We are faced with a paradox in shoulder anatomy. Knowledge regarding the details of shoulder anatomy has increased along with knowledge regarding its clinical significance. However, our exposure is decreasing. The proportion of the medical school curriculum dedicated to anatomy is shrinking, and at the same time, a number of open procedures, where residents and students might observe the anatomy firsthand, have been replaced by arthroscopic procedures ( Fig. 3.1 ). This decreased exposure has been offset by new imaging techniques that enable a deeper understanding of the current anatomy, both normal and abnormal, of the patient. Magnetic resonance imaging (MRI) allows a more detailed anatomic study of the patient, and ultrasound adds the ability to dynamically observe some structures.
This chapter details the anatomic structures of the shoulder. According to the concept of a layered portrait, the material is arranged in a layered fashion. The discussion begins with the innermost layer, the bones and joints, which are the most palpable and least deformable structures of the shoulder. They are the easiest to visualize and are the best understood anatomic landmarks. We then reveal the muscle layers that enable motion of the shoulder and the nerves that direct the muscles and provide sensation. We will discuss the vessels that control the internal environment of the tissues of the shoulder and, finally, the skin that encloses the shoulder.
The central theme of the shoulder is motion. The amount of motion in the shoulder sets it apart from all other joints and accounts for the manner in which the shoulder differs from all other regions of the body.
Bones and joints
The orthopedic surgeon considers bones primarily as rigid links that are moved, secondarily as points of attachment for ligaments, muscles, and tendons, and finally as the base on which important relationships with surrounding soft tissue are maintained. Treatment of fractures has been called the treatment of soft tissues surrounding them. In relation to pathology, bones are three-dimensional objects of the anatomy that must be maintained or restored for joint alignment. Bones exist in a positive sense to protect soft tissue from trauma and to provide a framework for muscle activity. In a negative sense, they can act as barriers to dissection for a surgeon trying to reach and repair a certain area of soft tissue, and as a barrier to ultrasound visualization. Loss of bone position may endanger soft tissue in an acute sense, and loss of bone alignment may endanger the longevity of adjacent joints.
Joints have two opposing functions: to allow desired motion and to restrict undesirable motion. The stability of joints is the sum of (1) their bony congruity and stability, (2) the stability of ligaments, and (3) the dynamic stability provided by adjacent muscles. The shoulder has the greatest mobility of any joint in the body and has the greatest predisposition to dislocation.
This great range of motion is distributed to three diarthrodial joints: the glenohumeral, acromioclavicular, and sternoclavicular. The last two joints, in combination with the fascial spaces between the scapula and chest, are collectively known as the scapulothoracic articulation . Because of the lack of congruence in two diarthrodial joints (the acromioclavicular and sternoclavicular joints), motion of the scapulothoracic articulation is mainly determined by the opposing surfaces of the thorax and scapula. Approximately one-third of the total elevation occurs in this part of the shoulder; the remainder occurs in the glenohumeral cavity. The three diarthrodial joints are constructed with little bony stability and mainly rely on their ligaments and on adjacent muscle at the glenohumeral joint. The large contributions of the scapulothoracic joint and axial body mechanics to the shoulder function have been emphasized over the past decade.
The division of motion over these articulations has two advantages. First, it allows the muscles crossing each of these articulations to operate in the optimal portion of their length-tension curve. Second, the glenohumeral rhythm allows the glenoid to be brought underneath the humerus to bear some weight of the upper limb, which decreases the demand on the shoulder muscles to suspend the arm. Such a division of motion is particularly important when the muscles are operating near the maximal abduction, the point in their length-tension curve at which they produce less force. , The study of the ultrastructure of ligaments ( Fig. 3.2 ) and tendons concerning the shoulder is in its infancy; however, preliminary studies reveal little difference in terms of collagen biochemistry and fiber structure. ,
Discussion regarding the bones and joints will proceed from the proximal to the distal portion of the shoulder and will include the joint surfaces, ligaments, and special intra-articular structures. Joint stability and the relative importance of each ligament to that stability are elaborated. The morphology of bones and their important muscle and ligament attachments are discussed. Finally, the relationship of bones and joints to other important structures in the shoulder is demonstrated.
Sternoclavicular joint
The sternoclavicular joint, which is composed of the upper end of the sternum and the proximal end of the clavicle, is the only skeletal articulation between the upper limb and axial skeleton. In both the vertical and anteroposterior dimensions, this portion of the clavicle is larger than the opposing sternum and extends superiorly and posteriorly relative to the sternum. , The prominence of the clavicle superiorly helps create the suprasternal fossa. The sternoclavicular joint has relatively little bony stability, and the bony surfaces are somewhat flat. The ligamentous structures provide the stability of the joint. The proximal surface of the clavicle is convex in the coronal plane but somewhat concave in the transverse plane. The joint angles from the anteromedial to the posterolateral in the axial plane. In the coronal plane, the joint surface is medially angled toward the superior end; the joint surfaces are covered with hyaline cartilage. In 97% of cadavers, a complete disk is observed to separate the joint into two compartments ( Fig. 3.3 ). The disk is rarely perforated. , The intra-articular disk is superiorly attached to the first rib below and to the superior surface of the clavicle through the interclavicular ligament. The disk rarely tears or dislocates by itself.
The major ligaments in the joint are the anterior and posterior sternoclavicular or capsular ligaments ( Fig. 3.4 ). The fibers superiorly run from their attachment to the sternum to their superior attachment on the clavicle. The most important ligament of this group, the posterior sternoclavicular ligament, is the strongest stabilizer to the inferior depression of the lateral end of the clavicle. The paired sternoclavicular ligaments are primary restraints so that minimal rotation occurs during depression of the clavicle.
The interclavicular ligament runs from clavicle to clavicle, with attachment to the sternum, and may be absent or nonpalpable in up to 22% of the population. The ligament tightens as the lateral end of the clavicle is depressed, thereby contributing to joint stability. The anterior and posterior costoclavicular ligaments attach from the first rib to the inferior surface of the clavicle. The anterior costoclavicular ligament resists lateral displacement of the clavicle on the thoracic cage, and the posterior ligament prevents medial displacement of the clavicle relative to the thoracic cage. Cave considered that these ligaments acted as a pivot around which much of the sternoclavicular motion occurs. Bearn found that they were not the fulcrum in depression until after the sternoclavicular ligaments were cut. They are the “principal limiting factor” in passive elevation of the clavicle and are a limitation on protraction and retraction. Perhaps the costoclavicular ligaments enable good results to be reported for proximal clavicle resection.
In the classic study regarding stability of the sternoclavicular joint, Bearn found that the posterior sternoclavicular or capsular ligament contributed most to resisting the depression of the lateral end of the clavicle. He performed serial ligament releases on cadaver specimens and carefully observed the mode of failure and shifting of fulcra. This qualitative observation is a useful addition to computerized assessment of joint stability.
Although reliable electromyographic studies demonstrate that the contribution of the upward rotators of the scapula is minimal in standing posture, permanent trapezius paralysis often leads to an eventual depression of the lateral end of the scapula relative to the other side, although this depression may be only a centimeter or two. Bearn’s experiment should probably be replicated using more sophisticated equipment to produce length-tension curves and quantitatively test the response of the joint to rotational and translational loading in the transverse and vertical axes and the anteroposterior axis that Bearn qualitatively tested.
Motion occurs in both the sections of the sternoclavicular joint: elevation and depression occur in the joint between the clavicle and disk, and anteroposterior motion and rotatory motion occur between the disk and sternum. The range of motion in living specimens is approximately 30 to 35 degrees of upward elevation. Movement in the anteroposterior direction is approximately 35 degrees, and rotation along the long axis is 44 to 50 degrees. Most sternoclavicular elevation occurs between 30 and 90 degrees of arm elevation. Rotation occurs after 70 to 80 degrees of elevation. Estimation of the limitation of range of motion as a result of fusion is misleading because of the secondary effects on the length-tension curve of the muscles of the glenohumeral joint and the ability of the glenoid to support the weight of the arm. Fusion of the sternoclavicular joint limits abduction to 90 degrees. ,
The blood supply to the sternoclavicular joint is derived from the clavicular branch of the thoracoacromial artery, with additional contributions from the internal mammary and suprascapular arteries. The nerve supply to the joint arises from the nerve to subclavius, with some contribution from the medial supraclavicular nerve.
Immediate relationships of the joint are the origins of the sternocleidomastoid in front and the sternohyoid and sternothyroid muscles behind the joint. Of prime importance, however, are the great vessels and trachea ( Fig. 3.5 ), which are endangered during posterior dislocation of the clavicle from the sternum—a rare event that may precipitate a surgical emergency. , ,
An open epiphysis is a structure not commonly observed in adults. However, the epiphysis of the clavicle does not ossify until the late teens and may not fuse to the remainder of the bone in men until the age of 25 years. , Therefore the clavicular epiphysis is a relatively normal structure within the age group at greatest risk for major trauma. The epiphysis is very thin and not prominent, which makes differentiation of physeal fractures from dislocations difficult. Instability of the sternoclavicular joint may result from trauma; however, in some individuals, it develops secondary to constitutional laxity.
Clavicle
The clavicle is a relatively straight bone when anteriorly viewed, whereas, in the transverse plane, it resembles an italic S ( Fig. 3.6 ). The greater radius of curvature occurs at its medial curve, which is anteriorly convex; the smaller lateral curve is posteriorly convex. The bone is somewhat rounded in its midsection and medially and relatively flat laterally. DePalma described an inverse relationship between the degree of downward facing of the lateral portion of the clavicle and radius of curvature of the lateral curve of the clavicle.
The obvious processes of the bone include the lateral and medial articular surfaces. The medial end of the bone has a 30% incidence of a rhomboid fossa on its inferior surface where the costoclavicular ligaments insert and a 2.5% incidence of actual articular surface facing inferiorly toward the first rib. The middle portion of the clavicle contains the subclavian groove, where the subclavius muscle has a fleshy insertion ( Fig. 3.7 ). The lateral portion of the clavicle has the coracoclavicular process when present.
The clavicle has three bony impressions for attachment of ligaments. At the medial side is an impression for the costoclavicular ligaments, which at times may be a rhomboid fossa. At the lateral end of the bone is the conoid tubercle and at the posterior portion of the lateral curve of the clavicle and the trapezoid line, which lies in an anteroposterior direction just lateral to the conoid tubercle. The conoid ligament attaches to the clavicle at the conoid tubercle, while the trapezoid ligament attaches at the trapezoid line. The relative position of these ligament insertions is important in their function. , , Rios et al. reported that the distance from the lateral edge of the clavicle to the medial edge of the conoid tuberosity in male and female specimens was approximately 45 mm, while the distance to the center of the trapezoid tuberosity was approximately 25 mm. These findings have potential implications in surgical reconstruction of the acromioclavicular joint.
Muscles that insert on the clavicle are the trapezius on the posterosuperior surface of the distal end and subclavius muscle, which has a fleshy insertion on the inferior surface of the middle third of the clavicle. Four muscles originate from the clavicle: the deltoid originates on the anterior portion of the inner surface of the lateral curve; the pectoralis major originates from the anterior portion of the medial two-thirds; the sternocleidomastoid largely originates on the posterior portion of the middle third; and contrary to its name, the sternohyoid to a small extent originates on the clavicle, just medial to the origin of the sternocleidomastoid.
Functionally, the clavicle mainly acts as a point of muscle attachment. Some literatures suggest that with good repair of the muscle, the only functional consequences of surgical removal of the clavicle are limitations in heavy overhead activity, , and thus its function as a strut is less important. This concept would seem to be supported by the relatively good function of individuals with congenital absence of the clavicle. However, others have found that sudden loss of the clavicle in adulthood has a devastating effect on shoulder function.
Important relationships to the clavicle are the subclavian vein and artery and brachial plexus posteriorly. In fact, the medial anterior curve is often described as an accommodation for these structures and does not form in Sprengel deformity, a condition wherein the scapula does not descend. Therefore the attached clavicle does not need to accommodate. The curve is a landmark for finding the subclavian vein. This relationship is more a factor in surgery than in trauma because the bone acts as an obstruction to surgeons in reaching the nerve or vessel tissue that they wish to treat. In trauma, clavicular injury usually does not affect these structures despite their close relationship, and nonunion is rare. Most cases of neurovascular trauma are of the following two groups: injury to the carotid artery from the displaced medial clavicle and compression of structures over the first rib.
Acromioclavicular joint
The acromioclavicular joint is the only articulation between the clavicle and scapula, although few individuals, as many as 1%, have a coracoclavicular bar or joint. , Lewis reported in his study that approximately 30% of cadavers had articular cartilage on the opposing coracoid and clavicular surfaces, with no bony process on the clavicle directed toward the coracoid.
The capsule of the acromioclavicular joint contains a diarthrodial joint incompletely divided by a disk that, unlike that of the sternoclavicular joint, usually has a large perforation at its center. , The capsule tends to be thicker on its superior, anterior, and posterior surfaces than on the inferior surface. Upward and downward movement allows rotation of approximately 20 degrees between the acromion and clavicle, which occurs in the first 20 degrees and last 40 degrees of elevation. It is estimated that many individuals have an even narrower range of motion; in some cases, fusion of the acromioclavicular joint does not decrease shoulder motion. DePalma found degenerative changes of both the disk and articular cartilage to be the rule rather than the exception in specimens in the fourth decade or older.
Blood supply to the acromioclavicular joint is mainly derived from the acromial artery, a branch of the deltoid artery of the thoracoacromial axis. Rich anastomoses are present between this artery, the suprascapular artery, and the posterior humeral circumflex artery. The acromial artery comes off the thoracoacromial axis anterior to the clavipectoral fascia and perforates back through the clavipectoral fascia to supply the joint. It also directs branches anteriorly up onto the acromion. Innervation of the joint is supplied by the lateral pectoral, axillary, and suprascapular nerves.
The ligaments about the acromioclavicular articulation and the trapezoid and conoid ligaments have been extensively studied ( Fig. 3.8 ). Traditional and more recent studies have reported that anteroposterior stability of the acromioclavicular joint is controlled by the acromioclavicular ligaments and that vertical stability is controlled by the coracoclavicular ligaments. , A serial-cutting experiment involving 12 force-displacement measurements was performed using more sophisticated equipment than that used in previous studies. Three anatomic axes of the acromioclavicular joint were used, and translation and rotation on each axis in both directions were measured. The results of the experiment confirmed the previously held views, particularly when displacements were large.
The acromioclavicular ligaments were found to be responsible for controlling posterior translation of the clavicle on the acromion. (In anatomic terms, this motion is really an anterior translation of the scapula on the clavicle.) The acromioclavicular ligaments were responsible for 90% of anteroposterior stability, and 77% of stability for the superior translation of the clavicle (or inferior translation of the scapula) was attributed to the conoid and trapezoid ligaments. Distraction of the acromioclavicular joint was limited by the acromioclavicular ligaments (91%), and compression of the joint was limited by the trapezoid ligament (75%), as discussed later.
The unique findings of the study were the contributions of these ligaments during small displacements. The acromioclavicular ligaments played a much larger role in many of these rotations and translations than in larger displacements, which may reflect the shorter lengths of the acromioclavicular ligaments. At shorter displacements, a greater load is applied to the fibers of the acromioclavicular ligaments for the same displacement.
Interpretation of the stability that is attributed to the acromioclavicular ligaments should reflect the additional role that they play in maintaining the integrity of the acromioclavicular joint. Although we would expect the linear arrangement of collagen in the acromioclavicular ligaments to resist distraction, it makes little sense that the acromioclavicular ligaments would resist compression with these fibers; yet in the study, 12% to 16% of compression stability was attributed to the acromioclavicular ligament. Maintaining the integrity of the acromioclavicular joint, particularly the position of the interarticular disk, might explain this discrepancy. We would not expect the acromioclavicular ligament to resist superior translation of the clavicle were it not for the presence of an intact joint below it, creating a fulcrum against which these ligaments can produce a tension-band effect.
These ligaments are seldom called on to resist trauma, and their usual function is to control joint motion. As noted earlier, this joint has relatively little motion, and muscles controlling scapulothoracic motion insert on the scapula. To a large extent, the ligaments function to guide the motion of the clavicle. For example, the conoid ligament produces much of the superior rotation of the clavicle as the shoulder is elevated in flexion.
The distal end of the clavicle does not have a physeal plate. Using microscopic dissection, Todd and D’Errico found a small fleck of bone in some individuals, which appeared to be an epiphysis, but it united within 1 year. We have not observed this structure during surgery or using roentgenogram. The articular cartilage probably functions in longitudinal growth as it does in a physis.
Scapula
The scapula is a thin sheet of bone that mainly functions as a site of muscle attachment ( Fig. 3.9 ). It is thicker at its superior and inferior angles and at its lateral border, where some of the more powerful muscles are attached ( Figs. 3.10 and 3.11 ). The scapula is also thick at the sites of formation of its processes: the coracoid, spine, acromion, and glenoid. The posterior surface of the scapula and the presence of the spine create the supraspinatus and infraspinatus fossae. Three processes, the spine, coracoid, and glenoid, create two notches in the scapula. The suprascapular notch is at the base of the coracoid, and the spinoglenoid or greater scapular notch is at the spine base. The coracoacromial and transverse scapular ligaments are two of the several ligaments that attach to two parts of the same bone. Sometimes an inferior transverse scapular ligament is found in the spinoglenoid notch. This transverse ligament and the ganglia of the labrum may all be factors in suprascapular nerve deficits. The coracoglenoid ligament, which originates on the coracoid between the coracoacromial and coracohumeral ligaments and inserts on the glenoid near the origin of the long head of the biceps, is rarely studied. The major ligaments that originate from the scapula are the coracoclavicular, coracoacromial, acromioclavicular, glenohumeral, and coracohumeral.
The coracoid process comes off the scapula at the upper base of the neck of the glenoid and anteriorly passes before hooking to a more lateral position. The coracoid process functions as the origin of the short head of the biceps and coracobrachialis tendons and serves as the insertion of the pectoralis minor muscle and the coracoacromial, coracohumeral, and coracoclavicular ligaments. Several anomalies of the coracoid have been described. As much as 1% of the population has an abnormal connection between the coracoid and clavicle, a bony bar or articulation. Some surgeons have seen impingement in the interval between the head of the humerus and the deep surface of the coracoid. The coracohumeral interval is smallest in the internal rotation and forward flexion.
The spine of the scapula functions as part of the insertion of the trapezius on the scapula and as the origin of the posterior deltoid. It also suspends the acromion in the lateral and anterior directions and thus serves as a prominent lever arm for the function of the deltoid. The dimensions of the spine of the scapula are regular, with less than 1.5-cm variation from the mean in any dimension. Sacrifice of the entire spine, including the acromion, has a predictably devastating effect on the shoulder function. ,
Because of the high occurrence of pathology involving the acromion and head of the humerus, the acromion is the most studied process of the scapula. Tendinitis and bursitis have been related to the impingement of the head of the humerus and coracoacromial arch in an area called the supraspinatus outlet. When viewed from the front, a 9- to 10-mm gap (6.6 to 13.8 mm in men and 7.1 to 11.9 mm in women) can be observed between the acromion and humerus. Recent advances in radiographic positioning allow better visualization of the outlet from the side or sagittal plane of the scapula.
Several methods of describing the capaciousness of this space or its tendency for mechanical discontinuity have been devised. Aoki et al. used the slope of the ends of the acromion relative to a line connecting the posterior acromion with the tip of the coracoid of the scapula to determine the propensity for impingement problems. Bigliani et al. divided the acromia into three types (or classes) on the basis of their shape and correlated the occurrence of the rotator cuff pathology in cadavers with the shape of the acromion on supraspinatus outlet radiographs. Their classification is generally easy to use; however, the transition between the types is smooth, so some inter-interpreter variability will occur in those cases close to the transitions. Type I acromia are those with a flat undersurface and the lowest risk for impingement syndrome and its sequelae. Type II has a curved undersurface, while type III has a hooked undersurface. As one would expect, a type III acromion with its sudden discontinuity in shape had the highest correlation with subacromial pathology. A report by Banas et al. comments on the position of the acromion in the coronal plane (i.e., the lateral downward tilt). In their series of 100 MRI procedures, an increasing downward tilt was associated with a greater prevalence of cuff disease. The remainder of the roof of the supraspinatus outlet comprises the coracoacromial ligament, which connects two parts of the same bone. It is usually broader at its base on the coracoid, tapers as it approaches the acromion, and has a narrower but still broad insertion on the undersurface of the acromion; it covers a large portion of the anterior undersurface of the acromion and invests the tip and lateral undersurface of the acromion ( Fig. 3.12 ). The ligament may not be wider at its base and often has one or more diaphanous areas at the base. Because of the high incidence of impingement in elevation and internal rotation, the acromia in individuals older than the fifth decade frequently have secondary changes such as spurs or excrescences.
In addition to static deformation of the acromion, one would expect an unfused acromion epiphysis to lead to the deformability of the acromion on an active basis and decrease the space of the supraspinatus outlet. However, Neer found no increased incidence of unfused epiphyses in his series of acromioplasties. Liberson classified the different types of unfused acromia as pre-acromion, meso-acromion, meta-acromion, and basi-acromion centers ( Fig. 3.13 ). In his series, an unfused center was noted on 1.4% of roentgenograms and bilaterally in 62% of cases. The meso-acromion–meta-acromion defect was most frequently found ( Fig. 3.14 ).
The glenoid articular surface is within 10 degrees of being perpendicular to the blade of the scapula, with the mean being 6 degrees of retroversion in one study. Strauss et al. reviewed multiple studies on various glenoid measurements. The mean glenoid height was found to range between 32.6 and 39 mm, with women having smaller glenoid heights. Mean glenoid width ranged from 23.6 mm in females to 28.3 mm in males. The mean glenoid version was found to range from 2 degrees anteversion to 9 degrees retroversion. The glenoid tends to go into more retroversion with osteoarthritis. Churchill et al. found the glenoids of men to be slightly more retroverted than those of women (1.9 vs. 0.9 degrees, respectively) and those of white patients significantly more retroverted than those of black patients (2.7 vs. 0.2 degrees, respectively). Churchill et al. also found the inclination to range from 3.6 degrees to 5.3 degrees superior inclination, with no significant difference based on race or gender.
The glenoid articular surface is inferiorly wider in the shape of a pear. The inferior glenoid has also been described as circular. The more caudad portions face more anteriorly than the cephalad portions. This perpendicular relationship, combined with the complementary orientation of the scapula and relationships determined by the ligaments of the scapulohumeral orientation, makes the plane of the scapula the most suitable coronal plane for physical and radiologic examination of the shoulder. The plane of the glenoid defines the sagittal planes, whereas the transverse plane remains the same.
The blood supply to the scapula is derived from vessels in the muscles that take fleshy origin from the scapula (see the “Muscles” section). Vessels cross these indirect insertions and communicate with bony vessels. The circulation of the scapula is metaphyseal in nature; the periosteal vessels are larger than usual, and they freely communicate with the medullary vessels rather than being limited to the outer third of the cortex. Such an anatomy may explain the reason for the subperiosteal dissection being bloodier here than over a diaphyseal bone. The nutrient artery of the scapula enters into the lateral suprascapular fossa or infrascapular fossa. The subscapular, suprascapular, circumflex scapular, and acromial arteries are contributing vessels.
Muscles that were not previously mentioned that originate from the scapula are the rotator cuff muscles: the supraspinatus, infraspinatus, teres minor, and subscapularis. At the superior and inferior poles of the glenoid are two tubercles for tendon origin, the superior for the long head of the biceps and the inferior for the long head of the triceps. At the superior angle of the scapula, immediately posterior to the medial side of the suprascapular notch, is the origin of the omohyoid, a muscle that has little significance for shoulder surgery but is an important landmark for brachial plexus and cervical dissection. The large and powerful teres major originates from the lateral border of the scapula. Inserting on the scapula are all the scapulothoracic muscles: trapezius, serratus anterior, pectoralis minor, levator scapulae, and major and minor rhomboids.
Humerus
The articular surface of the humerus at the shoulder is spheroid, with a radius of curvature of approximately 2.25 cm. As one moves down the humerus in the axis of the spheroid, one encounters a ring of bony attachments for the ligaments and muscles that control joint stability. The ring of attachments is constructed of two tuberosities, the intertubercular groove and the medial surface of the neck of the humerus. Ligaments and muscles that maintain glenohumeral stability do so by contouring the humeral head so that the tension in them produces a restraining force toward the center of the joint ( Fig. 3.15 ). In this position, the spheroid is always more prominent than the ligamentous or muscle attachments. For example, when the shoulder is in neutral abduction and the supraspinatus comes into play, the greater tuberosity, which is the attachment of this tendon, is on average 8-mm less prominent than the articular surface, and thus the tendon contours the humeral head. In the abduction and external rotation position, contouring of the supraspinatus is lost. The anterior inferior glenohumeral ligament now maintains the joint stability, and its attachments are less prominent than the articulating surface.
With the arm in the anatomic position (i.e., with the epicondyles of the humerus in the coronal plane), the head of the humerus is retroverted in relation to the transepicondylar axis. In addition, the average retrotorsion is less at birth than at maturity. The degree of retroversion has been a topic of debate. Boileau and Walch used three-dimensional computerized modeling of cadaveric specimens to analyze the geometry of the proximal humerus. They found a wide variation of retroversion ranging from −6.7 to 47.5 degrees. These findings have helped revolutionize shoulder arthroplasty and have brought about the development of the third-generation shoulder prosthesis or the concept of anatomic shoulder replacement. This concept centers on the great range in retroversion that is observed within populations. The surgical goal is to restore the patient’s own anatomy. Setting prosthetic replacements at an arbitrary 30 degrees to 40 degrees may not be optimal and does not account for individual anatomic variability. The intertubercular groove lies approximately 1 cm lateral to the midline of the humerus. , The axis of the humeral head crosses the greater tuberosity at approximately 9 mm posterior to the bicipital groove ( Fig. 3.16 ). The lesser tubercle (or tuberosity) lies directly anterior, and the greater tuberosity lines up on the lateral side. In the coronal plane, the head-shaft angle is approximately 135 degrees. Interestingly, this angle is less for smaller heads and greater for larger ones. The head size (i.e., radius of curvature) most strongly correlates with the patient’s height.
The space between the articular cartilage and ligamentous and tendon attachments is referred to as the anatomic neck of the humerus ( Fig. 3.17 ). It varies in breadth from approximately 1 cm on the medial, anterior, and posterior sides of the humerus to essentially undetectable over the superior surface where no bone is exposed between the edge of the articular cartilage and the insertion of the rotator cuff. The lesser tuberosity is the insertion for the subscapularis tendon, and the greater tuberosity bears the insertion of the supraspinatus, infraspinatus, and teres minor in a superior to inferior order. Because of its distance from the center of rotation, the greater tuberosity lengthens the lever arm of the supraspinatus as elevation increases above 30 degrees. It also acts as a pulley by increasing the lever arm of the deltoid below 60 degrees. The prominence of the greater tuberosity can even allow the deltoid to act as a head depressor when the arm is at the side. Below the level of the tuberosities, the humerus narrows in a region that is referred to as the surgical neck of the humerus because of the frequent occurrence of fractures at this level.
The greater and lesser tubercles constitute the boundaries of the intertubercular groove through which the long head of the biceps passes from its origin on the superior lip of the glenoid. The intertubercular groove has a peripheral roof that is referred to as the intertubercular ligament or transverse humeral ligament , which has varying degrees of strength. , Recent study has demonstrated that the coracohumeral ligament is the primary restraint to tendon dislocation. The coracohumeral ligament arises from the coracoid as a V-shaped band, the opening of which is posteriorly directed toward the joint. In most cases, the ligament histologically represents only a V-shaped fold of the capsule and has no distinct ligamentous fibers. Tightening this area does affect the shoulder function. The ring of tissue constituting the pulley constraining the biceps tendon comprises the superficial glenohumeral ligament (floor) and the coracohumeral ligament (roof). Because the biceps tendon is a frequent site of shoulder pathology, attempts have been made to correlate the anatomy of its intertubercular groove with a predilection for pathology ( Fig. 3.18 ). Biceps tendinitis was considered to result from dislocation of the tendon secondary to a shallow groove or supratubercular ridge and an incompetent transverse humeral ligament. Meyer attributed the greater number of dislocations of the biceps tendon on the left to activities in which the left arm is in the external rotation, a position that should have been protective. Current opinion is that the dislocation of the tendon is a relatively rare etiology of bicipital tendinitis and that most cases of bicipital tendinitis can be attributed to impingement ; dislocation of the tendon is not observed except in the presence of the rotator cuff or “pulley” damage. Walch et al. analyzed long head of biceps dislocations and found that in 70% of cases, the dislocation of the long head of the biceps was associated with massive rotator cuff tears. In particular, the subscapularis was intact in only two of 46 cases. It is possible that the “variable depth of the intertubercular groove” theory may also apply to the impingement syndrome as an etiology. A shallow intertubercular groove makes the tendon of the long head of the biceps and its overlying ligaments more prominent and thus more vulnerable to impingement damage.
The intertubercular groove has a shallower structure as it distally continues, but its boundaries, referred to as the lips of the intertubercular groove, continue to function as sites for muscle insertion. Below the subscapularis muscles, the medial lip of the intertubercular groove is the site of insertion for the latissimus dorsi and teres major, with the latissimus dorsi insertion being anterior, often on the floor of the groove. The pectoralis major has its site of insertion at the same level but on the lateral lip of the bicipital groove. At its upper end, the intertubercular groove also functions as the site of entry of the major blood supply of the humeral head, the ascending branch of the anterior humeral circumflex artery. This artery enters the bone at the top of the intertubercular groove or one of the adjacent tubercles. ,
Two shoulder muscles insert on the humerus near its midpoint. On the lateral surface is the bony prominence of the deltoid tuberosity, over which is located the large tendinous insertion of the deltoid. On the medial surface, at approximately the same level, is the insertion of the coracobrachialis.
The essential relationships to be maintained in surgical reconstruction are the retrograde direction of the articular surface and this surface’s prominence relative to the muscle and ligamentous attachments. Longitudinal alignment and the distance from the head to the deltoid insertion should be maintained. In fractures above the insertion of the deltoid that heal in the humerus varus or in cases of birth injury that cause humerus varus, the head-depressing effect of the supraspinatus will be ineffective in the neutral position when the shear forces that are produced by the deltoid are maximal. Interestingly, patients with congenital humerus varus rarely complain of pain but have limitation of motion.
Muscles
In this section, individual muscles are discussed in terms of origin and insertion, with comments on the type of attachment to bone. The discussion proceeds to the boundaries of muscles and their function that have been described to date. Innervation of muscles is discussed in terms of the nerve or nerves and the most common root representation. The vascular supply and its point or points of entry into the muscle are discussed, with brief mention of anomalies. The muscles are separated into four groups for purposes of discussion. First are the scapulothoracic muscles that control the motion of the scapula. The second group consists of the strictly glenohumeral muscles that work across that joint. Third are muscles that cross two or more joints. Finally, four muscles are reviewed that are not directly involved in shoulder function but are important anatomic landmarks.
For more advanced techniques such as muscle transfers, a functional comparison of the muscles is convenient. To that end, Herzberg et al. have listed the relevant properties of muscles as follows: (1) related groups, (2) internal structure (e.g., pennate vs. longitudinal), (3) excursion, and (4) relative tensions anticipated ( Tables 3.1–3.3 ). Muscles are grouped as axial-scapular, scapular-humeral, and axial-humeral. The potential excursion is measured as the resting muscle fiber length in relation to the number of sarcomeres in series without reference to the state of connective tissue restraints. The mass fraction is determined after the fleshy part of each muscle unit is weighed, and the measurements for all shoulder girdle muscles are totaled. The mass fraction is the percentage of the total muscle weight contributed by a given muscle. The relative tension is measured in a physiologic cross-sectional area of muscle fibers and expressed as the percentage of a group of muscles.
| Muscle Unit | Muscle Fiber Arrangement | Potential Muscle Excursion (cm) | Mass Fraction (%) | Relative Tension (%) |
|---|---|---|---|---|
| Upper trapezius (clavicular) | Longitudinal | 13.8 | 2.9 | 2.6 |
| Upper trapezius (acromial) | Longitudinal | 10.1 | 2.8 | 3.5 |
| Middle trapezius | Longitudinal | 10.4 | 2.4 | 2.9 |
| Lower trapezius | Longitudinal | 14.8 | 3.2 | 2.7 |
| Levator scapulae | Longitudinal | 15.3 | 2.1 | 1.7 |
| Rhomboids | Longitudinal | 12.5 | 4.0 | 4.0 |
| Serratus anterior (upper part) | Longitudinal | 11.1 | 3.5 | 3.9 |
| Serratus anterior (lower part) | Longitudinal | 17.6 | 7.9 | 5.6 |
| Pectoralis minor | Longitudinal | 13.2 | 2.3 | 2.1 |
| Muscle Unit | Muscle Fiber Arrangement | Potential Muscle Excursion (cm) | Mass Fraction (%) | Relative Tension (%) |
|---|---|---|---|---|
| Supraspinatus | Pennate | 6.7 | 2.8 | 5.2 |
| Subscapularis | Multipennate | 7.3 | 8.6 | 14.5 |
| Infraspinatus | Pennate | 8.6 | 6.7 | 9.7 |
| Teres major | Longitudinal | 8.8 | 1.8 | 2.6 |
| Teres minor | Longitudinal | 14.9 | 5.1 | 4.3 |
| Anterior deltoid | Longitudinal | 11.5 | 3.2 | 3.4 |
| Middle deltoid (anterior part) | Multipennate | 9.2 | 2.2 | 3.0 |
| Middle deltoid (posterior part) | Multipennate | 9.0 | 7.8 | 10.8 |
| Posterior deltoid | Longitudinal | 13.9 | 4.1 | 3.7 |
| Muscle Unit | Muscle Fiber Arrangement | Potential Muscle Excursion (cm) | Mass Fraction (%) | Relative Tension (%) |
|---|---|---|---|---|
| Latissimus dorsi | Longitudinal | 33.9 | 15.9 | 5.9 |
| Pectoralis major (clavicular) | Longitudinal | 14.5 | 2.7 | 2.3 |
| Pectoralis major (sternal) | Longitudinal | 18.8 | 8.0 | 5.4 |
Scapulothoracic muscles
Trapezius
The largest and most superficial of the scapulothoracic muscles is the trapezius ( Fig. 3.19 ). This muscle originates from the spinous processes of the C7 through T12 vertebrae. The lower border can be as high as T8 or as low as L2. The upper portion of the trapezius (above C7) takes its origin off the ligamentum nuchae, with two-thirds of specimens having an upper limit of origin as high as the external occipital protuberance. Insertion of the upper fibers is over the distal third of the clavicle. The lower cervical and upper thoracic fibers have their insertion over the acromion and the spine of the scapula. The lower portion of the muscle takes insertion at the base of the scapular spine. On the anterior or deep surface, the muscle is bounded by a relatively avascular space between it and other muscles, mostly the rhomboids. Posteriorly, the trapezius muscle is bounded by fat and skin.
As a whole, the muscle acts as a scapular retractor, with the upper fibers used mostly for elevation of the lateral angle. Although some of the other fibers may come into play, only the upper fibers were found by Inman et al. to be consistently active in all upward scapular rotations. The muscle follows a cephalocaudal activation as more flexion or abduction is obtained. In forward flexion, the middle and lower trapezius segments are less active because scapular retraction is less desirable than in abduction. Suspension of the scapula is supposed to be through the sternoclavicular ligaments at rest; electromyographic studies show no activity unless there is a downward tug on the shoulder. The muscle must provide some intermittent relief to the ligaments of the sternoclavicular joint because paralysis of the trapezius produces a slight depression of the clavicle, although not as much as one might expect. The major deformity is protraction and downward rotation of the scapula. The amount of depression may depend on the amount of downward loading of the limb with a paralyzed trapezius. There appears to be a characteristic deficit seen in trapezius paralysis in which the shoulder can be brought up only to 90 degrees in coronal plane abduction but can be brought much higher in forward flexion. In one case of congenital absence of the trapezius and the rhomboids, the patient compensated by using forward flexion to elevate the arm and lordosis of the lumbar spine to bring the arms up. When his arms had reached the vertical position, he would then release his lumbar lordosis and hold the elevation with the serratus anterior. Acquired loss of trapezius function is less well tolerated. , A triple muscle transfer of the levator scapulae, rhomboideus major, and rhomboideus minor can be performed to treat trapezius palsy.
The accessory spinal nerve (cranial nerve XI) is the motor supply, with some sensory branches contributed from C2, C3, and C4. The nerve runs parallel and medial to the vertebral border of the scapula, always in the medial 50% of the muscle ( Fig. 3.20 ). The arterial supply is usually derived from the transverse cervical artery, although Salmon found the dorsal scapular artery to be dominant in 75% of his specimens. The blood supply is described as type II, a dominant vascular pedicle with some segmental blood supply at other levels. Huelke reported that the lower third of the trapezius is supplied by a perforator of the dorsal scapular artery and the upper fibers are supplied by arteries in the neck other than the transverse cervical artery. Other authors have attributed the blood supply of the lower pedicle to intercostal vessels. Trapezius muscle transfers are based on supply by the transverse cervical artery.
Rhomboids
The rhomboids are similar in function to the midportion of the trapezius, with an origin from the lower ligamentum nuchae at C7 and T1 for the rhomboid minor and T2 through T5 for the rhomboid major ( Fig. 3.21 ). The rhomboid minor inserts on the posterior portion of the medial base of the spine of the scapula. The rhomboid major inserts into the posterior surface of the medial border from the point at which the minor leaves off down to the inferior angle of the scapula. The muscle has, on its posterior surface, an avascular plane between it and the trapezius. The only crossing structure here is the transverse cervical artery superiorly or a perforator from the dorsal scapular artery. On the deep surface is another avascular fascial space that contains only the blood vessel and nerve to the rhomboids. On the muscle’s deep surface inferiorly, the rhomboid major is bounded by the latissimus at its origin. Superiorly, the rhomboid minor is bounded by the levator scapulae.
The action of the rhomboids is retraction of the scapula, and because of their oblique course, they also participate in elevation of the scapula. Innervation to the rhomboid muscle is the dorsal scapular nerve (C5), which may arise off the brachial plexus in common with the nerve to the subclavius or with the C5 branch to the long thoracic nerve. The nerve may pass deep to or through the levator scapulae on its way to the rhomboids and may contain some innervation to the levator. The dorsal scapular artery provides the arterial supply to the muscles through their deep surfaces.
Levator scapulae and serratus anterior
Two muscles, the levator scapulae and serratus anterior, are often discussed together because of their close relationship indicated by comparative anatomy studies (see Fig. 3.21 ). The levator scapulae originate from the posterior tubercles of the transverse processes from C1 through C3 and sometimes C4. It inserts into the superior angle of the scapula. The muscle is bounded in front by the scalenus medius and behind by the splenius cervicis. It is bounded laterally by the sternocleidomastoid in its upper portion and by the trapezius in its lower portion. The spinal accessory nerve crosses laterally in the middle section of the muscle. The dorsal scapular nerve may lie deep to or pass through the muscle. In specimens in which the dorsal scapular artery comes off the transverse cervical artery, the parent transverse cervical artery splits, the dorsal scapular artery passes medial to the muscle, and the transverse cervical artery passes laterally. Ordinarily, the dorsal scapular artery has a small branch that passes laterally toward the supraspinatus fossa. In at least one-third of dissections, these vessels supply the levator with circulation.
The levator acts to elevate the superior angle of the scapula. In conjunction with the serratus anterior, it produces upward rotation of the scapula. That the levator ( Fig. 3.22 ) has a mass larger than the upper trapezius is illustrated properly only by comparing the two muscles in cross-section; in most illustrations, it is obscured by the overlying musculature. Some authors speculate that this muscle may also act as a downward rotator of the scapula. Innervation is from the deep branches of C3 and C4, and part of the C4 innervation is contributed by the dorsal scapular nerve.
The serratus anterior originates from the ribs on the anterior lateral wall of the thoracic cage. This muscle has three divisions ( Fig. 3.23 ). The first division consists of one slip, which originates from ribs 1 and 2 and the intercostal space and then runs slightly upward and posteriorly to insert on the superior angle of the scapula. The second division consists of three slips from the second, third, and fourth ribs. This division inserts along the anterior surface of the medial border. The lower division consists of the inferior four or five slips, which take origin from ribs 5 to 9. They run posteriorly to insert on the inferior angle of the scapula, thus giving this division the longest lever and most power for scapular rotation.
The serratus anterior is bounded medially by the ribs and intercostal muscles and laterally by the axillary space. Anteriorly, the muscle is bounded by the external oblique muscle with which it interdigitates, where this muscle originates from the same ribs.
The serratus anterior protracts the scapula, and participates in upward rotation of the scapula. It is more active in flexion than in abduction because straight abduction requires some retraction of the scapula. Scheving and Pauly found that the muscle was activated by all movements of the humerus. The serratus operates at a higher percentage of its maximal activity than does any other shoulder muscle in unresisted activities. , Absence of serratus activity, usually because of paralysis, produces a winging of the scapula with forward flexion of the arm and loss of strength in that motion. , Muscle transfer to replace the inferior slips mainly restores only flexion.
Innervation is supplied by the long thoracic nerve (C5, C6, and C7). The anatomy of this nerve has been studied intensely because of events in which injury has occurred. The nerve takes an angulated course across the second rib, where it can be stretched by lateral head tilt combined with depression of the shoulder. The blood supply to the serratus is classically stated to be through the lateral thoracic artery. Often, however, the thoracodorsal artery makes a large contribution to the blood supply, especially when the lateral thoracic artery is small or absent. The lateral thoracic artery is the most frequently anomalous artery taking origin from the axillary artery. The thoracodorsal artery may supply up to 50% of the muscle. The upper slips are supplied by the dorsal scapular artery. There, additional contributions from the intercostal and internal mammary arteries take place.
Pectoralis minor
The pectoralis minor has a fleshy origin anteriorly on the chest wall, from the second through the fifth ribs, and inserts onto the base of the medial side of the coracoid with frequent (15%) aberrant slips to the humerus, glenoid, clavicle, or scapula ( Fig. 3.24 ). , , The most common aberrant slip is the continuation across the coracoid to the humerus in the same path as the coracohumeral ligament. The mean width of the pectoralis minor tendon approximates 2 cm with a length of 4 cm. It has been hypothesized that the pectoralis minor is an anatomically feasible option for use in acromioclavicular joint reconstructions. Its function is protraction of the scapula if the scapula is retracted and depression of the lateral angle or downward rotation of the scapula if the scapula is upwardly rotated.
Innervation is from the medial pectoral nerve (C8, T1). Its blood supply is through the pectoral branch of the thoracoacromial artery. Reid and Taylor reported in their injection studies, however, that this vessel does not provide a constant supply to the pectoralis minor; another source is the lateral thoracic artery. Salmon found multiple tiny arteries direct from the axillary that he called the short thoracic arteries. Absence of the muscle does not seem to cause any disability. This muscle was thought to be always present when the entire pectoralis major is present, but Williams reported one case, verified at surgery, in which the pectoralis minor was missing from beneath a normal pectoralis major. Bing reported three other cases in the German literature.
Subclavius
The subclavius muscle is included with the scapulothoracic muscles because it crosses the sternoclavicular joint where most of the scapulothoracic motion takes place ( Fig. 3.25 ). It has a tendinous origin off the first rib and cartilage and a muscular insertion on the inferior surface of the medial third of the clavicle. The tendon has a muscle belly that is pennate in structure. The tendon, 1 to 1.5 inches in length, mainly lies on the inferior surface of the muscle. Its nerve supply is from the nerve to the subclavius. The blood supply is derived from the clavicular branch of the thoracoacromial artery or from the suprascapular artery. , The action of this muscle is to stabilize the sternoclavicular joint while in motion—particularly with adduction and extension against resistance, such as hanging from a bar (i.e., stabilization in intense activity).
Glenohumeral muscles
Deltoid
The largest and most important of the glenohumeral muscles is the deltoid, composed of three major sections: the anterior deltoid taking origin off the lateral third of the clavicle, the middle third of the deltoid taking origin off the acromion, and the posterior deltoid taking origin from the spine of the scapula. Typical of broadly based muscles, the origin is collagen-poor throughout its breadth. Insertion is on the deltoid tubercle of the humerus. It is a long and broad insertion. Rispoli reported the mean length of the anterior insertion was 70 mm, the middle 48.4 mm, and the posterior 63.4 mm. Klepps found that the anterior, middle, and posterior deltoid muscle fibers merged into a broad V-shaped tendinous insertion with a broad posterior band and a narrow anterior band. They found that the deltoid insertion in the vast majority of specimens was separated from the pectoralis major insertion by less than 2 mm. The axillary and radial nerves were not very close to the deltoid insertion.
The deltoid muscle’s boundary on the external side is subcutaneous fat. Because of the amount of motion involved, the subacromial bursa and fascial spaces bound the deep side. The axillary nerve and posterior humeral circumflex artery, the only nerve and the major blood supply of the muscle, also lie on the deep side.
The three sections of the deltoid differ in internal structure and function ( Fig. 3.26 ). The anterior and posterior deltoid sections have parallel fibers and a longer excursion than the middle third, which is multipennate, is stronger, and has a shorter excursion (1 cm). The middle third of the deltoid takes part in all motions involved in humerus elevation. An abundance of collagen makes it the portion of the muscle most frequently involved in contracture.
Elevation in the scapular plane is the product of the anterior and middle thirds of the deltoid, with some action by the posterior third, especially above 90 degrees. Abduction in the coronal plane decreases the contribution of the anterior third and increases the contribution of the posterior third. Flexion is produced by the anterior and middle thirds of the deltoid and the clavicular portion of the pectoralis major, with some contribution by the biceps ( Fig. 3.27 ). The contribution of the last two muscles is so small that the arm cannot be held against gravity without the deltoid. In summary, the deltoid is active in any form of elevation, and loss of deltoid function is considered a disaster. The deltoid contributes only 12% of horizontal adduction. The lower portion of the posterior deltoid has been suggested to contribute to adduction. However, Shevlin and coworkers attributed this action to providing an external rotation force on the humerus to counteract the internal rotation force of the pectoralis major, teres major, and latissimus dorsi—the major adductors of the shoulder. The deltoid accounts for 60% of strength in horizontal abduction. The deltoid muscle’s relationship to the joint is such that it has its shortest leverage for elevation in the first 30 degrees, although in this position, leverage is increased by the prominence of the greater tubercle. Gagey and Hue have shown that the deltoid may contribute to head depression at the initiation of elevation.
The anterior third of the deltoid is bounded on its deep surface by the coracoid, the conjoint tendon of the coracobrachialis, and the short head of the biceps and the clavipectoral fascia. The posterior portion of the deltoid is bounded on its deep surface by the infraspinatus and teres minor and by the teres major muscle on the other side of the avascular fascial space. The deltoid has very dense fascia on its deep surface. The axillary nerve and the posterior humeral circumflex vessels run on the muscle side of this fascia.
Innervation of the deltoid is supplied by the axillary nerve (C5 and C6), which enters the posterior portion of the shoulder through the quadrilateral space and innervates the teres minor in this position. The nerve splits in the quadrilateral space, and the nerve or nerves to the posterior third of the deltoid enter the muscle very close to their exit from the quadrilateral space and travel in the deltoid muscle along the medial and inferior borders of the posterior deltoid. Interestingly, the posterior branch extends 6 to 8 cm in length after it leaves the quadrilateral space. The branch of the axillary nerve that supplies the anterior two-thirds of the deltoid ascends superiorly and then travels anteriorly, approximately 2 inches inferior to the rim of the acromion. Paralysis of the axillary nerve produces a 50% loss of strength in elevation, even though the full abduction range is sometimes maintained. The deltoid’s vascular supply is largely derived from the posterior humeral circumflex artery, which travels with the axillary nerve through the quadrilateral space to the deep surface of the muscle. , , , The deltoid is also supplied by the deltoid branch of the thoracoacromial artery, with rich anastomoses between the two vessels. The deltoid artery travels in the deltopectoral groove and sends branches to the muscle. Numerous additional arteries are also present. The venous pedicles are identical to the arterial pedicles, except that the cephalic vein is quite dominant, especially for the anterior third of the deltoid.
Rotator cuff
Before discussing the rotator cuff muscles individually, some remarks regarding the cuff as a whole are in order. Made up of four separate muscles, the rotator cuff is a complex arrangement. The muscles may appear separate superficially, but in their deeper regions they are associated with each other, with the capsule underneath, and with the tendon of the long head of the biceps.
In their deeper regions, the tendons send fascicles into their neighbors. This sharing is at its most complex at the bicipital groove, where the fascicles of the supraspinatus destined for the insertion of the subscapularis cross over the groove and create a roof. Conversely, the fascicles of the subscapularis tendon that are headed for the supraspinatus insertion create a floor for the groove by undergoing some chondrometaplasia. Also in their deeper regions, muscles and tendons attach to the capsule. Again, the most complex of these arrangements occurs at the rotator interval. In this region, the coracohumeral ligament contributes fibers that envelope the supraspinatus tendon. This relationship is most apparent on the deep surface, where it is visible to the arthroscopist as a curved cable running from the anterior edge to the back of the supraspinatus tendon and on into the infraspinatus to create a laterally based arch or suspension bridge. This arrangement creates a thicker region of the cuff visible on ultrasound.
Kask et al. described the anatomy of the ligamentum semicirculare humeri (rotator cable) in a cadaveric and MRI study. They divided the rotator cable into three segments—anterior, middle, and posterior. The anterior segment of the rotator cable forms the lateral part of the rotator interval. The segment under the supraspinatus tendon forms the middle portion of the rotator cable. The course of the ligament is perpendicular to the longitudinal axes of the supraspinatus and infraspinatus tendons in this area. The fibers of the rotator cable, covered by the infraspinatus tendon, form the posterior part of the ligament. The descending fibers curve latero-posteriorly and end at the insertion region between the infraspinatus and teres minor tendons on the posterior side of the greater tuberosity ( Fig. 3.28 ).
Supraspinatus
The supraspinatus muscle lies on the superior portion of the scapula. It takes a fleshy origin from the supraspinatus fossa and overlying fascia and inserts into the greater tuberosity. Its tendinous insertion is in common with the infraspinatus posteriorly and the coracohumeral ligament anteriorly. This complex tendon formation is common to the rotator cuff. The superficial fibers are longitudinal and give the tendon the appearance of a more discrete structure. These more superficial fibers have larger blood vessels than do the deeper fibers. The deeper fibers run obliquely and create a nonlinear pattern that holds sutures more effectively. This tendon sends fibers anteriorly with the coracohumeral ligament over the bicipital groove to the lesser tuberosity. The anterior edge of the tendon is enveloped by the coracohumeral ligament. The anterior portion of the supraspinatus is more powerful than the posterior half, with the muscle fibers inserting onto an extension of the tendon within the anterior half of the muscle. This tendon extension can be seen on MRI. Roh et al. found that the physiologic cross-section of the anterior muscle belly was much larger than the posterior muscle belly. However, the tendon cross-sectional areas of the anterior tendon were slightly smaller than the posterior tendon. Thus a larger anterior muscle belly pulls through a smaller tendon area.
A portion of the coracohumeral ligament runs on the articular surface of the supraspinatus tendon perpendicular to the orientation of the tendon. This creates a laterally based arch that is visible from within the joint and runs all the way to the infraspinatus insertion. Its tendon may have an asymptomatic calcium deposit in as many as 2.5% of shoulders. Inferiorly, the muscular portion is bounded by its origin off the bone, the rim of the neck of the glenoid, and the capsule itself, which is not divisible from the deep fibers of the tendon ( Fig. 3.29 ).
The function of the supraspinatus muscle is important because it is active in any motion involving elevation. Its length-tension curve exerts maximal effort at approximately 30 degrees of elevation. Above this level, the greater tubercle increases its lever arm. Because the muscle circumscribes the humeral head above and its fibers are oriented directly toward the glenoid, it is important for stabilizing the glenohumeral joint. The supraspinatus, together with the other accessory muscles (the infraspinatus, subscapularis, and biceps), contributes equally with the deltoid to the torque of scapular plane elevation and forward elevation when tested by selective axillary nerve block. , The supraspinatus has an excursion approximately two-thirds that of the deltoid for the same motion, indicative of a shorter lever arm.
Other muscles of the rotator cuff, especially the infraspinatus and subscapularis, provide further downward force on the humeral head to resist shear forces of the deltoid. If these muscles are intact, even with a small rotator cuff tear, enough stabilization may be present for fairly strong abduction of the shoulder by the deltoid muscle, although the endurance may be shorter. Some patients externally rotate their shoulder so that they can use their biceps for the same activity. Because the supraspinatus is confined above by the subacromial bursa and the acromion and below by the humeral head, the tendon is at risk for compression and attrition. Because of such compression, Grant and Smith’s series and others indicate that 50% of cadaver specimens from individuals older than 77 years have rotator cuff tears. A later study by Neer showed a lower incidence.
The boundaries of the path of the supraspinatus tendon are referred to as the supraspinatus outlet . This space is decreased by internal rotation and opened by external rotation, thus showing the effect of the greater tubercle. The space is also compromised by use of the shoulder in weight-bearing, as when using crutches or doing pushups in a wheelchair. Martin suggested that the external rotation of the arm during elevation is produced by the coracoacromial arch acting as an inclined plane on the greater tubercle. Saha attributed this limitation of rotation during elevation to ligamentous control. , More recent data suggest that this external rotation is necessary to eliminate the 45-degree angulation of the humerus from the coronal plane, adding 45 degrees to the limited elevation allowed by the glenoid ( Fig. 3.30 ). Innervation of the supraspinatus is supplied by the suprascapular nerve (C5 with some C6). The main arterial supply is the suprascapular artery. These structures enter the muscle near its midpoint at the suprascapular notch at the base of the coracoid process. The nerve goes through the notch and is bounded above by the transverse scapular ligament. The nerve has no motion relative to the notch. The artery travels above this ligament. The suprascapular vessels and nerve supply the deep surface of the muscle. A branch also runs between the bone of the scapular spine and the muscle. The medial portion of the muscle receives vessels from the dorsal scapular artery.
Infraspinatus
The infraspinatus is the second most active rotator cuff muscle ( Fig. 3.31 ; see Fig. 3.30 ). It takes a fleshy, collagen-poor origin off the infraspinatus fossa of the scapula, the overlying dense fascia, and the spine of the scapula. Its tendinous insertion is in common with the supraspinatus anterior superiorly and the teres minor inferiorly at the greater tuberosity. On its superficial surface, it is bounded by an avascular fascial space on the deep surface of the deltoid. The infraspinatus is one of the two main external rotators of the humerus and accounts for as much as 60% of the external rotation force. It functions as a depressor of the humeral head. Even in the passive (cadaver) state, the infraspinatus is an important stabilizer against posterior subluxation. , An interesting aspect of muscle action at the shoulder is that a muscle may have opposing actions in different positions. The infraspinatus muscle stabilizes the shoulder against posterior subluxation in internal rotation by circumscribing the humeral head and creating a forward force. In contradistinction, it has a line of pull posteriorly and stabilizes against anterior subluxation when the shoulder is in abduction–external rotation. , The infraspinatus is a pennate muscle with a median raphe covered by a fat stripe that can be mistaken at surgery for the gap between the infraspinatus and teres minor muscles. The infraspinatus is innervated by the suprascapular nerve. The nerve tunnels through the spinoglenoid notch, which is not usually spanned by a ligament. Its blood supply is generally described as coming from two large branches of the suprascapular artery. Salmon, however, found in two-thirds of his specimens that the subscapular artery through its dorsal or circumflex scapular branch supplied the greater portion of the circulation of the infraspinatus muscle.
Teres minor
The teres minor has a muscular origin from the middle portion of the lateral border of the scapula and the dense fascia of the infraspinatus (see Figs. 3.30 and 3.31 ). Rarely are individuals found in whom the teres minor overlies the infraspinatus as far as the vertebral border of the scapula. The teres minor inserts into the lower portion of the posterior greater tuberosity of the humerus. On its deep surface is the adherent posterior capsule, and on the superficial surface is a fascial plane between it and the deep surface of the deltoid. On the inferior border lie the quadrilateral space laterally and the triangular space medially. In the quadrilateral space, the posterior humeral circumflex artery and the axillary nerve border the teres minor. In the triangular space, the circumflex scapular artery lies just inferior to this muscle. On its deep surface, in the midportion, lies the long head of the triceps tendon, loose alveolar fat, and the subscapularis. The teres minor is one of the few external rotators of the humerus. It provides up to 45% of the external rotation force and is important in controlling stability in the anterior direction. , It also likely participates in the short rotator force couple in abduction along with the inferior portion of the subscapularis. The teres minor is innervated by the posterior branch of the axillary nerve (C5 and C6). Its blood supply is derived from several vessels in the area, but the branch from the posterior humeral scapular circumflex artery is the most constant.
Subscapularis
The subscapularis muscle is the anterior portion of the rotator cuff. This muscle takes a fleshy origin from the subscapularis fossa that covers most of the anterior surface of the scapula. It contains multiple interspersed tendinous bands that merge laterally into a flattened tendon in the superior two-thirds of the muscle, while the inferior third of the subscapularis has a muscular attachment almost directly onto the inferior aspect of the lesser tuberosity and the anterior aspect of the humeral metaphysis through a thin membranous structure.
The upper 60% of the subscapularis muscle inserts through a collagen-rich tendon into the lesser tuberosity of the humerus. The lower 40% has a fleshy insertion into the humerus below the lesser tuberosity, cupping the head and neck. The internal structure of the muscle is multipennate, and the collagen is so dense in the upper subscapularis that it is considered to be one of the passive stabilizers of the shoulder. The subscapularis muscle is bounded anteriorly by the axillary space and the coracobrachialis bursa. Superiorly, it passes under the coracoid process and the subscapularis recess, or bursa. The axillary nerve and posterior humeral circumflex artery and veins pass deep below the muscle into the quadrilateral space. The circumflex scapular artery passes into the more medial triangular space. Laterally, the anterior humeral circumflex vessels mark the division between the upper 60% and the lower 40%.
The subscapularis functions as an internal rotator and passive stabilizer to prevent anterior subluxation and, especially in its lower fibers, serves to depress the humeral head ( Fig. 3.32 ). Through this last function, it resists the sheer force of the deltoid to help with elevation. Compression of the glenohumeral joint also adds to this function. Another feature of the subscapularis is that its function may vary with the level of training. The function of the subscapularis in acceleration is less in amateur pitchers than in professional throwers, implying that a less-trained thrower is still adjusting the glenohumeral joint for stability, whereas a professional can use the muscle as an internal rotator.
