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Why is the (STJ) axis transverse plane projection(?) a line? why not a zig-zag as each additional joint is 'actively engaged' or a set of curves as additional joints are included and the tension from the restraining structures (proximally and distally) changes slightly as the assessor's thumb moves distally?

 

The easy answer is that the definition of an axis of rotation is a line. However, what I think you are alluding to is that when you try to move the STJ by pushing on bones other than talus and calcaneus there are intervening joints that complicate the situation. How is force transmitted through those bones and ligaments so that you create a moment at the STJ, or not, depending on your leverage at the STJ.

An interesting way to illustrate this problem is applying a dorsiflexion moment at the ankle joint at the first met head. At the same time you will be applying a dorsiflexion moment on the first ray. Interestingly, the ankle starts to move before the first met is fully dorsiflexed. So the force gets transmitted to the talus even though the ligaments are not fully tight. So for the STJ, the other joints are in the way, but the point of application of force, relative to the axis, is not changed by having other joints in the way.

 

Eric, thank you for the reply.
Apologies as I should have included at the beginning of my post that i realise that an joint's axis is linear. What i should have made clear was I am referring to is the series of dots placed on the plantar surface when assessing where a vertical GRF would not cause a net tri-planar moment at the STJ ( and frontal plane moment at the ankle).
Scenario: The thumb pressed 'vertically' beneath the cuboid or navicular and it results in a net, say, supination moment at the MTJ, which would impart a supination moment at the STJ; but if that MTJ was fixed and the same force applied at the same position, results in a net pronation moment at the STJ (or eversion moment at the ankle) isn't it relevant as to where design features may be placed to impart STJ moments.
Thanks again, mark

 

Mark:

When I first started doing the STJ axis palpation technique in 1984 during my Biomechanics Fellowship, I fully expected the STJ axis to deviate from a line that followed the STJ axis under the calcaneus to another line that followed the "oblique midtarsal joint axis" when I crossed the midtarsal joint (MTJ) with my palpation. I assumed this because we had been taught by the Root-based biomechanics faculty at CCPM that the "oblique midtarsal joint axis" was angulated 52 degrees from the sagittal plane and initially thought that, past the MTJ, I would be mapping the MTJ axis. However, in all the patients and podiatry students I palpated (probably close to 50 within a few weeks), the STJ axis "points of no rotation" always formed a perfectly straight line.

This led me to greatly change my opinion about how the foot moves under load. With the metatarsal head region of the forefoot loaded from plantar, the MTJ and midfoot joints are stabilized against each other so that, in effect, the foot became a relatively rigid body, rotating about the talus. In other words, the reason the "points of no rotation" on the plantar foot was always in a straight line was that by loading the plantar metatarsal heads forces distal to the MTJ, the loading force was acting directly on the STJ as a STJ moment. This plantar loading force acted through the bones, joint surfaces and plantar ligaments of the MTJ and midfoot joints to affect STJ kinetics.

This was quite a revelation for me at the time as a 27 year-old podiatry resident in my final year of post-graduate training, and made me realize that STJ axis location was critical to determining the kinetic effects of ground reaction force (GRF) acting not only on the rearfoot, but also to determining the kinetic effects of GRF acting on the structures of the plantar forefoot. In other words, pushing lateral to the STJ axis at the metatarsal head level produced a direct STJ pronation moment and pushing medial to the STJ axis at the metatarsal head level produced a direct STJ supination moment.

This is why you can assume, in a weightbearing foot, that any GRF lateral to the STJ axis will cause an external STJ pronation moment and any GRF acting medial to the STJ axis will cause an external STJ supination moment. Even though a GRF may be acting distal to the MTJ and, therefore, not pressing on the calcaneus that make up one of the two bones of the STJ, the GRF on the plantar forefoot, distal to the MTJ, still results in a direct STJ moment.

Hope this helps.

 

Mark:

Here is a video that I did about a month ago on this topic of finding the STJ axis and it's development.