value; why? Look at slide 3-4 and note that the adult ape is the same as the infantile human - straight to the gene pool!. Classic peramorphic heterochrony.

Now look at the figures for talar head torsion - I know they get bigger, not smaller, but the pattern is the same. Look at figures 8-9 and see the same trend. Now look at figure 10 & 11. Figure a mt restraining mechanism in 10, and the lack of it in 11. This is what would be without the huge talar head torsion we see in homo sapiens. Sure, this is not the whole story, but it will not go away just because currently it is not fashionable.

I first wrote this stuff about 20-25 years ago in podiatric literature - and true to form, was ignored.

It is late on Friday night - if the slides are not attached because I am old and stupid, please someone tell me and I will do exactly that. Rob

 

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Rob, I am not sure that I understand the point you are trying to make with the slides. Apes tend to have MTJ's that break during ambulation. Apes have a huge amount of DNA that similar to humans. People don't usually have there MTJ's break (bend) with ambulation. Humans have differently shaped tali than apes. Are you trying to imply that the shape of the talus is the reason for less MTJ "break"?

Eric

 

Eric, What I am saying is that one component of the formation of the so-called restraining mechanism - that is, a relationship between subtalar position and midtarsal range, is the high degree of talar head torsion in homo sapiens. In terms of MT break, sure, apes do have a break which in entirely normal. Their large ROM at the midtarsal joint is essentially down to the above, and at least, if not more important, their lack of a process calcaneus on the cuboid. In humans, and in homo habilis, this is present and, together with its secure articulation with the calcaneus, greatly reduced the ROM at the caclaneocuboid joint

 

Pictures speak a thousand words, see diagram attached and wiki link on springs in series and parallel. http://en.wikipedia.org/wiki/Series_and_parallel_springs

 

An interesting discussion. Not sure if you have debated this recent paper by Chris Nester’s team:

Kinematic coupling relationships exist between non-adjacent segments of the foot and ankle of healthy subjects

Gait & Posture, Volume 37, Issue 2, February 2013, Pages 159-164
R. Dubbeldam, C. Nester, A.V. Nene, H.J. Hermens, J.H. Buurke

Pathologies of foot and ankle structures affect the kinematics at the site of the impaired structure but also influence kinematics elsewhere in the foot and ankle. An understanding of kinematic coupling relationships in the foot could provide insight into mechanisms that explain differences in foot and ankle kinematics between healthy and pathological subjects. The aim of this study was to explore foot and ankle kinematic coupling relationships between adjacent and non-adjacent segments of healthy subjects and evaluate individual variability of and effect of walking speed on these relationships.

Gait of 14 subjects was recorded at comfortable and two slower walking speeds to assess individual foot kinematics during stance phase. A qualitative evaluation of the coupling relationships was made using angle–angle plots to determine their consistency, i.e. changes in movement direction of each segment occurred at the same time and the plot returned along the same line after the turning point. The Pearson correlation coefficient of determination (R2) was used to provide a quantitative evaluation of coupling. Individual variability was assessed with the coefficient of variation (CV). The Friedman-test was used to test the effect of walking speed.

Consistent coupling relationships were observed between hindfoot in/eversion and hallux plantar/
dorsiflexion (R2 0.7, CV 0.2), between hindfoot in/eversion and forefoot ab/adduction (R2 0.5, CV 0.3) and between leg rotation and midfoot collapse/elevation (R2 0.5, CV 0.4). Less or non-consistent coupling relationships were observed between the other studied segments. Walking speed significantly influenced coupling relationships between hindfoot and midfoot.

I think it was Huson who described how the orientation of the ligaments determine the direction of motion of a joint which, in turn determines the axis of the joint at that instance in time.

This goes some way to explain why the amount and direction of motion of each of the foot joints varies between individuals.

In the above paper they note the variability in the segmental relationships between subjects. Thus, it would strike me that some individuals may have more motion in one plane or other, some in two and some in three planes. At any instance in time, the combined position of the joints will determine the axes position at that time.

Is the reduced or increased motion due to the axis position or vice versa? Is the increased or decreased motion due to the orientation and resistance of the ligaments and other soft tissue structures?

I have observed (subjectively) that patients with an external hip position generally have lower ranges of motion within the foot – is this adaptive?

If rearfoot inversion / eversion couples with forefoot abduction / adduction, then perhaps this is the dominant motion to consider. The leg rotation will occur at the same time as ankle dorsiflexion and thus it may be the combined motion that affects arch height. In the more mobile foot, both height and forefoot motion may be affected and thus greater deformation.

As always, more questions than answers and if it sounds like a ramble it is because I was on the over night from New York and am trying to face out the time to bed.

Trevor