Vertebral rotation and radicular compression: vector analysis in the frontal and rotatory plane

Vertebral rotation is produced by asymmetric shortening of bilateral muscles. The vertical components of muscular forces compress the intervertebral discs, while rotation of the vertebral bodies reduces the intervertebral foramen on the side opposite the convexity, generating radicular compression. Vector analysis in the frontal plane identifies the responsible muscles and explains why neurological symptoms often appear on the opposite side from the muscular cause.

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

From sagittal to three-dimensional analysis

In the sagittal plane, clear vector dominances exist for each vertebral segment: alterations are predictable. In the frontal and rotatory plane, predictability is reduced. The issue is no longer which muscles are intrinsically dominant, but which anatomically identical muscles develop different tensions on the two sides of the body.

Examination must be performed in the supine position. In upright standing, the muscular system is constantly active to maintain equilibrium: what is observed is the result of activation of the contractile components, not the state of the connective tissue components. In the supine position, with equilibrium stable and no muscle activated, true structural shortenings are observed. It is not uncommon for skeletal elements to appear completely reversed compared with observation in standing.

Rotation and convexity: the same phenomenon

When a vertebra rotates to the right, the vertebral body rotates right and the spinous process shifts left. On the concave side, the discs undergo greater compression. Muscles with direct vertebral insertion produce, through active traction, homolateral convexity and contralateral rotation.

This has direct clinical consequences. Neurological symptoms in the right upper limb due to T2 root compression occur on the concave side — the right — but the cause lies in the muscles of the left side, which determine homolateral convexity. Treatment must be directed toward the muscles on the left side, the cause, not toward the right side where the symptom manifests.

Cervical vertebrae C1–C5

The levator scapulae and scalenes are the principal muscles responsible for cervical deviation. Their asymmetric shortening produces homolateral convexity. In clinical reality, they always shorten together. The combined effect is a roto-translation of the cervical vertebrae.

The vertical components of both groups compress the discs on the side of their action. Radicular compression follows brachial plexus distribution: C5–C6 affects the musculocutaneous and radial territories, C8–T1 the ulnar territory.

Cervico-thoracic vertebrae C6–T4

Rhomboids and middle trapezius fibres act directly on this segment. Their shortening determines homolateral convexity with contralateral vertebral body rotation. Compression at T2–T3 may produce elbow pain identical to epicondylitis but without a local cause. Mistaking its origin leads to treatments directed at the elbow when the real cause is vertebral.

The two patterns of the latissimus dorsi

The latissimus dorsi presents complexity proportional to its size, with five principal force lines. Vector analysis shows that many resultants act in opposite directions. Depending on which force lines are more shortened, different skeletal patterns are produced, classified as pattern A and pattern B.

Pattern A — the approximation pattern — predominantly involves the fascicles connecting the iliac crest to the humerus. Their shortening approximates the hemipelvis to the homolateral shoulder, producing scapular descent, hemipelvic elevation, and homolateral lateral thoracic concavity. The vertebral concavity from T7 to T12 does not derive from direct traction on the vertebrae but is the mechanical consequence of approximation between scapula and hemipelvis. Pattern A is clinically rarer.

Pattern B — the elevation pattern — is characterised by the associated action of the upper fascicles of the latissimus dorsi and the muscles that elevate the shoulder girdle. The global resultant determines scapular elevation and adduction, homolateral lateral thoracic convexity from T4 to T12, and hemipelvic elevation. Pattern B is clinically more frequent.

The four independent curves

In the frontal and rotatory plane, the spinal column is composed of four muscularly independent curves: cranio-cervical C1–C5, cervico-thoracic C6–T4, thoracic T4–T12, and lumbar L1–L5. These curves may appear contralateral to one another or merge into broader radii. For treatment, they must be considered separately.

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.