Types of Glenohumeral Instability

Last updated: December 13, 2013

Glenohumeral instability

What is it?

Glenohumeral instability is the inability to maintain the humeral head centered in the glenoid fossa. (Matsen, Fu and Hawkins, 1993; Matsen, Lippitt, Sidles et al, 1994) Clinical cases of instability can be characterized according to the circumstances under which they occur, the degree of instability, and the direction of instability.

Circumstances of instability

Congenital instability may result from local anomalies, such as glenoid dysplasia (Wirth, Lyons and Rockwood, 1993) or systemic conditions such as Ehlers-Danlos syndrome. Instability is acute if seen within the first days after its onset; otherwise, it is chronic. A dislocation is locked (or fixed) if the humeral head has been impaled on the edge of the glenoid, making reduction of the dislocation difficult. If a glenohumeral joint has been unstable on multiple occasions, the instability is recurrent. Recurrent instability may consist of repeated glenohumeral dislocations, subluxations, or both.

Instability may arise from a traumatic episode in which an injury occurs to the bone, rotator cuff, labrum, capsule, and/or a combination of ligaments. Recurrent traumatic instability typically produces symptoms when the arm is placed in positions near that of the original injury. Conversely, instability may arise from the atraumatic decompensation of the stabilizing mechanisms. The degree to which the shoulder was "torn loose" as opposed to "born loose" or just "got loose" is critical in determining the best management strategy.

We have found that most patients with recurrent instability fall into one of two groups. On one hand, patients with a traumatic etiology usually have unidirectional instability; often have obvious pathology, such as a Bankart lesion; and often require surgery when instability is recurrent, thus the acronym: TUBS. On the other hand, patients with atraumatic instability often have multidirectional laxity; which is frequently bilateral; usually responding to a rehabilitation program. However, should surgery be performed, the surgeon must pay particular attention to performing an inferior capsular shift and closing the rotator interval, thus the acronym: AMBRII. Rowe (Rowe, 1956) carefully analyzed 500 dislocations of the glenohumeral joint and determined that 96 per cent were traumatic (caused by a major injury) and the remaining 4 per cent were atraumatic. DePalma, (DePalma, 1983) Rockwood, (Rockwood, 1979) and Collins and Wilde (Collins and Wilde, 1973) also recognized the importance of distinguishing between traumatic and atraumatic instability of the shoulder.

Patients with atraumatic instability may have generalized joint laxity. Imazato (Imazato, 1992) and Hirakawa (Hirakawa, 1991) demonstrated that patients with loose shoulders have relatively immature, more soluble and less cross-linked collagen fibers in their capsule, muscles and skin than controls; presumably tissues like the glenoid labrum would contain immature collagen as well, making them more deformable under load. Further evidence of constitutional factors is gained from a number of reports of positive family histories and bilateral involvement among those individuals with shoulder dislocations. O'Driscoll and Evans (O'Driscoll and Evans, 1988) and Dowdy and O'Driscoll (Dowdy and O'Driscoll, 1993) found a family history of shoulder instability in 24% patients requiring surgery for anterior glenohumeral instability. Morrey and Janes (Morrey and Janes, 1976) reported a positive family history in approximately 15% of patients who were operated on for recurrent anterior shoulder instability. A positive family history was also noted twice as frequently in patients whose postoperative course was complicated by recurrent instability compared with patients with successful surgery. Rowe and colleagues (Rowe, Patel and Southmayd, 1978) reported a positive family in 27% of 55 patients with anterior shoulder instability who were treated with a Bankart procedure. Bilateral instability was noted in 50% of patients with a positive family history compared with 26% of patients with negative family history, which suggested the possibility of a genetic predisposition.

When instability develops with no or minimal injury (Garth et al, 1987; Protzman, 1980; Rowe and Zarins, 1981), the initial reason for the loss of stability is often unclear. However, it appears that once lost, the factors maintaining stability may be difficult to regain. Certain phenomena may be self perpetuating: when the humeral head rides up on the glenoid rim, the rim becomes flattened and less effective, allowing easier translation. Furthermore, when normal neuromuscular control is compromised, the feedback systems which maintain head centering fail to provide effective input. Thus the joint becomes launched on a cycle of instability leading to loss of the effective glenoid concavity and loss of neuromuscular control leading to more instability.

If a patient intentionally subluxates or dislocates his or her shoulder, instability is described as voluntary. If the instability occurs unintentionally, it is involuntary. Voluntary and involuntary instability may coexist. Voluntary anterior dislocation may occur with the arm at the side or in abduction/external rotation. Voluntary posterior dislocation may occur with the arm in flexion, adduction and internal rotation, or with the arm at the side. The association of voluntary dislocations of the shoulder with emotional instability and psychiatric problems has been noted by several authors. (Carew-McColl, 1980; Rowe et al, 1973) The desire to voluntarily dislocate the shoulder cannot be treated surgically. However, the fact that patients can voluntarily demonstrate their instability does not necessarily mean they are emotionally impaired.

Neuromuscular causes of shoulder instability have been reported as well. Percy (Percy, 1960) described a woman who, following an episode of encephalitis, developed a posterior dislocation. Kretzler and Blue (Kretzler and Blue, 1966) have discussed the management of posterior dislocations of the shoulder in children with cerebral palsy. Sever, (Sever, 1927) Fairbank, (Fairbank, 1913) L'Episcopo, (L'Episcopo, 1939) Zachary, (Zachary, 1947) and Wickstrom (Wickstrom, 1962) have reported techniques for the management of neurological dislocation of the shoulder caused by upper brachial plexus birth injuries. Stroke is another important neurological cause of instability. (Zorowitz et al, 1995)

Degree of instability

Dislocations, subluxations, and apprehensions

Recurrent instability may be characterized as dislocation, subluxation or apprehension.

Dislocation of the glenohumeral joint is the complete separation of the articular surfaces; immediate, spontaneous relocation may not occur. Glenohumeral subluxation is defined as symptomatic translation of the humeral head on the glenoid without complete separation of the articular surfaces. Subluxation of the glenohumeral joint is usually transient: the humeral head returning spontaneously to its normal position in the glenoid fossa. In a series of patients with anterior shoulder subluxation reported by Rowe and Zarins, (Rowe and Zarins, 1981) 87 per cent were traumatic and over 50 per cent were not aware that their shoulders were unstable. Like dislocations, subluxations may be traumatic or atraumatic, anterior, posterior, or inferior, acute or recurrent, or they may occur after previous surgical repairs that did not achieve complete shoulder stability. Recurrent subluxations may coexist with or be initiated by glenohumeral dislocation. Rowe and Zarins (Rowe, 1956; Rowe and Zarins, 1982) reported seeing a Hill-Sachs compression fracture in 40 per cent of the patients in their series on subluxation of the shoulder, an observation indicating that at some time these shoulders had been completely dislocated. Apprehension refers to the fear that the shoulder will subluxate or dislocate. This fear may prevent the individual from participating fully in work or sports.

Directions of instability

Anterior dislocations

Dislocations of the shoulder account for approximately 45% of all dislocations. (Kazar and Relovszky, 1969) Of these, almost 85% are anterior glenohumeral dislocations. (Cave et al, 1974) Subcoracoid dislocation is the most common type of anterior dislocation. The usual mechanism of injury that causes subcoracoid dislocations is a combination of shoulder abduction, extension, and external rotation producing forces that challenge the anterior capsule and ligaments, the glenoid rim, and the rotator cuff mechanism. The head of the humerus is displaced anteriorly with respect to the glenoid and is inferior to the coracoid process. Other types of anterior dislocation include subglenoid (the head of the humerus lies anterior to and below the glenoid fossa), subclavicular (the head of the humerus lies medial to the coracoid process, just inferior to the lower border of the clavicle), intrathoracic (the head of the humerus lies between the ribs and the thoracic cavity). (Glessner, 1961; Moseley, 1963; Patel et al, 1963; Saxena and Stavas, 1983; West, 1949) and retroperitoneal. (Wirth et al, 1996-in press) These rarer types of dislocation are usually associated with severe trauma and have a high incidence of fracture of the greater tuberosity of the humerus and rotator cuff avulsion. Neurological, pulmonary, and vascular complications can occur, as can subcutaneous emphysema. West (West, 1949) reported a case of intrathoracic dislocation in which with reduction the humerus was felt to slip out of the chest cavity with a sensation similar to that of slipping a large cork from a bottle. His patient, who had an avulsion fracture of the greater tuberosity and no neurological deficit, regained a functional range of motion and returned to his job as a carpenter.

Posterior dislocations

Posterior dislocations may leave the humeral head in a subacromial (head behind the glenoid and beneath the acromion), subglenoid (head behind and beneath the glenoid), or subspinous (head medial to acromion and beneath the spine of the scapula) location. The subacromial dislocation is the most common by far. Posterior dislocations are frequently locked. Hawkins and coworkers (Hawkins et al, 1987) reviewed 41 such cases related to motor vehicle accidents, surgeries, and electroshock therapy.

The incidence of posterior dislocations is estimated at 2 per cent but is difficult to ascertain because of the frequency with which this diagnosis is missed. Thomas (Thomas, 1937) reported seeing only 4 cases of posterior shoulder dislocation in 6000 x-ray examinations. The literature reflects that the diagnosis of posterior dislocation of the shoulder is missed in over 60 per cent of cases. (Engelhardt, 1978; Hehne and Hubner, 1980; Mestdagh et al, 1994; Pavlov et al, 1985; Verrina, 1975) A 1982 article by Rowe and Zarins(Rowe and Zarins, 1982) indicates that the diagnosis was missed in 79 per cent of cases! McLaughlin (McLaughlin, 1952) stated that posterior shoulder dislocations are sufficiently uncommon that their occurrence creates a "diagnostic trap."

One of the largest series of posterior dislocations of the shoulder (37 cases) was recorded by Malgaigne (Malgaigne, 1855) in 1855, 40 years before the discovery of x-rays. He and his colleagues made the diagnosis by performing a proper physical examination! Cooper (Cooper, 1839) stated that the physical findings are so classic that he called it "an accident which cannot be mistaken."

Posterior dislocation may result from axial loading of the adducted, internally rotated arm (Moeller, 1975) or from violent muscle contraction, by electrical shock or convulsive seizures. (See references Ahlgren et al, 1981; Carew-McColl, 1980; Fipp, 1966; Hawkins and Hawkins, 1985; Lindholm and Elmstedt, 1980; McFie, 1976; Mills, 1974-1975; Onabowale and Jaja, 1979; Protzman, 1980; Segal et al, 1979) In the case of involuntary muscle contraction, the combined strength of the internal rotators (latissimus dorsi, pectoralis major, and subscapularis muscles) simply overwhelms the external rotators (infraspinatus and teres minor muscles). Heller et al have recently proposed a classification for posterior shoulder dislocation. (Heller et al, 1994)

Inferior dislocations

Inferior dislocation of the glenohumeral joint was first described by Middeldorpf and Scharm (Middeldorpf and Scharm, 1859) in 1859. Lynn (Lynn, 1921) in 1921 carefully reviewed 34 cases, and Roca and Ramos-Vertiz (Roca and Ramos-Vertiz, 1962) in 1962 reviewed 50 cases from the world literature. Laskin and Sedlin (Laskin and Sedlin, 1971) reported a case in an infant. Three bilateral cases have been reported by Murrard,(Murrard, 1920) Langfritz, (Langfritz, 1956) and Peiro and coworkers.(Peiro et al, 1975) Nobel(Nobel, 1962) reported a case of subglenoid dislocation in which the acromion--olecranon distance was shortened by 1.5 inches.

Inferior dislocation may be produced by a hyperabduction force that causes abutment of the neck of the humerus against the acromion process, which levers the head out inferiorly. The humerus is then locked with the head below the glenoid fossa and the humeral shaft pointing overhead, a condition called luxatio erecta. The clinical picture of a patient with luxatio erecta is so clear that it can hardly be mistaken for any other condition. The humerus is locked in a position somewhere between 110 and 160 of adduction. Severe soft tissue injury or fractures about the proximal humerus occur with this dislocation. At the time of surgery or autopsy, various authors have found avulsion of the supraspinatus, pectoralis major, or teres minor muscles and fractures of the greater tuberosity. (Kubin, 1964; Laskin and Sedlin, 1971; Lynn, 1921; Middeldorpf and Scharm, 1859; Murrard, 1920; Roca and Ramos-Vertiz, 1962) Neurovascular involvement is common. (Gardham and Scott, 1980; Lev-EI and Rubinstein, 1981; Lynn, 1921; Meadowcroft and Kain, 1977) Lev-El and associates (Lev-EI and Rubinstein, 1981) reported a patient who had an injury to the axillary artery and subsequently developed a thrombus that required resection and vein graft. Gardham and Scott(Gardham and Scott, 1980) reported a case in 1980 in which the axillary artery was damaged in its third part and was managed by a bypass graft using the saphenous vein. Rockwood and Wirth found that in 19 patients with this condition, all 19 had a brachial plexus injury and some vascular compromise before reduction. The force may be so great as to force the head out through the soft tissues and the skin. Lucas and Peterson (Lucas and Peterson, 1977) have reported a case of a 16-year-old boy who caught his arm in the power take-off of a tractor and suffered an open luxatio erecta injury. Reduction of an inferior dislocation can often be accomplished by traction and countertraction maneuvers. When closed reduction cannot be accomplished, the buttonhole rent in the inferior capsule must be surgically enlarged before reduction can occur.

Superior dislocations

Speed (Speed, 1942) reported that Langier, in 1834, was the first to record a case of superior dislocation of the glenohumeral joint; Stimson (Stimson, 1912) reviewed 14 cases that had been reported in the literature prior to 1912. In current literature little is mentioned about this type of dislocation, but undoubtedly occasional cases do occur. The usual cause is an extreme forward and upward force on the adducted arm. With displacement of the humerus upward, fractures may occur in the acromion, acromioclavicular joint, clavicle, coracoid process, or humeral tuberosities. Extreme soft tissue damage occurs to the capsule rotator cuff, biceps tendon, and surrounding muscles. Clinically, the head rides above the level of the acromion. The arm is short and adducted to the side. Shoulder movement is restricted and quite painful. Neurovascular complications are usually present.

Bilateral dislocations

Mynter (Mynter, 1902) first described this condition in 1902; according to Honner, (Honner, 1969) only 20 cases were reported prior to 1969. Bilateral dislocations have been reported by McFie, (McFie, 1976) Yadav, (Yadav, 1977) Onabowale and Jaja, (Onabowale and Jaja, 1979) Segal and colleagues, (Segal, Yablon, Lynch et al, 1979) and Carew-McColl. (Carew-McColl, 1980) Most of these cases were the result of convulsions or violent trauma. Peiro and coworkers (Peiro, Ferrandis and Correa, 1975) reported bilateral erect dislocation of the shoulders in a man caught in a cement mixer. Bilateral dislocation of the shoulder secondary to accidental electrical shock has been reported by Carew-McColl (Carew-McColl, 1980) and Fipp. (Fipp, 1966) Nicola and coworkers (Nicola et al, 1981) have reported cases of bilateral posterior fracture-dislocation following a convulsive seizure. Ahlgren and associates (Ahlgren, Lorentzon and Larsson, 1981) reported three cases of bilateral posterior fracture-dislocation associated with a convulsion. Lindholm and Elmstedt (Lindholm and Elmstedt, 1980) reported a case of bilateral posterior fracture-dislocation following an epileptic seizure, which was treated by open reduction and internal fixation with screws. Parrish and Skiendzielewski (Parrish and Skiendzielewski, 1985) reported a patient with bilateral posterior fracture-dislocations after status epilepticus. The diagnosis was missed for over 12 hours. Pagden and associates (Pagden et al, 1986) reported two cases of posterior shoulder dislocation following seizures related to regional anesthesia. Costigan and coworkers (Costigan et al, 1990) reported a case of undiagnosed bilateral anterior dislocation of the shoulder in a 74-year-old patient admitted to the hospital for an unrelated problem. The patient had no complaints related to the shoulders and was able to place both hands on her head and behind her back.

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