Likely relates to laxity of ligaments that tether these bones at a universal joint, the Acromial-Clavicular (AC) Joint; one of bio-mechanical factors that potentiates abnormal clavicle shift during arm abduction.
When ipsilateral Superior trapezius & Pectoralis minor muscles are hypertoned & hyperdynamic, during arm abduction distal clavicle is drawn superior (with hyperdynamic S. trap.) & inferior (with hyperdynamic P. minor).
Basic geometry shows that a 3-4mm shift of the distal clavicle evokes ~30% closure of Thoracic Outlet (TO) (dimension vs. inferior clavicle & superior 1st rib). Excessive closure of TO arouses Thoracic Outlet Syndrome (TOS).
Excessive closure of TO causes bio-mechanical impingement of tissues contained within the TO, the neurovascular bundle; giving rise to distal-upper extremity paraesthesiss. When subclavian artery (SA) is impinged, this leads to coolness & cramps within distal extremity.
Subclavian artery impingement can be readily observed by Ultrasound exam during Addson’s Maneuvers. Attendant with subclavian impingement, subclavian vein impingement can occur; sometimes manifest as a cluster of vein dilations over Pectoralis region.
An interesting physical finding, when examining for TOS, is oft a bruit heard in the proximal sub-clavicle region. This is manifestation of blood flow vortices aroused by pinch of the arterial lumen.
My clinical observations indicate that S. trap. dominance over ipsilateral P. minor leads to draw of distal clavicle posterior-inferior; leading to impingement of that part of brachial plexus that gives rise to radial nerve & radial paresthesias.
Similarly, dominance of P. minor leads to anterior-inferior shift of distal clavicle; leading to impingement of that part of brachial plexus that gives rise to ulnar nerve & ulnar paresthesias. Ulnar paresthesias are more common than radial amongst p/w Thoracic Outlet Syndrome.
My recent clinical research of TOS has been to ask radiology to do anterior-posterior views of the acromio-clavicular joint before and during abduction; to view inferior shift of distal clavicle relative to acromion. This shift can be seen.
Another way to clinically observe asymmetrical acromial-clavicular shifts is to put one finger on each bone during abduction. Normally, both fingers elevate in concert. Abnormal shift occurs when acrimon bone uplifts as distal clavicle dips (either anterior or posterior).
The most sensitive instrument to diagnose soft tissue disorders & dysfunctions of joints, ligaments, & muscles is a trained physician using knowledge of anatomy & sensitive touch to evaluate body parts in real time during function in space & over time.
Modern clinical medicine over dependence on static 2-dimensional X-rays & scans for diagnosis is defective. Soft tissues are relatively radiolucent. Imaging studies often neglect effects of tissue masses within gravitational field; as patients recline for many of these studies.
IMO, the defective diagnostic measures I have outlined are major factors in the current epidemic of opiate bowdlerization and failure of the physician class to provide caring & humane care to millions of people with chronic pain syndromes.
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