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Compensation for forefoot deformities in light of the single axis MTJ model

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Aug 24, 2008.

  1. Members do not see these Ads. Sign Up.
    I wanted to pick up on something I intimated about in the biomechanical explanation thread http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=15319

    It has been taught, and may well still be being taught ( a quick search of t'interweb suggests it is ;)), that a flexible forefoot valgus will be "compensated" through supination at the midtarsal joint. Supination being the combined movement of adduction, inversion and plantarflexion. Yet if we view the work of Nester, Findlow and Bowker: http://www.rxlabs.com/upload/pdf/Scientific_MTJ(2001).pdf
    we see from figure 4. that it is physically impossible to achieve supination from forefoot loading through toe-off with the midtarsal joint axial positions obtained by these workers. We also note the movement available at the other joints of the midfoot, those distal to the talonavicular and calcaneocuboid joints (I always had a problem with the way Root seemed to forget about these) and ask the question: how does "compensation" for a flexible forefoot valgus et al. occur in light of modern biomechanics theories? And how does variable joints stiffness influence this?
  2. Craig Payne

    Craig Payne Moderator

    Geeez, you trying to tax the brain on a Monday morning!

    Why not just ignore whatever MTJ model we have and reconceptualise it based on stiffness of all the joints.

    Traditionally and simplistically, a forefoot valgus was considered to compensate either at the midtarsal joint (flexible forefoot valgus) or subtalar joint (rigid forefoot valgus) depending on what the range of motion was available at the putative longitudinal axis of the MTJ was.

    If we reconsider that what we traditionally call a "forefoot valgus" can really compensate by some motion at any of the joints from the MPJ's backwards. ..... as to which joint(s) it compensates at will depend totally on the ROM and STIFFNESS of all the joints in the mid and rearfoot, so there is really a possibility of multiple combinations of multiple joints involved in the compensation for that forefoot valgus. Just what type of compensation will depend on the ROM, STIFFNESS, DIRECTION OF MOTION of each joint that could be involved.

    As the compensation is just a motion, that does not imply its pathological (which traditional teaching says it is). To become symptomatic the forces through the tissues need to be at a pathological level.
  3. Craig,
    You've pretty much arrived at the same conclusions that I have. The reason I included Nester's single axis MTJ model is that these workers pointed out that the spatial orientation of the axis does not determine the motion of the joint, but rather the motion of the joint determines the spatial location of the axis and in their subjects at least, there was no "supination" about a "longitudinal MTJ axis". Your question: "why not just ignore whatever MTJ model we have and reconceptualise it based on stiffness of all the joints" seems to suggest that the spatial location of the axes is unimportant. So how does the stiffness of a joint relate to the axial position?

    Given the within and between subject variability in the step to step forces, the "compensation" mechanism is also likely to show step to step variability such that some steps, in some environments could be beyond the zone of optimal stress and within the pathological zone, while others steps may not. Assuming pain avoidance is a primary concern, and that stiffness of these joints can be controlled by the CNS through muscle activation, pathology should only result when the forces exceed the level that can normally be accommodated by the tissues, or when the tissues ability to withstand the forces is reduced, or both.

    I also think the sequential loading of the forefoot is important here, which leads us back to the Bojsen-moller high/ low gear thread...
    Last edited: Aug 25, 2008
  4. pgcarter

    pgcarter Well-Known Member

    Some of my students at Latrobe may well remember me teaching this kind of concept of multiple couplings of joints and the "tone" of the motion available...for want of better terminology I used tone as a way of describing a variable in stiffness that changes with alteration of other factors also. I agree that there is probably no mid tarsal lock as previously taught, but I do believe that with the right shaped orthoses you can alter the tone and magnitude of motion that seems to occur through the mid tarsal complex. Mostly to do with driving the C-C joint to a position where it just does not want to move any more, in some feet this seems to happen when you use a large cuboid and planterflex the lesser rays a fair bit, examination by hand suggests the quality of motion does change.
    regards Phill

  5. As long as we don't believe what we were taught that there are two simultaneously occuring midtarsal joint (MTJ) axes, we will be fine. For the past two decades, I have been viewing frontal plane motion of the metatarsal heads to the rearfoot not as being longitudinal MTJ axis motion but rather as variable dorsiflexion/plantarflexion of the individual metatarsal rays relative to each other.

    Here is something I wrote about 8 years ago on the subject of forefoot supinatus that discusses my view of the Root idea of "longitudinal axis supination" (Kirby KA: The Biomechanics of Forefoot Supinatus. December 2000. In Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 127-128.)


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