biomechanics of scoliosis

1. Biomechanics and
patho-mechanics of scoliosis
Rashmita dash
MPO
NILD, kolkata
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2. Introduction
• Scoliosis , ancient greek term means, “ a bending
“ or “crooked” .
• Consistent lateral deviations of a series of
vertebrae from the LOG in one or more regions of
the spine may indicate the presence of a lateral
spinal curvature in the frontal plane called a
Scoliosis.
OR
• Scoliosis is defined as an appreciable lateral
deviation in the normally straight vertical line of
the spine.
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3. 3

4. • Sagittal plane – lordosis & kyphosis
• Coronal or frontal plane – lateral curvature
• Axial or transverse plane – rotational deformity of vertebral
column
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5. Patho-mechanics of scoliosis:-
• Anatomical changes during scoliosis-
Wedging of vertebrae caused by the Asymmetric
pressure on the immature vertebrae causes
the vertebral section on the concave side of the
curve to decrease growth whereas the other
convex vertebral section where less pressure is
applied has normal or accelerated growth.
Fadzan et al., Etiological Theories of
Adolescent Idiopathic Scoliosis: Past and
Present, 2017
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6. Hueter-Volkmann’s law
• According to Hueter-Volkmann Law, bone
growth in the period of skeletal immaturity
is retarded by mechanical compression on
the growth plate and accelerated by growth
plate tension.
• Hueter-Volkmann law is generally used to
explain mechanism of scoliosis. Because of
the physiologic curvature in the normal
spine, compressive force is delivered on the
ventrally located part of the vertebral
column, whereas distractive force is
delivered on the dorsally located part.
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7. Shortening of the following soft tissues on the concave side:-
• the intervertebral joint capsule, which may lead to facet joint
compression and ultimately osteoarthritis.
• the intervertebral muscles, the erector spinae, the quadratus
lumborum, the psoas major and minor and the oblique
abdominals shortening.
• The anterior and posterior longitudinal ligaments, the
ligamenta flava and the interspinous ligaments also shorten to
this side, and limit flexion towards the convex side.
Fadzan et al., Etiological Theories of Adolescent
Idiopathic Scoliosis: Past and Present, 2017
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8. • As the vertebrae rotate, the ribs, which
are attached to the vertebrae by the
musculoskeletal system, follow the
rotational torque applied by the spine.
They are pushed downwards as well as
forwards on the concave side.
• This causes a crowding of ribs posteriorly
on the concave side as well as a small
hump on the anterior chest wall of the
same side.
• Conversely, the ribs on the convex side
become widely separated and are pushed
backwards, creating a rib hump on the
posterior chest wall.
• Associated with the posterior movement
of the ribs is a narrowing of the rib cage
on the convex side. The ribs on the convex
side then push against the scapula and
make it more prominent .
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9. • In patients with structural scoliosis, the anterior elements of
the spine are indeed longer than the posterior elements.
• This condition is commonly called
‘relative anterior spinal overgrowth’ (RASO)
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10. Vicious cycle theroy :-
Stoke’s Vicious Cycle of
Pathogenesis: A lateral spinal
curvature produces asymmetrical
loading of the skeletally immature
spine, which in turn, causes
asymmetrical growth and a
progressive wedging deformity.
•Spinal loading asymmetry was
dependent on neuromuscular
activation strategy.
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11. 11

12. Classification of scoliosis:-
• Structural scoliosis – scoliosis with a component
of permanent deformity, vertebrae with sideways
tilt and rotated along their long axis.
1. Idiopathic scoliosis
2. Congenital scoliosis
3. Neuromuscular scoliosis
4. others
ref: Essential of orthopeadics, J Maheshwari
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13. Classification of IS
• CHRONOLOGICAL
CLASSIFICATION- JAMES
proposed that scoliosis should be
classified based on the age of the
child at which the deformity was
diagnosed.
• ANGULAR CLASSIFICATION- acc.
to the cobb’s angle.
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14. TROPOLOGICAL CLASSIFICATION-
based on the anatomical site of the
spinal deformity in
the frontal plane.
ref: SOSORT 2016 guidelines
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15. Idiopathic scoliosis
• Idiopathic scoliosis - It is the commonest type of
structural scoliosis. It may begin during infancy,
childhood or adolescence.
• Infantile scoliosis begins in the first year of life, and is
different from the other in that, it can be a progressive
type.
• Scoliosis beginning later in life progresses at a
variable rate, and leads to an ugly deformity.
The deformity is most obvious in thoracic
scoliosis because of the formation of a rib hump.
• Idiopathic curves progress until the cessation of skeletal
growth.
Ref: Essential of orthopeadics, J Maheshwari15

16. Congenital scoliosis
• This type is always associated with some form of radiologically
demonstrable anamoly of vertebral bodies.
(i) hemi-vertebrae (only one-half of the vertebra grows)
(ii) block vertebrae (two vertebral bodies fused)
(iii) an un-segmented bar (a bar of bone joining two adjacent
vertebrae on one side, thereby reventing growth on that
side).
• These curves grow, often at a very fast rate. Sometimes, there
are associated anomalies in the growth of the neural
structures, leading to a neurological deficit in the lower limb.
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18. Neuromuscular scoliosis
• Neuromuscular scoliosis is an irregular spinal curvature due
to abnormalities of the myo-neural (muscle-nerve) pathways
of the body. It is generally most severe in non-ambulatory
patients.
• Curve progression is much more frequent than idiopathic
scoliosis, and may continue into adulthood.
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19. • Neuromuscular
(1) Myopathic
arthrogryposis
muscular dystrophy
(2) neuropathic
upper motor neuron
lower motor neuron
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20. Others classification
• Marfan’s syndrome
• Myelomeningocele
• Syringomyelia
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21. • Non structural scoliosis - This is a mobile or
transient scoliosis.
1. Postural scoliosis: It is the commonest overall
type, often seen in adolescent girls. The
curve is mild and convex, usually to the left.
The main diagnostic feature is that the curve
straightens completely when the patient bends
forwards.
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22. 2. Compensatory scoliosis: In this type, the scoliosis
is a compensatory phenomenon, occurring in
order to compensate for the tilt of the pelvis (e.g., in a
hip disease or for a short leg). The scoliosis disappears
when the patient is examined in a sitting position (in case
the leg is short) or when the causative factor is removed.
3.Sciatic scoliosis: This is as a result of unilateral
painful spasm of the para-spinal muscles, as may
occur in a case of prolapsed inter-vertebral disc.
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23. • Normal spinal alignment
- Spinous processes all line up in a
straight line over the sacrum.
• Scoliosis is a combination of
- Angular displacement
- Lateral displacement
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25. Descriptive terms
• The side towards which the convexity of the curve is directed
is designated as right or left.
• The involved location of the curve is as follows
1. Cervical
2. Cervico thoracic
3. Thoracic
4. Thoracolumbar
5. lumbar
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26. • Simple curve- single spinal deviation
• Compound curve- displacement in right and left direction
• Primary curve- curve that develops first
• Secondary or compensatory curve- develops as a balancing response
to the primary curve.
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27. • Non structural curve – curve is flexible and corrects by bending
towards convex side
• Structural curve – curve is not corrected on bending on convex side.
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28. Lenke classification
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30. Biomechanical considerations
involved in prognosis
• Measurement of rib vertebral angles at
the apex of the curve would be useful in
determining the prognosis of idiopathic
scoliosis.
• For infantile idiopathic scoliosis, the rib
vertebral angle difference (RVAD ) was
>20° in 80% of those curves which
progressed and <20° in the remaining
curves.
• The rib-vertebrae angles on the
convexity and concavity of the spinal
curve.
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32. Clinical biomechanics
• Cobb’s angle and its significance
• Furgeuson angle
• Facet apposition, interface distance,
percentage canal occulusion, vertebral body
height loss and maximum fragment
retropulsion.
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33. BIOMECHANICS OF SCOLIOSIS
Creep and Relaxation: Creep is the deformation that follows the
initial loading of a material and that occurs as a function of time
without further increase in load.
•When a force is applied to correct a spinal deformity, and the
force continues to work after the initial correction, the
subsequent correction that occurs over a period of time as a
result of the same load is due to creep.
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34. • Creep in scoliosis. F is a constant force
applied with axial traction. The original
length of the scoliotic segment L
corrects and increases to L+ D as a
function of time. D is the deformation or
change in the length of the curved
segment of spine.
• When a load is applied to a visco-elastic
material and the deformation remains
constant, the observed subsequent
decrease in load with time is relaxation.
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35. Comparison of Axial, Transverse, and Combined
Loads for Scoliosis Correction
• The comparative efficiency of different types
and combinations of loads applied to a
scoliotic spine for correction. The scoliotic
spine is modelled by three components:
• Two rigid links AC and BC, connected by way
of a torsional spring C .
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36. A) The scoliotic spine under axial load
B) A simplified model of the spine being subjected to
axial distraction force F.
C) Free body diagram of the model link BC and the
joint C.
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37. • An axial force is applied at the two ends of the
spine segment, represented by points A and B in
the model, to elongate and straighten the spine.
• The mechanism of angular correction by
elongation is not due to tensile stresses in the
spine but rather to the bending moments
(stresses) created at the various disc spaces. It is
these bending moments that correct the angular
deformity.
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38. A) The scoliotic spine under transverse loads.
B) A simplified model of spine being subjected to
three-point transverse forces.
C) Free body diagram of the model link in BC and the
joint C.
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39. • The lateral force is applied at C, and reactive
forces half its size are taken up at points A and
B. The angular correction is again obtained by
creating corrective bending moments at the
disc spaces.
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40. • The corrective bending moment at the apex of
the curve is the axial force F multiplied by its
perpendicular distance D to the apex of the
curve. It is easily seen that the greater the
deformity, the greater is the distance D .
• In other words, the correctional ability of the
force increases with the severity of the
deformity.
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41. • The components lie and move in the frontal
plane. The links are oriented to simulate spine
deformity in theta(°) degrees as measured by
Cobb's method. The static behavior of this
model is studied under three separate loading
conditions- axial force, transverse force, and a
combination of axial and transverse forces.
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42. • The corrective bending moment at the apex of
the curve equals half of the force at the apex
(the other half works on the other half of the
spine) multiplied by D , the perpendicular
distance to the apex of the curve. In contrast
to the axial force, the corrective bending
moment for the lateral force decreases as the
deformity of the spine increases.
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43. A) The scoliotic spine under combined axial and
lateral loads.
B) A simplified spine model being subjected to
combined loading.
C) Free body diagram of the model link BC and the
joint C.
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44. • COMBINED LOAD-Using equal loads at the
three loading points, the two end forces will
have to be tilted 30 degree toward the centre
force for the equilibrium of the spine
• Comparison of the efficiency of the three
loading types can be made on the basis of the
corrective bending moment produced at the
disc space. The greater the bending moment,
the greater is the angular correction obtained.
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45. In conclusion of above loadings :
• Axial components provide most of the
corrective bending moment when deformity is
severe
• Transverse component takes over the
corrective function when deformity is mild
• The combined load is most benificial for all
situations
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46. • A graphic representation of "relative corrective moment ML"
as a function of spine deformity in degrees (Cobb's method)
for the three loading types. According to the theoretical
model studied here, we note the following: The combined
load is the most efficient for any degree of deformity; the
axial load efficiency increases with the angular deformity; and
the transverse load efficiency decreases with angular
deformity. The deformity angle of 53° is a break-even point for
the axial and transverse loads. Examples of two theoretical
patients with mild (30°) and severe (70°) curves are shown.
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48.  Considering two patients whose curve measure 30 and 70
degrees as per cobbs method.
For Ө = 30 degrees , the values of M/FL for:
 Axial load = 0.26
 Transverse load = 0.48
 Combined load = 0.71
For Ө = 70 degrees , the values of M/FL for:
 Axial load = 0.57
 Transverse load = 0.41
 Combined load = 0.91
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49. • Based on this theoretical consideration, it can
be concluded that patients with severe
deformity should be treated with axial loading
in the beginning and as the deformity
decreases the loading should be changed to
transverse type assuming that axial and
transverse loadings can not be combined and
applied simultaneously and vice versa.
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50. Thank you
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