Applied Surgical Anatomy of the‌

Posterior Approach to the Shoulder Joint

Overview

The posterior aspect of the shoulder is similar to the anterior and lateral aspects, being covered by two muscular sleeves. The posterior part of the deltoid muscle forms the outer sleeve of muscle, as it does for all other approaches to the shoulder joint. The inner sleeve consists of two muscles of the rotator cuff, the infraspinatus, and the teres minor (Figs. 1-70 and 1-71).

Landmark and Incision

Landmark

The spine of the scapula is a thick, bony ridge projecting from the back of the blade of the scapula. Its base runs almost horizontally, and its free lateral border curves forward to form the acromion. The spine separates the supraspinous fossa from the infraspinous fossa. The trapezius muscle inserts into it from above; part of the deltoid muscle originates from its inferior border (see Fig. 1-70).

Figure 1-70 The superficial muscles of the posterior aspect of the shoulder. The posterior portion of the deltoid as it takes origin from the spine of the scapula is aponeurotic, and the plane between it and the underlying infraspinatus is difficult to identify.

Figure 1-71 The posterior portion of the deltoid is detached from the spine of the scapula, revealing the infraspinatus, teres minor, and teres major muscles, as well as the long and lateral heads of the triceps muscle. The boundaries of the quadrangular space are, superiorly, the lower border of the teres minor; laterally, the surgical neck of the humerus; medially, the long head of the triceps; and, anteriorly, the upper border of the teres major. Through this space run the axillary nerve and the posterior circumflex humeral artery. Infraspinatus. Origin. Medial three-fourths of infraspinous fossa of scapula. Insertion. Central facet on greater tuberosity of humerus. Action. Lateral rotator of humerus. Nerve supply. Suprascapular nerve. Teres Minor. Origin. Axillary border of scapula. Insertion. Lowest facet on greater tuberosity of humerus. Action. Lateral rotator of humerus. Nerve supply. Axillary nerve.

Incision

Because the transverse skin incision runs across the relaxed skin tension line, the resultant scar usually is broad. A vertical incision at the lateral end of the scapular spine is more cosmetic, but provides very poor

exposure of the joint and the neck of the glenoid.

Superficial Surgical Dissection

In the posterior approach, only those fibers of the deltoid muscle that arise from the spine of the scapula are detached. Because the fibers are straight and blend intimately with the periosteum of the scapula, the muscle can be removed subperiosteally. During closure, the good, tough tissue that remains attached to the muscle provides an excellent anchor for sutures, in contrast to the anterior and lateral portions of the muscle. Drill holes may need to be placed through the spine, however, to anchor the muscular sutures.

Deep Surgical Dissection

The deep dissection in this approach lies between the infraspinatus and teres minor muscles (see Fig. 1-71).

Infraspinatus Muscle

The fibers of the infraspinatus muscle are multipennate; numerous fibrous intramuscular septa give attachment to them.

The infraspinatus forms its tendon just before crossing the back of the shoulder joint; a small bursa lies between the muscle and the posterior aspect of the scapular neck to help the tendon glide freely over the bone. The muscle also inserts into the capsule of the shoulder joint, mechanically increasing the capsule’s strength (Fig. 1-72).

Figure 1-72 The lateral portion of the infraspinatus and the teres minor has been removed to reveal the joint capsule. The suprascapular nerve and the circumflex scapular artery are seen curving medially and distally around the lateral border of the spine of the scapula. The axillary nerve is seen emerging through the quadrangular space and splitting into many branches. The medial branch splits to supply the teres minor muscle. The radial nerve is seen crossing through the triangular interval and entering the spiral groove in the upper portion of the humerus. The triangular interval is formed superiorly by the lower border of the teres major muscle, medially by the long head of the triceps, and laterally by the shaft of the humerus.

Teres Minor Muscle

The teres minor runs side by side with the infraspinatus. Its fibers run parallel with one another, in contrast to the multipennate fibers of the infraspinatus; this difference may help in identification of the interval between the two muscles.

The axillary nerve enters the muscle from its inferior border. The superior border (the boundary between the infraspinatus and teres minor muscles), therefore, is the safe side of the muscle and a true internervous

plane (see Fig. 1-71).

Dang

Axillary Nerve

The axillary nerve is a branch of the posterior cord of the brachial plexus. It runs down along the posterior wall of the axilla on the surface of the subscapularis, far from the incision made in that muscle during the anterior approach to the shoulder (see Fig. 1-26). The nerve then runs through the quadrangular space, where it touches the surgical neck of the humerus. At that point, it can be damaged easily by surgery, by fractures of the surgical neck of the humerus, or by anterior dislocation of the shoulder.

The boundaries of the quadrangular space differ when viewed from the front and from the back (see Fig. 1-71).

Posterior View. The boundaries from the posterior view are as follows: Superiorly, the lower border of the teres minor; laterally, the surgical neck of the humerus; medially, the long head of the triceps; and inferiorly, the upper border of the teres major.

Anterior View. The boundaries from the anterior view are as follows: Superiorly, the subscapularis; laterally, the surgical neck of the humerus; medially, the long head of the triceps; and inferiorly, the upper border of the teres major (see Fig. 1-26).

The axillary nerve disappears beneath the lower border of the subscapularis and, after traversing the quadrangular space, emerges in the back of the shoulder beneath the lower border of the teres minor. The posterior circumflex humeral vessels run with it (see Fig. 1-71).

Dissections carried out above the teres minor do not damage the axillary nerve; however, if the dissection strays out of the correct plane and below the teres minor, the axillary nerve can be damaged. Because the axillary nerve is the sole nerve supply to the deltoid muscle, any damage will produce severe functional impairment.

Within the quadrangular space, the axillary nerve divides into two branches after giving off a twig to the shoulder joint. The deep branch enters and supplies the deep surface of the deltoid (see Fig. 1-71). The superficial branch supplies the teres minor muscle and sends a cutaneous branch to the lateral aspect of the upper arm, namely, the upper lateral cutaneous nerve of the arm, which supplies the skin over the insertion of

the deltoid muscle (see Fig. 1-70).

The upper lateral cutaneous nerve of the arm is of clinical importance in cases of traumatic axillary nerve palsy following, for instance, an acute anterior dislocation of the shoulder. Examination of the paralyzed deltoid and teres minor muscles may be difficult or impossible because of the pain that follows this injury. Diminution of sensation over the insertion of the deltoid is good presumptive evidence of the presence of an axillary nerve palsy.

The axillary nerve is the best example of Hilton’s law, which states that the motor nerve to a muscle tends to send a branch to the joint that the muscle moves and another branch to the skin over the joint.51 Pain in the shoulder is perceived via the axillary nerve and, therefore, may be referred to the cutaneous distribution of that nerve.

Radial Nerve

The radial nerve, which is the other major branch of the posterior cord of the brachial plexus, leaves the axilla by passing backward through a triangular space that is defined superiorly by the lower border of the teres major, laterally by the shaft of the humerus, and medially by the long head of the triceps (see Figs. 1-61 and 1-72).

The chances of endangering the radial nerve by this approach are remote. It cannot be damaged during the posterior approach to the shoulder unless dissection is carried out not only below teres minor but below teres major as well.

Circumflex Scapular Vessels

Yet another triangular space exists when the inner sleeve of shoulder muscles is viewed from the back. Its boundaries are as follows: Superiorly, the lower border of the teres minor; laterally, the long head of the triceps; and inferiorly, the upper border of the teres major (see Fig. 1-71).

This triangular space contains the circumflex scapular vessels, which form part of the extremely rich blood supply to the scapula. Dissection carried out between the teres minor and teres major muscles should not be carried out in elective surgical procedures because damage to these vessels will cause profuse hemorrhage that is difficult to control (see Fig. 1-71). Because the scapula has such a rich blood supply, fractures of the scapula are often associated with profuse blood loss. The hematoma is constrained within the fascia surrounding the scapula muscles and is not obvious. Potential blood loss from a fractured scapula always must be considered during vascular assessment of a polytraumatized patient.