|Necessary for large range of joint positions
The capsule and its associated ligaments are lax in most of the common functional positions of the glenohumeral joint.
To see this compare the common functional positions with the envelope of motion in which there is no capsular tension (see figures). This laxity is necessary for the joint to achieve its large range of positions. Because of this midrange laxity the capsule cannot stabilize the joint in many important functional positions. Instead centering of the humeral head in the glenoid fossa must depend on other mechanisms such as concavity compression.
To demonstrate the degree of laxity present in eight normal shoulders electromagnetic sensors were pinned to the humerus and scapula to allow accurate measurement of the magnitude of translation on standard clinical tests of glenohumeral laxity: the anterior and posterior drawer tests the sulcus test and the push-pull test. The anterior and posterior drawer tests were performed by stabilizing the scapula and clavicle with one hand while grasping the proximal humerus with the other hand. The arm was placed in a relaxed position at the subject's side. The humeral head was pushed forward to assess maximal anterior translation and then pushed posteriorly to assess maximal posterior translation. In the sulcus test downward traction was applied to the subject's arm while the shoulder girdle was stabilized with the other hand.
The push-pull test was performed with the subject supine and the arm elevated 90 degrees in the plus 20 degrees thoracic plane. The examiner pushed down on the proximal humerus with one hand while pulling up on the subject's wrist with the other.
The shoulders were stressed to the clinical end point. Even though the force applied was not quantified the amount of translation was highly reproducible. The results of these tests indicate that this group of normal shoulders demonstrated substantial translations on these clinical laxity tests. These data indicate that in the positions tested the capsule and ligaments were lax and were not contributing to the centering of the humeral head in the glenoid fossa. We conclude that in these midrange positions the head is centered by stabilizing mechanisms other than the capsule and ligaments.
|Because laxity is a feature of stable shoulders it is of interest to ask whether unstable shoulders have more laxity than stable shoulders. Of greater clinical relevance are the questions: Are laxity tests useful in discriminating stable from unstable shoulders? Do laxity tests reveal the primary pathology in glenohumeral instability? As a step toward answering these questions we measured the laxity of 16 patients requiring surgery because of symptomatic recurrent instability that was refractory to non-operative management. We then compared these results with those of normal shoulders presented earlier. Eight of these patients had classic anterior traumatic instability and eight had classic atraumatic instability. Each patient was studied under anesthesia just prior to surgical repair with our electromagnetic position sensors rigidly attached to the humerus and scapula. The laxity tests were carried out exactly as described earlier for the normal subjects. The magnitudes of the passive glenohumeral translations measured in the unstable shoulders were remarkably similar to those measured in the normal subjects.
These results suggest that glenohumeral laxity is not the preponderant factor in determining the clinical stability of the shoulder. Shoulders that are quite lax may be completely stable while those without major laxity may be clinically unstable. These data further serve to caution against using the magnitude of translation on these laxity tests to distinguish between clinically stable and unstable shoulders. As we will see the diagnosis of instability must rest on a careful history and physical examination which endeavor to define the problem that is symptomatic for the patient.
In conclusion substantial translational laxity is allowed by the normal glenohumeral joint capsule especially in midrange positions. The wide variance in translation among normal shoulders precludes the definition of a "normal" amount of translation on laxity tests. Translation on clinical laxity tests is not an indication of instability. Stability of the glenohumeral joint especially in midrange positions must be due to factors other than tension in the capsular structures.