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View Full Version : Re: Locating Tibialis Anterior tendon's action line using MRI



asolimene53
05-19-2008, 12:54 PM
Dr. Kirby,

A few thoughts .....

"Further complicating the line of action of the anterior tibial muscle
> is the fact that it is bound down at the ankle joint by the superior
> and inferior extensor retinacula ..."

I don't know why the retinacula would, in any manner, "complicate"
the line of action.

"The anterior tibial muscle has sufficient cross-sectional area and
> dorsiflexion moment arm to the ankle joint axis [which has a
> pronation-supination axis just like the subtalar joint]..."

I don't understand your usage of the term pronation-supination axis
for the talo-crural joint / upper ankle joint. Unless of course your
point of reference is to the anatomical planes of the body. Are you
using the terms Pure and Compound motion as I previously mentioned?
To the best of my knowledge there is one (anatomically determined -
screw like) axis at the talo-crural joint. I don't understand why the
cross sectional area of a muscle would have any effect on the
direction of movement in this case.

"At the STJ, the actions of the anterior tibial muscle are more
> complicated due to the inherent variability of the STJ axis relative
> from one foot to another."

I realize that it may seem that I am "nit" picking - but - why would
the variability of location of the STJ axis complicate the line of
action of the TA muscle? The line of action / force of the TA muscle
is what it is - the dynamics of motion is what may change (the
direction of (?)) movement - e.g. the reversal of muscle action.


Nevertheless a fine presentation Kevin !!!!!

I think there should be some attempt to define actions and movement in
a more uniform manner.


Cheers

Al


Quoting Kevin Kirby :

> Colleagues:
>
> I have been reading the discussion anterior tibial muscle line of
> action with great interest and thought I might be able to offer some
> information based on my clinical experience and research on subtalar
> joint axis location.
>
> Determining the line of action of the anterior tibial muscle/tendon
> unit is not a simple task since it originates lateral to the tibia and
> inserts as a tendon on the medial-plantar aspects of the first
> cuneiform and first metatarsal. Therefore the anterior tibial tendon
> passes from proximal lateral to distal-medial obliquely across four
> joints of the foot and lower extremity, not two joints as previously
> suggested. The four joints that the anterior tibial tendon cross are
> the ankle joint, subtalar joint (STJ), talo-navicular joint and
> navicular-first cuneiform joint.
>
> Further complicating the line of action of the anterior tibial muscle
> is the fact that it is bound down at the ankle joint by the superior
> and inferior extensor retinacula which have sufficient strength to
> alter the three dimensional course of the anterior tibial tendon
> especially when the anterior tibial muscle is relaxed or under low
> levels of contractile activity. These retaining structures of the
> anterior ankle exert a passive posteriorly directed force on the
> anterior tibial tendon which will keep the tendon closer to the ankle
> joint axis (i.e. more posteriorly located), especially when the muscle
> is relaxed.
>
> The anterior tibial muscle has sufficient cross-sectional area and
> dorsiflexion moment arm to the ankle joint axis [which has a
> pronation-supination axis just like the subtalar joint] to have the
> capacity to generate the greatest magnitude of ankle joint dorsiflexion
> moment than any other muscle of the foot and lower extremity. The
> anterior tibial is therefore a very important muscle at decelerating
> the rapid ankle joint plantarflexion that occurs during the contact
> phase of walking or heel-striking running due to the high magnitudes of
> external ankle joint plantarflexion moment that occur due to ground
> reaction force acting on the posterior aspect of the plantar calcaneus
> at heel-strike. The anterior tibial muscle also is important at
> dorsiflexing the ankle during the swing phase of walking to minimize
> the hip and knee flexion angles required to allow the toes to clear the
> ground during swing phase and to help prevent tripping and falling.
>
> At the STJ, the actions of the anterior tibial muscle are more
> complicated due to the inherent variability of the STJ axis relative
> from one foot to another. Most anatomy textbooks claim that the
> anterior tibial muscle is an invertor or supinator of the foot, while,
> in fact, there are times that the anterior tibial muscle may exert a
> STJ pronation moment or be a STJ pronator. This less-recognized action
> of the anterior tibial muscle will occur if the STJ axis is more
> medially deviated than normal due to a pes planus/valgus deformity and
> the STJ is close or in its maximally pronated rotational position. The
> medial translation and internal rotation of the talar head relative to
> the anterior tibial tendon insertion points on the first cuneiform and
> first metatarsal cause a concomitant medial translation and internal
> rotation of the STJ axis. This abnormal medial position of the STJ
> axis causes the STJ axis to lie medial to the anterior tibial tendon,
> making the anterior tibial muscle a STJ pronator. In the hundreds of
> patients I have examined and treated with posterior tibial tendon
> dysfunction (i.e. adult acquired flatfoot deformity), where an
> abduction deformity of the forefoot to the rearfoot has occurred over
> time, I have noticed that the anterior tibial muscle is a pronator of
> the STJ when the foot is maximally pronated at the STJ. This clinicial
> observation, along with evaluation the spatial location of the STJ
> axis, indicates that the anterior tibial muscle can be classified
> either as a supinator or pronator of the STJ, depending on the relative
> position of the tendon to the STJ axis spatial location.
>
> Finally, even though the ankle joint and STJ are separate anatomical
> joints, and therefore independent anatomically due to their separate
> and distinct synovial compartments, to say the ankle joint and STJ are
> independent joints functionally, in my opinion, would be a significant
> error. Since none of the extrinsic muscles of the foot have
> significant insertions on the talus, then all of the extrinsic muscles
> of the foot cross both the ankle and STJ axes and, therefore, when any
> of these muscles exert tensile forces on their pedal insertion points,
> they will affect the kinetics of both the ankle and subtalar joints
> simultaneously. Therefore, the kinetics of the ankle and STJ are
> dependent on each other functionally, sharing the talus as a common
> articular element, and should not therefore, I believe, be considered
> to be independent functional joints, but rather should be considered
> to be functionally dependent joints where the function of one directly
> affects the function of the other.
>
> Cheers,
>
> Kevin
>
> ************************************************** **************************
> Kevin A. Kirby, DPM
> Adjunct Associate Professor
> Department of Applied Biomechanics
> California School of Podiatric Medicine at Samuel Merritt College
>
> Private Practice:
> 107 Scripps Drive, Suite 200
> Sacramento, CA 95825 USA
>
> Voice: (916) 925-8111 Fax: (916) 925-8136
> ************************************************** **************************
>
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