The action of the short rotators on the normal abduction of the arm : with a consideration of their action in some cases of subacromial bursitis and allied conditions / by James H. Stevens.

  • Stevens, James H.
Date:
1909
Catalogue details

Licence: In copyright

Credit: The action of the short rotators on the normal abduction of the arm : with a consideration of their action in some cases of subacromial bursitis and allied conditions / by James H. Stevens. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    of the humerus. For a demonstration of this contraction of the sub- scapularis, infraspinatus, and teres minor, it is only necessary to stand behind a model who is abducting his arm, and note with the hand the tenseness of these muscles, especially after the humerus has swung past the horizontal line, in contrast with the flabby relaxed belly and tendon of the pectoralis major in the same position. Beyond a certain point, however, the infraspinatus and teres minor behind and the subscapularis in front, which have acted together up to this point solely as tractors to hold in place the articular surface of the humerus, may possibly take on an added action, although it is probably to a very limited extent, inasmuch as abduction of the humerus is by this time a nearly completed act, namely, that of abductors also, as aids of the deltoid. The action of the supraspinatus is practically little by this time, and the short arm of the lever has ceased to be of much advantage. Its force is probably expended more as a tractor to help hold firmly the articular surface of the humerus from slipping on its fulcrum, and it is in this possibly aided by the tense, firm inferior surface of the capsule, as suggested by Piersol. A new lever, therefore, takes the place of the old (Fig. 4). Considering the articular surface of the humerus as the segment of a circle, turning constantly as the arm abducts and forming a constantly changing fulcrum or point on the glenoid, the moment that the lines of pull of the short rotators have swung upward through the fulcrum of the old lever they cease to act simply as tractors, and a new lever, or a new short arm, is formed instantly. Or, whenever the line of pull of these short rotators transects the segment of the circle above the fulcrum they become the power on the short arm of a new lever, the long arm of which remains the same (Fig. 4, M, B, C), and their action would be to raise the arm to a still higher level, assisting the deltoid. The reverse of this is also true to a limited extent. Thus, the short rotators act in abduction of the humerus when the arm has passed a certain level, and act to pull it downward, adduct, after it has fallen below another fixed level on its downward course, depend- ing on the relation of the line of pull to the fulcrum of the lever at B. To recapitulate, it would seem that from a theoretical and prac- tical consideration of the shoulder-joint, always taking into considera- tion the origin and insertion of its various muscles, and considering them as the power applied to the lever (the humerus), whose fulcrum in the normal shoulder would be a constantly changing point, it would seem that the necessary movements would be as follows: (1) Supra- spinatus contraction would abduct the arm and tend to tip the articu- lar surface of the humerus, so as to bring it against the glenoid, but its action and its strength are slight (30 degrees). The powerful deltoid, contracting, forces the tuberosity against the under surface of the acromion, where interposes the well-lubricated subacromial