Scoliosis: vertebral rotation, lateral deviation, and biomechanical intervention criteria

Scoliosis can be analysed through the same vector principles applied to vertebral deviations in the frontal plane. Analysis of the relationship between rotation and vertebral deviation provides an interpretative criterion for evaluating response to muscular treatment. Once established, scoliosis develops self-perpetuating mechanisms involving both convex-side and concave-side muscles.

The attached PDF document, available for free download, develops the complete vector analysis with images and bibliographic references.

Aetiology remains unknown

The aetiology of idiopathic scoliosis remains largely unknown despite decades of research. Genetic, neurological, biomechanical, metabolic, hormonal, and other theories have been proposed, but none fully explains the phenomenon. The analysis that follows does not aim to explain the causes of scoliosis: it is limited to describing the vector mechanisms through which vertebral deviations manifest and can be interpreted from a muscular perspective.

The rotation-deviation criterion

Physiologically, rotation of the vertebral bodies is contralateral to the lateral deviation relative to the midline axis. In some scolioses, this relationship is reversed: convexity and rotation become homolateral.

This clinical observation suggests an interpretative criterion. When rotation of the vertebral bodies remains opposite to the lateral deviation — physiological pattern — there may be room for improvement through treatment directed at the muscular system. When rotation is homolateral to the lateral deviation — non-physiological pattern — the scoliotic curve may be so structured that it does not respond to direct work on the muscles.

This criterion is an empirical observation requiring scientific validation and does not replace standard radiological classifications.

Growth phase and adult phase

During active growth, when the pattern is physiological, there is theoretically the possibility of intervening on the muscular component. However, the observed increase in Resistant Force may represent a compensatory mechanism developed by the nervous system to contain curve progression. Interfering with this equilibrium without adequate evaluation could alter a spontaneous defensive strategy of the system. Intervention requires a multidisciplinary approach.

After skeletal maturity, work is directed at the results of the disease. With a physiological pattern, the possibility of achieving effective curve reduction is greater: the system is no longer subjected to the unknown evolutionary forces of the active pathology. With a non-physiological pattern, the objective is limited to symptom control.

Scoliosis as sagittal saturation

The appearance of lateral spinal deviations may be interpreted as the expression of saturation of the possibility of further altering the sagittal course without producing mechanical conflict. The system, no longer able to compensate in the sagittal plane, begins to use the frontal plane. Clinically, in the majority of scoliosis cases the spine presents as rigid and with significant sagittal alterations.

Self-perpetuating mechanisms

Once an oblique muscle prevails over the contralateral one, laterally deviating the spine, the directions of the vector components change. When the longitudinal components of the antagonist muscle project beyond the midline, instead of opposing they add themselves to the horizontal and vertical components of the agonist muscle. They contribute to stabilisation and worsening of the scoliosis.

The force lines of the paravertebral muscles also change direction. Following the deviated spine, they lose their verticality and, by adding themselves to the dominant oblique forces, contribute to fixation of the vertebral deviation.

Concave-side muscles: paradoxically shortened

The muscles on the concave side are lengthened relative to their initial position. Their lengthening, however, is only apparent: it does not exceed the maximal potential lengthening of the muscle. These muscles increase their tension in an attempt to balance vertebral lateralisation and, over time, this excess tension determines shortening of the connective tissue portion of the muscle fibre. The muscles on the concave side are therefore in relative lengthening compared with the starting position, but globally in shortening.

Treatment cannot be directed only at the dominant oblique vectors of the convex side. Excess tension also affects the contralateral muscles and the paravertebral muscles. A bilateral approach is essential to interrupt the self-perpetuating mechanism.

Physical foundations of the model.
This article applies the AIFIMM biomechanical model.
Its physical foundations are developed in three sequential articles, best read in order:
1. How muscle shortening generates joint conflict — why muscles shorten and the Resistant Force / Working Force model
2. Do antigravity muscles really oppose gravity? — how segmental malalignment raises Resistant Force
3. Why joint conflict develops: vector analysis of muscular forces — how the responsible forces are identified and predicted

This topic is part of the online course Systemic and Segmental MSK Biomechanics.