The shoulder is notable for being the most mobile joint in the body. Optimal function requires a delicate balance of stability and flexibility. This balance is achieved by contributions from both static and dynamic stabilizers. It is important to understand that some athletes, such as swimmers and dancers, inherently require greater amounts of flexibility, while others, such as overhead athletes, depend more on stability. Shoulder instability is a pathologic spectrum in which the motion of the humeral head exceeds the containment of the glenoid labral complex resulting in pain and dysfunction. Instability of the shoulder can occur in the anterior, posterior, or inferior direction, or in a combination of these directions.
Shoulder instability is a common problem in the young athletic population. , Interestingly, studies have reported that young males are 2.7 times more likely to present with shoulder instability than young females. , Therefore male participants comprise the majority of patients in research cohorts related to shoulder instability. However, when comparing shoulder instability rates between young males and females in gender-comparable sports (e.g., soccer, basketball, and rugby), the rates of shoulder instability are nearly equal. ,
Stability of the shoulder is conferred by both static and dynamic restraints. The static stabilizers of the glenohumeral joint include the bony anatomy (i.e., the humeral head and glenoid), as well as the glenoid labrum, capsule, and glenohumeral ligaments. The dynamic stabilizers of the shoulder include the surrounding muscles, specifically the rotator cuff (i.e., supraspinatus, infraspinatus, subscapularis, and teres minor) and scapular stabilizers.
The proximal humerus consists of the humeral head and articular surface; the greater and lesser tuberosities, which are the attachment sites for the rotator cuff muscles; and the humeral shaft. The scapula is a relatively flat, triangular bone that sits on the posterior lateral aspect of the chest wall at the level of the second to the seventh rib. The superolateral corner of the scapula gives rise to the glenoid fossa, which is angled slightly posterior and inferior relative to the body ( Fig. 15.1 ).
The glenoid labrum is a fibrocartilaginous ring that is located circumferentially around the rim of the glenoid and is continuous with the articular surface of the glenoid face. It has a bumper effect, limiting abnormal translation of the humeral head on the shallow glenoid bone ( Fig. 15.2 ). On cross section, the labrum is wedge shaped and functionally increases the depth of the glenoid cavity by up to 50%. The glenohumeral joint capsule encapsulates the articular portion of the humeral head and glenoid. Three main thickenings in the capsule, the superior glenohumeral ligament (SGHL), the middle glenohumeral ligament (MGHL), and both the anterior and posterior bands of the inferior glenohumeral ligament (IGHL), also provide restraint against glenohumeral translation. The IGHL is a hammocklike structure, as its anterior and posterior bands extend along the inferior aspect of the glenohumeral joint. The IGHL predominantly stabilizes the arm in the abducted, externally rotated position. The MGHL can demonstrate variable anatomy, and in general, it originates from the anterior superior labrum or glenoid rim and inserts on the anatomic neck of the humerus, providing stability against anterior translation when the arm is in 45 degrees of abduction. Finally, the SGHL arises from the anterosuperior glenoid, runs parallel to the biceps tendon in the rotator interval, and inserts on the lesser tuberosity. In some studies, it is thought to be the most important stabilizer against inferior translation, while other studies have shown it is the coracohumeral ligament (CHL) and/or the IGHL that resist inferior translation ( Fig. 15.3 ).
The glenohumeral joint is the most mobile joint in the body and allows range of motion with 6 degrees of freedom. Additionally, the humeral head can translate as much as 8–14 mm anteriorly, posteriorly, or inferiorly on the glenoid fossa. Given the significant mobility and wide range of motion inherent to the glenohumeral joint, it is not surprising that the shoulder is not only the most mobile joint in the body but also the most unstable. While shoulder joint stability is predominantly achieved through a combination of the static and dynamic stabilizers, there is also inherent negative intra-articular pressure due to the glenoid concavity “plunger” effect on the humeral head. The glenohumeral joint capsule is important in maintaining this intra-articular vacuum effect.
Pathoanatomy of Anterior Instability
When the humeral head dislocates from the glenoid, the resulting pathoanatomy can be capsulolabral, osseous, or both. In anterior shoulder instability, the “essential lesion” involves an injury to the anteroinferior glenoid labrum and the anterior band of the IGHL. This is often referred to as the “Bankart lesion,” which is seen in 97% of cases of acute, first-time, traumatic, anterior dislocations. When there is an osseous component (i.e., an associated fracture of the anteroinferior glenoid rim), it is referred to as a “bony Bankart lesion.” Recurrent anterior instability can lead to anterior glenoid bone loss due to medial displacement of a bony fracture or erosion of the anterior glenoid.
Concomitant osseous injury to the posterosuperior humeral head (i.e., Hill-Sachs lesion) can occur via a compression fracture as the softer humeral head impacts the anterior glenoid rim during anterior shoulder subluxation or dislocation. , Hill-Sachs lesions are seen in approximately 40% of patients with recurrent subluxation, in 70%–90% of patients with a single dislocation, and in almost 100% of patients with recurrent shoulder dislocations. , The majority of Hill-Sachs lesions are small and are likely clinically insignificant. However, they can be large and clinically significant. Additional intra-articular and extra-articular pathology that needs to be identified include tears of the superior labrum anterior to posterior (SLAP tears), circumferential labral lesions, humeral avulsion of the glenohumeral ligament (HAGL) lesions, anterior labral periosteal sleeve avulsion (ALPSA) lesions, glenoid labrum articular disruption (GLAD) lesions, and rotator cuff injuries. These lesions may alter surgical management as well as rehabilitation for the patient.
As males are more commonly afflicted with shoulder instability, females are underrepresented in the majority of studies describing the pathoanatomy. In an attempt to better understand the unique pathology associated with shoulder instability in collegiate female athletes, Patzkowski et al. performed a retrospective analysis of a consecutive series of female students at a National Collegiate Athletic Association (NCAA) Division I military service academy, who were treated operatively for shoulder instability. The authors described the pathoanatomy seen at the time of arthroscopy in 36 female student athletes with an average age of 20 years. They reported that shoulder instability in female athletes presents commonly as multiple subluxation events. Soft tissue Bankart lesions were found with a frequency similar to those published in previous mixed-gender studies; however, bony Bankart lesions were much less common in females. Additionally, the presence of combined anterior and posterior labral tears and HAGL lesions in females was noted to be much more common in this group than previously reported.
Pathoanatomy of Posterior Instability
Posterior shoulder instability can be thought of as a spectrum of pathology. The posterior labrum, capsule, and posterior band of the IGHL are the primary stabilizers to posterior translation of the humeral head when the arm is between 45 and 90 degrees of abduction. A frank posterior dislocation may occur in the setting of a traumatic posteriorly directed blow to an adducted, internally rotated, and forward-flexed upper extremity (e.g., fall on an outstretched arm), which can result in an acute posterior capsulolabral detachment (i.e., a reverse Bankart tear). On the other end of the spectrum, because the posterior capsule is the thinnest segment of the shoulder capsule and does not contain any supporting ligamentous structures like that seen anteriorly and inferiorly, it is more susceptible to attenuation from repetitive stress. Therefore in overhead athletes (e.g., throwers and swimmers), in whom there is repetitive direct stress on the posterior capsular labral complex, athletes can develop recurrent posterior shoulder subluxation in the absence of frank dislocation. It is thought that the progressive laxity of the posterior capsule and fatigue of both static and dynamic stabilizers lead to attenuation and deformation of the posterior capsule, resulting in a patulous posterior inferior capsular pouch and labral tearing. This can be seen on both magnetic resonance arthrography (MRA) and arthroscopy.
Pathoanatomy of Multidirectional Instability
Multidirectional instability (MDI) of the shoulder refers to anterior and posterior instability associated with involuntary inferior subluxation or dislocation. While glenohumeral stability results from a complex interplay of static and dynamic stabilizers, anatomically it has been noted that the depth of the glenoid cavity in patients with MDI is more shallow than that of age-matched control subjects. However, the characteristic pathologic entity in patients with MDI is increased capsular redundancy. The redundancy may be congenital and associated with a connective tissue disorder (e.g., Ehlers-Danlos syndrome, Marfan syndrome, benign joint hypermobility syndrome, and osteogenesis imperfecta). Alternatively, it can be acquired from repetitive microtrauma and/or repetitive overuse during activities that result in stretching of the capsuloligamentous restraints. The most significant restraint to inferior su bluxation of the humeral head is the rotator interval complex, which is made up of the SGHL, the CHL, and the superior joint capsule. Incompetence of the rotator interval is often seen in patients with MDI. ,
Evaluation of the Female Athlete with Shoulder Instability
The importance of taking a good history is both to establish the diagnosis of shoulder instability and to obtain information that will help direct treatment. Unidirectional, anterior shoulder instability most often results from a discrete traumatic event, and joint manipulation is often required to obtain reduction. If the athlete has had a true dislocation event, it is helpful to determine the mechanism of injury, use radiographs to determine the direction of the dislocation, establish if there was a need for manual reduction versus if the patient experienced a spontaneous reduction, and determine if there is any associated disability resulting from the dislocation (e.g., paresthesias or weakness in the involved extremity). If the patient has had numerous dislocations, it is important to ascertain if the mechanism of injury has changed; for example, if the instability events are becoming more frequent and occurring with less traumatic force. Additionally, it is important to document the age at the time of the first dislocation and at the time of the subsequent dislocations, as age is a significant prognostic indicator for recurrence.
Symptoms of shoulder subluxation without dislocation tend to be more vague and difficult to evaluate. Patients may report an insidious onset of symptoms and a feeling of looseness and achiness in the shoulder. Occasionally, these complaints can occur with concomitant transient neurologic symptoms. The precipitating factors may be unpredictable and symptoms may occur with activities of daily living or even during sleep.
To distinguish between anterior instability, posterior instability, and MDI, it is helpful to determine the arm position that makes the patient feel uncomfortable. Most patients with anterior instability will be apprehensive with the arm in an abducted and externally rotated position. Patients with posterior instability will report pain with the arm in the forward-flexed, internally rotated, and adducted positions (i.e., reaching forward to open a door). Patients with MDI may report pain with the arm in several different positions; however, by definition, they should have symptoms, with inferior translation among their constellation of complaints.
In addition to understanding the direction and frequency of the instability events as well as concomitant symptoms, it is important to understand the demands the patient puts on the affected shoulder. This includes inquiring about which sports they participate in; if they throw, hit, or swing with the affected extremity; and about their occupation and if it is affected by their symptoms. Understanding their social history as well as their instability history will help guide treatment for the patient.
A general physical examination of the cervical spine and scapula is performed followed by a thorough examination of the shoulder. Once inspection, palpation, range of motion, and strength testing are carefully performed, specific provocative maneuvers can be done to elicit symptoms suggestive of the different instability patterns. Ligamentous laxity should be evaluated in all patients presenting with complaints of shoulder instability. Beighton scoring is the most common way to quantify generalized hypermobility. The patient is awarded one point for each positive test result for a possible total of 0–9. The higher the score, the more generalized joint hypermobility the patient has. The components of this test include passive dorsiflexion of the small fingers beyond 90 degrees, passive apposition of the thumbs to the volar forearm, hyperextension of the elbows greater than 10 degrees, hyperextension of the knees beyond 10 degrees, and touching the floor with the palms of the hands while flexing forward with the trunk and keeping the knees extended. Female athletes have been noted to have more signs of generalized joint hyperlaxity than male athletes as measured by the Beighton criteria.
Provocative Tests for Anterior, Posterior, and Multidirectional Shoulder Instability
Anterior apprehension test
With the patient in the supine position and the scapula stabilized on the examination table, the shoulder is passively brought into 90 degrees of abduction and then into maximal external rotation, while slight anterior pressure is applied to the posterior aspect of the humeral head ( Fig. 15.4 ). A positive test result is signified by a sense of impending anterior shoulder dislocation.
Jobe relocation test
In this test, a posteriorly directed manual force is applied to the humeral head with the arm in 90 degrees of abduction and maximal external rotation. A positive relocation test result is signified by relief of the patient’s sense of impending shoulder dislocation ( Fig. 15.5 ).
Anterior and posterior drawer test
The anterior drawer test is performed with the patient in the supine position on an examination table. The scapula is again stabilized on the table and the examiner brings the arm to 80–120 degrees of abduction, slight forward flexion, and slight external rotation. The arm is then translated anteriorly. The movement can be graded by the modified Hawkins scale, with type 1 being translation to the glenoid rim, type 2 as translation over the glenoid rim, and type 3 as translation over the glenoid rim that does not spontaneously reduce. Any apprehension noted during the anterior drawer test should be considered positive for anterior shoulder instability. Translation should also be compared to the contralateral shoulder to determine what is normal for the patient.
The posterior drawer test is also performed with the patient in the supine position. In this case, the examiner holds the patient’s proximal forearm and flexes the elbow to 120 degrees. The arm is brought into 80–120 degrees of abduction and 20–30 degrees of forward flexion. With the contralateral hand, the examiner stabilizes the scapula. The arm is then internally rotated and flexed to 60–80 degrees, and the examiner’s thumb is used to subluxate the humeral head posteriorly. Any apprehension is considered a positive test result for posterior instability.
Load and shift test
The patient can be positioned in the seated position, with the examiner standing behind the patient. The scapula and shoulder girdle are stabilized with one hand, and the humeral head is then loaded and shifted anteriorly and posteriorly with the other hand. The amount of translation is noted and graded with the same modified Hawkins scale of translation. Alternatively, the patient can be placed in the supine position and the arm brought into 20 degrees of abduction and 20 degrees of forward flexion so that it is centered on the glenoid fossa. The examiner then applies an anterior and posterior force to assess the translation of the humeral head relative to the glenoid room ( Fig. 15.6 ). The affected shoulder should always be compared to the contralateral shoulder to establish the pathologic translation relative to the patient’s physiologic laxity.
The jerk test is used to evaluate for posterior instability. With the patient in the sitting position, the examiner stabilizes the patient’s scapula with one hand while the arm is abducted to 90 degrees and internally rotated to 90 degrees. The examiner’s other hand then axially loads the patient’s arm at the elbow and applies a horizontal adduction force. A positive test result is defined by the presence of a sharp pain with or without a clunk in the posterior aspect of the shoulder. ,
The Kim test is another test for posterior shoulder instability. It is performed with the patient in the sitting position and the arm in 90 degrees of abduction. The examiner holds the patient’s elbow and lateral upper arm and applies an axial force and 45 degrees of upward diagonal elevation, while also pushing inferiorly and posteriorly on the upper arm. Posterior shoulder pain with or without a clunk is considered a positive test result for posterior labral pathology , ( Fig. 15.7 ).
When evaluating for inferior instability or MDI, it is important to evaluate for a sulcus sign. With the patient in the upright position and the arm relaxed by the side, downward traction is applied to the patient’s upper arm ( Fig. 15.8 ). A positive sulcus sign is when this reproduces pain or instability and a depression is created between the lateral edge of the acromion and the humeral head. The depression can be measured and reported in centimeters. Type I is less than 1 cm of depression, type II is 1–2 cm of depression, and type III is greater than 2 cm of depression. Additionally, a positive sulcus sign noted with the arm at 30 degrees of external rotation can suggest pathologic laxity of the rotator interval ( Fig. 15.9 ).
An additional test evaluating glenohumeral laxity is the Gagey test. The examiner’s forearm pushes down firmly on the shoulder girdle. The examiner then lifts the relaxed upper limb of the patient with the elbow flexed to 90 degrees into abduction with his/her other hand. Excessive abduction (i.e., >105 degrees) or apprehension limiting passive abduction is a positive finding for laxity of the IGHL.
Initial workup of any patient presenting with shoulder instability should include obtaining plain radiographs (true anteroposterior, scapular Y, axillary lateral). Variations of the axillary lateral include the Velpeau view as well as the West Point view. The scapular Y view is more easily obtained in the setting of pain or acute trauma. It is obtained with the arm at the side and the patient seated upright or in the prone position and rotated 30–45 degrees toward the cassette. The projection that results is the Y-shaped intersection of the scapular body, coracoid process, and scapular spine, with the humeral head superimposed on top of the glenoid fossa. This view can also be used to visualize anterior or posterior shoulder dislocations.
Advanced imaging with magnetic resonance imaging (MRI) or MRA is often warranted to evaluate for labral or other soft tissue pathology. MRI also allows for the evaluation of bony impactions, fractures, and osseous edema, which may be present in the setting of shoulder instability ( Fig. 15.10 ). Computed tomography may be warranted to obtain more detailed evaluation of bony anatomy, especially in the setting of trauma or significant glenoid bone loss.