Glenohumeral instability is defined as an
ability to maintain the humeral head centered in the glenoid fossa. This problem typically is caused by a lesion of the soft tissue, either a traumatic rupture of the capsule and ligaments (usually following a complete dislocation or partial dislocation, also known as subluxation), or by generalized laxity of the soft tissue. In the case of post-dislocation instability, the shoulder is not secure in only one plane (the movement that should have been stopped by the injured tissue), but with generalized laxity the instability is multidirectional. Patients with glenohumeral instability may suffer repeat episodes of subluxation in which the joint surfaces are damaged. In very broad terms, instability from trauma is amenable to surgical repair, whereas multidirectional is not, and requires physical therapy to strengthen the rotator cuff which can then actively stabilize the joint.
Structure and Function
The shoulder joint is inherently unstable, allowing for the increased possibility of injury. Only a small portion of the humeral head articulates with the glenoid at any given time, due to the inherent shallowness of the glenoid (
Figure 1: A CT scan of the shoulder, showing the mismatch between the size of the glenoid (yellow) and the size of the humeral head (red). (https://radiopaedia.org/cases/normal-ct-shoulder-2)
The shoulder has both dynamic (active) and static (passive) stabilizers. The static stabilizers of the shoulder are the ligaments and capsular tissue. Unlike the femoroacetabular joint of the hip and humeroulnar joint at the elbow, the glenohumeral joint has little static stability provided by the bony anatomy. Some additional stability is provided in the shoulder by the labrum, a lip of cartilage around the glenoid that deepens the socket.
There are condensations of the capsule which are identified as distinct ligaments or ligament complexes: the superior, middle and inferior glenohumeral ligaments (
figure 2). The superior ligament provides stability with the arm at the side. The middle provides restraint with the arm partially abducted and the inferior glenohumeral ligament is the primary stabilizer with the arm fully abducted.
Figure 2: The ligaments connecting the humerus to the scapula. The coracohumeral ligament is shown in faint black; the capsule is outlined in red. The capsule itself comprises the superior (green) middle (pink) and inferior (yellow) gleno-humeral ligaments. (Annotations on x-ray courtesy of https://radiopaedia.org/cases/external-and-internal-rotation-views-of-the-shoulder)
The rotator cuff along with the long head of the biceps stabilize the glenohumeral joint actively (
figure 3). Paired contraction of the muscles of the rotator cuff hold the humeral head in close approximation to the glenoid cavity.
Figure 3: As seen from the front of the body, the subscapularis (red), supraspinatus (blue) and long head of the biceps (green) all dynamically stabilize the gleno-humeral joint. (On the posterior side, the infra-spinatus and teres minor, not shown, provide stability as well)
The axillary nerve is often injured with shoulder dislocations. The axillary nerve wraps around the surgical neck of the humerus and runs on the inferior side of the humeral head.
A thorough history is essential to characterize the patient’s shoulder instability. The patient may commonly recall a specific traumatic instability event, have numerous incomplete instability events, or describe generalized laxity of both shoulders and multiple other joints.
Anterior instability is often reported with an injury when the arm is abducted and externally rotated. In contrast, posterior instability is often reported when injury occurs with the arm in an adducted, flexed, and internally rotated position. Inferior instability is accompanied by a history of instability with carrying heavy loads. Although anterior instability is significantly more common than inferior, posterior, or multidirectional instability, all pathologies must be identified.
The acute presentation of anterior shoulder dislocations is notable for a palpable prominence of the humeral head anterior and inferior to the shoulder and a lack of shoulder contour over the deltoid. The arm is generally held in a position of adduction and internal rotation, and abduction of the arm is limited to less than 90°. The acute presentation of posterior shoulder instability is more subtle, and a lack of external rotation compared to the contralateral normal shoulder may be the only presenting sign.
The physical exam should begin with evaluation of the asymptomatic shoulder for comparison. Two simple tests may be used even by inexperienced examiners to evaluate shoulder instability.
The sulcus sign can be elicited by applying longitudinal inferior traction of the humerus – pulling down on the patient’s wrist with the arm held at the side. Excessive inferior displacement of the humerus relative to the lateral border of the acromion creates a sulcus and reflects laxity of the capsule
. (FIGURE 4)
Figure 4: The sulcus sign. The examiner pulls down on the arm at the elbow. If there is laxity of the shoulder, an indentation on the skin, aka a “sulcus”, will be seen between the acromion and the humeral head (with permission of https://www.shoulderdoc.co.uk/article/798 (email to JB.) NOTE: if fotios has a better one, use it!)
The anterior apprehension test is performed by asking the patient to abduct the shoulder and external rotate the arm–as if cocking the arm to throw a ball. A patient with anterior instability will be apprehensive or completely decline to perform this maneuver
( fig 5).
More subtle anterior instability can be detected by having the supine patient abduct the shoulder and external rotate the arm, and the examiner can then push on the arm as if to glide the humeral head out of the joint.
Signs of generalized ligamentous laxity include the ability to touch the palms to floor while bending at waist; hyperextension of the elbows, metacarpophalangeal or knee joints (genu recurvatum) and the ability to abduct the thumb to the forearm. The presence of these
so-called Beighton criteria is indicative of laxity.
To assess patients with suspected shoulder instability, obtain both an anteroposterior (AP) view (
figure 6) and an axillary lateral view of the shoulder. It is important to obtain two orthogonal views. Shoulder dislocations might look like a normal shoulder on the AP view, and this diagnosis may be missed if both views are not obtained.
Figure 6: Anterior shoulder dislocation (Case courtesy of Dr Jeremy Jones, Radiopaedia.org, rID: 7132)
The use of magnetic resonance imaging (MRI) can be used to visualize ligaments that can be torn with shoulder dislocation (
figure 7). MRI is more useful with the injection of contrast into the joint before evaluation. MRI is especially useful for the diagnosis of older patients who are more likely to tear the rotator cuff muscles or tendons concurrently with a shoulder dislocation.
Figure 7: An MRI of the shoulder after reduction of a dislocation. Damage to the anterior labrum and edema within the humeral head, both caused by the dislocation, are noted with arrows. (Case courtesy of Dr. Mandakini Siwach, Radiopaedia.org, rID: 53957)
CT scans may be helpful for evaluation of the bony anatomy and should be obtained if there is suspicion of a large Hill-Sachs lesion or glenoid fracture.
The estimated incidence rate of shoulder dislocations in the overall population of the United States is 24 per 100,000 person-years. The incidence rates in military personnel and athletes are considerably higher than the general population. Anterior shoulder dislocations account for over 95% of shoulder dislocations, while posterior dislocations account for about 4%, and inferior shoulder dislocations the scant remainder, 1% or less.
Multidirectional shoulder instability is frequently bilateral. Its incidence peaks in young adulthood (approximately late teens) and can be found in overhead athletes (pitchers, swimmers and gymnasts) or with generalized ligamentous laxity found in connective tissue disorders such as Ehlers-Danlos or Marfan’s syndrome.
When patients present with suggested glenohumeral instability, physicians also exclude the following concomitant conditions.
- A Bankart lesion (avulsion of the anterior labrum)
- A “bony Bankart” lesion (a fracture of the anterior glenoid)
- A Hill Sachs defect (impaction fracture of the humeral head, caused by contact against the glenoid during dislocation)
- Fracture of the greater tuberosity fracture (especially in older patients)
- Fracture of the lesser tuberosity fracture (with posterior dislocation)
- Nerve injuries, especially axillary nerve
- Generalized ligamentous laxity
Seizures and electrocution can cause posterior shoulder dislocations and should be considered in any patients with that history and shoulder complaints. Likewise, the presence of a posterior shoulder dislocation raises the suspicion of an unreported seizure.
Posterior shoulder dislocation are also associated with small lesser tuberosity fractures. These fractures might be missed if the films are not examined closely.
Multidirectional shoulder instability may suggest a connective tissue disorder such as Ehlers-Danlos or Marfan’s syndrome.
Abnormal passive motion suggests that the shoulder is not reduced.
In patients older than 40 years, the orthopaedic surgeon should have a high index of suspicion for concomitant rotator cuff pathology.
Although axillary nerve injuries are seen in only a small minority of cases, they are frequent enough (~5% of cases) that the presence of dislocation itself is a “red flag” for a nerve injury and must be excluded on exam.
Treatment Options and Outcomes
Needless to say, acute shoulder dislocations must be reduced. A variety of reduction techniques described can be performed under either conscious sedation or administration of an intra-articular injection. The most crucial aspect of reduction is relaxation of the shoulder musculature. Radiographs after the procedure are required to verify reduction. It is crucial to differentiate between a posterior and anterior location due to the differences in reduction maneuver.
The Hippocratic method for anterior dislocations uses gentle longitudinal traction applied with a counterforce (e.g., a sheet placed in the patient’s axilla). The Milch maneuver has the patient lying prone on the exam table with both abduction and external rotation applied to the arm as the physician’s thumb attempts to push the humeral head into place.
Thereafter, a short period of immobilization is reasonable, followed by range of motion exercises and then a strengthening program.
Physical therapy alone may be adequate treatment for traumatic instability. Surgery may be considered especially in younger patients with a Bankart lesion or patients with a history of recurrent dislocations. Surgery is typically a repair of the capsule with a “shift” to tighten the tissue (OPTIONAL FIGURE 8).
Patients with a shoulder dislocation are considered cleared to play when strength and full range of motion have returned to normal.
Multidirectional shoulder instability is treated with physical therapy and then more physical therapy if that does not work. Operative stabilization procedures are chosen with reluctance, i.e. only if there is instability that interferes with critical activities after an extensive round of non-operative management. Surgery attempts to plicate redundant capsule. It is key that this is done in a balanced fashion, as too much tightening on one side (anteriorly, say), will just create more instability in the other direction.
The natural history of initial shoulder dislocations remains controversial. Recurrent dislocation is most strongly predicted by the age of the patient. In athletes younger than 20 years of age treated non-operatively, recurrence rates in excess of 50% have been reported. Recurrent dislocation following an initial anterior shoulder dislocation is less common in older patients, but in this cohort rotator cuff tears are more common and may be limiting.
Athletes who have had a shoulder stabilization procedure after traumatic dislocation report a far lower dislocation recurrence rate (less than 15%) than would be expected from non-operative treatment. Most patients also report excellent subjective and objective clinical outcome scores.
Risk Factors and Prevention
Patients might be at increased risk for shoulder instability for reasons under one’s control (choice of sports) and beyond individual control (glenoid anatomy and ligamentous laxity).
Risk may be minimized by assuring adequate strength, including scapular stabilizing muscles, and that repetitive shoulder stress is kept to a minimum.
Hippocrates not only invented a method of reducing the shoulder (
see figure 9), he also devised a means of repairing the tissue injured by dislocation: namely, burning the capsule with a hot poker placed in the axilla. This method is frowned upon by modern authorities.
Figure 9: Hippocrates’ method of reducing a dislocation of the shoulder. (https://en.wikipedia.org/wiki/Hippocrates#/media/File:GreekReduction.jpg)
Glenohumeral instability, Anterior dislocation, Posterior dislocation, Inferior dislocation, Subluxation, Humeral head, Labrum, Bankart Lesion, Hill-Sachs defect
Recognize history and signs suggesting dislocation. Recognize dislocation on imaging, and in particular, whether imaging is inadequate.