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kkirby52
08-12-2007, 11:58 PM
I have followed this thread for some time and have been asked by Greg
Lewis if I could add any more to this very interesting discussion on
subtalar joint (STJ) axis location. As an introduction, I first became
interested in the clinical effects of deviations in STJ axis location in
1984 while doing my Biomechanics Fellowship at the California College of
Podiatric Medicine. It was at this time that I developed the clinical
technique for palpating the plantar aspect of the foot to approximate the
plantar representation of the STJ axis (Kirby KA: Methods for
determination of positional variations in the subtalar joint axis. JAPMA,
77: 228-234, 1987). The performance of this technique when combined with
clinical examination and gait examination of thousands of patients has led
me to the conclusion that STJ axis spatial location (i.e. 3D location of
STJ axis) has very significant effects on the kinetics of the STJ, and
other joints of the foot and lower extremity, during weightbearing
activities.

In regards to podiatric research on STJ axis location, Craig Payne and
coworkers have developed a device that measures the resistance force to
supination motion that has found a positive correlation between supination
resistance and STJ axis spatial location (Payne C, Munteaunu S, Miller K:
Position of the subtalar joint axis and resistance of the rearfoot to
supination. JAPMA, 93(2):131-135, 2003). Most recently, Simon Spooner, PhD
and I have developed an apparatus, the STJ Axis Locator, that clamps onto
the calcaneus and appears to be able track the talar head and neck fairly
well, without directly contacting the talus, during range of motion of the
STJ both in a non-weightbearing and weightbearing situation (Spooner SK,
Kirby KA: The subtalar joint axis locator: A preliminary report. JAPMA,
96:212-219, 2006). There is some possibility that the ideas around which
the STJ Axis Locator are based could also be used along with a 3D motion
analysis system to track specific translations and rotations of the
calcaneus during weightbearing activities.

If it is true, as VanLangelaan's landmark research (Van Langelaan EJ: A
kinematical analysis of the tarsal joints. Acta Orthop. Scand., 54:Suppl
204, 135-229, 1983) seems to support, that there is a very tight bundle of
STJ axes passing through the dorsal neck region of the talus, then it
should be feasible to better track the 3D movements of the STJ axis just
by tracking the 3D movements of the calcaneus during weightbearing
activities. Eventually, it is my hope that this type of 3D STJ axis
tracking system could be synchronized along with force plate analysis to
provide a more real-time clinical method, within a gait analysis lab, to
determine STJ kinetics during gait.

In regards to the clinical significance of STJ axis location, over the
past 23 years of performing clinical techniques to assess STJ axis
location, there is no question in my mind that the location of the STJ
axis relative to the plantar structures of the foot has a very significant
impact on both the kinetics and kinematics of gait and also on the types
of pathologies that are produced within the human foot during
weightbearing activities. To generalize, a medially translated or
internally rotated position of the STJ axis relative to the plantar foot
will tend to cause a maximally pronated STJ in stance and gait and will
tend to cause pathologies related to increased demand on the anti-
pronation structures of the foot and ankle (e.g. posterior tibial tendon,
sinus tarsi region of talo-calcaneal joint) whereas a laterally translated
or externally rotated position of the STJ axis relative to the plantar
foot will tend to cause a foot that suffers from pathologies related to
increased demand on the anti-supination structures of the foot and ankle
(e.g. peroneal tendinopathy, lateral ankle ligaments due to chronic
inversion ankle sprains). Foot orthosis treatments for these pathologies
may therefore be based on STJ axis location with a high level of
therapeutic success since STJ axis location will tend to affect the
internal forces and moments acting within the anatomical components of the
foot and lower extremity (Kirby KA: The medial heel skive technique:
improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992).

In my discussions with many biomechanics researchers, there seems to be a
tendency within the international biomechanics community to only be
concerned with the angular orientation of the STJ axis location relative
to the cardinal body planes, with very little attention being paid to the
actual location of the STJ axis relative to the anatomical structures of
the foot. I believe that this lack of attention to medial-lateral
location of the STJ axis by biomechanics researchers is a very important
oversight that needs to be corrected within presentations, papers and
discussions on this subject.

The medial-lateral location of the STJ axis is extremely important when
one considers the kinetics of the STJ and that, by far, the largest
magnitudes of forces that contribute to the pronation and supination
moments acting across the STJ axis during weightbearing activities come
from ground reaction forces acting through the plantar structures of the
foot. For example, if the STJ axis is found to be 23 degrees medially
angulated from the longitudinal bisection of the foot, does this mean that
the STJ axis passes 1 cm medial to the first metatarsal head, 2 cm medial
to the first metatarsal head or 3 cm medial to the first metatarsal head?
To my knowledge, the current biomechanical conventions using kinematics to
determine STJ axis location do not account for medial-lateral positioning
of the STJ axis and its possible significant effects on STJ kinetics. It
is very important for the clinician to understand which anatomical
structures the STJ passes above or through in order for them to better
understand the clinical significance of the research.

The current research that is being done by Greg Lewis and Steve Piazza at
Penn State in which we are trying to develop better methods by which to
isolate the STJ axis spatial location in live subjects via non-invasive
methods by restriction of talocrural joint motion, may indeed be a
promising avenue for further research on this important subject (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). This collaborative research between clinician and researcher, I
believe, will lead us to a much better understanding and to better
treatments for the mechanically-based clinical pathologies that affect
millions of individuals in our respective countries. Thanks for your time
in reading this rather long discussion on this fascinating subject.



Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

Private Practice:
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