3D Nature of AIS
Course on the LYON METHOD for Scoliosis  Bangalore India 2017
First presentation: SOSORT Wiesbaden 2014
Second Presentation: SOSORT Katowice 2015
3D nature of AIS: implications in nonsurgical treatment
From the frontal to the sagittal plane
The fight against gravity starts at Devonian ( 450 million years ago), when
vertebrates leave the water (which almost completely nullified the force of
gravity by buoyancy) to venture on the ground floor.
Breathing requires taking off the chest of the floor with mobility in the
sagittal plane. Coupled movements in the frontal plane and in the sagittal plane
will automatically cause a rotation. With bipedia, the sagittal plane has even
become the function plane of the spine.
The torso column
The torso column is the mathematical representation of a circled helicoid with
generating circle in a horizontal plane perpendicular to the vertical axis of
the spine.
Movement of a solid on an axis
Only two movements are possible between a solid and an axis: Translation and
Rotation. So relative to an axis, a solid has two degrees of freedom.
In the frontal plane
Motion is the sum of the rotation on the sagittal axis and the translation on
the transversal axis.
The Charleston overcorrecting night brace uses this biomechanical action.
Some braces are built on this 3 points system.
In the sagittal plane
Motion is the sum of the translation on the sagittal axis and the rotation on
the transversal axis in the frontal plane.
To correct the sagittal plane, it's necessary to use a 5point system because
both physiological lumbar lordosis and thoracic kyphosis must be corrected
simultaneously.
This is easier with segmental molding and overlapping shapes.
On the vertical axis
Motion is the sum of the vertical translation or decoaptation and rotation of
the apical vertebral.
To facilitate rotation, we must push in the horizontal plane on the
posteromedial border of the rib hump convexity and on the sternocostal
cartilage on the opposite concave side. Axial elongation will facilitate
derotation.
Evolution of translation along the vertical axis during last century
Coupled motion of the spine
In scoliosis, we are seeing usually a right thoracic inflexion and an opposite
rotation to the left of the apical vertebra.
Lateral bending of the thoracic spine does not involve pure lateral bending but
accompanies rotation. Coupled axial rotation in the same direction as lateral
bending was observed in the neutral, flexed, and extended postures of the
thoracic spine. Coupled axial rotation in the same direction as lateral bending
were observed in the neutral and flexed postures, while coupled axial rotation
in the opposite direction was observed in an extended posture.
This means achieving at the same time convex bending to the right and thoracic
kyphosis if we want to correct the scoliosis initial rotation to the left.
From 3D plane geometry to the 3D solid geometry
Segmental derotation and Global detorsion
The segmental derotation at the apex of scoliosis is achieved through a mostly
multiple 3point system. The more complex system is the Chêneau brace with
multiple 3 points system.
The overall untwisting or global derotation requires an elongation along the
vertical axis between the scapular and pelvic belts, a push up effect on the
horizontal plane at the thoracolumbar junction and a design of the brace with
torsion opposite to scoliosis.
Due to the vertebral deformation, it is often impossible to reverse scoliosis
and the segmental molding can indicate the maximal possible correction.
4D dimension is the time
The active correction is always maintained during breathing and motion.
Expansions in the concavity must allow deep breath inspiration with concave ribs
contact.
From 3D Deviation to 3D vertebral Deformation
The apical vertebra is not only the most rotated, it’s also the most distant
from the gravity line with a very strong mechanical compression in the
concavity. Ian Stokes describe the vicious circle with asymmetrical growth,
vertebral and disc wedging, increasing the vertebral deviation and so one.
3D disc deformation
Is maximum at the lumbar level. Bracing is more efficient in remodelling disc
than vertebral body, which explain the best correction of lumbar curves with the
brace.
Isostatic balance
The obligation of the contact in 3 zones of 2 adjacent vertebrae, whatever the
motion, requires the combination of several movements of translation and
rotation at the Functional Spinal Unit. In adults, this mechanism explains the
ante, retro listhesis and the rotational dislocations. In children, this
mechanism explains the instability of flat back and hyperkyphosis.
3D muscular chaining
In the frontal plane, lateral chains are crossing at the thoracolumbar level
and for the anterior chain, at knee level. Which explains that children often
have trouble with knees in scoliosis.
Sagittal chains are crossing at the pelvic level (reciprocation). It is always
difficult to determine whether the muscle is a motor or brake. Therefore unlike
polio which requires analytical care, it is often global in idiopathic
scoliosis.
Sagittal flexion and apical rotation
The instantaneous centre of rotation (ICR) of the vertebral body is near the
posterior wall. During a forward sagittal bending of the spine, concave and
convex muscle vectors are oriented towards the concavity. Below 25° of rotation,
the ICR is between the two vectors and ensures stability. Above 25° of rotation,
the ICR is outside of the two vectors and increases the rotation.
This is why the sport activity with forward bending of the trunk is sometimes
contraindicated.
The delivery tricycle speed effect
Usually the weight is in front of the tricycle. For the same major curve, if the
turn is approached at low speed, the tricycle will remain more stable that if it
is approached at high speed.
