Biomechanics of the Spine: Frontal and Rotatory Planes. Scoliosis

Biomechanics of the spine in the frontal and rotational planes. Scoliosis

Dr. PT Mauro Lastrico

Excerpt from:
"Musculoskeletal Biomechanics and the Mézières Method"
Author: Dr. Mauro Lastrico
Marrapese Publisher

In the lateral deviations of the spine, similarly to what occurs in the sagittal plane, the G and R forces applied to each individual center of gravity can cause focused and asymmetric compressions on the intervertebral discs and moments of force.
The overall G and R forces, applied at the lumbosacral junction, can produce significant compressions on the disc as well as an overall moment of force.
The compressive forces on the intervertebral discs are also determined and accentuated by the vertical vector components of all muscles, both anterior and posterior, acting directly or indirectly on the spine.
These compressions can eventually affect the roots of the plexuses, leading to radiculopathies.

Latissimus Dorsi Patterns
Having numerous insertion points, the shortening of this muscle produces various skeletal effects.
Schematically, two main patterns can be distinguished: "A" and "B".
These patterns are not always "pure" and mixed patterns are often observed.

* Pattern "A"
In this pattern, primarily the bundles of the latissimus dorsi that run from the iliac crest to the humerus are involved. By bringing the hemipelvis and the shoulder closer together, they cause a direct lowering of the shoulder and elevation of the hemipelvis, and as a mechanical result, the lateral concavity of the thoracic spine on the same side. These bundles are also responsible, due to mechanical results, for the contralateral convexity of the thoracic spine and the elevated contralateral shoulder.

If the mechanical result predominates, the spine will present a long-radius scoliosis with contralateral convexity, which is actually a consequence of the ipsilateral concavity caused by the approximation of the shoulder and the hemipelvis. 

The latissimus dorsi has insertions on the spinous processes from T12 to T7 and, through the thoracolumbar fascia, on the costiform processes of the lumbar vertebrae. The quadratus lumborum, in addition to the twelfth rib, has insertions on the costiform processes of the first three lumbar vertebrae. Their line of force, therefore, is not only able to raise the hemipelvis but also to draw the vertebrae from L5 to T7.
The latissimus dorsi and the quadratus lumborum can oppose the mechanical result caused by the elevation of the hemipelvis, keeping the spine vertical or even creating an ipsilateral convexity.
In this case, a double curvature occurs, where the latissimus dorsi and the quadratus lumborum are directly responsible for the lumbar and lower thoracic vertebral convexity by direct traction on the vertebrae, and the upper thoracic concavity due to the approximation of the shoulder and the hemipelvis.

* Pattern "B"
This pattern is characterized by the combined action of the upper bundles of the latissimus dorsi and the muscles primarily responsible for elevating the shoulder girdle. Together, these muscles have a resultant force that causes adduction and elevation of the scapula and clavicle. If the omohyoid is also involved, the hyoid bone will be laterally deviated.
The thoraco-humeral bundles of the latissimus dorsi produce a lateral thoracic convexity in the lower quadrant, while the lower bundles raise the hemipelvis and rotate it backward. 

Moreover, the lateral thoracic convexity is also a mechanical consequence of the elevation of the shoulder girdle, which is directly caused by the rhomboids and the middle portions of the trapezius. These muscles, by elevating and adducting the scapula, can produce an ipsilateral convexity of the thoracic vertebrae. This convexity can extend into the upper lateral quadrant of the chest and, to further accentuate the distortion, the serratus anterior may also be engaged. The serratus, by trying to oppose the adduction and upward displacement of the scapula, has its insertion point fixed, and its line of force acts upon the ribs, causing them to shift laterally. 

The combined pattern may potentially result in the following skeletal characteristics, although not necessarily all present, due to direct muscular action:
* Elevated and adducted scapula
* Ascending clavicle
* Lateral displacement of the hyoid bone
* Lateral thoracic convexity in the upper quadrant (above T7)
* Lateral thoracic convexity in the lower quadrant (below T7)
* Elevated hemipelvis
* Posterior rotation of the hemipelvis

In the lumbar area, if the mechanical result caused by the elevation of the hemipelvis predominates over the vertebrae, it can lead to a double scoliotic curve, with lumbar concavity and thoracic convexity. The lower bundles of the latissimus dorsi elevate the hemipelvis, while the upper bundles, combined with the rhomboids and the middle and lower portions of the trapezius, determine the vertebral convexity of the thoracic spine. 

In the case where the latissimus dorsi bundles that connect the pelvis and the thoracolumbar spine, together with the quadratus lumborum, are shortened, they counteract the mechanical result caused by the elevation of the hemipelvis. This allows the thoracolumbar spine to maintain a vertical or ipsilateral convex alignment. 

This results in a long-radius curvature where the thoracolumbar convexity is caused directly by the traction forces of the latissimus dorsi and the quadratus lumborum. Meanwhile, the upper thoracic convexity is induced by the adductors of the scapula and the mechanical result of the elevated shoulder. 

Cranium
The position of the skull in space is ensured by postural reflexes through the cocontraction of all cranio-cervico-scapular muscles. Since a horizontal gaze is a dominant function, significant alterations due to cranial rotation or inclination are rare. It is more common for the underlying skeletal structures to adapt in order to allow a good positioning of the head. In this context, the hyoid bone plays a fundamental role due to its numerous connections.
If cranial rotation and inclination are observed as dominant elements, one should suspect the interference of disturbances arising from other systems, such as the visual (ocular suppression, strabismus, latent strabismus, ocular torticollis, etc.) or ENT (ear, nose, throat), making it important to carry out specific examinations in collaboration with specialists.

Cervical Vertebrae
Lateral displacement of the cervical vertebrae can be caused by direct or indirect muscular action.
Direct action is performed by the levator scapulae and scalene muscles, with vertebral convexity occurring on the same side as the line of tension. In association with vertebral convexity, an elevated and adducted scapula (levator scapulae) and protrusion of the first ribs (scalene muscles) can also be observed.
Indirect action is caused by the upper bundle of the trapezius which, despite having no vertebral insertions, can cause a vertebral concavity on the same side due to the mechanical result of head tilt and scapular elevation.
Both the scalenes and the levator scapulae, through their horizontal vector components, cause contralateral vertebral body rotation, while their vertical vector components cause asymmetric compression of the intervertebral discs. This compression is exacerbated by the forces G and R acting upon each vertebra. When prolonged over time, such compressions can affect the roots of the plexuses, leading to radiculopathies. 

Cervicothoracic Vertebrae (C6-T4)
In this segment of the spine, direct action is exerted by the rhomboids (both major and minor) and the middle portions of the trapezius. In addition to causing vertebral convexity, these muscle groups rotate the vertebrae contralaterally.
Their longitudinal vector components, which are more prominently expressed by the rhomboids, stiffen the segment C6–T4.
These effects help maintain postural stability and define the dynamics of this area of the spine.

Thoracic Vertebrae (T2–T12)
Lateral displacement can be induced:
* Directly by the lower bundles of the trapezius and the portion of the latissimus dorsi extending between the thoracic vertebrae (T7–T12), the inferior angle of the scapula, and the humerus.
* By the portion of the latissimus dorsi extending between the thoracic vertebrae and the iliac crest.
The combined action of these muscle groups produces vertebral convexity with contralateral vertebral body rotation.
The vertical vector components of both muscles, combined with the paravertebrals, stiffen the spine.
The individual G and R forces applied to each vertebra, through their g and r components, cause localized and asymmetric compressive forces upon the intervertebral discs.
These compressions, if prolonged, can affect the root nerves, leading to radiculopathies. 

Indirectly, lateral displacement can also be caused by the latissimus dorsi bundles that connect the humerus and the iliac crest. In this case, the action of the muscle produces concavity. As an associated pattern, the shoulder appears lower, and the hemipelvis appears elevated. 

Lumbar Vertebrae
Lateral displacement is caused directly by the bundles of the latissimus dorsi inserted in the thoracolumbar fascia (L1–L5) and by the quadratus lumborum. Additionally, it is influenced by the diaphragm and the psoas muscles, which, through vertebral body rotation, contribute to the lateral displacement. In combined action, these muscles create an ipsilateral convexity. 

Contralateral lateral displacement can also arise from the mechanical result caused by the elevation of the pelvis, due to the action of the latissimus dorsi and the quadratus lumborum when acting as a base of support. 

Scoliosis
The etiology of scoliosis is still unknown. There are many hypotheses, including the psychosomatic theory.
From a muscular point of view, in the sagittal plane, the balance of forces is vectorially asymmetric, leading to dominances that manifest as alterations in the skeletal trajectory of the vertebral sinusoid.
In the frontal plane, however, the horizontal components of the muscles inserting directly into the spine can be balanced by contralateral muscles with equal intensity but opposite direction. For example, the traction exerted on the thoracic vertebrae by the rhomboids can be balanced by the rhomboids of the contralateral side, potentially expressing an equal and opposite force.
In contrast to the lateral deviation of the spine, the paravertebrals, through their longitudinal vectors, stiffen the spine.
Moreover, all the oblique muscles, in addition to their horizontal components, have vertical components that, when combined with those of the paravertebrals, can modify the sagittal alignment and stiffen the spine.
In this context, the balance between the muscular forces acting upon the spine is vital for maintaining postural integrity and proper curvature across planes. 

In the chapter addressing the neurophysiological model, it is explained how the "system" distributes muscular shortening to prevent, for as long as possible, mechanical conflicts within the endoarticular structures that might generate pain and debilitating motor limitations.
In this sense, the appearance of lateral deviations in the spine can be interpreted as an expression of reaching the limits of compensatory potential within the sagittal sinusoidal alignment.
In the majority of cases, when scoliosis is present, the spine is stiff and shows significant sagittal alterations.
Once an oblique muscle prevails over its antagonist, deviating the spine laterally, the direction of the vector forces changes.
If the longitudinal vector components of the antagonist muscle cross the midline, those forces will combine with the horizontal and vertical components of the agonist muscle, contributing to the stabilization and worsening of the scoliotic curve.
Similarly, the lines of force of the paravertebrals, when scoliosis has already developed, change direction and, combined with the dominant oblique forces, further fix the vertebral deviation.
The muscles located on the concave side are elongated relative to their starting position, but this elongation does not exceed their maximum physiological stretch. These muscles will experience increased tension as they attempt to balance the lateralization of the vertebrae, ultimately leading over time to a shortening of their connective tissue component. Thus, the muscles on the concave side end up being relatively elongated compared to their starting point, but overall shortened due to the tension accumulated over time. 

It is, therefore, necessary to identify which oblique force vectors dominate in determining the observed scoliotic pattern, but treatment must not be directed exclusively towards these. The excess tension will also involve the contralateral muscles and the paravertebrals.
Considering the individual muscles with dominant vectors, these can have a direct effect on the spine — through rotation and translation (or, in reality, roto-translation) due to their insertions — or cause lateral vertebral deviation as a mechanical result of their action on the girdles and/or the skull.

Once an oblique muscle prevails over its antagonist, deviating the spine laterally, the directions of the vector forces change.
If the longitudinal vector components of the antagonist muscle extend beyond the midline, these forces combine with the horizontal and vertical forces of the agonist muscle, contributing to the stabilization and progression of the scoliotic curvature.
Even the lines of force of the paravertebrals, when scoliosis has already developed, change direction and, combined with the dominant oblique forces, further fix the vertebral deviation.

Muscles located on the concave side are elongated relative to their starting position, but this elongation does not exceed their maximum physiological stretch. These muscles experience increased tension as they attempt to balance the lateralization of the vertebrae, which over time leads to shortening of their connective tissue. In this way, the muscles on the concave side end up being relatively elongated compared to their starting point, but overall shortened due to the tension accumulated over time. 

It is, therefore, necessary to identify which dominant oblique vectors are responsible for the scoliotic pattern observed. However, treatment must not be focused exclusively on these muscles, as the resulting tension will also affect the contralateral muscles and the paravertebrals.
Each individual muscle with dominant vector forces can have a direct effect on the spine through its insertion points, causing rotational and translational forces (or roto-translation), or it can cause a lateral vertebral deviation as a result of its action on the girdles and/or the skull. 

In the presence of scoliosis, the muscles acting upon the spine can be responsible for both concavity and convexity, or both, as these muscles operate in cocontraction and in summation.
When planning a therapeutic approach, it is necessary to first identify the primary dominant vector forces (first level) and act upon them. Gradually, as the dominant first-level forces diminish, second-level forces will arise, and so on, until a satisfactory muscular balance is achieved. This allows the spine to regain a proper positioning in both the sagittal and lateral planes — in other words, a three-dimensional balance.
The therapeutic potential for acting upon scoliosis depends greatly on the patient’s age.
In adults, treatment is generally simpler, unless structural deformities have progressed to the point of causing mechanical blockages. In adolescence, the situation is more complex.
The etiology of scoliosis is still unknown, although for adolescent cases, the psychosomatic theory is gaining increasing attention.
Excess systemic muscle tension may arise from an exaggeration of the ego’s defense mechanisms, which form the body’s armor, from unconscious messages seeking expression through the body (secondary benefits of illness), or from difficulty in coherently interpreting external environmental messages.
For example, an adult has a normative role towards an adolescent, and if this is perceived by the adolescent as a limitation to their autonomy ("I am not capable... I am unable...") or as a denial of affection ("no one loves me..."), self-doubt and anxiety can arise. The system may respond by elevating muscle tension, creating a physical armor.
This body armor provides a functional role by allowing the adolescent to bypass the conscious perception of psychological discomfort, but it also alters the axial balance of the skeleton.
Similarly, when an adult conveys truly destructive messages ("you are incapable, you are worthless, you don’t understand..."), the adolescent objectively perceives this communication as destructive and disstructive.
This perception generates internal conflict between the desire to escape the frustrating situation and the impossibility of doing so due to logistical constraints (as adolescents typically lack independence). In such cases, the system defends itself by suppressing feelings of helplessness and low self-worth, using the body’s muscular armor as a defense mechanism.
To date, there is no comprehensive clinical evidence supporting the psychosomatic theory, but in clinical practice, the only successes observed in treating adolescent scoliosis have been achieved when treatment was conducted in collaboration with a psychotherapist.
If therapy is conducted in isolation, it is already an accomplishment to maintain the existing degree of scoliosis until skeletal maturity.
In adulthood, scoliosis treatment becomes comparatively more manageable and potentially more successful, as the emotional conflicts characteristic of adolescence have usually been resolved by this point.