Chapter 1: Extracapsular Anatomy for Shoulder Arthroscopy

Introduction

The shoulder is the root of the upper limb. Because of this, the anatomy surrounding the joint is much more complex than the knee, and the hazards correspondingly greater for the arthroscopist. The knee is a simple joint to arthroscope as the soft tissue envelope around it is thin, the joint space can easily be felt, and landmarks are simple to distinguish. There are only two hazards: the neurovascular bundle and the lateral popliteal nerve, helpfully located far from the usual portal sites. This should be contrasted with the shoulder which has a thick, soft tissue envelope, where the joint space cannot be felt, landmarks may be difficult to distinguish, particularly in obese or muscular patients, and the joint is surrounded by six major nerves - the axillary artery, and five of its six branches, as well as the cephalic vein. Thus, not only is the anatomy more complex but major nerves are situated only millimetres from the two major portals. The suprascapular nerve passes 1 cm from the posterior joint line, and the musculocutaneous nerve enters the cora-cobrachialis directly in front of the anterior joint line. Nerve injury to the brachial plexus,1 the musculocutaneous nerve2 and the median nerve3 have all been reported. For these reasons, an intimate knowledge of gross shoul­der anatomy is an absolute prerequisite to the aspiring shoulder arthroscopist.

Various dissections of the shoulder may be performed in order to examine the relationship of the nerves and vessels to the normal arthroscopic portals. This chapter is based on such observations.

Posterior Portal

Figure 1.1 shows the muscular anatomy of the right shoulder, as seen from behind. The only constant and useful landmark is the posterior angle of the acromion. The posterior portal is placed 2 cm inferior and medial to this constant point (Figures 1.2 and 1.3).4~7 The first muscle layer that the arthroscope will traverse is the deltoid muscle (Figure 1.4). If the dissection is taken further so that deltoid is detached from the acromion and spine of the scapula, and folded forward (Figure 1.5), the next anatomical layer can be seen.

The first structure to note is the axillary nerve emerging, along with the posterior circumflex humeral vessels, from below teres minor. This neurovascular bundle is only 3 cm below the posterior portal (Figure 1.6), a point of great importance if a second, accessory posterior portal is made in order to perform arthroscopic surgery (for instance, the removal of loose bodies from the infraglenoid recess). The axil­lary nerve has a singularly inappropriate name, for the first thing it does on leaving the posterior cord of the brachial plexus is to pass below the inferior recess of the shoulder capsule and leave the axilla through the quadrilateral space. As can be seen, this is more a slit than a space, with teres major below, then the long head of triceps medially, humerus laterally, and finally teres minor above it.

SHOULDER ARTHROSCOPY

Figure 1.1 Muscular anatomy of the right shoulder seen from behind. The constant landmark is the posterior angle of the acromion, which you can feel on your own shoulder.

Figure 1.2 The posterior portal is situated 2 cm medial to and 2 cm inferior to the posterior angle of the acromion, as marked on this patient.

As the axillary nerve skirts the inferior border of the shoulder capsule it gives off branches to the joint, and divides into its two terminal branches, the deep and superficial branches. The superficial branch supplies teres minor and then appears behind the posterior border of deltoid to become the upper lateral cutaneous nerve of the arm (not shown on the dissection).

Figure 1.8 In this dissection, the spine of the acromion has been osteotomized and removed along with trapezius and deltoid. Supraspinatus and infraspinatus have then been lifted from their origins on the blade of the scapula to show the suprascapular nerve passing around the edge of the spine to supply both muscles.

Figure 1.9 Infraspinatus and teres minor have now been reflected back and the posterior capsule of the shoulder has been incised at its origin from the neck of the glenoid and reflected laterally. The 'bare area' of the humeral head can be seen.

Figure 1.10 The head of the humerus has now been osteotomized and removed exposing the glenoid, the glenoid labrum, the long head of biceps and the anterior glenohumeral ligaments.

Figure 1.11 A simplified interpretation of the capsular structures as seen at arthroscopy.

The head of the humerus has now been osteotomized to expose the anterior capsular struc­tures arthroscopically (Figure 1.10). It is impor­tant at this stage to notice the close relationship of the axillary nerve and posterior circumflex artery as they pass directly under the inferior aspect of the joint. The glenoid can clearly be seen along with its central grey spot and surrounding labrum. The long head of biceps can be seen running across the top of the joint cavity, and below it the superior surface of the subscapularis tendon (Figure 1.11).

Anterior Portal

Figure 1.12 Muscular anatomy of the left shoulder seen from the front.

Figures 1.13 and 1.14 The anterior portal is made half way between the anterior edge of the acromion and the coracoid process, as shown on the skeleton.

Figure 1.12 shows the anterior aspect of the left shoulder. The deltopectoral groove can be seen, containing the cephalic vein. The anato­mical landmarks for the anterior portal are the anterior acromion, the acromioclavicular (AC) joint and the tip of the coracoid process (Figures 1.13 and 1.14). Matthews3 has shown how the anterior portal should be above and lateral to the tip of the coracoid process. In no circumstances should any instrument be pla­ced inferior or medial to the coracoid process, as this would jeopardize the brachial plexus and the vascular supply of the whole upper limb. Figure 1.15 shows the position of the standard anterior portal which passes through deltoid and lateral to the cephalic vein.

Figure 1.15 &1.16 The arthroscope passes through the anterior deltoid, lateral to the cephalic vein. The deltoid is now dissected from its origin on the acromion and folded laterally to show the path of the anterior portal. In this dissection, the arthroscope is shown passing the edge of the coracoacromial ligament. Usually, however, it passes through the ligament. Note the position of the brachial plexus.

Deltoid is now dissected free from its anterior origin on the clavicle and anterior border of the acromion. Pectoralis major is folded out later­ally to expose the tendons of pectoralis minor and the conjoined tendons of coracobrachialis and short head of biceps originating from the coracoid process (Figure 1.16). The brachial plexus can be seen 5cm below the coracoid. The musculocutaneous nerve leaves the lateral cord of the brachial plexus and enters the coracobrachialis muscle at a variable site 2-5 cm distal to the coracoid process. The mus­culocutaneous nerve supplies coracobrachialis and the short head of biceps as it passes through these muscles; it then supplies the rest of biceps and brachialis to emerge in the cubital fossa of the elbow as the lateral cuta­neous nerve of the forearm.

The brachial plexus emerges from the axilla as three large mixed motor and sensory nerves (median, ulnar and radial) and two purely sensory nerves (medial cutaneous nerves of arm and forearm). The radial nerve is a con­tinuation of the posterior cord and passes through the triangular space below teres major, with the humeral shaft laterally and the long head of triceps medially. The remaining nerves surround the axillary artery and vein and run on to the medial side of the arm.

Figure 1.17 Arteriogram showing the major branches of the axillary artery.

Figure 1.18 Arteriogram showing a close-up of the axillary artery. The patient had a traumatic false aneurysm of the posterior circumflex humeral artery.

The axillary artery (Figures 1.17 and 1.18) has six branches. The first is the superior thoracic artery which leaves the upper third of the main artery to run forward and supply the pectoral muscles. The next two branches - the acromiothoracic and lateral pectoral arteries -leave the middle third of the axillary artery as it passes under pectoralis minor. The acro­miothoracic artery divides into four branches: the pectoral branches pierce the clavipectoral fascia and pass down to the muscle; the deltoid branch pierces the fascia and passes along within or under the deltoid paralleling the clavicle; and the acromial branch passes up alongside the coracoacromial ligament. The lateral thoracic artery passes down the chest wall on serratus anterior, supplying the pectoral muscles (and in women the breast). The final three branches leave the third part of the axillary artery. The largest branch of this artery is the subscapular artery whose circum­flex scapular branch anastomoses with the suprascapular artery. The anterior circumflex humeral artery runs along the inferior border of the subscapularis tendon to the humerus; here it gives an ascending branch which supplies the long head of biceps as it passes up the bicipital sulcus and goes on to supply the capsule and humeral head. The trunk of the anterior circumflex humeral artery passes around the humerus to anastomose with its larger partner, the posterior humeral circumflex artery, which has accompanied the axillary nerve through the quadrilateral space.

The coracoacromial ligament can be seen passing from coracoid to acromion. It is a large triangular ligament which inserts onto the undersurface of the anterior acromion, an important point to note when it comes to subacromial decompression. Having passed through deltoid, the instruments pass through the coracoacromial ligament or skirt its free edge to enter the joint just above the superior edge of the subscapularis muscle. Detrisac and Johnson8 describe an occasional anterior capsular artery in close proximity to the super­ior glenohumeral ligament which can be dam­aged as the instrument traverses the anterior capsule at this point.

Superior Portal

A superior portal has been described (Neviaser portal9) (Figure 1.19) which is useful for irrigat­ing the shoulder joint. A needle is placed through the trapezius muscle, medial to the acromion, in the triangle bordered anteriorly by the clavicle and posteriorly by the spine of the scapula.

Figure 1.19 The superior portal: the needle is inserted medial to the acromion to enter the superior portion of the joint, as shown on the skeleton.

In the dissection, viewed from behind, trape­zius has been dissected free from its insertion into the spine of the scapula and the acromion (Figure 1.20). The acromion has then been osteotomized from the spine of the scapula and tilted forward to expose the subacromial space. This portal has been criticized by some who say that the instruments damage the tendon of supraspinatus. Figure 1.21 shows this not to be
the case - the instrument passes through the muscle fibres of supraspinatus, not the tendon, and this should not cause irreparable damage. The suprascapular artery and nerve are under the muscle at this point, about 2.5 cm medial to the portal.

Figure 1.20 Trapezius has been dissected free and the acromion osteotomized to show the subacromial space.

Figure 1.21 The arthroscope passes through the muscle fibres of supraspinatus.

Subacromial Space

To the arthroscopic surgeon, the subacromial bursa is another cavity for exploration. This is helped by the fact that this is the largest bursa in the body and readily accepts 15 ml saline, just under half the capacity of the glenohumeral joint itself. Moreover the subacromial bursa is of particular interest to the shoulder surgeon since it is found at the impingement point, allowing the pathology of impingement to be seen. In fact the subacromial space is only a potential space, the bursa being two membra­nous surfaces separated by a thin film of lubricating fluid. It is only when the surgeon distends the bursa with irrigating fluid that a measurable space is created.

Figure 1.22 In this dissection of a left shoulder, seen from above, the subacromial bursa has been filled with blue gelatin. The forceps point to the centre of the free margin of the coracoacromial ligament. Note that the bursa hardly extends under the acromion at all.

The first thing that the arthroscopic surgeon must understand is how far anterior the bursa is, for it is misnamed and should be called the subcoracoacromial ligament bursa. Figure 1.22 shows a dissection of the bursa which was prefilled with latex. As can be seen, the centre of the bursa is under the junction of the upper and middle thirds of the coracoacromial liga­ment, and the bursa hardly extends under the acromion at all. This is of vital importance to the arthroscopist, as the most common arthrosco­pic error is failure to enter the bursa, usually due to inability to visualize where the bursa ends.

Codman10 dissected over 500 subacromial bursae and his detailed description is essential reading for arthroscopists. Prior to Codman's dissections, anatomists believed that there were separate subacromial, subdeltoid and subcoracoid bursae. Codman showed that these three were one, although sometimes the subacromial bursa could be partly loculated with plicae, just as the suprapatellar pouch may be subdivided by the suprapatellar plicae.

Strizak et al11 found that three distinct com­ponents of the bursa could be identified in 15 dissections: subacromial, subdeltoid and sub­coracoid. In 14 of these dissections, the sub­acromial and subdeltoid portions were con­fluent, but in one a septum separated the two portions. The subcoracoid portion was only identified in three dissections, extending more interiorly and lying between subscapularis and coracoid, but never connecting with the gleno­humeral joint.

Figure 1.23 Note how the coracoacromial ligament inserts into the undersurface of the acromion.

Figure 1.24 An arthroscopic view of the subacromial space showing the deltoid nsertion.

The floor of the subacromial bursa consists of the tendon of insertion of supraspinatus, the shoulder capsule at the rotator interval, under which runs the tendon of the long head of biceps and the coracohumeral ligament. This corresponds to Laumann's middle zone of the subacromial space12.

The roof corresponds to the middle and upper thirds of the coracoacromial ligament,
the anterior third of the acromion and the undersurface of the acromioclavicular joint. It should be noted how the coracoacromial liga­ment inserts onto the undersurface of the anterior third of the acromion (Figure 1.23). Both the roof and the floor firmly adhere to these tissues.The walls can only be described when the bursa is inflated, an unnatural condition. Moreover, if the bursa is inflated it changes shape from two flat surfaces in apposition to a sphere. As the height of the space increases, its diameter must decrease correspondingly, changing the position of the walls. Perhaps it would be better to call the walls the 'reflections' of the bursa. The lateral reflection is under the deltoid muscle (Figure 1.24)11; the posterior reflection, at the junction of the anterior and middle thirds of the acromion; the anterior reflection, under the coracoacromial ligament or, if a subcoracoid extension is present, under the coracoid; the medial reflection, under the acromioclavicular joint.

For more detailed descriptions, the reader should study the works of Codman,10 Strizak,11 Laumann12 and Matthews et al.13,14