Mechanism of the Anterior Shoulder Dislocation.

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There are many articles on the capsular mechanism of preventing anterior shoulder dislocation, and few reports on the dynamics of the dislocation. This study investigates the dislocation based on the dynamics.The anterior dislocation usually occurs when an object bumps posteriorly against the forearm during the abduction position. If the force acts posteriorly at distal place from the gravity center of the upper extremity, the center will move posteriorly at a speed of F/M (F: force, M: mass of the upper extremity) and the humeral head will rotate anteriorly as an angular velocity (ω) (ω=F·h/I, h: distance between gravity center and action point, I: moment of inertia). So the head will move anteriorly at a speed of ω·x-F/M (x: distance between gravity center and humeral head). Because of the gyration radius of the extremity is larger than the radius of glenoid, Bankert's lesion will be produced.The volume of upper extermity of a young male was measured at same length rate and the weight was calculated as the specific gravity was one. The moment of inertia was 0.105kgm. If the force (10N) acts to the distal place (h=40cm), humeral head will move 5.4cm anteriorly during 0.01 secoed. The capsule and rotator cuff muscles will be stretched, but stretch reflex of these muscles will not prevent the movement of the head, because the reflex will not happen during 0.01 second.If the moment of inertia is made larger, the incidence of the anterior dislocation may decrease.

Mechanism of the Anterior Shoulder Dislocation.

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