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kkirby52
05-17-2008, 01:05 AM
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

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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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