eMedicine Specialties > Radiology > Musculoskeletal

Shoulder, Dislocations

Author: Gavin Yeh Tseng, MBBS, FRCR, FAMS, Consultant Radiologist, Department of Diagnostic Radiology, Raffles Hospital
Coauthor(s): Wilfred CG Peh, MD, MBBS, FRCP(Glasg), FRCP(Edin), FRCR, Clinical Professor, Faculty of Medicine, National University of Singapore; Senior Consultant Radiologist, Alexandra Hospital, Singapore
Contributor Information and Disclosures

Updated: Jul 31, 2009

Introduction

Background

The shoulder joint is inherently unstable, and injury to the shoulder is commonly encountered. Shoulder dislocations can be classified either as an acute single event or as recurrent episodes (glenohumeral instability).^1,2,3,4,5,6

Glenohumeral instability can be classified further by underlying causes: atraumatic (congenital laxity), macrotraumatic (resulting from an event), and microtraumatic (repetitive injury). They can also be categorized by the various degrees of subluxation or dislocation.

Other subdivisions include direction and voluntary/involuntary mechanisms. A first-time acute shoulder dislocation may also represent the initial presentation of recurrent dislocation.

Patients can be classified into the following 2 clinical categories, which represent the 2 ends of a broad spectrum:

• Patients with traumatic unidirectional instability with a Bankart lesion who require surgical intervention (TUBS)
• Patients with no history of trauma (atraumatic type) who have multidirectional and bilateral instability and who either undergo rehabilitative therapy or are treated surgically with inferior capsular imbrication (AMBRI).

Traumatic anterior glenohumeral instability accounts for 95% of glenohumeral instability seen in clinical practice.

Recent studies

Ng et al investigated the usefulness of MRI in assessing capsular laxity in 64 patients with recurrent shoulder dislocation who underwent MR arthrography between October 2002 and May 2008. The patients were divided into three groups: group A, no shoulder dislocation; group B, first dislocation; and group C, recurrent dislocation. The sensitivity and specificity to detect capsular laxity were 92% and 100%, respectively, for clinical tests and 85% and 96% for MRI. The authors concluded from findings that MRI is a useful and objective method to assess capsular laxity in patients with recurrent shoulder dislocation.¹

Saupe et al retrospectively evaluated lesions of osseous and soft-tissue structures of the glenohumeral joint on magnetic resonance (MR) images after first-time traumatic posterior shoulder dislocation in 36 male patients. MR arthrography was performed in 18 patients and conventional shoulder MR imaging in 18 patients. Hill-Sachs lesions, rotator cuff tears, biceps tendon abnormalities, posterior labrocapsular complex lesions, humeral head translation, and osseous glenoid version angle were evaluated.³

In the Saupe study, the MR appearance of traumatic posterior shoulder dislocation was characterized by reverse Hill-Sachs lesions in 86% of patients and posterocaudal labrocapsular lesions in nearly 60% of patients. Full-thickness rotator cuff tears were seen in approximately 20% of patients. In 31 of the 36 patients, a reverse Hill-Sachs lesion was found; 11 had a reverse osseous Bankart lesion; 12 full-thickness rotator cuff tears were seen in 7 patients; 6 patients had biceps tendon abnormalities; 21 had posterior labrocapsular complex tears; and 27 had a retroverted scaphoglenoid angle.³

Magee et al reported on the diagnostic sensitivity of 3-T conventional MRI versus MR arthrography of the shoulder in 150 consecutive conventional shoulder MRI and MR arthrography examinations on patients 50 years or younger who subsequently underwent arthroscopy. All patients selected for arthroscopy had abnormal findings on clinical examination and MRI or MR arthrography. Three full-thickness and 9 partial-thickness supraspinatus tendon tears, 7 superior labral anterior-to-posterior (SLAP) tears, 6 anterior labral tears, and 2 posterior labral tears were seen on MR arthrography but not on conventional MRI. In the Magee study, MR arthrography showed statistically significant increased sensitivity for detection of partial-thickness articular surface supraspinatus tears, anterior labral tears, and SLAP tears compared with conventional MRI at 3-T.⁵

For excellent patient education resources, visit eMedicine's Breaks, Fractures, and Dislocations Center. Also, see eMedicine's patient education article Shoulder Dislocation.

Pathophysiology

Stability of the glenohumeral joint is maintained by passive and active mechanisms.

• Passive stabilizing structures
  • Size, shape, and tilt of the glenoid fossa
  • Vacuum effect (negative pressure adhesion and cohesion of 2 articular surfaces)
  • Ligamentous structures (ie, superior glenohumeral ligament [SGHL], middle glenohumeral ligament [MGHL], inferior glenohumeral ligament [IGHL], coracohumeral ligament)
  • Capsular structures (anterior and posterior)
  • Glenoid labrum (cartilaginous and fibrocartilaginous glenoid labrum)
  • Osseous (acromion, coracoid process)
• Active stabilizing structures
  • Rotator cuff muscles
  • Long head of biceps tendon

The vacuum effect may be disrupted by avulsion, rupture, or stretching by injury. The labrum accounts for 50% of the socket area of the joint; therefore, loss of labrum can reduce the socket depth substantially.

Repairing labral detachments, such as Bankart lesions, is important because labrectomy in the mildly unstable glenohumeral joint may lead to progressive symptomatic deterioration and instability. Dislocation often occurs because of these anatomic characteristics and because of the frequency of injuries to the shoulder region.

The 3 glenohumeral ligaments (GHLs) represent condensations of the shoulder capsule and are variable in appearance on imaging. The ligaments act to strengthen the joint capsule and course from the humerus to their glenoid attachments. The capsule and its insertions are depicted well on MRIs. The posterior capsule always inserts into the posterior labrum. The anterior capsule has a more variable medial insertion. The 3 types of anterior capsule insertion have been described as follows:

• Type 1 - Insertion into the labrum
• Type 2 - Insertion into the glenoid neck adjacent to the labrum
• Type 3 - Insertion into the medial scapula remote from the labrum

Most capsules are inserted anteriorly on the labrum (47%) or glenoid neck (49%). In the past, anterior capsular insertions of the shoulder were regarded as being contributory toward shoulder instability, but studies have shown that the different types of capsular insertions are statistically similar in both stable and unstable shoulders. Furthermore, capsular abnormalities have little role in influencing surgical treatment.

Anterior dislocation

Anterior dislocations are usually the result of direct or indirect trauma, with the arm forced into abduction and external rotation. This is by far the most frequent type of shoulder dislocation and represents more than 90% of injuries. Of single acute dislocations, 40% become recurrent as a result of associated damage of the surrounding ligamentous and capsular structures that stabilize the joint.

The most important structure stabilizing the shoulder—one that limits gross anterior-inferior subluxations and dislocation—is the IGHL. This ligament forms a sling with discrete anterior and posterior bands. It is lax when the humerus is in the neutral position, and it allows normal shoulder movement. The ligamentous complex becomes taut in abduction and external rotation and, thus, stabilizes the joint at the end range of shoulder movement in the abduction external rotation (ABER) direction.

During a dislocation, forces exceed the threshold that the ligamentous complex can bear, leading to tears or stretching. This may lead to laxity and instability. Failure of the IGHL can occur at the insertion site (40%), in the ligamentous substance (35%), and at the humeral insertion site (25%). Avulsions are seen more frequently in the anterior band and the anterior aspect of the axillary pouch, whereas ligamentous substance tears are more common in the posterior aspect of the axillary pouch.

Bankart lesions represent failure of the IGHL at the glenoid insertion. IGHL capsule laxity represents intrasubstance ligamentous failure, whereas humeral avulsions of the GHL (HAGL) represent failure of the IGHL at its humeral insertion. The MGHL is often absent or underdeveloped and plays a minor role. Tears of the GHL can occur without associated labral tears and also can cause shoulder instability.

The following lesions may be seen in anterior dislocation:

• Labral lesions (ie, Bankart lesions)
• Bony glenoid lesions - Osseous anterior glenoid rim fractures (44%), bony Bankart lesions, fracture of the greater tuberosity
• Ligamentous lesions - Anterior band of the IGHL
• Capsular abnormalities - Separation of the capsule from the anterior glenoid rim (85%)
• Hill-Sachs lesions (77%)
• Intra-articular loose body
• Rotator cuff lesions - Supraspinatus or subscapularis tears

A subset of these lesions initially were defined using arthroscopy (ie, anterior labroligamentous periosteal sleeve avulsion [ALPSA] lesion, Perthes lesion, glenoid labral articular disruption [GLAD], HAGL lesion) but are also well demonstrated by magnetic resonance (MR) arthrography.

Regarding labroligamentous lesions, in 1938, Bankart identified an "avulsed fibrous and fibrocartilaginous fragment" from the glenoid margin as the essential lesion of recurrent shoulder dislocation and described a highly successful surgical procedure. The avulsion of the labroligamentous complex from the anteroinferior aspect of the glenoid is termed a Bankart lesion (or a bony Bankart lesion if it is accompanied by a fracture).

A Bankart lesion is the most common lesion in anterior instability. The tear is usually large enough to involve not only the labrum, where the anterior band of the IGHL inserts, but also the middle labrum and, sometimes, the superoanterior labrum. Tears of the anteroinferior labrum are the most common subtype. The second most common subtype involves tears of the entire anterior labrum.

The following classification system was devised to aid in surgical planning:

• Type 1 - Represents a partial Bankart lesion in which there is a small detachment of the capsulolabral complex but no stripping of the capsule from the glenoid labrum
• Type 2 - Moderate detachment of the labrum and the capsule from the glenoid, with preservation of the labral shape.
• Type 3 - Severe detachment of the capsulolabral complex, with attenuation of the glenoid labrum
• Type 4 - Includes a fracture of the glenoid margins

Computed tomography (CT) arthrography and magnetic resonance imaging (MRI) can help in classifying the lesions for surgical planning.

In a Bankart lesion, the scapular periosteum ruptures as the labroligamentous ligaments are avulsed from the glenoid. In Bankart variants, the scapular periosteum remains intact relative to the labroligamentous complex. If the labroligamentous complex is displaced medially and shifted inferiorly, rolling up on itself, the lesion is called an ALPSA lesion. An ALPSA lesion is associated with more severe injury.

The diagnosis of this lesion in the acute setting is usually a straightforward procedure employing arthrographic findings. In chronic cases, healing and resynovialization occurs. Therefore, the ALPSA lesion may be difficult for surgeons to visualize during arthroscopy.

Avulsion of the inferior glenohumeral joint from its attachment at the anatomic neck of the humerus is known as an HAGL lesion. It is often associated with tears of the subscapularis tendon and results from a shoulder dislocation. A bony humeral avulsion of the glenohumeral ligament (BHAGL) can be similar to an HAGL lesion, but it also involves the bone. This lesion occurs significantly less frequently than classic Bankart lesions. HAGL lesions are seen in approximately 9% of anterior shoulder instabilities. HAGL lesions are treated with surgical reattachment of the GHL to its humeral avulsion site.

The IGHL complex may also tear at its midportion. The IGHL complex should be examined along its entire course, from its humeral origin to its labral insertion, since defects have been found at the humeral origin and within the substance of the ligament. If the labroligamentous avulsion occurs with an intact scapular periosteum and if the periosteum is stripped medially, becoming redundant, the lesion is called a Perthes lesion.

A Perthes lesion is distinguished from an ALPSA lesion via the redundant periosteum versus the rolled-up, medially displaced periosteal labroligamentous mass. The avulsed labrum resumes a normal position at the glenoid margin, in which partial healing takes place.

A GLAD lesion is a tear of the anteroinferior labrum (nondisplaced) with avulsion of the adjacent glenoid cartilage. A glenoid chondral defect is therefore visualized. The labrum is not detached, and there is no capsular stripping. This lesion is clinically stable. The mechanism is glenohumeral impaction in the ABER position. Clinically, these patients complain of pain rather than instability. The lesion can be treated with arthroscopic debridement without need for a stabilization procedure.

A Bennett lesion is an extra-articular, posterior, capsular avulsive injury associated with a posterior labral injury and posterior undersurface rotator cuff damage. This injury is seen most commonly in baseball pitchers. The diagnosis of this lesion should raise suspicion for associated labral and rotator cuff abnormalities.

The mechanism is from traction of the posterior band of the IGHL during the decelerating phase of pitching. Clinically, the throwing athlete presents with posterior shoulder pain during pitching, with posterior point tenderness. If left untreated, patients progress from functional to anatomic instability.

Regarding osseous lesions, infractions or fractures of the glenoid rim (osseous Bankart lesions) are diagnostic of anterior instability when they are demonstrated on radiographs. When these lesions are detected, no other imaging is needed. However, not all anterior instabilities or recurrent dislocations are associated with an osseous Bankart lesion.

The sole presence of a Hill-Sachs lesion is pathognomonic of anterior instability and is seen in 50% of patients. The need to detect the lesion is reduced if other pathognomonic findings exist (ie, anterior instability found on physical examination or Bankart lesion). The Hill-Sachs lesion describes a characteristic defect of the posterolateral surface of the humeral head and represents a compression fracture.

The resultant lesion is influenced by the patient's age at dislocation and the length of time since the initial dislocation. This is demonstrated as follows:

• Young patients (young adults in their teens and 20s)
  • Anteroinferior labroligamentous complex avulsion (Bankart lesion)
  • ALPSA lesion
  • Perthes lesion
  • HAGL lesion
  • Bennett lesion
  • Surgical reconstruction
• Older patients
  • Paucity of Bankart lesions
  • Supraspinatus tendon (30% of tears in older patients)
  • Fracture of the greater tuberosity (one third of older patients)
  • Avulsion of the subscapularis and capsule from the lesser tuberosity (one third of older patients)
  • HAGL lesion
  • Conventional imaging (no need for arthrography)
  • Treatment usually conservative

Intra-articular loose bodies are not uncommon and require surgical intervention. Most loose bodies are composed of bone, cartilage, or both. They may also consist of fibrous tissue, fibrin, fat, or blood.

Posterior dislocation

Posterior dislocations and posterior glenohumeral joint instabilities are rare (approximately 2-4%). They may result from a fall on an outstretched hand, direct trauma to the shoulder, or violent muscle contractions from electric shocks or seizures. As a result of acute dislocation, instability may occur, and surgical correction of the underlying damage may be indicated.

Posterior instability caused by repeated microtrauma without frank dislocations may result in persistent shoulder pain in young athletes. Abduction, flexion, and internal rotation (eg, swimming, throwing, punching) are the mechanisms involved in these cases. When injuries occur in this position, the capsulolabral structures are taut, and the patterns of injury that occur most often are the reverse of those of anterior dislocation.

Most patients with atraumatic posterior instability can be treated with conservative measures involving strengthening of the posterior stabilizing muscles. If this fails, surgery may be required.

A fall on an outstretched hand with the arm in abduction is another mechanism of posterior dislocation. Posterior instability may also occur as an operative complication in patients with multidirectional instability after a misdirected anterior capsular procedure. An acute posterior dislocation may remain unrecognized in 50% of patients and subsequently may present as a frozen shoulder.

The posterior band of the IGHL complex is primarily responsible for capsuloligamentous restraint to a posterior translation of 90° of abduction. The anterosuperior capsule, or rotator interval capsule, also has been shown to be important in limiting the posterior and inferior translation.

Imaging findings of posterior dislocation, which typically are the reverse of those found in anterior instability, are as follows:

• Labral lesions
• Posterior labral tear or detachment (reverse Bankart lesion)
• Ligamentous – Tear of the posterior band of the IGHL
• Capsule - Capsule tear or laxity
• Osseous lesions - Fractures, erosions, sclerosis, or ectopic calcification of the posterior glenoid rim; reverse Hill-Sachs lesion (impacted fracture of the anterior aspect of the humeral head)
• Abnormal glenoid, including hypoplasia, and excessive retroversion
• Teres minor lesions, including partial tear and edema in the tendon or muscle
• Increased retroversion of humerus
• Intra-articular loose bodies

The most common finding is a tear or shredding of the posterior labrum. The most likely explanation for a high incidence of teres minor tears is that the underbelly of the teres minor muscle is inseparable from the posterior capsule of the GHL.

Multidirectional instability

Imaging is usually not necessary because multidirectional instability is mostly a diagnosis of exclusion. MRI is used principally to exclude conventional causes of instability (ie, Bankart lesion).

The patient is usually a young female with bilateral joint laxity. No visible labroligamentous lesions are seen in patients with true multidirectional instability. The capsular mechanism is redundant, and the labrum is often hypoplastic. Degenerative changes of the glenohumeral joint in association with labral degeneration or tearing may be seen.

Physical examination with traction applied on a patient's abducted arm causes inferior subluxation of the humeral head. This results in a visible sulcus (sulcus sign) between the prominence of the acromion and the inferiorly subluxed humeral head.

Luxatio erecta

Luxatio erecta (or inferior dislocation) is uncommon. This dislocation usually occurs when a direct axial force is applied to a fully abducted arm or when a hyperabduction force leads to leverage of the humeral head across the acromion, resulting in inferior dislocation of the humerus. In luxatio erecta, the inferior capsule almost always is torn.

Associated bony injuries include fractures of the greater tuberosity, acromion, clavicle, coracoid process, and glenoid rim. Brachial plexus and axillary artery injuries are possible serious complications. Long-term complications include adhesive capsulitis and recurrent subluxations or dislocations.

Frequency

International

Incidence of traumatic dislocations is as follows:

• Anterior dislocation (96%)
• Posterior dislocation (2-4%)
• Inferior dislocation/luxatio erecta (1-2%)
• Superior dislocation (<1%)

Posterior instability is most commonly bidirectional (posterior and inferior or multidirectional).

Mortality/Morbidity

Shoulder instability has a significant impact on young patients, especially on professional athletes. Evidence suggests that primary repair at the time of injury prevents recurrence.

Age

Initial dislocations have been recognized as occurring at 2 age peaks: at 10-30 years and at 50-70 years.

Anatomy

The shoulder joint is a synovial joint of the ball-and-socket variety. The bones involved are the hemispherical head of the humerus and the shallow glenoid cavity of the scapula. A 4:1 disproportion exists in the articulating surface areas of the humeral head and the glenoid cavity. This arrangement permits considerable movement.

The joint is protected above by an arch formed by the coracoid process, the acromion, and the coracoacromial ligament. The articular cartilage covering the head of the humerus is thicker at the center than at the periphery. The reverse is true in the case of the articular cartilage of the glenoid cavity, with thicker articular cartilage at the periphery than at the center. The soft-tissue supporting structures of the shoulder include the following:

• Articular capsule
  • The articular capsule completely encircles the joint.
  • The articular capsule is attached above to the circumference of the glenoid cavity beyond the glenoid labrum and supraglenoid tubercle and attached below to the anatomic neck of the humerus (except inferiorly, where its attachment to the surgical neck of the humerus is a finger's breadth below the articular margins).
  • The articular capsule is thick above and below and is considerably loose and lax; therefore, it allows the bones to be separated from each other by more than 2.5 cm, an evident provision for that extreme freedom of movement peculiar to this articulation.
  • The articular capsule is strengthened above by the supraspinatus muscle, below by the long head of the triceps brachii, behind by the tendons of the infraspinatus and teres minor, and in front by the tendon of the subscapularis.
  • Usually, the capsule has 3 openings.
    
    
    • The first is located anteriorly, below the coracoid process, and establishes a communication between the joint and a bursa adjacent to the subscapularis tendon.
    • The second, which is not constant, is at the posterior part, where an opening sometimes exists between the joint and a bursal sac under the tendon of the infraspinatus.
    • The third is between the tubercles of the humerus for the passage of the long head of the tendon of the biceps brachii.
• Coracohumeral ligament
  • This ligament is a broad band that strengthens the upper part of the capsule.
  • The coracohumeral ligament arises from the lateral margin of the coracoid process and passes obliquely downward and laterally to the front of the greater tubercle of the humerus, blending with the tendon of the supraspinatus.
  • The coracohumeral ligament is united intimately with the capsule posteroinferiorly.
  • The anterior and superior borders of the ligament present a free edge, which overlaps the capsule.
  • The coracohumeral ligament restrains external rotation of the humerus and, in conjunction with the supraspinatus muscle, supports the dependent arm and acts as a support for the long head of the biceps tendon.
  • The coracohumeral ligament strengthens the upper part of the capsule.
• GHLs
  • In addition to the coracohumeral ligament, 3 supplemental bands strengthen the capsule; these are called the GHLs. In addition to these, the capsule is also strengthened in front by 2 bands derived from the tendons of the pectoralis major and teres major.
  • The SGHL extends from the superior glenoid margin, just anterior to the origin of the long head of biceps tendon, to the lesser tubercle, where it blends with the coracohumeral ligament. The SGHL may originate with the biceps tendon, either alone or with the MGHL. Normally thin, the SGHL may also be thick, in which case the MGHL may be absent or underdeveloped.
  • The MGHL has the greatest variation of the 3 GHLs. It arises more commonly from the anterosuperior aspect of the labrum but may originate with the IGHL. The MGHL inserts into the humerus at the base of the lesser tubercle and may blend with the capsule before reaching the tubercle. The MGHL may be entirely absent in about 25% of patients.
  • The IGHL complex consists of an anterior band, a posterior band, and the axillary recess of the capsule between these bands. The anterior band extends from the anteroinferior aspect of the labrum to the surgical neck of the humerus. The posterior band extends from the posteroinferior aspect of the labrum to the surgical neck of the humerus. The anterior band usually is thicker than the posterior band. The insertion of the anterior and posterior bands into the neck of the humerus creates a jagged appearance on axial MRIs.
• Transverse humeral ligament
  • The transverse humeral ligament is a broad band of the capsule that passes from the lesser tubercle to the greater tubercle of the humerus.
  • It is always limited to that portion of the bone that lies above the epiphyseal line. The transverse humeral ligament converts the intertubercular groove into a canal and is a true extension of the subscapularis tendon.
• Glenoid labrum
  • The glenoid labrum is a fibrocartilaginous rim attached around the margin of the glenoid cavity.
  • The glenoid labrum is triangular on cross section; its base is fixed to the circumference of the cavity, while the free edge is thin and sharp.
  • The glenoid labrum is continuous above with the tendon of the long head of the biceps brachii, which gives off 2 fasciculi to blend with the fibrous tissue of the labrum.
  • It deepens the articular cavity and protects the edges of the bone.
  • Since the entire labrum is pliable, its shape depends on the position of the humeral head.
  • The glenoid labrum is often rounded anteriorly and triangular posteriorly. A cleft may exist, or the glenoid labrum may be crescentic, thin, or absent.
  • The anterosuperior labrum may be separated from the glenoid margin (anterosuperior sublabral foramen). Superiorly, the labrum and long head of biceps tendon are intertwined. A normal recess can be seen between the 2 structures.
  • The hyaline cartilage covering the glenoid fossa can extend under the labrum superiorly and anteriorly.

The synovial membrane is reflected from the margin of the glenoid cavity over the labrum and then reflected over the inner surface of the capsule, covering the lower part and sides of the anatomic neck of the humerus as far as the articular cartilage on the humeral head. The tendon of the long head of the biceps brachii passes through the capsule and is enclosed in a tubular sheath of synovial membrane.

This membrane is reflected on the long head of the biceps brachii from the summit of the glenoid cavity and is continued around the tendon into the intertubercular groove distally as far as the surgical neck of the humerus. The tendon thus traverses the articulation, but it is not contained within the synovial cavity.

Bursae in the neighborhood of the shoulder joint include the following:

• A constant bursa is situated between the tendon of the subscapularis muscle and the capsule. It communicates with the synovial cavity through an opening in the front of the capsule. This recess demonstrates many variations that depend on the configuration of the GHLs. Absence of the MGHL is often associated with a large subscapular recess (subscapularis bursa).
• A large bursa exists between the deltoid muscle and the capsule, but it does not communicate with the joint. This bursa is prolonged under the acromion and coracoacromial ligament and intervenes between these structures and the capsule (subacromial bursa).

The muscles related to the joint include the following:

• Above - The supraspinatus
• Below - The long head of the triceps brachii
• In front - The subscapularis
• Behind - The infraspinatus and teres minor
• Within - The tendon of the long head of the biceps brachii

The deltoid covers the articulation in front, behind, and on the sides.

The arteries supplying the joint are articular branches of the anterior and posterior humeral circumflex and transverse scapular arteries. The nerves are derived from the axillary and suprascapular nerves.

Shoulder stability is maintained by articulator and periarticular tissues that collectively are termed the capsular mechanism. The combination of the glenoid labrum and the superior, middle, and inferior GHLs are called the labroligamentous (or capsulolabral) complex.

Presentation

Anterior dislocation

Typically, a patient with a single dislocation has a history of acute trauma. A patient may describe a feeling of the shoulder popping out. On physical examination, the dislocated shoulder demonstrates a characteristic prominent humeral head anteriorly and a hollow below the acromion.

Associated damage to the axillary nerve may occur, with loss of sensation on the lateral aspect of the shoulder, producing the sergeant's-stripe pattern of sensory loss. In the clinical definition of instability, the humeral head slips out of the glenoid socket during activity, causing symptoms. Varying degrees of instability are recognized, ranging from subluxations to dislocations.

Anterior instability

In posttraumatic instability, the patient usually reports a specific incident, and the shoulder has not returned to its normal position. In many posttraumatic types, a true dislocation has never occurred, and the symptoms are related to recurrent subluxations. The atraumatic type is common in people who play sports, especially those engaging in overhead activities, such as baseball pitchers, javelin throwers, swimmers, and tennis players. The anterior capsule becomes stretched, leading to development of instability.

The athlete may present with shoulder pain, an initial episode of subluxation, or episodes of dead-arm syndrome. In dead-arm syndrome, or pain-induced subjective paresis, a patient may experience sharp pain when the shoulder is placed in external rotation or when a force is delivered to the shoulder (as when throwing a hard ball). The patient may then lose control of the arm and even drop an object in hand. After the acute episode, the shoulder may remain sore.

The shoulder pain in chronic anterior instability is a result of impingement of rotator cuff tendons due to narrowing of the subacromial space during recurrent anterior translation of the humeral head. This is aggravated by eventual weakening of the rotator cuff muscles, which in turn may lead to failure of humeral depression.

Failure of humeral depression in this instance is probably due to imbalance between the deltoid muscle and humeral head stabilizers (rotator cuff muscles). Each deltoid contraction then moves the humeral head superiorly and narrows the space through which the supraspinatus tendon passes. The resultant impingement results in rotator cuff tendinitis.

Multidirectional instability

Multidirectional instability is usually of the atraumatic type and is often associated with generalized ligamentous laxity throughout the whole body. Multidirectional instability of the glenohumeral joint involves a combination of 2 or 3 instabilities — namely, anterior, posterior, and/or inferior instabilities.

Clinically, inferior instability is diagnosed by eliciting the sulcus sign. This is tested with inferior traction on the arm as it is held straight by the side. A positive test result is when the humeral head translates inferiorly so that a visible sulcus is seen between the acromion and the humeral head.

Treatment of multidirectional instability involves strengthening exercises of the shoulder stabilizers. Stretching the muscles around the shoulder joint should be avoided. If conservative treatment fails, surgical treatment can be attempted. However, surgical results in patients with generalized ligamentous laxity have been disappointing.

Posterior dislocation

Acute, traumatic posterior dislocation is far less common than anterior dislocation. Posterior dislocation results from direct trauma to the shoulder or a fall on an outstretched arm in some degree of internal rotation or adduction. Posterior dislocation may also be caused by an electric shock or epileptic seizure.

On examination, the patient's shoulder may reveal loss of the normal rounded appearance anteriorly. The shoulder is held in internal rotation and adduction. Limitation of external rotation is marked. Treatment consists of shoulder reduction by applying traction and forward pressure on the humerus.

The most common type of posterior instability seen in athletes is the atraumatic type that is part of a multidirectional instability. The shoulder may be voluntarily posteriorly subluxed.

Luxatio erecta

Luxatio erecta (inferior dislocation) is an uncommon condition. It usually occurs when a direct axial force is applied to a fully abducted arm or when a hyperabduction force leads to leverage of the humeral head across the acromion, resulting in inferior dislocation of the humerus.

The patient usually has the arm in a fixed elevated position over the head because the humeral head is immobilized below the inferior aspect of the glenoid. Treatment is reduction (usually under general anesthesia) and evaluation of associated injuries.

Preferred Examination

In general, the modality chosen depends on its availability and the treatment plan for a particular patient.^1,2,3,4,5,6

Radiography is inexpensive and is readily available. It should be performed as the initial imaging investigation in patients presenting with a clinical problem related to the shoulder. It complements the other advanced techniques and provides an overview of the bony components of the shoulder joint. In some patients, radiography obviates further imaging.

MR arthrography is the imaging modality of choice to evaluate the labrum. It has the highest sensitivity and specificity of all available modalities. However, it is invasive and may not be necessary in patients in whom surgery is not being considered as a treatment option.^{7,8,9,10,11,12,13,14,15,16,17,18}

Conventional MRI provides a good overview of shoulder lesions and anatomy, particularly the soft-tissue structures. However, it is less accurate than MR arthrography for depiction of small labroligamentous lesions associated with shoulder dislocation.^{7,19,20,11,21,22,23,24,25,26,15,27,16}

CT arthrography largely has been superseded by MR arthrography. CT arthrography usually is used when MRI is not available. It is useful in showing bony lesions and anterior and posterior labral and capsular lesions.^{7,8,9,21,28,29,26,30,15,31}

Limitations of Techniques

Radiography cannot demonstrate labral, ligamentous, or capsular lesions. MRI is more sensitive and specific and has superseded CT arthroscopy for demonstrating intra-articular and periarticular soft-tissue structures.

Although conventional MRI and MR arthrography are the modalities of choice, their use is limited by their cost and limited availability. In regions with fewer resources, CT arthrography is a good alternative.

Limitations of arthrography include discomfort to patients, risk of septic arthritis, and the need for contrast administration. Use of gadopentetate dimeglumine in intra-articular injections has not been approved by the Food and Drug Administration (FDA).

Differential Diagnoses

Shoulder, Glenoid Labrum Injury (MRI)
Shoulder, Rotator Cuff Injury (MRI)
Shoulder, Rotator Cuff Injury (Ultrasonography)

Other Problems to Be Considered

Impingement and biceps subluxation may occasionally accompany glenohumeral joint instability.

Rotator cuff disease, superior labral anteroposterior (SLAP) lesions, and glenoid labrum cyst have been associated with glenohumeral instability.

Rotator cuff tears are found in more than 25% of patients with shoulder dislocations.

Ganglion cysts often are associated with instability. In particular, a strong association exists with superior labrum tears; however, these cysts can arise from virtually all aspects of the glenohumeral joint.

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