肩関節外転の運動力学的考察

When abducting the arm, the scapula and the humerus move cooperatively. This cooperation is known as “scapulohumeral rhythm”, and a constant relationship is said to exist between the abduction angle of the humerus and that of the scapula.<BR>But since the glenoid is too small to support the humeral head, it may happen that the humeral head will be dislocated or the soft tissue, like tendons or capsule, will be exposed to a excessive load, if the resultant force through the humeral head does not directly face to the glenoid. So we guess that a change of the resultant force which is consist of load and muscle power to the humerus may change the incline of the glenoid even at the same humeral abducting angle.<BR>In this study, we investigated the relationship between the glenoid inclination angle and the humeral abduction angle with or without block of the supraspinatus muscle power by the suprascapular nerve block.<BR>The subjects of this experiment was 5 healthy men, and the angle was measured by X-ray films which were taken at every 45 degrees abduction position with voluntary effort to abduct isometrically before and after the nerve block, and also the abduction torque was measured simultaneously by the Cybex- II.<BR>The result was that the inclination angle of the scapula was decreased after block, as the maximum defference at 90 degree abduction by 6 degree. And the torque was decreased by the mean of 25 %.<BR>From our hypothesis that the resultant force faces the glenoid perpendicularly, these results could be explained by the functional anatomy of the supraspinatus tendon. At 0 and 135 degree abduction the supraspinatus runs perpendicularly to the glenoid, so the resultant force does not change the direction after block. But at 90 degree abduction the position of the insertion of the supraspinatus is the highest of all abduction angle, so it acts to pull down the head. The resultant that force after block changes the direction upward, and so the glenoid inclination angle decreases to face the head.<BR>From these results, we guess a resultant force to the humeral head always faces to the glenoid by controlled constraction of the muscles around the shoulder.