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