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Anatomy and Function of the Glenohumeral Ligaments in Anterior Shoulder Instability

Burkart, Andreas, C.; Debski, Richard, E.

Clinical Orthopaedics and Related Research: July 2002 - Volume 400 - Issue - p 32-39
SECTION I SYMPOSIUM: Recent Basic Science and Clinical Advances in Anterior Glenohumeral Instability
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The anatomy of the glenohumeral ligaments has been shown to be complex and variable and their function is highly dependent on the position of the humerus with respect to the glenoid. The superior glenohumeral ligament with the coracohumeral ligament was shown to be an important stabilizer in the inferior direction, even though the coracohumeral ligament is much more robust than the superior glenohumeral ligament. The middle glenohumeral ligament provides anterior stability at 45° and 60° abduction whereas the inferior glenohumeral ligament complex is the most important stabilizer against anteroinferior shoulder dislocation. Therefore, this component of the capsule is the most frequently injured structure. An appropriate surgical procedure to repair the inferior glenohumeral ligament complex after shoulder dislocation must be considered. In addition, a detached labrum can lead to recurrent anterior instability and a compromised inferior glenohumeral ligament complex. However, additional capsular injury usually is necessary to allow anterior dislocation.

From the Musculoskeletal Research Center, Department of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, PA.

Reprint requests to Richard E. Debski, PhD, Musculoskeletal Research Center, Department of Orthopaedic Surgery, University of Pittsburgh, P.O. Box 71199, Pittsburgh, PA 15213.

Shoulder stability is dependent on many factors that can be separated into two categories: active and passive. The active stabilizers (deltoid, biceps, and rotator cuff muscles) and the passive restraints (bony geometry, 19 labrum, capsule, and glenohumeral ligaments) contribute during normal shoulder function. However, no one structure stabilizes the glenohumeral joint throughout the range of motion (ROM).

The capsuloligamentous complex at the glenohumeral joint first was described in 1829 and consists of the superior, middle, and inferior glenohumeral ligaments and the coracohumeral ligament. 13 The glenohumeral ligaments do not act as traditional ligaments that carry a pure tensile force along their length and become taut at varying positions of abduction and humeral rotation. In addition, the glenohumeral ligaments do not have the strength characteristics of the ligaments at the knee. 43–45

After a traumatic episode, more than 97% of patients with anterior shoulder instability were found to have a Bankart lesion. 23,25 This lesion is defined as the detachment of the anterior and inferior glenohumeral capsule with involvement of the anterior band of the inferior glenohumeral ligament. 2 Clinical and experimental findings suggest that this site is the most common location of failure and typically includes the labral attachment to the glenoid. 3,14,21,34 The initial lesion found after shoulder dislocation also includes the periosteal insertion of the labrum. 1 Recurrent dislocations can lead to detached capsular structures and additional tissue damage. 15 However, tearing and avulsion from the humerus of the capsule also can occur. 1,6,12 Based on these findings, the anterior band of the inferior glenohumeral ligament is commonly accepted to be the primary anterior stabilizer of the glenohumeral joint. 40

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Anatomy of the Glenohumeral Ligaments

Coracohumeral Ligament

The coracohumeral ligament originates from the dorsolateral base of the coracoid process and extends as two bands, blending with the capsule, to the greater tuberosity and to the lesser tuberosity with only a few fibers 20 (Fig 1). Portions of the coracohumeral ligament form a tunnel for the biceps tendon on the anterior side of the joint. 11,20 The rotator interval, the region of the capsule between the anterior border of the supraspinatus and the superior border of the subscapularis muscle, is reinforced by the coracohumeral ligament. 20 This ligament also blends inferiorly with the superior glenohumeral ligament.

Fig 1.

Fig 1.

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Superior Glenohumeral Ligament

The superior glenohumeral ligament originates from the supraglenoid tubercle, just anterior to the origin of the long head of the biceps, and inserts on the humerus near the proximal tip of the lesser tuberosity on the medial ridge of the intertubercular groove (Figs 2, 3). It forms an anterior cover around the long head of the biceps tendon. 20 The superior glenohumeral ligament also is part of the rotator interval. 20 In an anatomic study, the superior glenohumeral ligament was missing in 6% of the specimens, whereas in 17% of the specimens, the superior glenohumeral ligament had a common origin with the middle glenohumeral ligament at the 1 o’clock position on the glenoid labrum. 38

Fig 2.

Fig 2.

Fig 3.

Fig 3.

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Middle Glenohumeral Ligament

The origin of the middle glenohumeral ligament is found on the supraglenoid tubercle and anterosuperior region of the labrum between the 1 and 3 o’clock positions 38 (Figs 2, 3). Therefore, the anatomy of the middle glenohumeral ligament is similar to the superior glenohumeral ligament. However, the fibers of the middle glenohumeral ligament blend with portions of the subscapularis tendon approximately 2 cm medial to its insertion on the lesser tuberosity. 40 The width and thickness of this ligament has been found to be as much as 2 cm and 4 mm, respectively.

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Inferior Glenohumeral Ligament Complex

The inferior glenohumeral ligament complex is a hammocklike structure with anchor points on the anterior and posterior sides of the glenoid (Fig 4) and includes the anterosuperior, inferior, and posteroinferior regions of the capsule. The anterior band of the inferior glenohumeral ligament originates primarily from the anterior labrum and attaches to the glenoid through two separate mechanisms: (1) the collagen fibers directly attach to the glenoid labrum; and (2) collagen fibers attach at an acute angle along the neck of the glenoid and some fibers run parallel to the surface and blend with the periosteum. At the 3 o’clock location on the face of the glenoid, the labral attachment of the anterior band of the inferior glenohumeral ligament is similar to the direct insertion of a tendon. At the 5 o’clock position, the insertion consists mainly of fibrous tissue. The length, width, and thickness of the anterior band of the inferior glenohumeral ligament have been reported to be 37 ± 2 mm, 13 ± 1 mm, and 3 ± 0 mm, respectively. 26,27

Fig 4.

Fig 4.

The humeral insertion site of the anterior band of the inferior glenohumeral ligament is located on the inferior margin of the articular surface and around the anatomic neck, below the lesser tuberosity (Fig 3). Two distinct attachments also can be observed on the humeral side: (1) a collarlike insertion that attaches the entire complex to the articular edge of the humeral head; and (2) a V-shaped attachment with the anterior and posterior bands attaching at the apex of the V, distal to the edge of the articular cartilage. 29 The axillary pouch runs from the inferior ⅓ of the humeral head to the inferior ⅔ of the anterior glenoid.

Histologically, the inferior glenohumeral ligament complex consists of closely packed collagen bundles, which tighten with abduction and external rotation of the glenohumeral joint. 29 In the midsubstance, the collagen bundles are oriented in a radial fashion. However, near the glenoid they change direction and blend into the labrum circumferentially. 14 The posterior band of the inferior glenohumeral ligament is the least commonly found portion of this region and only could be identified in 63% of the specimens examined. 14

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Function of the Glenohumeral Ligaments

Coracohumeral Ligament and Superior Glenohumeral Ligament Complex

Based on a selective sectioning study, the superior glenohumeral ligament was found to be an important inferior stabilizer of the adducted shoulder 41 whereas the coracohumeral ligament also has been shown to contribute to inferior stability. 5,20,32 In addition, these ligaments limit external rotation of the adducted arm. 17,18,31 The superior glenohumeral ligament and anterior band of the coracohumeral ligament also seem to be more important restraints up to 50° abduction and external rotation. Furthermore, the posterior band of the coracohumeral ligament elongates with internal rotation and adduction. 33

While examining the tensile properties of the superior glenohumeral ligament and coracohumeral ligament, the coracohumeral ligament was found to have twice the stiffness and three times the ultimate tensile load of the superior glenohumeral ligament. 5 The coracohumeral ligament absorbed six times the amount of energy to failure but only elongated 1.5 times as much as the superior glenohumeral ligament. During these tests, all of the coracohumeral ligaments failed in the proximal ligament substance whereas the superior glenohumeral ligament failed in the distal ligament substance near the insertion on the humerus. The cross-sectional area of the coracohumeral ligament also was significantly greater than that of the superior glenohumeral ligament at their midportions (coracohumeral ligament 54 ± 3 mm; superior glenohumeral ligament 11 ± 2 mm). 5,43 The cross-sectional area measured at the midportion of the coracohumeral ligament was approximately fivefold that of the superior glenohumeral ligament.

The in situ force distribution of the glenohumeral capsule and the resulting joint kinematics attributable to application of external loads at different abduction angles have been determined using a robotic and universal force moment sensor testing system. 10 The superior glenohumeral ligament—coracohumeral ligament complex carried force during anterior loading (26 ± 16 N at 0°) at all abduction angles and during posterior loading in anatomic rotation at 0° abduction. However, no forces were detected in this complex during posterior loading at 30°, 60° and 90° abduction. This finding confirms the role of the superior glenohumeral ligament in the adducted arm. Using a computational model, the superior glenohumeral ligament was predicted to carry up to 71 N at the position of maximum anterior translation 8,9 (Fig 5).

Fig 5.

Fig 5.

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Middle Glenohumeral Ligament

The middle glenohumeral ligament has been shown to become taut at 45° abduction, 10° extension and external rotation. 11,40 Therefore, this ligament has been assumed to contribute to anterior stability. Using a robotic and universal force moment sensor testing system, this hypothesis was confirmed by finding that the middle glenohumeral ligament was the primary anterior stabilizer in anatomic rotation and adduction 10 (Fig 6). The middle glenohumeral ligament carried force during only anterior loading at 30°, 60°, and 90° abduction with the maximum force of 34 N being achieved at 60° abduction. 10 As the abduction angle was increased, the middle glenohumeral ligament and inferior glenohumeral ligament become more important. Selective sectioning of the middle glenohumeral ligament resulted in increased translation at the joint, 35 and abducting the arm to 45° increased the strain in the middle glenohumeral ligament. 31 Others showed that the contribution of the superior glenohumeral ligament together with the middle glenohumeral ligament was similar to the inferior glenohumeral ligament in abduction and external rotation, but the contribution of the inferior glenohumeral ligament becomes more important with increasing external rotation. 4,31 Based on these observations, the overall function of the middle glenohumeral ligament could be summarized as: (1) to support the arm; (2) to restrain external rotation from 0° to 90° abduction; and (3) to provide anterosuperior stability.

Fig 6.

Fig 6.

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Inferior Glenohumeral Ligament Complex

Based on anatomic studies, the anterior band of the inferior glenohumeral ligament spans the midportion of the anterior glenohumeral joint at 90° abduction and external rotation and restrains anterior and inferior translation of the humerus. 36,40 Measurement of the in situ force in the anterior band of the inferior glenohumeral ligament revealed that it carried force at 60° and 90° (30 ± 21 N) abduction and reached a maximum at 90° abduction. A computational model found that at 90° abduction the anterior band of the inferior glenohumeral ligament had the highest forces (45 N) during anterior translation. However, during application of a posterior load at all abduction angles, the posterior band of the inferior glenohumeral ligament experienced only minimal forces. 9

At the neutral position (0° abduction and 30° horizontal extension), the anterior band of the inferior glenohumeral ligament becomes the primary stabilizer. Sectioning the anterior band of the inferior glenohumeral ligament and the anterior half of the axillary pouch resulted in significant increases in anterior, posterior and total translation at −30° and 0° flexion and extension. 30 On the opposite side of the capsule, the posterior band of the inferior glenohumeral ligament is considered the primary stabilizer with the arm in flexion and internal rotation, providing posterior stability. 28

Tensile testing of the inferior glenohumeral ligament with the shoulder oriented in the clinical apprehension position revealed that 66% of the specimens failed at the glenoid attachment site whereas 34% failed at the midsubstance and humeral insertion. 26,27 The failures at the glenoid attachment can be separated into two categories: (1) the labrum was avulsed from the glenoid bone (63%); and (2) the labrum remained attached to the glenoid and the ligament alone was avulsed, representing failure at the ligament-to-labral junction (37%). In other studies, failure occurred at the glenoid insertion site, the ligament midsubstance and the humeral insertion site in 40%, 35%, and 25% of the specimens, respectively. 3,16,39 Before failing, all regions of the inferior glenohumeral ligament experienced a significant amount of strain. 37 High strain rates also increased the percentage of failures in the midsubstance of the ligaments (54%) whereas failures at the humeral insertion decreased substantially to 8%. 39

The ultimate load of the anterior band of the inferior glenohumeral ligament was not significantly different when failure occurred either at the glenoid insertion (353 ± 32 N), in the midsubstance of the ligament (213 ± 64 N) or at the humeral insertion (250 ± 28 N). 26,27 The elongation at failure was greater with rupture at the glenoid insertion site (9.1 ± 0.5 mm) than with failure at the midsubstance (6.4 ± 0.3 mm) or the humeral insertion site (7.6 ± 0.7 mm). However, the amount of irrecoverable elongation was found to be 0.8 mm at the glenoid insertion site, 0.2 mm in the midsubstance of the ligament, and 0.9 mm at the humeral insertion site. The yield strain at the glenoid insertion site (12% ± 1%) and humeral insertion site (11% ± 1%) was larger than midsubstance failure (5% ± 0%). 26,27 Therefore, permanent stretching of the anterior band of the inferior glenohumeral ligament could lead to an increase in length that never exceeds 4% strain.

The midsubstance strain reported in other studies has been found to be between 9% and 11%. 3,39 The elongation at failure of the bone-ligament-bone complex reported by McMahon and coworkers 27 was only 6% although others have reported a range of 24% to 30%. 3,39 These differences might be caused by the use of different strain rates and joint positions. The failure mode of the anterior band of the inferior glenohumeral ligament also has been shown to be age-dependent. 22 In younger individuals, disruption of the anterior band of the inferior glenohumeral ligament occurred most frequently at the glenoid insertion site, whereas in older individuals, the midsubstance of the ligament was the most common failure site. The load at failure of the anterior band of the inferior glenohumeral ligament was higher in the younger group (164% of older group). 22

The strain field in the inferior glenohumeral ligament during anteroinferior subluxation of the glenohumeral joint also has been examined. 24 The maximum principal strains on the glenoid side of the inferior glenohumeral ligament were found to be significantly greater than on the humeral side. However, the strain fields were highly variable over the region studied. The principal strain vectors tended to be oriented diagonally across the region and did not correlate with any anatomic directions. This finding supports the hypothesis that the capsule functions as a membranous structure.

These experimental findings suggest that patients with initial glenohumeral instability have only a small irrecoverable elongation of their capsuloligamentous structures so that plication of the capsule, in addition to repair of the Bankart lesion, may not be necessary. In addition, repetitive episodes of subluxation or dislocation with permanent stretching of the anterior band of the inferior glenohumeral ligament must be considered when determining treatment options because they may lead to increased soft tissue elongation. Furthermore, even though healing may occur quickly after injury to the capsule or its humeral attachment site, the components of the capsule still could be in an elongated state.

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Glenohumeral Joint Capsule

Posterior dislocation of the glenohumeral joint did not occur until the capsule was sectioned from the 1 o’clock to 3 o’clock positions in addition to the entire posterior capsule. 30,42 In addition, sectioning of the posterior band of the inferior glenohumeral ligament and the posterior half of the axillary pouch resulted in a significant increase in anterior, posterior, and total translation at 30° flexion. Furthermore, sectioning the inferior capsule resulted in a significant increase in total translation in all humeral positions. 30 The posterior capsule does not have a significant role in anteroposterior stability.

The components of the capsule provide significant contributions at 0° and 90° glenohumeral abduction in anatomic rotation. Therefore, as the contribution of the capsule to stability decreases at 30° and 60° abduction, the amount of translation increases and the rotator cuff’s contribution to joint stability must increase. The capsule also has been shown to carry no force when the humeral head is centered in the glenoid with the humerus in anatomic rotation. 7–9

The function of the glenohumeral ligaments is dependent on the position of the humerus with respect to the glenoid. The superior glenohumeral ligament and coracohumeral ligament were shown to be important stabilizers in the inferior direction from 0° to 50° abduction whereas the middle glenohumeral ligament provides anterior stability between 45° and 60° abduction. The inferior glenohumeral ligament complex was found to be the most important stabilizer against anteroinferior shoulder dislocation and the most frequently injured component of the capsule. After shoulder dislocation, an appropriate surgical procedure to repair the inferior glenohumeral ligament complex must be considered. Repetitive episodes of subluxation or dislocation could be indications of a detached labrum and a compromised inferior glenohumeral ligament complex and injuries to other capsular components.

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Patrick J. McMahon, MD; and Thay Q. Lee, PhD—Guest Editors

© 2002 Lippincott Williams & Wilkins, Inc.