STJ axis location and rotational equilibrium Flashcards
What is a MOMENT/torque/moment of force
the turning effect produced by a force acting at some distance from an axis
vector - has a direction
Sum of moments
not in equilibrium induce a change in rotational velocityy
Rotational equilibrium
if the sum of moments = 0Nm = object will continue in its state of rotational motion (ROTATIONAL EQUILIBRIUM)
STATIC ROTATIONAL EQUILIBRIUM
if object is not rotating and moments are in rotational equilibrium =
what allows joints to rotate
Moments generated by the forces acting around an axis is a force:
muscle forces
ligaments
interosseous compression forces
GRFs
What is rotational equilibirum dependent on
STJ axis location
Origins - for STJ to reach equilibrium:
Sum of the STJ pronation moments must exactly equal sum of stj supination moments
pathological foot types in relation to position of stj axis:
medially deviated stj axis
laterally deviated stj axis
interally generated forces - stj roational equilbrium
ligamentous and tendon tensile forces
joint compression forces
externally generated forces - stj roational equilbrium
GRF
orthosis and footwear reaction forces
what happens the longer the distance between the forces and the stj axis
= the greater the moment
Effect of forces - grf and orthotics
those that act medial to stj will exert stj supination moment
those that act lateral to stj will exert stj pronation moment
effect of forces - muscle and lig tensile forces
- force medial = stj supination
lateral = stj pronation
effect of forces - interosseous compression forces
lateral = stj supination moment
medial = stj pronation moment
Moment arm of forces - in laterally deviated stj, short moment arm 5th mtpj
low pronation moment
moment arm of forces - in medially deviated stj, large moment arm 5th mtpj
high pronation moment
moment arm of forces - in laterally deviated stj, lays medial to the axis 1st mtpj
supination moment
moment arm of forces - in medially deviated stj, lays lateral to the axis 1st mtpj
pronation moment
Medially deviated stj axis
talar head and neck are positioned medially in relation to plantar surface
During relaxed stance:
1. medial border of foot will demonstrate a convex shape
2. ST contour of the talar head and neck at anterior ankle will be more medially positioned and internally rotated in relation to calc
from post: increase in convexity in the medial midfoot, just anterior and inferior to the med mall
Laterally deviated stj axis
less common than medially deviated
talar head and neck postioned more laterally in relation to the plantar foot
increased magnitudes of stj supination moments and decreased stj
Laterally deviated stj axis have increased frequency of:
lateral ankle sprains
peroneal tendinopathy
Medially deviated stj and orthotic prescription
- induce modifications that will apply supination moments/ move grf medially
- medial heel skive, blake inverted device
laterally deviated stj + orthotic prescription
modifications that will apply a pronation moment/grf laterally
- lateral heel skive
- everted balancing techniques
- valgus wedge
RF structure and STJ on normal foot
- positioned lateral to the plantar WB area of the calc (medial tubercle)
- grf on medial tubercle = supination moment
- magnitude of the supination moment dependent on length of the lever arm and magnitude
RF structure and stj - RF varus
medial calc tubercle more medially positioned relative to stj axis
increase stj supination
clinically:
- higher degrees of RFVR have lateral instability - common to see have stj pronation during gait
RF and stj - RFVL
medial tubercl is more laterally in relation to stj axis
increase in pronation moment
CLinically:
usually maximally pronated at stj stance (WB) - no sufficient supination moment to counterbrace the prontion moments
RF position and motion and STJ axis
position of stj alters relative position of medial calc tubercle and stj axis
