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Department
of Podiatry
Other
Biomechanical Theories: A
Introduction
Refer back to the second lecture of first semester on all the problems
with the traditional or 'Root' based approach to foot biomechanics
Refer back to the lectures on sagittal plane theory on the role of
theory in informing clinical practice and the role of theory in understanding
foot biomechanics
What are some of the problems that a new approach needs to address?
Classification
proposed by Shearer based on the traditional approach (Valmassey,
1996)
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FOREFOOT
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Inverted
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Perpendicular
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Everted
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Inverted
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1
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2
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3
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REARFOOT
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Perpendicular
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4
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5
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6
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Everted
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7
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8
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9
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What
are some of the problems with this classification?
They compare the foot to an idealised norm
They look at the foot in stance
They rely on measurements
Do not account for rigidity of MTJ or joint axes variations
Do not predict dynamic function
Any others?
To solve
some of these problems an approach has been put forward by Eric Fuller
from the California College of Podiatric Medicine (CCPM), based on
the following:
"Pronation does not necessarily hurt - it's what stops it that hurts"
It is the stress on the structure that stops pronation that is a problem
- can we calculate that stress?
It
is essentially a further development of Kirby's work on the STJ axis
No information is yet available in the literature
Principles
that it is based on:
The establishment of rotational equilibrium about joint axes of motions
moment = force x distance
to determine the moment from ground reaction forces you need to know
the location of force and the location of the axis
Where
is the location of force and the location of the axis?
The axis is located by determining the position of the subtalar joint
axis (Kirby)
The location of force is found by determining where the centre of
pressure (CoP) is located
There are 3 possible combinations of the above - the CoP is lateral
to the axis; the CoP is beneath the axis; the CoP is medial to the
axis
CoP Lateral
to the Axis
Ground reaction forces will want to excessively pronate the foot
The pronatory moment from GRF will need to be countered by a supinatory
moment from internal to the foot
What can provide this supinatory moment? :
1) Plantar
fascia
2) Muscle
(posterior tibial)
3) Bone
(sinus tarsi)
CoP Lateral
to the Axis: The Plantar Fascia Foot
Provides a supinatory moment via the windlass mechanism
Pathology: plantar fasciitis; hallux limitus; hallux valgus (due to
the imbalance of moments about the joint)
Clinical observations: tight plantar fascia during static stance (palpate);
some pronation is available as the STJ is not maximally pronated;
there is a high medial forefoot loading or a low lateral loading
CoP Lateral
to the Axis: The Muscle Foot
The supinatory moment is provided by the posterior tibial muscle
Related pathology: posterior tibial tendonitis and dysfunction (during
stance, the tendon is tight and "pops" out from behind the medial
malleolus
Clinical observations: Forefoot loading is variable - get a high force
on lateral column when the muscle is active; the plantar fascia is
variable as the muscle is being used; STJ is not maximally pronated
CoP Lateral
to the Axis: The Sinus Tarsi Foot
The supination moment is provided by the osseous structures of the
sinus tarsi
Related pathology: sinus tarsi syndrome
Clinical observations: no STJ pronation is available as the STJ is
maximally deviated; the plantar fascia is 'slack'; there will be high
lateral forefoot loading - it will be high on the medial side if the
STJ axis is medially deviated
Other:
Combination of the 3 types
CoP Beneath
the Axis:
This will be a balanced foot in rotational equilibrium
No consistent related pathology - except possible lateral instability
Clinical observations: even forefoot weight bearing (maybe higher
on lateral); plantar fascia 'slack'; STJ pronation is available
CoP Medial
to the Axis:
Rare
Very unstable laterally
Recurrent ankle sprains
Peroneal tendonitis
Clubfoot
Clinical
Observations:
Is the STJ maximally pronated?
Location of forefoot force
Palpate arch/plantar fascia
Palpate posterior tibial tendon
Advantages
Of This Classification System
Does not require a heel bisection or measurements
Ease of predicting pathology - pathology correlates with physical
findings
Easier to teach
Treatment is aimed at reducing the stress on anatomical structure
(the tissue stress model)
Where
do we go next with this approach?
References
Valmassey,
R. (1996) Clinical biomechanics of the lower extremity
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