Anterior glenohumeral instability remains enigmatic. The pathoanatomy is unclear, treatment with rehabilitation is inconsistent, and there currently are many operative indications and surgical treatments. Part of the reason is the spectrum of degree, direction, frequency, and preexisting laxity in patients with glenohumeral instability. In the absence of significant bony restraint, stability to the glenohumeral joint is provided by the articulating surfaces, capsular and ligamentous structures, and the synchronous activity of the rotator cuff and deltoid muscle groups. 2 The glenohumeral ligaments serve as static stabilizers preventing excessive translation of the humeral head, especially at the extremes of motion. 5,18
The glenohumeral joint stabilizers commonly are categorized into two groups: static and dynamic. 20 The static stabilizers refer to bony, cartilaginous, capsular, and ligamentous structures. The dynamic stabilizers include all the musculature around the shoulder. Instability is a pathologic condition in which there is an inability to maintain the normal relationship of the humeral head on the glenoid fossa. Laxity changes with the position of the arm. At the extreme ranges of motion (ROM), the capsuloligamentous structures, which primarily are capsular thickenings, become taut and laxity decreases. In the midranges of motion, none of these structures are taut, and their contribution to stability therefore is limited. Instead, dynamic stabilizers and the configuration of articular surfaces, labrum, and intraarticular pressure play a role in stabilization in the midrange.
Overall, how the static and dynamic stabilizing mechanisms interact is not well understood. In the past several years, new technologies have been introduced that have enabled investigators to make some progress in elucidating the causes of glenohumeral joint instability. The current authors will review recent progress in researching the static and dynamic restraints to anterior instability of the glenohumeral joint.
The role of any specific component of the stabilizing system varies with glenohumeral joint position and direction of the opposing force. A functional interplay or interdependence exists between anterior and posterior, and superior and inferior components of the capsuloligamentous system. This concept has been referred to as the circle concept of capsuloligamentous stability of the shoulder, which implies that excessive translation in one direction may require damage to restraints on the same and opposite sides of the joint. 12 The magnitude of injury required to cause instability varies based on the inherent capsuloligamentous laxity of a given shoulder.
Selective sectioning studies in conjunction with available data from anatomic studies, strain gauge analysis, and clinical studies have confirmed this complex interaction among the various regions of the glenohumeral joint capsule and their labral attachment sites. 8 In a landmark study by Turkel et al 51 the stabilizing mechanisms of the glenohumeral joint that prevent anterior dislocation were investigated. They showed that at 0° abduction the subscapularis muscle stabilizes the joint to a large extent; at 45° abduction the subscapularis, middle glenohumeral ligament, and anterosuperior fibers of the inferior glenohumeral ligament provide primary stability; and as the glenohumeral joint approaches 90° abduction, the inferior glenohumeral ligament prevents dislocation during external rotation. This study has inspired many investigations into the anatomic and biomechanical characteristics of the glenohumeral joint.
The Capsuloligamentous Complex
The inferior glenohumeral ligament complex consists of an anterior band arising from the 2 to 4 o’clock positions (right shoulder), the anterior portion of the axillary pouch, and the posterior portion of the pouch 3,34,40 (Fig 1). Considerable variation in the glenohumeral ligaments exists, ranging from three well-defined distinct entities to barely discernible thickenings in the anterior capsule. 3,15,40,42
The primary stabilizer of the abducted glenohumeral joint is the inferior glenohumeral ligament complex. As the glenohumeral joint approaches 90° abduction with external rotation, the superior band of the inferior glenohumeral ligament prevents anterior dislocation. 41 The total translation varies with the position of the humeral head relative to the glenoid. Therefore, to restore stability to an unstable shoulder, the competency of the inferior glenohumeral ligament complex must be restored.
Inferior Glenohumeral Ligament Mechanical Properties
The inferior glenohumeral ligament has the ability to stretch considerably before ligament or insertion failure suggesting that lateral translation of the humeral head is possible under loads that would allow the head to override the glenoid rim. 3 This supports the clinical finding that certain patients can sublux without disruption of either the capsule or its insertion sites. The occurrence of considerable capsular stretching, be it the result of one traumatic event or of a repetitive nature, also represents the possibility that the ligament may be stretched without insertion site failure. Repetitive loading of the inferior glenohumeral ligament at increasing levels of subfailure strain has shown a dramatic increase in peak force with elongation suggesting that the capsule may be subject to plastic deformation especially after repetitive trauma. 44
Anterior translation produced after creation of a Bankart lesion in specimens from cadavers results in only small increases in anterior and inferior translation at all positions of abduction. 3,47 It seems that the amount of humeral head translation needed to clinically produce anterior glenohumeral dislocation requires inferior glenohumeral ligament plastic deformation in addition to the Bankart lesion. This suggests that in surgical repair for recurrent instability the possibility of capsular laxity produced by the initial traumatic event, and detachment of the glenoid insertion of the inferior glenohumeral ligament must be considered. 44 However, others have shown that traumatic unidirectional anterior dislocations result in minimal irrecoverable elongation of the inferior glenohumeral ligament. 35,48 Therefore, meaningful imbrication in addition to repair of the Bankart lesion may be unnecessary. Clearly, this is an area in need of additional clarification.
Three failure sites for the inferior glenohumeral ligament are the glenoid insertion site, midsubstance, and the humeral insertion site. Glenoid insertion site failure and midsubstance failure are more common. The humeral avulsion of the glenohumeral ligament although uncommon still needs to be ruled out in patients with documented anterior instability without a demonstrable primary Bankart lesion. 52 At fast strain rates, failures predominantly are ligamentous. However, at slow strain rates, the inferior glenohumeral ligament is less likely to fail within the ligamentous substance. Failure at the humeral insertion decreases substantially at fast strain rates compared with the slow strain rates. At the glenoid insertion, failures at fast and slow strain rates do not change dramatically. 50 This failure pattern of the bone-ligament-bone specimens at different strain rates has shown viscoelastic behavior that lends additional support to the functional role of the inferior glenohumeral ligament as a stabilizer of the glenohumeral joint when the arm is placed in vulnerable positions associated with anterior instability. 3,50
Although greater tensile forces are needed to rupture the more massive superior band of the inferior glenohumeral ligament, when cross-sectional area differences are accounted for, the three regions of the inferior glenohumeral ligament behave similarly with respect to failure stress. 3 This suggests a fairly homogeneous composition for the collagen fibers of the three regions of the inferior glenohumeral ligament. At higher strain rates, the superior band of the inferior glenohumeral ligament has been shown to have greater stiffness than either the anterior axillary pouch or the posterior axillary pouch. 50 In addition, for the superior band, higher tensile stress at failure and higher tensile modulus are found at faster strain rates. The anterior axillary pouch shows greater tensile stress at failure and tensile modulus at faster strain rates. In comparing this with the behavior of the inferior glenohumeral ligament at slower strain rates, the superior band and the anterior axillary pouch have shown the viscoelastic effects of increased stiffness and failure stress. The viscoelastic stiffening characteristics of the superior band and anterior axillary pouch function to restrain the humeral head from rapid abnormal anterior displacement when dynamic restraints are overwhelmed.
A novel approach, measuring the in situ force distribution in the glenohumeral joint capsule, has shown that the glenohumeral capsule carries no force when the humeral head is centered in the glenoid with the humerus in anatomic rotation. 10 The superior glenohumeral ligament carries force during anterior loading at all abduction angles and during posterior loading at 0° abduction. The middle glenohumeral ligament experiences force during anterior loading at 30°, 60°, and 90° abduction. Finally, the anterior band of the inferior glenohumeral ligament carries force only at 60° and 90° abduction and reaches a maximum at 90°, whereas the posterior band of the inferior glenohumeral ligament experiences minimal force during all loading tests. This reinforces the concept that the glenohumeral ligaments do not act as traditional ligaments that carry pure tensile forces along their length, but rather a complex interaction is present between the various structures. Also supporting this complex interaction, two-dimensional strain field measurements using stereoradiogrammetry have shown that maximum principal strains are not aligned along the predominant direction of the inferior glenohumeral ligament. In addition, peak strains typically are higher on the glenoid side providing evidence for common failures at this site. 32
The glenohumeral articular surfaces have been shown to provide static stability to the joint. 46 Congruence can be defined as the difference in the radii of the humeral head and the glenoid articulating surfaces. 2 The closer this difference is to 0 the more congruent is the joint. 46 The articulating cartilage surfaces are much more conforming than the underlying bone surfaces. As a result, the glenoid appears misleadingly flat based on radiographs as compared with its true anatomic shape when the articular cartilage layer is included. Kinematic analysis using bone surfaces yield erroneously large translation values compared with those obtained using the articular cartilage surfaces. This is better understood when one considers that the center of rotation during abduction is close but not equal to the center of curvature of the humeral head bone surface. Therefore, even a pure rotation about the cartilage center would result in some translation of the center of the bone surface (Fig 2).
Concavity compression refers to the stability afforded a convex object that is pressed into a concave surface. This mechanism is active in all glenohumeral positions but is particularly important in the functional midrange, in which the capsule and ligaments are slack. 29 The specialized anatomy of the rotator cuff muscles and the intraarticular long head of the biceps are situated ideally to actively compress the humeral head into the glenoid concavity. 27 The outer sleeve of the shoulder muscles such as the deltoid, pectoralis major, and latissimus, also contribute significantly to this compression in certain glenohumeral joint positions. Shoulders with weakened or deficient rotator cuff mechanisms are likely to have compromised stability from concavity compression.
The glenoid labrum primarily is fibrous throughout with a fibrocartilaginous transition zone at its attachment with the glenoid articular cartilage. The labrum increases the depth of the glenohumeral socket by 50% on average, but does not significantly alter the curvature of the articulating surface. 6 Regional variation in the labrum exists with the inferior portion being immobile and firmly attached to the glenoid and the superior portion being attached more loosely allowing for substantial range of motion of the glenohumeral joint.
Recurrent instability tends to erode the articular cartilage of the anterior inferior glenoid rim and additionally decreases concavity. The relative contribution of the glenoid concavity and the labrum to joint restraint has been examined by measuring the force necessary to dislocate the humeral head under constant compressive force. 20 With the labrum intact, the humeral head resists tangential forces as much as 60% of the compressive load. 30 Resection of the glenoid labrum reduces the effectiveness of compression-stabilization by approximately 20%. Chondral-labral defects reduce the height of the glenoid which in turn significantly reduces the stability ratio. 27 Anatomic reconstruction of these defects can restore normal stability. However, ligament repairs that involve fixation on the medial glenoid neck may not restore concavity compression and repair to the glenoid rim.
A Bankart repair often is done to reduce abnormal translations of the humeral head on the glenoid caused by a Bankart lesion. 15,17,39 However, this procedure sometimes is accompanied by a loss of rotational ROM that may lead to decreased function and osteoarthritis. One study has shown that a Bankart repair significantly affects external rotation of the shoulder. 39 The reduction in anterior translation that is the goal of the repair is significant only with the larger imbrication (5 mm) of the capsule but also is accompanied by more severe limitations in external rotation, extension, and abduction-adduction rotations. 39 This raises serious questions about the loss of rotation, coupled to the goal of reducing humeral translations, which may be a result of the amount of imbrication of the capsule during repair. This loss of rotational ROM is deleterious because of its effect on activities of daily living and sports and likely is related to development of osteoarthritis.
Various-sized osseous defects are associated with Bankart lesions and these affect the extent of glenoid concavity. 21,26 The repair of these Bankart lesions can result in a change of the ligamentous tension around the glenohumeral joint. 21 With the arm in abduction and external rotation, the stability of the glenohumeral joint after Bankart repair does not change significantly regardless of the size of the osseous defect. However, with the arm in abduction and internal rotation, the stability decreases significantly as the size of the osseous defect increases (Fig 3). The range of external rotation in shoulders with an osseous defect (at least 21%) of the glenoid is significantly less than that in shoulders without a defect because of the pretensioning of the capsule caused by closing the gap between the detached capsule and the glenoid rim. The average loss of external rotation associated with these repairs is 25° per centimeter of defect.
Scapulohumeral balance refers to the principle that the humeral head is balanced in the glenoid if the net joint reaction force passes through the fossa. 29 As long as the scapula is positioned such that the glenoid fossa encloses the net forces acting on the humeral head, the glenohumeral joint will remain stable. The larger the arc subtended by the glenoid concavity, the larger the range of directions of net force acting through the humeral head that can be stabilized. A redundant capsule may allow excessive glenohumeral angles that exceed the scapulohumeral balancing mechanism. 36 This allows instability to occur before the capsuloligamentous structures are sufficiently tight to provide restraint.
Active stabilization of the glenohumeral joint has been described. 9,53 However, dynamic muscular control is far more difficult to investigate experimentally than passive stability. Dynamic glenohumeral joint stability occurs through the action of the shoulder musculature. 20 The contribution of the shoulder musculature to joint stability may be caused by the following mechanisms: passive muscle tension from the bulk effect of the muscle; contraction causing compression of the articular surfaces; joint motion that secondarily tightens the passive ligamentous constraints; barrier or restraint effect of the contracted muscle; and redirection of the joint force to the center of the glenoid surface by coordination of muscle forces. 1–4,25,30
Anterior dislocation of the glenohumeral joint occurs either by disruption of the glenohumeral ligament or by rupture of the rotator cuff. Rupture of the musculotendinous cuff, particularly the supraspinatus, infraspinatus, and teres minor can permit anterior dislocation of the humeral head on an intact anterior soft tissue hinge. 7 The rotator cuff acts as part of a force couple around the joint, either by controlling joint motion or by controlling and directing force through the joint. 24,38 Two types of force couples work around a joint. The first force couple is coactivation or simultaneous activation of agonist and antagonist muscles about a joint. Coactivation creates low net torque around the joint with increased control of motion. The second force couple is a coordinated activation of the agonist and inhibition of the antagonist muscle. This force couple increases joint torque and motion, increases forces through the joint, and allows transfer of forces through the joint. 37 This sequential coordinated muscle activation is necessary to produce the torques and accelerations necessary for using the glenohumeral joint.
Displacement of the humeral head increases with an increase in the rotator cuff tear size. 19 With loading, a large and anteriorly located defect has the most influence on stability. Tear size has the greatest effect on stability in the inferior direction for a tear centered at the critical area (supraspinatus tear with extension into the infraspinatus muscle) and in the anterior direction for a tear centered at the rotator interval. The tear location has the most significant effect on stability in the inferior and anterior directions for smaller tears and on the anterior direction for larger tears.
Significant changes in glenohumeral joint motion occur only if paralysis or anatomic deficiency violates the transverse force couple (subscapularis, infraspinatus, and teres minor tendons). 49 With simulated supraspinatus muscle paralysis, glenohumeral kinematics have not been disrupted. 13 This suggests that joint compression through the remaining rotator cuff muscles is adequate to provide a stable fulcrum for concentric compression of the glenohumeral joint during abduction. These observations help clarify what often is reported clinically in patients with massive rotator cuff tears. A significant extension of a rotator cuff tear into the infraspinatus tendon disrupts the transverse force couple and the stable fulcrum for glenohumeral abduction is lost. This supports the fact that if the transverse force couple remains functionally intact, sufficient force maintains concentric reduction of the humeral head on the glenoid and normal kinematics are preserved.
The infraspinatus and teres minor control external rotation of the humerus and reduce anteroinferior capsuloligamentous strain. 5,24 The subscapularis is of primary importance in stabilizing the glenohumeral joint anteriorly with the arm in abduction and neutral rotation but becomes less important with external rotation, where the posterior cuff muscles reduce anterior strain. 33 Combined contraction of the subscapularis and infraspinatus forms a force couple providing stability throughout the midranges of elevation. An electromyographic study showed that the subscapularis and infraspinatus contract to stabilize the glenohumeral joint in abduction at 60° to 150°. 11 In baseball pitchers, researchers showed that during late cocking, as the glenohumeral joint reaches extreme external rotation, the subscapularis has the most activity, followed by infraspinatus and teres minor. 16 The supraspinatus has the least activity.
It has been hypothesized that dynamic factors potentially can stabilize the glenohumeral joint throughout the entire range of glenohumeral motion. 28 This is in contrast to current thoughts that capsuloligamentous restraints primarily are responsible for the end ROM. Combining the force components with concavity-compression mechanics, a new term called the dynamic stability index was calculated. This index was designed to facilitate comparisons of the stabilizing and destabilizing roles of the rotator cuff muscles. However, using the dynamic stability index, the stability provided by the four rotator cuff muscles in the end ROM is approximately 20% less than that in the midrange. This difference has been attributed to a decrease in the dynamic stability index for the subscapularis in the end range.
The long head of the biceps, which runs along the surface of the humeral head within the glenohumeral joint, has been thought to be a depressor of the humeral head. 20 However, increased activity of the biceps in anteriorly unstable shoulders during throwing motion has been observed, suggesting that the biceps can compensate for glenohumeral joint instability. 14 With biceps loading, there is significantly decreased anteroposterior translation, particularly with external rotation 22 (Fig 4). The biceps becomes more important than the rotator cuff as an anterior stabilizer as the stability from the capsuloligamentous structure decreases. 23 When an artificial Bankart lesion is present, the biceps is more important than any of the cuff muscles in stabilizing the glenohumeral joint against anterior displacement.
The superior labrum serves as an attachment site for the long head of the biceps, and the superior and middle glenohumeral ligaments. 8 Although the superior labrum often is loose and mobile in normal shoulders, frank detachment can occur from trauma or overuse, and has been implicated as a source of shoulder pain, instability, or both, particularly in athletes who throw overhead. Injury may represent a traction phenomenon (secondary to activity in the biceps) or possibly a compression phenomenon. Superior labral defects (from 10 to 2 o’clock positions) decrease torsional rigidity and increase inferior glenohumeral ligament strain. 45 Torsional rigidity increases with increasing biceps force; however, mean values are lower in shoulders with superior labral detachment. Strain in the inferior glenohumeral ligament is significantly greater in shoulders with labral detachment compared with normal shoulders. The combination of decreased torsional rigidity and increased inferior glenohumeral ligament strain resulting from superior labral detachment may contribute to anterior instability. A complete lesion of the superior labrum (biceps detached) often leads to significant multidirectional increases in translation, particularly in the lower middle ranges of abduction. 43
Concomitant Evaluation of Static and Dynamic Stabilizers
Only a few studies have tried to address the simultaneous contributions of the static and dynamic stabilizers. 4,23,31 In a study by Malicky et al, 32 passive and active factors providing glenohumeral joint stability were evaluated simultaneously throughout a 10 mm range of subluxation. Stability was evaluated under varying configurations of capsule cuts, humeral rotation, and muscular loads. Among the muscles, the biceps was the most important stabilizer in neutral rotation, with the subscapularis providing the greatest degree of stabilization in external rotation. In external rotation, the superior, middle, and inferior glenohumeral ligaments were the most effective ligamentous stabilizers and all provided progressively more stabilization as higher displacements were reached. The findings were not unexpected; however, the simultaneous design of this experiment lends more validity to previous studies. More experiments need to be done which try to simultaneously simulate the static and dynamic restraints of the shoulder.
Basic science studies have been pivotal in increasing the understanding of the static restraints. Limitations exist and include variability in normal capsuloligamentous anatomy, differences in the loads applied to cadaveric joints and, alteration in the synergy of static and dynamic restraints from sequential cutting of the static restraints. Recent study has included the shoulder muscles to simulate the dynamic restraints. Complex interplay of the static and dynamic restraints results in glenohumeral joint stability in vivo and should be included in future basic science studies.
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Patrick J. McMahon, MD; and Thay Q. Lee, PhD—Guest Editors