David Eldringhoff, MD; Barry I. Shafer, PT, DPT, ATC; Gregory J. Adamson, MD; and Thay Q. Lee, PhD
Shoulder motion is provided by several articulations, including the glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic joints. The glenohumeral joint has the greatest range of motion of any joint in the body. This unique ability is required so that the upper extremity can be positioned in space for hand function. The downside to this tremendous mobility is that it puts the glenohumeral joint at greater risk for instability. This chapter will review the clinical anatomy and biomechanics of the shoulder including the bony passive stabilizers, soft-tissue passive, and soft-tissue active stabilizers that play a role in allowing substantial range of motion while providing stability.
The bony passive stabilizers of the shoulder include the scapula, humeral head, and the clavicle. Specifically, the glenohumeral bony stabilizers are composed of the glenoid and the humeral head.
The glenoid is on the lateral aspect of the scapular body and functions as a shallow socket for the glenohumeral joint. The glenoid face has been described as pear shaped because the superior aspect is narrower, from anterior to posterior, than the inferior aspect (Figures 4-1A and 4-1B).
The average superior-inferior dimension of the glenoid has been reported to be 39 ± 3.7 mm and the average anterior-posterior dimension of the lower half 29 ± 3.1 mm.1 The glenoid is typically thought to be in a slight position of inclination and retroversion, which helps contribute to the bony stability provided. In the coronal plane, the glenoid is inclined superiorly, with a mean inclination of 4.2 degrees and a range from –7 to 15.8 degrees.2 In the sagittal plane, the glenoid is retroverted 7.79 ± 4.85 degrees.3 For a surgeon, the large variation in glenoid version can become a complex challenge. Excessive retroversion has been described as a cause of posterior instability.4 An inferior tilt may lead to a higher risk for multidirectional instability (MDI) and inferior dislocations.5 Furthermore, the glenoid height-to-width ratio (glenoid index) was found to be a significant risk factor for instability. Specifically, Owens et al showed that a glenoid height-to-width ratio of greater than 1.58 (taller and thinner glenoids) had 2.64 times the risk of injury compared with those with a ratio less than 1.58.6
As with the glenoid, the humeral head has a wide degree of anatomical variation. The humeral head is essentially hemispherical and articulates with the glenoid cavity to form the glenohumeral joint (Figures 4-2A to 4-2C). Robertson and colleagues7 reviewed and reported on the morphology of 30 pairs of proximal humeri. They found the humeral head retroversion averaged 19 degrees with a range from 9 to 31 degrees. The head inclination, otherwise known as the neck shaft angle, averaged 41 degrees with a range from 34 to 47 degrees. The radius of curvature of the humeral head averaged 23 mm with a range from 17 mm to 28 mm. The center of the humeral head is medial to the center of the longitudinal axis of the humeral shaft. This is known as the medial humeral head offset and on average it measures 7 mm with a range of 4 to 12 mm. The humeral head also has a posterior head center offset with an average of 2 mm with a range from 1 to 8 mm. Just as the glenoid variation has a clinical impact, so does the humeral head variation.
The articulation between the glenoid and the humeral head provides minimal restraint to the shoulder because of a large humeral head and a small, shallow glenoid. This inherent instability is related to the fact that only 25% to 30% of the humeral head is in contact with the glenoid surface at any given anatomic position (Figures 4-3A and 4-3B).2,8,9 Some have compared the glenoid and humeral head to a golf ball on a tee, demonstrating the glenohumeral joint’s propensity for instability. This anatomic relationship between the humeral head and glenoid can be thought of as a ratio of the diameter of each. This is known as the glenohumeral index. It has been reported that the ratio is approximately 0.75 in the sagittal plane and 0.6 in the transverse plane.10 Glenohumeral stability was quantitatively characterized by Lippitt and Matsen11 as the stability ratio. This is the force necessary to dislocate the humeral head from the glenoid divided by compressive force.11,12 Many factors influence the stability ratio, including the labrum, the depth of the glenoid, and the presence of any glenoid and/or chondrolabral defects.13 The clavicle and scapula are also important contributors to the bony anatomy because they provide muscular attachments and contribute to the total shoulder range of motion. The following sections will review the pertinent anatomy of these important bony structures.
The scapula is part of both the glenohumeral joint and the acromioclavicular joint, and is the interposed bony linkage between the humerus and the clavicle/axial skeleton.14 As with the clavicle, the scapula serves as an attachment site for many of the glenohumeral stabilizing structures. The scapula serves as the attachment site or origin for 17 different muscles (Figure 4-4A and 4-4B). The scapula allows motion along the rib cage and is not a true joint with the thorax but functions similarly to a sliding joint. It articulates with ribs 2 to 7. It has a resting-position angulation of about 10 to 20 degrees’ anterior angulation, 30 to 45 degrees’ internal rotation in the coronal plane, and with a slight upward tilt of about 3 degrees.15 Abduction in the scapular plane, also known as Scaption, is the result of both glenohumeral and scapulothoracic motion. This has been termed scapulohumeral rhythm, by which approximately one-third of the Scaption is from the scapulothoracic motion and the other two-thirds are from the glenohumeral joint.16
The scapula is the link in proximal to distal sequencing of velocity, energy, and forces in shoulder function.17 The shoulder motion, force development, force regulation, and ligamentous tension require coupling of scapular motion and humeral motion.18 In the last 20 years, the importance of the scapula in shoulder mechanics has become a vastly discussed topic in the shoulder. Extensive work by Kibler et al14,18 and others has influenced the way we think about shoulder mechanics, kinetics, and the interplay in pathology.
Scapular dyskinesis has been defined as movement of the scapula that is dysfunctional and may create a possible impairment of the overall shoulder function.19 There is now an extensive body of literature demonstrating that in the painful shoulder it is possible that scapular dyskinesis is a major contributor to shoulder pain14,17 and therefore should be evaluated as part of the routine shoulder exam.17 Scapular dyskinesis has also been implicated as a causative factor of shoulder injuries or other pathologies.14
Acromion and Coracoid Process
The acromion is the lateral-most extension of the scapula and has its medial connection to the clavicle through the acromioclavicular joint. The acromion has a somewhat triangular shape that is flattened, projects laterally, and then curves forward and upward. Its superior surface is convex and the inferior surface is concave, providing an overhang to the glenoid cavity. The acromion forms the point of attachment for the trapezius and deltoid muscles. Medially, the acromion articulates with the lateral end of the clavicle immediately behind the attachment of the coracoacromial ligament (Figure 4-5). The coracoacromial ligament, in conjunction with the acromion and the coracoid process, forms an arch over the glenohumeral joint, preventing its upward dislocation and limiting the upward rotation of the humerus.
The coracoid process is an anterior projection from the scapular neck. It is known as the lighthouse of the shoulder because it is a vital landmark for the surgeon. It provides a medial boundary to the subacromial space and rotator interval. It serves as the attachment site for the conjoint tendon and the coracoclavicular ligaments. Coracoid impingement against the subscapularis tendon and bursa can lead to anterior shoulder pain, subscapularis degeneration, or rupture.20 The coracohumeral distance has also been implicated in shoulder instability by Owens and colleagues. They reported the coracoid location to be an independent risk factor for instability, with each millimeter increase in distance leading to a 20% increase in injury risk6 (Figure 4-6).
The humerus provides leverage for upper extremity strength and range of motion for hand positioning. The humeral head is an ellipsoidal shaped structure on the proximal end of the humerus, where it articulates with the glenoid. The proximal humerus also includes the greater and lesser tuberosities, which are bisected by the bicipital groove (see Figure 4-2). The anatomic neck is defined as the junction of the cortical bone and the articular surface. The anatomic neck is located medial to the tuberosities. The greater tuberosity is the insertion point for the supraspinatus, infraspinatus, and teres minor tendons from anterior to posterior, respectively. The lesser tuberosity is the insertion point for the subscapularis tendon. The bicipital groove is flanked between the tuberosities and stabilizes the long head of the biceps.
The clavicle is an S-shaped bone that functions to position the upper extremity laterally from the body axis and provides the only diarthrodial joint connection to the thorax at the sternoclavicular joint. (Figures 4-7A and 4-7B) On the lateral end of the clavicle is the acromioclavicular joint, where the clavicle articulates with the scapula. This joint is stabilized by the acromioclavicular joint capsule and the conoid and trapezoid ligaments of the coracoclavicular ligament complex. The acromioclavicular joint capsule and ligaments provide mostly horizontal (anterior-posterior) stabilization and the coracoclavicular ligaments provide mostly vertical (superior-inferior) stabilization. The clavicle facilitates shoulder elevation by allowing clavicular rotation of approximately 40 to 50 degrees throughout the shoulder range of motion.21 The clavicle also serves as the attachment site for many of the active stabilizers of the shoulder, such as the upper trapezius and deltoid muscles.
PASSIVE SOFT-TISSUE STABILIZERS
The passive soft-tissue stabilizers of the shoulder include the glenoid labrum, the glenohumeral ligaments, and the glenohumeral joint capsule (Figures 4-8A and 4-8B). These soft-tissue stabilizers limit glenohumeral joint rotation and translation. The glenohumeral ligaments limit translation in a position-dependent manner. The following section will review the specific anatomy and function of these passive soft-tissue stabilizers of the shoulder.
The labrum is defined as a fibrocartilaginous tissue that surrounds the glenoid. Its function is to deepen the glenoid socket for stability. Specifically, the glenoid articular surface and labrum combine to create a socket that is approximately 9 mm deep in the superoinferior direction and 5 mm deep in the anteroposterior direction, where the labrum contributes approximately 50% of the total depth of the socket.22 It deepens the glenoid an average of 9 mm in the superior-interior plane and 5 mm in the anterior-posterior plane. The long head of the biceps tendon (LHBT) attaches intra-articularly to the supraglenoid tubercle along with the origin of the superior labrum. The labrum and biceps at this location function as a passive stabilizer of the humeral head.
The labrum has been extensively studied, especially regarding shoulder instability. Often, patients who have had a traumatic dislocation of the shoulder sustain an injury to their labrum. This in turn, increases the risk of further dislocations. In a cadaveric study, the stability ratio decreases by 20% with resection of the labrum and decreases even more when chondral injury is added.13 Another cadaveric study echoed these results by removing the labrum but leaving the capsule in place. That study demonstrated an increase in laxity in the adducted position due to the labral resection.23
The glenohumeral ligaments consist of thickenings of the glenohumeral joint capsule and are typically divided into 4 different regions: superior, middle, anterior inferior, and posterior inferior (see Figures 4-8A and 4-8B). Each of these is an important passive stabilizer of the shoulder. They function to prevent translation of the humeral head off the glenoid in a position-dependent manner. Capsular stretch injury without injury to the labrum also results in decreased force required for further dislocation events.24
The superior glenohumeral ligament (SGHL) originates on the superior aspect of the glenoid and coracoid process and runs to the fovea capitis just superior to the lesser tuberosity. The superior tilt of the glenoid and the SGHL both provide a passive restraint to inferior subluxation of the humeral head.25–27
The middle glenohumeral ligament (MGHL) originates just inferiorly to the SGHL on the anterior superior glenoid and runs to the anterior aspect of the anatomic neck of the humerus. The MGHL provides substantial constraint to anterior humeral head translation. It is taut in external rotation in the first 45 degrees of abduction.28 However, the MGHL has not been shown to affect anterior instability in a significant manner on its own. It was shown in a sectioning study that the MGHL alone did not result in instability but did increase the excursion of the humeral head.26
The inferior glenohumeral ligament (IGHL) is a complex that can be broken down into an anterior (AIGHL), a posterior (PIGHL), and an inferior sling that connects the two. The IGHL originates on the inferior two-thirds of the glenoid labrum and periosteum medially and runs to about 2 cm from the articular surface of the lateral humerus. It is the primary stabilizer of anterior translation of the humeral head in the apprehension (90 degrees’ abduction and 90 degrees’ external rotation) position. Functionally, in the abducted position the AIGHL is taut in external rotation and the PIGHL is taut in internal rotation.29 There is a positive linear correlation between the length of the AIGHL and anterior translation.30 Anatomically, there are 2 types of attachments of the IGHL on the glenoid. One is a direct, dense, collagen-fiber attachment to the labrum. The other is a dense, collagen-fiber attachment to the labrum and the front of the glenoid neck (Figures 4-9A and 4-9B).31 AIGHL and PIGHL lesions both have been implicated in the pathology of the unstable shoulder. The AIGHL is commonly injured or stretched during a traumatic anterior dislocation. One study suggested that the capsule injury is even more important to the risk of subsequent dislocations than is the labrum.24 The PIGHL and capsule have been implicated in glenohumeral internal rotation deficit (GIRD). In this condition, the posterior capsule tightens in athletes and can lead to an imbalance in internal and external rotation of the effected shoulder. A cadaveric study with induced GIRD showed increased glenohumeral contact pressures, rotator cuff impingement, and posterior subluxation of the humeral head.32 These findings help reinforce the importance of sleeper stretches for the treatment of GIRD because it may help to prevent abnormal throwing kinematics.