Shoulder

Scapula

Ossification centers of the scapula begin to form during the eighth week of intrauterine life, but complete fusion does not occur until the end of the second decade. The acromial apophysis develops from four separate centers of ossification: the basi-acromion, meta-acromion, meso-acromion, and pre-acromion. Failure of complete fusion in a skeletally mature individual, referred to as an os acromiale, is estimated to occur in 8% of the population, with one third of cases being bilateral. The proximal humeral epiphysis is composed of three primary ossification centers (the humeral head, the greater tuberosity, and the lesser tuberosity) that coalesce at approximately age 6 years. Eighty percent of longitudinal growth of the humerus is achieved through the proximal physis. Physeal closure occurs at the end of the second decade.

The top of the humerus has a large, nearly spherical articular surface surrounded at its articular margin (anatomic neck of the humerus) by two tuberosities. The humeral head articulates with the glenoid surface, which is only a little more than one third of its size. The great freedom of movement of the glenohumeral joint is inevitably accompanied by a considerable loss of stability.

The insertion of the supraspinatus portion of the rotator cuff is superiorly on the greater tuberosity, and the infraspinatus and teres minor insert on the posteriormost part of the greater tuberosity. All of the four rotator cuff muscles take origin from the body of the scapula. The scapula is a thin sheet of bone that provides the site of attachment for several important muscles of the shoulder girdle. The lateral end of the clavicle articulates with the medial aspect of the acromion to form the acromioclavicular joint.

The large deltoid muscle has its broad origin from the spine of the scapula posteriorly around the lateral acromion and then from the lateral third of the clavicle. Likewise, the trapezius muscle takes its insertion over a very similar area superior and medial to the deltoid origin. The trapezius has its primary function in scapula retraction and elevation of scapula. The deltoid origin on the humerus at the deltoid tuberosity is approximately one third the distance from the shoulder to the elbow. The levator scapulae and rhomboid major and minor insert on the medial border of the scapula and function to retract the scapula toward the spine.

Between the anterior portion of the scapula and the chest wall (not shown) is the scapulothoracic articulation. This articulation is another important component of proper shoulder function. In addition to its contribution to overall shoulder motion, rotation of the scapula brings the glenoid underneath the humeral head so it can bear a portion of the weight of the upper extremity, thus decreasing the necessary force generated by the muscles of the shoulder girdle. Bony and soft tissue pathologic processes can result in bursitis and possibly crepitus at this articulation, leading to a “snapping scapula.”

The body of the scapula has a large concavity on its costal surface, the subscapular fossa, for the subscapularis muscle. The dorsum is convex and is separated by the prominent spinous process into a supraspinatous fossa above, for the supraspinatus muscle, and an infraspinatous fossa below, for the infraspinatus muscle. The suprascapular notch is immediately medial to the coracoid process at the superior aspect of the scapular body. The spinous process is a large triangular projection of the dorsum of the bone, extending from the medial border to just short of the glenoid process. It increases its elevation and weight as it progresses laterally and ends in a concave border, the origin of which is the neck of the scapula. The spinous process continues freely to arch above the head of the humerus as the acromion, which overhangs the shoulder joint. Its lateral surface provides origin for the posterior and middle thirds of the deltoid muscle.

The coracoid process projects anteriorly and laterally from the neck of the scapula. It gives attachment to the pectoralis minor, the short head of the biceps brachii, the coracobrachialis, the coracoacromial ligament, and the coracoclavicular ligaments. The lateral angle of the scapula broadens to form the glenoid, which has minimal bony concavity. It is pear shaped, with a wider inferior aspect. The fibrocartilaginous glenoid labrum attaches circumferentially to the margin of the glenoid, and the long head of the biceps brachii attaches directly to the supraglenoid tubercle.

Humerus

The humerus is a long bone composed of a shaft and two articular extremities. Proximally, the head is roughly one third of a sphere, although the anteroposterior dimension is slightly less than the superoinferior distance. The anatomic neck is the slight indentation at the margin of the articular surface where the capsule attaches. The surgical neck is the narrowed area just distal to the tubercles, where fractures frequently occur. The greater tubercle serves as the attachments for the supraspinatus, infraspinatus, and teres minor tendons. The lesser tubercle is the insertion of the subscapularis tendon. Each of the tubercles is prolonged downward by bony crests, with the crest of the greater tubercle receiving the tendon of the pectoralis major muscle and the crest of the lesser tubercle receiving the tendon of the teres major muscle. The intertubercular groove, lodging the long tendon of the biceps brachii muscle, also receives the tendon of the latissimus dorsi muscle into its floor. The shaft of the humerus is somewhat rounded above and prismatic in its lower portion. The deltoid tuberosity is prominent laterally over the midportion of the shaft, with a groove for the radial nerve that indents the bone posteriorly, spiraling lateralward as it descends.

Clavicle

The clavicle is the first bone to ossify in the developing embryo; however, complete ossification does not occur until the third decade of life. When viewed from above, the clavicle has a gentle S shape with a larger medial curve that is convex anteriorly and a smaller lateral curve that is convex posteriorly. The medial two thirds of the bone is roughly triangular in section, whereas the lateral third is flattened. Several bony prominences are present on the inferior surface of the clavicle. The undersurface of the lateral third of the bone demonstrates the conoid tubercle and trapezoid line, which correspond to the attachment of the two parts of the coracoclavicular ligament. Centrally, the subclavius groove receives the subclavius muscle. Medially, there is an impression where the costoclavicular ligament attaches. The sternal extremity of the bone is triangular and exhibits a saddle-shaped articular surface, which is received into the clavicular fossa of the manubrium of the sternum. The acromial extremity has an oval articular facet, directed lateralward and slightly downward, for the acromion.

In addition to functioning as a strut that keeps the shoulder in a more lateral position, it also serves as a point of attachment for several muscles. Medially, the clavicular head of the pectoralis major originates anteriorly while the sternohyoid muscle originates posteriorly. The subclavius muscle originates from the inferior surface of the middle third of the clavicle. Laterally, the anterior third of the deltoid originates anteriorly, a portion of the sternocleidomastoid originates superiorly, and a portion of the trapezius inserts posteriorly. Resection of portions of the clavicle is typically well tolerated as long as the integrity of the muscular attachments is not compromised. The sternoclavicular joint represents the only true articulation between the trunk and the upper limb. Rotation of the clavicle at this joint allows the arm to be placed in an over-the-head position. An articular disc is interposed between the joint surfaces, which greatly increases the capacity for movement. Joint stability is conveyed through static stabilizers.

Ligaments

Stability of the shoulder is highly dependent on numerous static stabilizers. The superior, middle, and inferior glenohumeral ligaments are thickenings in the anterior wall of the articular capsule. Really visible only on the inner aspect of the capsule, they radiate from the anterior glenoid margin adjacent to and extending downward from the supraglenoid tubercle of the scapula. These ligaments are best seen on arthroscopic photographs.

Sternoclavicular Joint

The sternoclavicular joint represents the only true articulation between the trunk and the upper limb. Rotation of the clavicle at this joint allows the arm to be placed in an over-the-head position. An articular disc is interposed between the joint surfaces, which greatly increases the capacity for movement. Joint stability is conveyed through static stabilizers. The articular capsule is relatively weak but is reinforced by the capsular ligaments. The anterior sternoclavicular ligament is a broad anterior band of fibers attached to the upper and anterior borders of the sternal end of the clavicle, and, below, it is attached to the upper anterior surface of the manubrium of the sternum. This strong band is reinforced by the tendinous origin of the sternocleidomastoid muscle. The posterior sternoclavicular ligament has a similar orientation on the back of the capsule and has similar bony attachments. The costoclavicular ligament is a short, flat band of fibers running between the cartilage of the first rib and the costal tuberosity on the undersurface of the clavicle. The interclavicular ligament strengthens the capsule above. It passes between the right and left clavicles with additional attachment to the upper border of the sternum. The anterior supraclavicular nerve gives the sternoclavicular joint its nerve supply. Blood supply is derived from branches of the internal thoracic artery, the superior thoracic artery, and the clavicular branch of the thoracoacromial artery.

Glenohumeral Joint

Given the lack of bony constraint, the glenohumeral joint is circumferentially surrounded by static and dynamic stabilizers. Arthroscopic examination of these structures is essential to accurately identify a pathologic process in a symptomatic shoulder. The anatomic structures and their relationship can be visualized by arthroscopy of the joint (see Plates 1-5 and 1-6 ). The long head of the biceps must be visualized along its entire intra-articular course. The integrity of the biceps anchor should be examined, as should the stability of the biceps sling at the superior aspect of the bicipital groove. The attachment of the glenoid labrum should be inspected circumferentially, although a sublabral foramen in the anterosuperior quadrant can be a normal variant. An attached labrum is seen in the arthroscopic views and art. The condition of the articular cartilage on the glenoid and humeral head can be characterized according to its appearance on arthroscopic examination. Grade 1 changes are seen as softening of the cartilage without loss of the smooth cartilage surface. Grade 2 changes show loss of the smooth cartilage surface and luster with a cobblestone appearance yet no loss of cartilage thickness. Grade 3 indicates loss of cartilage thickness and fissuring of the cartilage, giving it a velvet appearance when mild and the end of a mop appearance when severe. Grade 4 is characterized by complete loss of cartilage down to the subchondral bone. The axillary pouch must be visualized because this is a common location of loose bodies within the joint.

The insertion sites of the four rotator cuff tendons should be noted. Superiorly the footprint is adjacent to the articular margin, but posteriorly there is a bare area of bone between the articular cartilage and infraspinatus/teres minor insertion. The subscapularis tendon is located anteriorly, and complete visualization of its insertion can be challenging when there is a well-defined middle glenohumeral ligament. Medial subluxation of the long head of the biceps brachii tendon from being centered in the bicipital groove is a sign that the insertion of the subscapularis is compromised or there is damage to the medial pulley and soft tissue wall of the biceps groove.

Deltoid Muscle

The deltoid muscle is triangular with a semicircular origin along the lateral third of the clavicle, the lateral border of the acromion, and the lower lip of the crest of the spine of the scapula. All fasciculi converge to be inserted on the deltoid tuberosity of the humerus. The deltoid muscle is a principal abductor of the humerus, an action produced primarily by its powerful central portion. Because of their position and greater fiber length, the clavicular and scapular portions of the deltoid muscle have different actions from those of the central portion of the muscle. The clavicular portion assists in flexion and internal rotation of the arm, while the scapular portion assists in extension and external rotation.

The axillary nerve (C5, C6) from the posterior cord of the brachial plexus supplies the deltoid muscle. An upper branch curves around the posterior surface of the humerus and courses from behind forward on the deep surface of the muscle, sending offshoots into the muscle. A lower branch supplies the teres minor muscle by ascending onto its lateral and superficial surface. It then becomes the superior lateral brachial cutaneous nerve. The posterior circumflex humeral artery serves this muscle.

Pectoralis Muscle

The pectoralis major muscle originates from the medial half of the clavicle on its anterior surface and the anterior surface of the manubrium and body of the sternum. Additional fascicles arise from the cartilages of the second to sixth ribs as well as from the anterior layer of the sheath of the rectus abdominis muscle. The muscular fibers converge to insert on the crest immediately distal to the greater tubercle, lateral to the bicipital groove. The tendon folds on itself to form a bilaminar U -shaped tendon with the fold of the tendon below. Thus, the fibers of the clavicular part insert as the upper part of the anterior lamina; the lower sternal and abdominal fibers reach up into the superior part of the posterior limb; and the sternal fibers distribute into the anterior lamina, the fold, and the lower part of the posterior lamina.

The pectoralis major muscle flexes and adducts the humerus; it is also capable of medial rotation of the arm but usually becomes active only when this action is resisted. The clavicular portion of the pectoralis major muscle elevates the shoulder and flexes the arm, while the sternocostal portion draws the shoulder downward. The muscle is innervated by the lateral and medial pectoral nerves from both the lateral and medial cords of the brachial plexus, involving all the roots (C5 to T1). The pectoral branches of the thoracoacromial artery accompany the nerves to the muscle.

The deltopectoral triangle is a separation just below the clavicle of the upper and adjacent fibers of the deltoid and pectoralis major muscles. Distally, the separation of these adjacent fibers is made by the cephalic vein and the deltoid branch of the thoracoacromial artery.

The pectoralis minor muscle arises from the outer surfaces of the third, fourth, and fifth ribs near their costal cartilages, with a slip from the second rib a frequent addition. The muscle fibers converge to an insertion on the medial border and upper surface of the coracoid process. The pectoralis minor muscle draws the scapula forward, medially, and strongly downward. With the scapula fixed, the muscle assists in forced inspiration. The muscle is innervated by the medial pectoral nerve (C8, T1), which completely penetrates the muscle to pass across the interpectoral space into the pectoralis major muscle. Pectoral branches of the thoracoacromial artery are distributed with the nerve. Deep to the tendon of the pectoralis minor muscle pass the axillary artery and the cords of the brachial plexus.

Serratus Muscle

The serratus anterior muscle originates laterally from the first eight ribs. The muscle fibers converge to insert on the deep surface of the lateral border of the scapular body. Contraction of the muscle protracts the scapula and participates in upward rotation of the scapula. Weakness results in scapula winging (see Plates 1-20 and 1-52 ). Innervation is supplied by the long thoracic nerve (C5 to C8), which can easily be injured during axillary lymph node dissection. The blood supply is primarily through the lateral thoracic artery.

Subclavius Muscle

The subclavius muscle is a small, pencil-like muscle that arises from the junction of the first rib and its cartilage. It lies parallel to the underside of the clavicle and inserts in a groove on the underside of the clavicle, between the attachments of the conoid ligament laterally and the costoclavicular ligament medially. The muscle assists by its traction on the clavicle in drawing the shoulder forward and downward. The nerve to the subclavius muscle is a branch of the superior trunk of the brachial plexus, with fibers from the fifth cervical nerve, which reaches the upper posterior border of the muscle. There is a small, special clavicular branch of the thoracoacromial artery to the muscle.

Trapezius Muscle

The trapezius muscle is divided into upper, middle, and lower divisions with a broad origin from the occipital protuberance superiorly to the spinous process of the T12 vertebrae inferiorly. It inserts onto the posterior border of the lateral third of the clavicle, the medial border of the acromion, and the upper border of the crest of the spine of the scapula. The directionality of the upper and lower divisions allows it to rotate the scapula so the glenoid faces superiorly, which allows full elevation of the upper extremity. The middle division serves to retract the scapula. When the function of the trapezius is absent, the scapula wings laterally owing to unopposed contraction of the serratus anterior (see Plate 1-52 ). The nerves reaching the trapezius muscle are the spinal accessory (cranial nerve XI) and direct branches of ventral rami of the second, third, and fourth cervical nerves. The accessory nerve perforates and supplies the sternocleidomastoid muscle and then crosses the posterior triangle of the neck directly under its fascial covering, coursing diagonally downward to reach the underside of the trapezius muscle. The transverse cervical artery of the subclavian system supplies the trapezius muscle; it is supplemented in the lower third of the muscle by a muscular perforating branch of the dorsal scapular artery.

Levator Scapulae Muscle

The levator scapulae originates from the transverse processes of the first three or four cervical vertebrae. It inserts into the medial border of the scapula from the superior angle to the spine. It is overlapped and partially obscured by the sternocleidomastoid and trapezius muscles. It functions to elevate and adduct the scapula. Innervation is provided by the dorsal scapular nerve (C3 to C5), and blood supply is from the dorsal scapular artery.

Rhomboideus Muscle

The rhomboideus minor muscle originates from the lower part of the ligamentum nuchae and the spinous processes of C7 to T1. It lies parallel to the rhomboideus major muscle, directed downward and lateralward, and it is inserted on the medial border of the scapula at the root of the scapular spine. The rhomboideus major muscle arises from the spinous processes of T2 to T5 and inserts on the medial border of the scapula below its spine. Both rhomboideus muscles draw the scapula upward and medially and assist the serratus anterior muscle in holding it firmly to the chest wall. Their oblique traction aids in depressing the point of the shoulder. The innervation and blood supply is the same as for the levator scapulae.

Latissimus Dorsi Muscle

The latissimus dorsi muscle originates from the inferior thoracic vertebrae, the thoracolumbar fascia, the iliac crest, and the lower third to fourth ribs. It inserts onto the floor of the intertubercular groove of the humerus. Contraction of this muscle extends the humerus, drawing the arm downward and backward and rotating it internally. The muscle is innervated by the thoracodorsal nerve from the posterior cord of the brachial plexus, with fibers from the seventh and eighth cervical nerves. The thoracodorsal artery, a branch of the subscapular artery, and a vein of the same name accompany the nerve.