What is Glenohumeral (Shoulder) Stability?
Glenohumeral (Shoulder) stability is mainly achieved through dynamic factors. Active contraction of the rotator cuff contributes to joint stabilization by coordinated muscular activity and by secondary tightening of the ligamentous constraints. This effect works in combination with the concavity-compression mechanism, in which muscle contraction causes compression of nearly congruent articular surfaces into one another.
The rotator cuff consists of the subscapularis, supraspinatus, infraspinatus, and teres minor muscles. The tendons of the rotator cuff muscles almost completely surround the humeral head as they blend together at their insertions on the greater and lesser tuberosities of the humerus. Because the rotator cuff muscles insert very close to the center of rotation of the axis of rotation of the humerus, the rotator cuff provides a joint compression force at the glenohumeral articulation.
This coordinated compressive function of the rotator cuff muscles is required to counteract the upward shearing force of the strong deltoid muscle during abduction and/or flexion. The rotator cuff, therefore, maintains stability and allows for a spinning motion of the humerus on the glenoid by counteracting the shearing motion of the deltoid.
Ligament and capsulolabral sectioning studies have demonstrated that muscle action of the rotator cuff and deltoid provided joint stability even after a large Bankart lesion was created or the entire capsule and glenohumeral ligaments were sectioned. Dynamic stability of the glenohumeral (shoulder) joint is also provided for by the contraction of the long head of the biceps, coordinated scapulothoracic rhythm, and the proprioceptive modulation of all dynamic factors.
The scapular rotators include the trapezius, rhomboids, the serratus anterior, and the levator scapulae. Of these, scapulothoracic rotation is primarily achieved by the serratus anterior and trapezius muscles, which provide the overall rhythm of the shoulder motion. The ratio of normal glenohumeral to scapulothoracic motion is 2:1 (glenohumeral rotation to scapulothoracic rotation).
This normal relationship is important because it maintains the glenoid as a stable platform underneath the humeral head as the shoulder rotates into positions required for overhead motions such as throwing. Because the axioscapular muscles (scapular rotators) tend to fatigue first during repetitive overhead motions such as throwing and swimming, a lag in normal scapulothoracic rotation has been implicated as a contributing factor for instability and pain during shoulder motion in these sports activities.
The serratus anterior acts as a scapular rotator and protractor, therefore normal positioning of the scapula and glenoid is dependent on proper serratus anterior function. Several authors have noted an association between altered serratus anterior function and anterior instability.
Decreased serratus anterior function in unstable shoulders may cause decreased upward scapular rotation with shoulder abduction, scaption, or flexion which would alter the position of the humerus relative to the glenoid. This could be important in unstable shoulders where this altered scapulothoracic rhythm may act to increase shear forces across the glenohumeral joint via inefficient glenoid positioning.
The possible association between glenohumeral and scapulothoracic problems has been investigated by several authors, who have demonstrated scapular malpositiong or abnormal scapular function in patients with instability.
However, although scapular malpositioning, or muscle dysfunction, is associated with shoulder instability, whether this represents a primary phenomenon or if painful conditions such as instability inhibit the scapulothoracic muscles secondarily is still unknown.
The association between the scapular dyskinesias and instability form the rationale for supplementing rotator cuff strengthening with an axioscapular strengthening program when treating shoulder instability.