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Shoulder motion is a result of the complex interactions of the individual joints and muscles of the shoulder girdle.
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Scapulothoracic motion significantly affects measurements of glenohumeral motion.
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Shoulder motion is measured and described in multiple planes of motion.
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Stability of the shoulder is conferred by dynamic and static constraints.
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The clavicle serves as a strut and suspension between the thorax and scapula
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Scapular motion is a complex interaction of motion in three planes.
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The coordinated movement of the clavicle, scapula, and humerus involves a complex interaction of more than twenty muscles.
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Scapulohumeral rhythm is a dynamic state adapting to varying speed, load, and stability.
The shoulder is the most mobile joint in the body. Motion occurs through complex interactions of the individual joints of the shoulder girdle, including the glenohumeral joint, the sternoclavicular joint, the acromioclavicular joint, and the scapulothoracic articulation. Together the coordinated interaction of these structures allows for an extraordinary freedom of movement and function.
Range of Motion
Measuring Normal Range of Motion
Traditionally, shoulder motion has been described by measuring the angle formed by the arm relative to trunk. Forward elevation, or flexion, of the shoulder is in the sagittal plane and may in some individuals reach 180 degrees. The normal range varies, but has been reported to be on average 165 to 170 degrees in men, and 170 to 172 degrees in women. Posterior elevation or extension in the sagittal plane has been found to be on average 62 degrees. Axial rotation of the arm is described by degrees of internal and external rotation. With the arm at the side an average external rotation is 67 degrees. Estimates of total axial rotation (the sum of internal and external rotation) with the arm at the side range from 150 degrees to 180 degrees. Total axial rotation with the arm abducted to 90 degrees is reduced to about 120 degrees. In the horizontal plane, when the arm is perpendicular to the trunk, motion is commonly described as horizontal abduction and adduction (or horizontal extension and flexion).
Range of motion is influenced by several factors, including the determination of the end-point, the plane in which the motion is tested, and whether the scapula is stabilized. By comparing the relative contribution of passive and active arcs of motion, McCully and colleagues concluded that scapulothoracic motion significantly influences glenohumeral range-of-motion measurements.
Factors such as age, gender, and hand dominance also affect shoulder range of motion. Normal shoulder range of motion decreases with age. Boone and Azen reported on two groups of males with an age difference of 12.5 years. The younger group averaged 3.4 degrees more flexion and internal rotation, 8.4 degrees more external rotation, and 10.2 degrees more extension.
Codman’s Paradox
Shoulder motion is rarely limited to one plane. Therefore, in describing the position of the arm in space it is necessary to use multiple planes of reference. The traditional methods of motion description are inadequate for complex motion because the final position of the arm is dependent on the motion sequence. This is illustrated by a concept known as the Codman’s paradox. If the arm is raised forward to the horizontal, then horizontally abducted, followed by a return adduction to the side, the final resting position of the arm is externally rotated axially 90 degrees, yet the arm was never specifically externally rotated. Serial angular rotations about orthogonal axes are not additive, but sequence-dependent. Rotation about the x -axis, followed by rotation about the y -axis results in a different end resting position from the reverse sequence.
Three-Dimensional Joint Motion
A central feature to the understanding of joint kinematics is the ability to measure and describe motion in a consistent and reproducible manner. One method of describing complex joint kinematics is to use a system of vertical planes of elevation, similar to the degrees of longitude used to describe global positioning ( Fig. 4-1 ). Pure coronal abduction is defined as 0 degrees, and pure sagittal flexion as 90 degrees. At the horizontal, the maximum adduction is 124 degrees whereas the maximum abduction is −88 degrees, producing a total of 212 possible vertical planes of elevation. Humeral elevation is measured by the angle formed between the elevated arm and the unelevated arm. Isolated forward flexion to 90 degrees in this coordinate system is described as (90,90), whereas isolated abduction to 90 degrees is described as (0,90). Finally, axial rotation is described in reference to the plane of elevation by an angle formed by the forearm with the elbow flexed to 90 degrees. If the forearm is perpendicular to the plane of elevation, the rotation is 0 degrees. External rotation is positive, internal rotation is negative. A classic military salute in this system would be described as (+30, +80, −406).
Glenohumeral Anatomy
Glenoid
The glenoid arises laterally from the scapular neck at the junction of the coracoid, scapular spine, and lateral border of the scapular body. It is a pear-shaped structure forming a shallow socket that is retroverted on average 7 degrees with respect to the scapular plane, but maintains an overall anteversion of about 30 degrees with respect to the coronal plane of the body. The glenoid also maintains a superior tilt of about 5 degrees in the normal resting position of the scapula. It is thought that this superior inclination contributes to inferior stability via a cam effect that is a function of the tightening superior capsular structures.
The glenoid surface area is about one third that of the humeral head. The depth of the glenoid measures about 9 mm in a superoinferior direction, but only 5 mm in an anteroposterior direction, half of which is constituted by the labrum. In addition, the glenoid cartilage is thicker peripherally than centrally, further deepening the socket. The glenoid socket is therefore significantly more concave and congruous with the humerus than the bony anatomy would suggest.
Humeral Head
The humeral head is oriented with an upward tilt of about 45 degrees from the horizontal, and it is retroverted about 30 to 40 degrees with respect to the intercondylar axis of the distal humerus. The articular surface forms approximately one third of a sphere. Utilizing stereophotogrammetric studies, Soslowsky and associates demonstrated that the glenohumeral joint congruence is within 2 mm in 88% of cases, with a deviation from sphericity of less than 1% of the radius. Retroversion is greater in young children, with an average of 65 degrees between the ages of 4 months to 4 years. By 8 years of age most of the derotation has occurred with a more gradual derotation continuing until adulthood.
Glenohumeral Biomechanics
Laxity
Glenohumeral laxity is a normal finding to varying degrees in all shoulders. In cadaveric shoulders, average passive humeral translation of 13.4 mm anteriorly and 10.4 mm posteriorly has been demonstrated with a 20-N force. In a study of healthy unanesthetized volunteers, passive humeral translation averaged 8 mm anteriorly, 9 mm posteriorly, and 11 mm inferiorly. Far less translation occurs with normal glenohumeral kinematics. Radiographic analysis of normal volunteers demonstrates that the humeral head is maintained precisely centered in the glenoid in all positions except simultaneous maximal horizontal abduction and external rotation. In this extreme position an average of 4 mm of posterior translation occurred. These studies demonstrate that despite the great potential for translational motion in the shoulder, the combined stabilizers of the glenohumeral joint act in concert to maintain centricity.
Laxity Versus Instability
Shoulder laxity is a normal property that varies widely within the general population. It is often measured as increased passive translation of the humeral head on the glenoid and may be affected by several factors, including age, gender, and congenital factors. Instability is a pathologic condition involving active translation of the humeral head on the glenoid ( Fig. 4-2 ). Unlike laxity, instability is usually symptomatic. It represents a failure of static and dynamic constraints to maintain the humeral head precisely centered within the glenoid. Instability may occur in one direction, such as anterior instability following a traumatic anterior dislocation, or it may be multidirectional, occurring in any combination of anterior, posterior, or inferior directions. Patients with multidirectional instability often have asymptomatic laxity of the contralateral shoulder. What distinguishes these shoulders from normally functioning shoulders is a complex interaction of muscular, neurologic, and structural factors.
Glenohumeral Stabilizers
Shoulder dislocations are the most common form of joint dislocation, with an average incidence of 1.7%, demonstrating the great potential for instability that exists in the shoulder. The critical constraints for the control of shoulder stability may be divided into static and dynamic elements. The interaction of these constraining elements is complex. In the pathologic state, where one or more constraining factors is abnormal, instability may occur. Restoring these normal anatomic constraints is critical to the successful treatment and rehabilitation of the shoulder.
Static Stabilizers
Early investigators focused on the articular components of glenohumeral stability. The humeral head retroversion roughly matches the glenoid orientation on the chest wall. Saha emphasized the contribution of this articular version to stability, noting that individuals with congenital anteversion of the glenoid had a greater tendency for recurrent dislocation. Subsequent studies have not confirmed this hypothesis, finding instead considerable variability in articular version and inclination. In any position of rotation only about 25% to 30% of the humeral head surface is in contact with the glenoid. The glenohumeral index, calculated by measuring the diameter of the humeral head relative to the glenoid, has been measured. It was hypothesized that individuals with larger heads relative to their glenoid would be unstable; however, investigators have made no such correlation.
The relatively smaller surface area of the glenoid relative to the humeral head emphasizes the importance of soft tissues surrounding the joint, including the labrum, capsular, and ligamentous structures. The labrum is composed of dense collagen fibers that surround and attach to the glenoid rim, creating a deeper and broader glenoid surface. Functionally the labrum increases the articular contact of the glenoid with the humerus to about one third and improves the articular conformity and thereby stability ( Fig. 4-3 ). Lippitt and Matsen demonstrated the contribution of the labrum to joint stability in cadavers. Excision of the labrum resulted in a 20% decrease in stability as measured using the stability ratio defined by Fukuda and coworkers.
Slight mismatch of the articular surface diameter of curvature between the glenoid and humeral head may have a significant effect on glenohumeral stability. Saha initially described three types of glenohumeral articulations: type A had a shallow glenoid, type B had conforming surfaces, and type C had a humeral radius greater than that of the glenoid. Soslowsky and associates, using stereophotogrammetric studies of fresh frozen cadaveric shoulders, found that 88% of glenohumeral articulations are perfectly congruent. Kim and colleagues, however, recently analyzed MRI scans in patients with multidirectional instability (MDI) and compared them with normal MRIs. They determined that the diameter of curvature of the glenoid surface in the MDI patients was greater than the humeral diameter, suggesting that this loss of conformity may play a role in instability.
A slightly negative intra-articular pressure of the glenohumeral joint also acts to maintain joint stability. Normally the shoulder contains about 1 mL of synovial fluid, which is maintained at a lower atmospheric pressure by high osmotic pressures in the surrounding tissues. Warner and coworkers demonstrated that in normal shoulders an inferiorly directed force of 16 N generated an inferior translation of 2 mm; however, if the capsule of the same shoulder is vented, the same force generates an inferior translation of 28 mm.
The conformity of the glenohumeral joint combined with the presence of synovial fluid generates adhesion and cohesion between the humeral head and the glenoid in much the same fashion as a moist glass sticks to a coaster. Adhesion is due to the material properties of the synovial fluid, but cohesion is due to the conformity of the joint. The compliant labrum further potentiates these stabilizing effects.
The glenoid geometry and labrum in concert with muscle contractions of the rotator cuff are responsible for stability in the midranges of motion. The capsuloligamentous structures that remain lax during the midrange of motion are mainly responsible for stability at the end ranges of motion when all other stabilizing mechanisms have been overwhelmed. ,
The Glenohumeral Ligaments
The superior ligaments comprise the coracohumeral ligament (CHL) and the superior glenohumeral ligament (SGHL) ( Fig. 4-4 ). The CHL is broad, thin, extra-articular structure originating on the coracoid process and inserting broadly on the greater and lesser tuberosities, intermingling with the fibers of the supraspinatus and subscapularis. The SGHL, which lies deep to the CHL, is present in over 90% of cases, originating on the superior tubercle of the glenoid and inserting anteriorly just medial to the bicipital groove. Together these structures resist inferior translation with the arm in adduction.
The middle glenohumeral ligament (MGHL) has the greatest variation both in size and presence of all glenohumeral ligaments. It is absent in up to 30% of shoulders. The morphology of the MGHL may be sheetlike or cordlike, and it usually originates along with the SGHL on the superior glenoid tubercle, inserting just medial to the lesser tuberosity. Although the MGHL limits inferior translation in the adducted and externally rotated shoulder, the ligament primarily functions to limit anterior translation of the humerus on the glenoid with the shoulder abducted 45 degrees. In individuals with a more cordlike MGHL it may also function to limit anterior translation in the 60- to 90-degree abduction range with the arm externally rotated. ,
The inferior glenohumeral ligament (IGHL) is likely the most important ligament complex of the glenohumeral joint. The IGHL is composed of thickened bands that form a sling, or “hammock,” that cradles the humerus inferiorly in what is referred to as the axillary pouch. Typically, the IGHL originates broadly at the equatorial to inferior half of the anterior glenoid adjacent to the labrum and inserts just inferior to the MGHL medial to the lesser tuberosity. In a histologic and anatomic study by O’Brien and coworkers, they demonstrated that the posterior and anterior portions of the IGHL contain thickened bands of dense collagen fibers. Gohlke and coworkers confirmed the existence of the anterior band, but found the posterior band to be present in only 62.8% percent of individuals. The stabilizing function of the IGHL complex increases as the arm is elevated in abduction. With external rotation of the arm in 90 degrees of abduction, the anterior band broadens and tightens, forming a taut sling that prevents anterior translation. Similarly, with internal rotation of the abducted arm, the posterior band fans out and tightens. The IGHL is also the primary restraint to inferior translation of the humerus with the arm in 90 degrees of abduction.
The Interplay Between Static and Dynamic Constraints
Ligaments only function under some degree of tension. However, during normal motion of the glenohumeral joint the ligaments remain under little to no tension. In addition a large amount of passive translation is commonly possible in multiple directions. Yet the humeral head remains perfectly centered in the glenoid during normal active motion. Therefore, it seems that factors other than the capsule and ligaments must be contributing to the lack of translation observed with normal motion. The factor responsible for maintaining the humeral head centered in the glenoid is therefore the interplay between the remaining static stabilizers (adhesion, cohesion, negative intra-articular pressure, and the congruency of the joint) and the dynamic stabilizers (the rotator cuff muscles, biceps brachii, the scapular rotators, and coordinated proprioceptive feedback) of the shoulder.
Despite the complex dynamic and static constraints that maintain the humeral head centered in the glenoid, pathologic translation of the humeral head does occur. In all but the most severe cases of laxity, shoulder dislocations result in tearing or fracturing of the glenohumeral architecture. Selective cutting experiments have demonstrated the potential instability that may result from sectioning individual capsular and ligamentous structures of the glenohumeral joint.
Cadaveric experiments have demonstrated the primary ligamentous constraints to translation of the humeral head on the glenoid in the anterior, inferior, and posterior directions. The anterosuperior band of the IGHL is the primary ligamentous constraint to anterior translation with the arm abducted and externally rotated. As abduction decreases, the MGHL is of increasing importance in resisting anterior translation. The primary constraints to inferior translation in the adducted arm are the superior structures, particularly the SGHL which is maximized by external rotation. , With increasing abduction to 90 degrees, the IGHL becomes the primary constraint to inferior translation. The primary constraint to posterior translation is the posterior band of the IGHL. Although resection of the posterior capsule increases posterior translation, it is not sufficient for a posterior glenohumeral dislocation to occur. However, posterior dislocation is possible if the same shoulder is incised anterosuperiorly, cutting the SGHL and MGHL. Posterior translation increases with the arm in 30 degrees of extension if the anterior band of the IGHL is incised or detached from its glenoid insertion. ,
Dynamic Stabilizers
The rotator cuff muscles improve joint stability by increasing the load necessary to translate the humeral head from its centered position in the glenoid. Lippitt and Matsen found that tangential forces as high as 60% of the compressive force were required to dislocate the glenohumeral joint in a cadaveric study, finding also that joint stability was reduced with removal of a portion of the anterior labrum. Similarly, Wuelker and associates noted a nearly 50% increase in anterior displacement of the humeral head in response to a 50% reduction in rotator cuff forces. The glenohumeral joint reaction force has been calculated in a cadaveric model to reach a maximum of 0.89 times body weight. Glenohumeral joint contact pressures measure a maximum of 5.1 MPa in cadavers using pressure-sensitive film with the arm in 90 degrees of abduction and 90 degrees of external rotation. Joint reaction forces increase with increasing abduction angle and peak at 90 degrees abduction. Increasing joint compression appears to increase the centering of the humeral head, thereby providing a stable fulcrum for arm elevation. ,
Other factors may also contribute to dynamic glenohumeral stability. Ligament dynamization through attachment to the rotator cuff muscles has been postulated, whereby rotator cuff contraction may affect tensioning of the glenohumeral capsuloligamentous complex. Similarly, Pagnani and colleagues hypothesized that biceps contracture may tension the relatively mobile labrum and thereby tension the associated SGHL and MGHL, potentially enhancing stability. They also conclude that the long head of the biceps may itself stabilize the joint depending on shoulder position. Whether this is a true dynamic function is controversial. Yamaguchi and coworkers observed no biceps muscle activity with normal arm motion in both normal rotator cuffs and deficient cuffs.
The capsuloligamentous structures may also provide propioceptive feedback on joint position. Vangsness and associates found low-threshold, rapid-adapting pacinian fibers in the glenohumeral ligaments. Others have found diminished proprioception in shoulders with instability, with subsequent improvement after repair. Proprioceptive feedback likely helps not only to tension the rotator cuff muscles, but also to position the scapula and clavicle appropriately in space.
