12.1 Introduction

The miraculous functional capacity of the hand detailed elsewhere in this book is of less benefit without the ability to position the hand in space with a stable platform. It is the dual requirements of maximum range of motion and stability that make the shoulder joint so complex. In contrast to the hip, which primarily requires stability for locomotion with range of motion as a secondary consideration, the shoulder has little inherent bony stability. The demanding functional requirements of the shoulder are achieved through the interaction of the sternoclavicular joint, clavicle, acromioclavicular joint, scapulothoracic articulation, and, most important glenohumeral joint.

12.1.1 Clavicle and Sternoclavicular Joint

Due to their subcutaneous nature, there is a danger of underestimating the potential risk associated with surgical procedures on the clavicle and sternoclavicular joint. It is critical to have a complete understanding of the relational anatomy.

It should also be appreciated that even in the absence of catastrophic blood loss following injury to subclavian vessels, venous injury may cause air embolus leading to circulatory compromise or even death. ¹ This is a particular consideration when surgery in this region is undertaken in an upright or semi-upright (beach chair) position and where spontaneous ventilation rather than positive pressure ventilation is utilized, with both of these technical aspects increasing the risk of air embolism.

The sternoclavicular joint is intimately related to the confluence of internal jugular and subclavian veins to form the brachiocephalic vein. This is readily appreciated in ▶Fig. 12.1, the joint capsule has been divided to allow the clavicle to be lifted superiorly. No tissue has been removed from between the posterior periosteum of the clavicle and the veins. The scant protection offered by the meager size of the subclavius muscle belly is evident.

The subclavian artery and axillary artery are divided into three parts by the scalenus anterior (▶Fig. 12.2) and pectoralis minor (▶Fig. 12.8), respectively. ^{2 – 4} The subclavian artery becomes the axillary artery at the lateral border of the first rib. The axillary artery then becomes the brachial artery at the lower border of the teres major. Iatrogenic injury to the artery from internal fixation of the clavicle would involve the subclavian artery at the sternoclavicular level, the first part of the axillary artery for midshaft fixation, and the second part of the axillary artery with coracoclavicular fixation.

The neurovascular bundle passes under the apex of the anterior curve of the S-shaped clavicle and traveled from superomedial to inferolateral (▶Fig. 12.1). Galley et al ⁵ assessed the relationship of the major vessels and the clavicle and noted considerable variability in the dimensions of the clavicle and the adjacent structures. As a consequence, the relative position of the vessels is best reported as a percentile of the entire length of the clavicle from the sternoclavicular joint. The mean length of the clavicle was 138 mm (range, 114–168). The subclavian vein lies under the junction of the medial and middle thirds of the clavicle at the 33rd percentile (range, 28–36) (▶Fig. 12.3), while the subclavian artery lies lateral to the vein but still medial to the midpoint of the clavicle, at the 42nd percentile (range, 37–46) (▶Fig. 12.4 and ▶Fig. 12.5). Therefore, from a surgical safety point of view, the surgeon needs to be aware that the vessels are at significant risk from the 28th to 46th percentile. ⁵ This represents 28 to 84 mm from the sternoclavicular joint. To make it easier to remember, at-risk zone is the second quarter, which is to say between the medial quarter and the midclavicle. ⁵

The distance from the superior surface of clavicle to the superior aspect of the subclavian artery was 26 mm (range, 22–34). This was due to the thickness of the clavicle (15 mm; range, 11–16) and the subclavius muscle (11 mm; range, 7–15).

Sinha et al ⁶ utilized three-dimensional CT arteriograms to study the relation between the clavicle and vessels. At the medial end of the clavicle, they noted the vessels lying posterior to the clavicle. In some of the scans, the subclavian vein was in direct contact with the posterior periosteum. This is a critical point, as any violation of this significant vein that lies adjacent on the posterior clavicle could cause profuse venous bleeding, which is very difficult to control.

In the middle third, the three-dimensional relationship was more variable, with the artery and vein lying at a mean angle of 50° (range, 12–80°) and 70° (range, 38–100°), respectively, to the horizontal. This posteroinferior relationship is demonstrated in ▶Fig. 12.6. Also seen in this figure is a branch from the subclavian vein passing superiorly, posterior to the clavicle, which may be tethered as a result of a clavicle fracture and cause tearing during surgical dissection, especially in subacute cases.

Sinha et al ⁶ also observed that at the lateral aspect of the clavicle, the vessels were inferior to the clavicle, with the artery and vein at mean distances of 64 mm (range, 47–97 mm) and 76 mm (range, 50–109 mm), respectively.

In summary, anatomic considerations would dictate the following precautions for safer clavicle surgery:

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  Extreme upright positioning should be avoided and positive pressure ventilation employed.

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  Breaching the periosteum of the posterior clavicle in the second quarter must be avoided.

  
  
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  Be mindful of the direction of drilling, specifically in relation to the foregoing three-dimensional relationships. In general terms, it is safest to direct medial screws from superior to inferior, to consider unicortical screws in the second quarter, and to direct lateral screws from anterior to posterior.

12.2 Acromioclavicular Joint

The clavicle serves as a stabilizing strut for the shoulder girdle via the scapula. The acromioclavicular joint is a synovial joint possessing an intra-articular fibrocartilaginous disc. The capsule of this synovial cavity may provide some degree of secondary stability; however, the main force transfer between the scapula and the clavicle occurs via the coracoclavicular ligaments. ⁴ The conoid ligament is more medial and posterior. It is an inverted cone with a smaller origin at the base of the coracoid and a larger insertion on the conoid tubercle on the posterior aspect of the lateral clavicle. The origin of the trapezoid ligament is more lateral on the coracoid and extends in a more horizontal fashion toward the trapezoid ridge on the clavicle (▶Fig. 12.7). This complex ligamentous arrangement allows the clavicle to perform a suspensory function and prevents medial collapse of the scapula, throughout the range of scapulothoracic positioning.

As already described, the risk of damage to vascular structures is less in relation to the lateral clavicle than more medially. However, surgical procedures in the vicinity of the coracoclavicular ligaments (such as passage of grafts or sutures around the coracoid process for acromioclavicular joint reconstruction) require greater caution with regard to nerves. ▶Fig. 12.8 demonstrates that the musculocutaneous nerve (a branch of the lateral cord of the brachial plexus) is closer to the coracoid than the axillary vessels. In this specimen, the nerve enters the coracobrachialis muscle 5 cm distal to the coracoid; however, this relationship is extremely variable and may be as little as 2 cm. In addition, care must be taken at the medial aspect of the base of the coracoid process due to the proximity of the suprascapular nerve (▶Fig. 12.9).

12.3 Glenohumeral Joint

The principal focus of surgical anatomy in the shoulder region is the glenohumeral joint.

The most common surgical approach to the glenohumeral joint is the anterior deltopectoral approach. Identification and development of the plane between the medial border of the deltoid muscle and the lateral border of the pectoralis major muscle is usually defined by the presence of the cephalic vein within the deltopectoral interval (▶Fig. 12.10). Depending on body habitus, there may be deposition of fat associated with the interval. As can be seen from this specimen, however, in the absence of significant adipose tissue, the interval may be hard to define, especially where the cephalic vein is deep, vestigial, or absent. The orientation of deltoid and pectoral muscle fibers converges distally and the interval is easier to define proximally, closer to the clavicle. Palpating the underlying coracoid process may also assist in locating the interval.

It is easier to dissect between the cephalic vein and the pectoralis major muscle, as most of the tributaries of the vein run between the lateral aspect of the vein and the deltoid muscle. There is, however, one shortcoming in taking the vein laterally, and that is if the procedure is more extensive, requiring an extensile exposure such as in complex trauma or major shoulder arthroplasty. In such a circumstance, at the superior aspect of the exposure, once the deltoid and pectoralis muscles are separated, the cephalic vein will be crossing the deltopectoral interval from lateral to medial, and it will be at risk of injury and may limit proximal exposure. For that reason, if it is anticipated that more extensive exposure is required, then it may be preferable to spend a little more time ligating the tributaries on the lateral aspect of the vein and separating the vein from the deltoid and taking it with the pectoralis major.

Having defined and developed the deltopectoral interval, the principal anatomy of the anterior aspect of the shoulder as it relates to the deltopectoral approach may be defined by two critical bony landmarks. One of these landmarks is fixed and the other mobile, and both are of equal importance in developing appropriate surgical exposures in this region.

The fixed bony landmark is the coracoid process. It is encountered in the proximal aspect of the deltopectoral interval as soon as the two muscles are separated. The coracoid process has four key attachments, one from each direction. From the medial aspect is the tendon of pectoralis minor (▶Fig. 12.8). Superiorly and more toward the root of the coracoid process are the conoid and trapezoid coracoclavicular ligaments (▶Fig. 12.7). Laterally is the coracoacromial ligament (▶Fig. 12.9), and inferiorly is the conjoint tendon of the short head of biceps and the coracobrachialis muscle (▶Fig. 12.8).

The mobile landmark which further facilitates identification of critical structures is the bicipital groove. The location of the groove depends on the rotation of the humerus; however, in general it is directly anterior when the humerus is in neutral rotation, as judged by the position of the forearm with the elbow flexed. The bicipital groove and related structures are in a deeper plane than the coracoid process, and their identification requires division of the clavipectoral fascia, particularly lateral to the coracoid and inferior to the coracoacromial ligament. The groove is occupied by the long head of biceps tendon, and this tendon may be traced superiorly to identify the rotator interval between the subscapularis and supraspinatus tendons and is one of the critical intervals for developing exposure to the glenohumeral joint. The medial aspect of the bicipital groove is formed by the lesser tuberosity, to which is attached the subscapularis tendon superiorly, with subscapularis muscular attachment to the humerus inferior to that (▶Fig. 12.11a). As the biceps tendon is traced more distally to the shallower portion of the bicipital groove, the tendon is covered by the pectoralis major tendon which inserts on the lateral lip of the bicipital groove. The pectoralis major tendon insertion is quite complex, with crossing over of the fibers such that the costal fibers tend to insert more superiorly and the sternal and clavicular fibers to insert more inferiorly. At this level within the floor of the bicipital groove is the insertion of the flat ribbon like latissimus dorsi tendon, and closely related to the latissimus dorsi insertion and just inferior to the subscapularis muscle insertion is the insertion of the teres major tendon into the medial lip of the bicipital groove. Although closely related and sometimes harvested together as a dual tendon transfer, these tendons are almost completely separate (▶Fig. 12.11b), and there is a well-defined bursa between the posterior aspect of the latissimus tendon and the medial aspect of the humeral shaft.

Most surgical approaches to the glenohumeral joint require some form of manipulation of the subscapularis tendon, whether through horizontal splitting of the muscle/tendon unit, tenotomy, or lesser tuberosity osteotomy. Exposure of the subscapularis is possible after division of the clavipectoral fascia. Mobilization of the subscapularis muscle and tendon requires dissection along the lateral border of the conjoint tendon. Loss of glenohumeral joint external rotation range of motion may be contributed by contracture around subscapularis in association with arthrosis or scar from previous surgery. Mobilization of subscapularis is advocated; however, experience in shoulder arthroplasty has suggested that aggressive mobilization may risk denervation. ⁷ The motor nerve supply to subscapularis is generally attributed to the upper and lower subscapular nerves, both branches of the posterior cord of the brachial plexus. This nerve supply is variable. ▶Fig. 12.12 demonstrates the multiple and vulnerable nature of this nerve supply and even shows a branch from the axillary nerve to the subscapularis. Due to concerns regarding the nerve supply, dissection along the anterior aspect of the subscapularis muscle belly should be minimized, particularly medial to the conjoint tendon. Most releases are performed along the superior aspect of the subscapularis tendon and the posterior surface of the tendon, where it is intimately related to the anterior capsule of the glenohumeral joint, as well as the posterior aspect of the muscle belly, where the plane between the muscle and the subscapularis fossa of the anterior scapula may safely be developed well medial to the glenoid. Indeed, development of this plane is an important adjunct to intraoperative orientation of glenoid version in shoulder arthroplasty. The interval between anterior capsule and the subscapularis tendon is more easily developed medially, where the two structures diverge at the level of the glenoid labrum.

When operating in the region of the glenoid neck and subscapularis, it is essential to appreciate the proximity of the axillary nerve. The axillary nerve is one of two terminal branches of the posterior cord of the brachial plexus. It passes from the axilla to the posterior aspect of the shoulder via the quadrilateral space. This space is defined by the subscapularis muscle superiorly, the teres major inferiorly, the long head of triceps medially, and the humeral shaft laterally (▶Fig. 12.13). The thickness of subscapularis separating the axillary nerve from the glenohumeral joint capsule is variable and decreases as the nerve runs posteriorly and closer to the glenoid. It may be directly related to the inferior capsule (▶Fig. 12.14). The axillary nerve is held to the shoulder capsule with loose areolar tissue in the zone between the 5 and 7 o’clock positions and is at risk in any arthroscopic or open procedure in this interval. ⁸ The nerve exits the quadrilateral space posteriorly, primarily to supply the deltoid muscle. The posterior boundaries of the quadrilateral space vary in one regard to the anterior boundaries: the superior margin is defined by teres minor instead of subscapularis (▶Fig. 12.15a).

The other terminal branch of the posterior cord is the radial nerve. It passes anterior to latissimus and teres major before reaching the posterior aspect of the shoulder by passing through a triangular space. The triangular space is bounded by the inferior aspect of teres major, the long head of triceps medially, and the shaft of the humerus laterally (▶Fig. 12.15b).

Once the axillary nerve enters the posterior aspect of the shoulder, it wraps around the proximal humerus, closely applied to the deep surface of the deltoid muscle (▶Fig. 12.16). It travels with the posterior circumflex humeral vessels. There is a constant vessel branching from the circumflex humeral vessels deep to the anterior third of deltoid (▶Fig. 12.17). The significance of this vessel is twofold. Most surgical procedures via a deltopectoral approach require definition and mobilization of the subdeltoid space. This vessel is a frequent cause of troublesome bleeding, which may be avoided if it is identified and controlled rather than incidentally disrupted during subdeltoid mobilization. Care should be taken, however, not to damage the axillary nerve when controlling this vessel, particularly when using electrocautery. In addition, this vessel can be used as a marker of the level of the axillary nerve on the deep surface of deltoid. The distance from the lateral margin of the acromion to the axillary nerve is variable but may be a little as 5 cm. ▶Fig. 12.18 demonstrates the nerve lying approximately 6 cm distal to the acromion, where it is at risk in surgical approaches that split the deltoid. The axillary nerve is certainly at risk if dissection between the deltoid and the lateral humerus proceeds in the wrong plane. This is particularly the case in revision surgery, where there is frequently scarring in the subdeltoid space. The correct plane for subdeltoid dissection may be best identified between the coracoacromial ligament and the supraspinatus medially, with dissection sweeping laterally from the subacromial to subdeltoid space. This plane may also be developed inferiorly if there is difficulty identifying the plane between subscapularis and the conjoint tendon, again, in revision surgery.

Another frequent contributor to loss of external rotation is contracture within the rotator interval. The rotator interval lies between the superior margin of the subscapularis tendon and the anterior margin of the supraspinatus. These two tendons diverge medially to pass either side of the base of the coracoid. The long head of biceps passes through this interval to enter the glenohumeral joint on its way to inserting on the superior glenoid (▶Fig. 12.19) and the interval is bridged by glenohumeral joint capsule. Thickenings in this capsule form the coracohumeral ligament, which passes between the humeral head laterally and the posterolateral surface of the coracoid process medially (▶Fig. 12.20). Pathological contracture of the coracohumeral ligament is one of the most potent causes of loss of external rotation, and release of this structure is an extremely safe and effective contributor to restoration of that motion. In the patient with a retracted rotator cuff tear, it tethers the cuff, and prevents reduction of the tendon. Releasing the coracohumeral ligament from the coracoid is an important aspect of mobilization of the cuff.