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Aircast walker and orthotic?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by JohnW35, Dec 28, 2011.

  1. JohnW35

    JohnW35 Member

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    Hello all,

    If using an aircast below knee walker will the posterier tibilialis tendon be completely protected from movement or will the arch still deflect?

    I am trying to allow enough time for a torn tibilias tendon to heal.


  2. efuller

    efuller MVP

    It's still possible to contract the muscle when it's in a cast/walker. The muscle will cause tension in the tendon. It's really hard to turn that part of the brain off (the part that tells the muscle to contract) when weight bearing. So, no it will not be "completely" protected in a weight bearing BK walker. It would probably be more protected non weight bearing, but that's not easy either.

    During healing passive range of motion is helpful to prevent adhesions. After healing gradual strengthening is important. However, starting those too soon could cause re-injury.

  3. David Smith

    David Smith Well-Known Member

    Like Eric said there will still be a load on the foot since the patient will be weight bearing however the load will be distributed differently and will reduce the total stress within the PT tendon, which is what you are looking for.

    How will the boot achieve that?

    First consider the nature of the stress on the PTT, GRF causes STJ pronation moments and longitudinal medial arch lowering/flattening caused by dorsiflexion moments about the midfoot joints. The PTT resists these moments by applying opposing forces.

    What does the boot do?

    1) The air compression balloons will clamp the frontal plane motion of the STJ and therefore the pronation moments from GRF are resisted by the heel pushing against the balloon that pushes against the stiff shell of the boot that pushes against the leg shank.

    2) The rocking action of the stiff sole reduces the dorsiflexion moment arm available to GRF by changing the position of the CoP/CoF, which will tend to move relatively posteriorly toward the midfoot.

    3) The CoP acting on the forefoot to dorsiflex the midtarsal joint (which already has a shorter lever) will act on the stiff shell of the boot foot and again this force is resisted by the stiff shell of the boot acting against the tibia of the shank, which only induced bending moments in the shell of the boot instead of usual scenario when the resisting force is being transmitted to the Achilles tendon and so inducing dorsiflexion moments about the midfoot.

    So, as you see, whereas there will still be some deformation or tendency to deform, of the structures usually supported by the PTT, most of the stress is transferred to the stiff shell of the boot, as is desirable.

    Regards Dave Smith
  4. I would like to add to what Dave and Eric said. As opposed to Eric, I believe these boot-brace walkers (BBW) do significantly affect how the central nervous system (CNS) sends efferent stimuli to the muscles of the foot and lower extremity. To my knowledge, there is no good research proof of this, but this idea certainly seems to me to make the most mechanical sense as to how these BBWs actually work to relieve pain in many foot and lower extremity conditions.

    I believe that one of the major effects of the taller BBWs (not the short ones that end at the mid-tibia) is that they will significantly reduce the efferent signals from the CNS to the gastrocnemius and soleus muscles during the latter 2/3rds of the stance phase of gait, and thus also significantly reduce the Achilles tendon tensile force during this phase of gait. This occurs since the CNS "senses" that the brace will help decelerate the forward rotation of the tibia once the center of mass (CoM) of the body is anterior to the ankle joint axis so the CNS doesn't need then to activate the gastrocnemius and soleus muscles to accomplish the task of helping slow down the speed of forward movement of the CoM.

    As a result, not only is the magnitude of Achilles tendon tension reduced by these braces but also, as Dave mentioned, the magnitude of forefoot dorsiflexion moments (i.e. longitudinal arch flattening moments) will also be reduced due to the reduction in Achilles tendon tension caused by these BBWs. I believe that this is just another example of the marvelous metabolic energy conservation mechanisms that are allowed by a properly functioning CNS where it "senses" that if the muscle is not needed for an activity, it will optimize the contractile strength of that muscle to reduce metabolic demand and "save energy" for the bipedal organisms that we call..........humans.
  5. efuller

    efuller MVP

    This is consistent with what I said. The question asked was does the walker completely shield the PT tendon. So, I agree with the above that the PT can get better in the walker even though it is not completely shielded.


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