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kkirby52
05-21-2008, 11:32 PM
Al:

Thanks for responding to my posting on anterior tibial (AT) tendon
location. The following are my answers to your queries.

The superior and inferior retinacula, by placing a passive posteriorly
directed force on the anterior tibial AT tendon, will change the course
of the AT tendon at the ankle. Therefore, if one is using an MRI scan
to determine AT tendon function (which was the original question of this
thread), but are doing that MRI scan while the subject is relaxed
without the foot being volitionally dorsiflexed by the subject, then the
AT tendon will be more posteriorly displaced by the retinacula than it
would if the AT muscle was undergoing contractile activity. This change
in position of the AT tendon from the relaxed position to the position
it would be in during contractile activity of the AT muscle is what I
was referring to when I stated that the superior and inferior retinacula
would "complicate" determining the line of action of anterior tibial
muscle tension.

Both the ankle joint and subtalar joint (STJ) have joint axes that are
pronation-supination axes. In other words, both of these joint axes are
directed from posterior-inferior-lateral to anterior-superior-medial
which will produce triplane motions of dorsiflexion-eversion-abduction
and plantarflexion-inversion-adduction with both STJ and ankle joint
motions. The terminology used within the podiatric community for the
motions that occur in joint axes that are significantly deviated from
all three cardinal body planes is "triplane motion".

The cross-sectional area of the AT muscle was only mentioned to
emphasize the fact that AT muscle contraction can create a strong ankle
joint dorsiflexion moment. In fact, I believe the AT muscle is the
strongest ankle joint dorsiflexor. Cross-sectional area of muscle
obviously has nothing to do with direction of motion, but is one of the
variables, along with the length of muscle moment arm to the joint axis,
which determines what we clinically call "muscle strength".

I don't believe you are "nit-picking" regarding your comments on STJ
axis location and AT tendon location. It is very important to point out
the fact that the AT muscle may be either a pronator or supinator of the
STJ depending on the relative location of the AT tendon to the STJ
axis. Since most biomechanists do not make any attempt to locate the
STJ axis during their lower extremity kinetics studies, being content to
use the longitudinal bisection of the foot to determine whether external
forces, such as ground reaction force, are causing an "external inverion
moment" or "external eversion moment", they are therefore missing out on
this important kinetic variable, that the STJ axis spatial location is
widely variable from one foot to another, and that it changes spatial
location relative to the plantar foot during closed kinetic chain STJ
rotational motions. The research I am involved in currently, and have
been involved with over the past two decades, has focused on determining
the spatial location of the STJ axis and discussing its effects on the
kinetics of the foot and lower extremity during weightbearing
activities. A bibliography is provided below for anyone who is
interested. Thanks again for your posting.

Kirby KA: Methods for determination of positional variations in the
subtalar joint axis. JAPMA, 77: 228-234, 1987.

Kirby KA: Rotational equilibrium across the subtalar joint axis.
JAPMA, 79: 1-14, 1989.

Ruby P, Hull ML, Kirby KA, Jenkins DW: The effect of lower-limb anatomy
on knee loads during seated cycling. J Biomech, 25 (10): 1195-1207, 1992.

Kirby KA, Green DR: Evaluation and Nonoperative Management of Pes
Valgus, pp. 295-327, in DeValentine, S.(ed), /Foot and Ankle Disorders
in Children/. Churchill-Livingstone, New York, 1992.

Kirby KA: The medial heel skive technique: improving pronation control
in foot orthoses. JAPMA, 82: 177-188, 1992.

Kirby KA: Biomechanics of the normal and abnormal foot. JAPMA,
90:30-34, 2000.

Kirby KA: Subtalar joint axis location and rotational equilibrium theory
of foot function. JAPMA, 91:465-488, 2001.

Roukis TS, Kirby KA: A simple intraoperative technique to accurately
align the rearfoot complex. JAPMA, 95:505-507, 2005.

Spooner SK, Kirby KA: The subtalar joint axis locator: A preliminary
report. JAPMA, 96:212-219, 2006.

Lewis GS, Sommer HJ, Piazza SJ: In vitro assessment of a motion-based
optimization method for locating the talocrural and subtalar joint
axes. J Biomech Eng. 128:596-603, 2006.

Lewis GS, Kirby KA, Piazza SJ: Determination of subtalar joint axis
location by restriction of talocrural joint motion. Gait and Posture.
25:63-69, 2007.

Sincerely,

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
************************************************** **************************


as572@columbia.edu wrote: 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