Multidirectional instability—epidemiology, history, physical examination, imaging, and nonoperative treatment

Epidemiology

Multidirectional instability (MDI) of the shoulder was first described by Neer and Foster in 1980 in their description of the inferior capsular shift procedure on 36 patients. Although the exact definition is debated among authors, MDI is most commonly defined as glenohumeral instability in more than one direction with minimal or no causative trauma. ^, Patients with MDI can be categorized in a variety of ways; however, the distinguishing feature of MDI that differentiates it from other types of instability is that it must include inferior instability. Success with surgical management of MDI therefore requires inferior capsular redundancy to be addressed. Thomas and Matsen addressed this by dividing patients with recurrent shoulder instability based on etiology into one of two categories: the first group had atraumatic MDI that was often bilateral and responded well to rehabilitation but required an inferior capsular shift for surgical management if nonoperative management failed (a description shortened to the acronym “AMBRI”). The second group had traumatic unidirectional instability resulting in a Bankart lesion requiring s urgical repair (a description shortened to the acronym “TUBS”). Many others have contributed towards characterizing MDI, such as Schenk and Brems, who described MDI as symptomatic global laxity of the shoulder with increased capsular volume and rotator interval (RI) deficiency. Although there is no universally accepted criteria for diagnosing MDI, it is important to recognize that surgical procedures used to treat traumatic anterior instability may not fully address the pathology involved in MDI. ^,

The incidence of anterior shoulder dislocations is approximately 57 dislocations per 100,000 people per year, with subluxations being five times more common. Surprisingly, the incidence of MDI is largely unknown. ^, ^, MDI typically affects patients between ages 12 to 35 years, and although females are more likely to have generalized ligamentous laxity, there do not appear to be any differences in the prevalence of MDI between the sexes. ^, ^, In several recent meta-analyses, most patients with MDI were noted to have a positive sulcus sign and a load-and-shift test. However, the literature reviewed in these meta-analyses was comprised largely of retrospective case series, highlighting the paucity of literature regarding MDI epidemiology.

The pathophysiology underlying MDI is multifactorial; however, the most common etiology is thought to be repetitive microtrauma on a congenitally lax and redundant glenohumeral joint capsule. ^, In particular, hyperlaxity of the RI and inferior capsuloligamentous complex are implicated in the development of MDI. Of note, patients with hyperlaxity or redundant joint capsule must be symptomatic to be considered to have MDI due to the fact that many patients with lax shoulders and accompanying physical exam findings such as a sulcus sign remain asymptomatic. ^, Most patients with generalized joint hyperlaxity do not have a named connective tissue disorder, but it is still important to identify patients with connective tissue disorders such as Ehlers-Danlos syndrome and Marfan syndrome because these patients tend to have worse surgical outcomes after surgical stabilization procedures. Although joint laxity is the most commonly agreed upon etiology of MDI, clearly there are other factors that influence the pathology of MDI given that generalized joint laxity is only noted in 40% to 70% of patients with MDI. Abnormal shoulder and scapular kinematics due to disruption of the dynamic stabilizers of the glenohumeral joint and scapulothoracic joint are also factors contributing to the development of MDI. ^, Rotator cuff weakness or periscapular muscle weakness can lead to muscular imbalance, altered kinematics of the shoulder girdle, and loss of the normal concavity-compression mechanism that stabilizes the humeral head within the glenohumeral joint leading to shoulder instability. ^, ^, Glenoid or labral hypoplasia or excessive glenoid retroversion can also lead to decreased static stability of the glenohumeral joint predisposing to MDI. ^, ^, ^, Lastly, patients with MDI may have acquired joint hyperlaxity. Repetitive microtrauma from athletic activities such as swimming, overhead throwing sports, gymnastics, rowing, and weight-lifting can stretch the normal capsuloligamentous structures of the glenohumeral joint over time. ^, These patients will present similarly to their congenitally lax counterparts, but they typically do not have generalized joint hyperlaxity, and the opposite shoulder is usually unaffected. ^, The key features of MDI are summarized in Box 37.1 .

BOX 37.1

Features of Multidirectional Instability of the Shoulder

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Instability in more than one direction (must include inferior direction)
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Affects younger patients, age 12–35 years
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Atraumatic (often bilateral) or attritional capsular laxity
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Subluxations far more common than dislocations
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Can be associated with global ligamentous laxity or connective tissue disorders
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Associated with rotator cuff weakness, abnormal scapular positioning, periscapular weakness, and glenolabral hypoplasia or excessive glenoid retroversion

History

The key to diagnosing and treating all shoulder pathology is by first obtaining a thorough history and physical exam. This is no different for patients with suspected MDI. The evaluating physician should begin by asking the patient to characterize the nature of the symptoms; in particular, the patient should be encouraged to identify whether he or she is experiencing pain, weakness, numbness/tingling, apprehension or feelings of instability with certain positions, and limitations in range of motion. In contrast to traumatic anterior shoulder instability, the clinical presentation of MDI is typically insidious, without a major traumatic or inciting event. ^, MDI typically affects younger patients aged 12 to 35 years, and they usually have symptoms of subluxation without a discrete inciting injury. ^, ^, These symptoms are often vague and difficult to assess, but patients will commonly complain of aching pain, discomfort, subjective feelings of instability, recurrent glenohumeral joint subluxations or dislocations, clicking or catching, apprehension in certain shoulder positions, traction paresthesias (particularly while the patient is holding a heavy object that pulls the arm inferiorly), or transient neurological deficits (“dead arm”). ^, ^, Any patients complaining of paresthesias or dysesthesias should also be evaluated for cervical radiculopathy and thoracic outlet syndrome, as these conditions can coexist with MDI or be the underlying pathology in an otherwise asymptomatic patient with global hyperlaxity. ^, ^,

Once the patient’s symptoms have been elucidated, they should be further characterized by determining the duration of symptoms, age of onset, presence of recent activity or lifestyle changes, whether the symptoms are unilateral or bilateral, existence of any prior shoulder trauma or injury, and whether the patient’s symptomatic shoulder has previously been treated (operatively or nonoperatively). Additionally, the treating physician should inquire about a family history of shoulder instability and connective tissue disorders (e.g., Ehlers-Danlos syndrome and Marfan syndrome) as well as the patient’s level of athletic activity. It is also important to identify patients with voluntary instability, in which they are able to subluxate or dislocate the glenohumeral joint at will, because treatment for these patients differs. ^, Patients with shoulder instability due to generalized hyperlaxity may have no inciting injury; however, it is important to recognize that sustaining a traumatic event does not preclude a diagnosis of MDI. ^, A discrete traumatic event such as with a labral tear in an overhead throwing athlete or with sustained repetitive microtrauma in a hyperlax shoulder may result in symptomatic unilateral or MDI. ^, ^,

Patients with unidirectional traumatic instability often only experience symptoms when the arm is held in certain positions at the limits of glenohumeral joint excursion. In contrast, patients with MDI may endorse symptoms with normal activities of daily living and with the arm in the mid-ranges of glenohumeral motion. Nevertheless, it is still important to ask about positions of the arm that exacerbate the symptoms. Reproduction of symptoms or pain with overhead motion of the arm in an abducted and externally rotated position, such as with the early acceleration phase of throwing, suggests anterior instability or posterosuperior internal impingement. ^, Pain with activities where the arm is forward flexed, adducted, and internally rotated, such as with pushing open a door, suggests posterior instability or anterosuperior internal impingement. ^, ^, With inferior instability, patients may complain of pain or traction paresthesias while carrying a heavy bag or any object that exerts inferiorly directed traction on the arm. ^, Lastly, it is important to evaluate for other causes of shoulder pain in patients with suspected MDI given that many patients with generalized hyperlaxity remain asymptomatic, as mentioned above.

Physical examination

The physical exam of the shoulder for patients with suspected MDI should be thorough and systematic to ensure accurate diagnosis as well as to address any concomitant pathology. The key components to the physical exam include inspection, palpation, range of motion, strength testing (including examination of the rotator cuff musculature), neurovascular examination, and special tests for laxity and instability. It is critical to identify which physical exam maneuvers re-create the patient’s symptoms or feelings of instability. Of all the physical exam techniques used to assess shoulder instability, the sulcus sign, load-and-shift test, and hyperabduction test are the most sensitive maneuvers for detecting MDI. ^, ^, ^,

Begin with inspection. Both of the patient’s shoulders should be adequately exposed to be able to visualize the entire shoulder girdle as well as to compare the symptomatic to the asymptomatic shoulder. The shoulders should be inspected for skin changes or scarring in skin folds, external signs of trauma, posture, muscle atrophy, scapular position, and symmetry. Ligamentous laxity should also be evaluated at this stage with the use of the Beighton scale, for which a score of 4 points or more out of a possible 9 points is representative of generalized ligamentous laxity ( Table 37.1 ). Patients with hyperlaxity according to this scale should be further evaluated for connective tissue disorders such as Ehlers-Danlos syndrome or Marfan syndrome because these patients tend to have worse outcomes with surgical management. ^,

TABLE 37.1

Beighton Scale for Diagnosis of Generalized Ligamentous Laxity

Passive dorsiflexion of the small finger beyond 90 degrees	1 point per hand, 2 total
Passive thumb apposition to the ipsilateral forearm	1 point per hand, 2 total
Active elbow hyperextension past 10 degrees	1 point per elbow, 2 total
Active knee hyperextension past 10 degrees	1 point per knee, 2 total
Flexion of trunk with knees fully extended and palms resting on the floor	1 point

A score of ≥4 points out of a possible 9 points is representative of generalized ligamentous laxity.

After inspection, several anatomic landmarks should be palpated to check for pain or reproduction of symptoms. Start by palpating the anterior and posterior glenohumeral joint line. Tenderness along the joint line is nonspecific but can be seen with capsular stretching, labral injuries, and rotator cuff pathology. In isolation, tenderness about the joint line can sometimes be used to differentiate anterior versus posterior instability. ^, Often patients with MDI will have scapular dyskinesis, which may cause tenderness to palpation about the medial angle of the scapula. ^, ^, Palpation of the acromioclavicular joint, supraspinatus fossa, and biceps tendon in the bicipital groove should also be performed on all patients to evaluate for other concurrent pathology.

Next, range of motion of both shoulders should be tested and compared. First examine forward elevation in both the frontal and scapular planes, which can be done by asking the patient to lift the arms straight forward and then again at 30 degrees from the frontal plane in line with the scapular body. Abduction is then assessed by asking the patient to lift the arms out to the side as high as they will go. Hyperabduction past 180 degrees is characteristic of MDI. The scapula is then stabilized by the examiner, and the arm is once again abducted. A difference of greater than 20 degrees of abduction between shoulders during this maneuver is considered a positive hyperabduction test described by Gagey and Gagey, which is a sign of inferior laxity and instability. Internal and external rotation with the arm at the side and at 90 degrees of abduction should then be assessed. Internal rotation can be documented as the highest vertebral level the patient can reach behind his or her back. External rotation greater than 90 degrees can be a sign of both MDI as well as anterior hyperlaxity due to a lax anterior capsule. If there is asymmetry in any of the above-mentioned active motion arcs, the examiner should check for symmetrical passive range of motion. At this point, the examiner should assess for scapular dyskinesis and abnormal scapular motion, which can be done by asking the patient to forward elevate the arms while observing from behind. The patient should then be asked to perform a standing pushup against the wall while the examiner observes from behind to note any signs of abnormal scapular motion or scapular winging. Any asymmetry in scapular motion should be noted ( Fig. 37.1 ). Winging of the inferior medial tip of the scapula (type I dyskinesis) or of the entire medial border of the scapula (type II dyskinesis) is frequently associated with patients with voluntary instability in MDI. ^, ^,

Strength testing should be performed next. Begin by assessing deltoid strength by testing shoulder abduction against resistance. The rotator cuff is then assessed with well-established tests for each particular part of the cuff. Jobe’s test or the “full can” test are used to test supraspinatus strength. Infraspinatus strength can be tested by examining external rotation strength (starting with the patient’s elbows in 90 degrees of flexion and arm at 45 degrees internal rotation) as well as by examining for any external rotation lag. A positive hornblower’s sign is indicative of teres minor weakness, but if this test is positive it is usually indicative of a more extensive posterior rotator cuff tear. Belly press, lift-off, and bear hug tests are all internal rotation tests used to gauge subscapularis strength, and any internal rotation lag should be noted. Although most patients with MDI do not have any underlying rotator cuff tears, it is still important to perform a full rotator cuff strength exam because rotator cuff weakness can lead to altered kinematics of the shoulder girdle leading to shoulder instability. ^, ^, The patient’s strength testing should be documented using the standard 5-point grading system for muscle strength. Next, a full neurovascular exam should be performed. This is particularly important for patients with suspected MDI who are complaining of sensory abnormalities, such as with traction paresthesias or transient neurologic deficits, because these may often be signs of other underlying pathology such as cervical radiculopathy or thoracic outlet syndrome. ^, ^, The Adson test can be used to evaluate for thoracic outlet syndrome: the patient slightly abducts and externally rotates the shoulder with the elbow extended while the examiner palpates the radial pulse. The patient is then asked to maximally extend and laterally rotate the neck toward the affected side while inhaling and holding the breath; a diminished radial pulse noted during this maneuver denotes a positive result.

The final part of the physical exam in patients with suspected MDI includes assessment of laxity and instability. It is important to reiterate that shoulder laxity itself is not necessarily diagnostic of MDI, as there is a wide spectrum of asymptomatic laxity. ^, If the patient’s symptoms are reproduced with laxity or instability testing, clinical suspicion for MDI should rise. Begin by assessing inferior laxity with the sulcus test, which is performed by putting downward traction on the patient’s arm in a seated upright position with the arm in neutral rotation. ^, ^, If there is sufficient inferior laxity, the humeral head will migrate inferiorly relative to the glenoid, creating the characteristic sulcus sign, which appears clinically as an indentation or sulcus inferior to the acromion ( Fig. 37.2 ). The sulcus sign is graded as 1+ for an acromiohumeral distance of less than 1 cm, 2+ for 1 to 2 cm, and 3+ for greater than 2 cm. A sulcus sign is pathognomonic for inferior instability when pain, paresthesias, or apprehension occurs during sulcus testing. ^, Persistence of the sulcus sign with the arm in external rotation suggests a deficient RI capsule.