|Last Updated: Thursday, February 10 2005|
|Glenohumeral balance is a stabilizing mechanism in which the glenoid is positioned so that the net humeral joint reaction force passes through the glenoid fossa.
The joint can be positioned so that the force is contained within the fossa. No other stabilizing mechanism is necessary as long as the humeroscapular position is such that the glenoid supports the net humeral joint reaction force. When the joint is in balance its stability is independent of the magnitude of net humeral joint reaction force.Movie
Directions and angles
Balance is sensitive to the direction of the net humeral joint reaction force vector with respect to the glenoid fossa. The larger the arc subtended by glenoid concavity the larger the range of directions of the net humeral joint reaction force vector that will be stabilized by it. This range of direction can be estimated from simple geometric calculations. A radian is the central angle of a circle which subtends an arc equal in length to the radius of the circle. There are 2 pi radians in a circle thus one radian equals 360/2 pi or almost 60. The angular range of stability resulting from glenohumeral balance can be predicted by dividing the length of the glenoid arc by the radius of the humeral head and multiplying the quotient by 360/2 pi. As an example if the anteroposterior arc length of the glenoid were equal to the radius of the humeral head the glenoid would balance net joint reaction force vectors through a range of directions of approximately 60 degrees (from about 30 degrees anterior to 30 degrees posterior to the glenoid center line). Hypoplasia erosion or fracture of the glenoid rim can diminish the arc available for balance stability.
In an experiment with a series of cadaver shoulders we demonstrated this balancing effect in a manner similar to the golf ball and tee demonstration. We found that the net humeral joint reaction force vector was balanced through a wide range of angles. The combined anteroposterior balance stability angle averaged 36 degrees and that for the superior-inferior directions averaged 57 degrees. In these cadaver shoulders balance stability was relatively symmetrical around the glenoid center line (the anterior and posterior stability angles were approximately equal as were the superior and inferior angles). Owing to the increased vertical extent of the glenoid the superior-inferior stability angle was greater than the anterior-posterior stability angle. The anterior stability angle was reduced by an average of over 25 percent by the presence of a relatively small glenoid rim defect.
The large number of component forces operating on the humerus makes it difficult to calculate the exact direction of the net humeral joint reaction force vector in vivo. A rough approximation is that in midrange humeroscapular positions the vector sum of the deltoid and cuff muscle forces lies close to the axis of the humerus. In many vigorous shoulder activities the scapula is positioned so that the glenoid center line is closely aligned with the humerus. During the critical moments of the boxer's knockout punch the bench press or the tennis stroke for example the humeroscapular position appears to be such that the glenoid center line and the humerus are aligned. Under these circumstances the glenohumeral joint is stabilized by balance so that muscle energy is preserved for power. This observation emphasizes two special features of balance stability:
Stability from balance is of particular interest because of its relationship to humeroscapular position. It is apparent that essentially identical humerothoracic positions can be achieved using different humeroscapular positions. Some of these positions will favor glenohumeral balance while others will not. Consider the arm elevated 90 degrees in the plus 90 degree thoracic plane. If the scapula is protracted the humerus is closely aligned with the glenoid center line. Alternatively if the scapula is retracted the humerus is almost at right angles to the glenoid center line. The essential point is that balance at the glenohumeral joint is determined by humeroscapular position rather than the more easily observed humerothoracic position. Which humeroscapular position is used to achieve a given humerothoracic position is a question of neuromuscular control habit and training. In analyzing patients with glenohumeral instability it is important to document the humeroscapular positions in which the instability occurs. Neuromuscular retraining may help the patient regain stabilizing balance.
The precision of neuromuscular control required for balance is inversely related to the size of the glenoid fossa. The angle through which the balancing mechanism can function is less in the anterior and posterior directions where the glenoid arc is relatively small than in the superior and inferior directions where it is larger. The balance stability angle is diminished in glenoid hypoplasia fracture or degenerative erosion. In prosthetic shoulder arthroplasty components with a large articular surface offer a greater balance stability angle.
The balance stability mechanism may fail in cases of severe muscle imbalance where the net humeral joint reaction force is not aligned with the glenoid even though the humerus is close to the glenoid center line. The balance stability mechanism may fail in the presence of abnormal glenoid version where the glenoid center line deviates substantially from the plane of the scapula and from the net.