Applied Surgical Anatomy of the Anterolateral and Lateral Approaches
Applied Surgical Anatomy of the Anterolateral and Lateral Approaches
Overview
Two muscular sleeves cover the lateral aspect of the shoulder joint: The outer sleeve consists of the lateral portion of the deltoid muscle, and the inner sleeve is the supraspinatus tendon (part of the rotator cuff) (Figs. 1-51 and 1-52).
Landmarks and Incision
Landmark
The acromion, which is the lateral continuation of the spine of the scapula, is the summit of the shoulder, overhanging the greater tuberosity of the humerus. Muscles either insert onto it or take origin from it, but no muscle crosses it. Thus, it is partially subcutaneous and can be palpated (see Fig. 1-51). The anatomical shape of the acromion has considerable variation, which may be associated with an impingement syndrome.37
Incision
Because the lateral skin incision crosses the lines of relaxed skin tension line almost transversely, it is likely to leave a broad scar.
Superficial Surgical Dissection
Deltoid Muscle
The lateral approach affects the portion of the deltoid muscle that arises from the lateral border of the acromion. The lateral deltoid consists of oblique fibers arising in a multipennate fashion from tough tendinous bands that originate from the acromion. These bands actually mark the bone with a series of notches. Similar bands arise from the insertion of the muscle onto the humerus halfway down the lateral border; the muscle fibers arising from these tendinous bands interdigitate in a herringbone pattern.
This multipennate arrangement provides the deltoid muscle with maximum strength, although it limits the degree to which it can contract. Nevertheless, despite the arrangement of the fibers, it is relatively easy to split the muscle in a longitudinal fashion. The tough tendinous bands also prevent excessive damage to the muscle when it is split during surgery (Fig. 1-53).
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Figure 1-51 The superficial muscles of the lateral aspects of the shoulder. The muscles take origin from or insert into the acromion and the spine of the scapula, but do not cross them.
Whether the deltoid muscle should be detached from the acromion still is in question, because reattachment is difficult and often unsuccessful.38 An acromial osteotomy and subsequent reattachment of the bone, with the muscle still attached to it, may be the best solution, although acromial nonunion may occur because the anterior and posterior portions of the deltoid tend to pull apart the site subjected to osteotomy. In the vast majority of cases, widespread detachment of the deltoid from the acromion
should not be necessary for the adequate exposure of the underlying structures.
Trapezius Muscle
See the section regarding the posterior approach to the cervical spine.
Axillary Nerve
See the sections regarding dangers in the lateral approach to the shoulder.
Acromial Branch of the Acromiothoracic Artery
The acromial branch of the acromiothoracic artery is a tributary of the acromiothoracic artery which arises from the second part of the axillary artery. Running immediately deep to the insertion of the deltoid muscle to the acromioclavicular joint, it is coagulated easily.
Subacromial (Subdeltoid) Bursa
The subacromial bursa separates the two sleeves of muscle that cover the lateral aspect of the shoulder joint. It helps them glide past each other and protects the rotator cuff (the inner sleeve) from the hard overlying bone and ligamentous complex—the acromial process (acromion), the coracoacromial ligament (which spans the gap between the coracoid process and the acromion), and the coracoid process of the scapula. Because the bursa lies between the supraspinatus and deltoid muscles, and between the supraspinatus and coracoacromial ligaments, it is called both the subacromial bursa and the subdeltoid bursa (Figs. 1-54 and 1-55).
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Figure 1-52 Portions of the deltoid and trapezius have been removed to reveal the underlying rotator cuff and the axillary nerve, usually beneath the teres minor in the quadrangular space.
The bursa is a large structure, extending anteriorly from beneath the coracoid process. At this point, it provides lubrication between the conjoined tendons of the coracobrachialis and biceps brachii muscles, and the underlying subscapularis muscle.
The bursa ordinarily does not communicate with the shoulder joint. Rupture of the supraspinatus tendon, however, can cause the two synovial-lined cavities to join; an arthrogram of the shoulder can reveal this
communication (Fig. 1-56).39,40
With the arm in the dependent position, the bursa lies under the deltoid muscle and the coracoacromial ligament. When the arm is abducted, the bursa retreats under the cover of this ligament. At this point, the patient feels pain if there is inflammation of the bursa, mainly because the bursa is compressed between the undersurface of the acromion and the humeral head (Fig. 1-57). Classically, this painful arc of movement occurs between
80 and 120 degrees of shoulder abduction. Paradoxically, there is no tenderness on the lateral aspect of the shoulder in this position, because the bursa now is protected from palpation completely by the coracoacromial ligament. When the arm is adducted again, the pain disappears, because the bursa no longer is compressed between the ligament and the supraspinatus. Tenderness on palpation may be elicited on the lateral aspect of the shoulder below the acromion, however, because the bursa now is accessible. Passive extension of the shoulder also brings the bursa out anteriorly from beneath the acromion and makes it palpable.
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Figure 1-53 The multipennate arrangement of the muscle fibers of the middle portion of the deltoid muscle.
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Figure 1-54 The subacromial bursa. Note the expansion of the subacromial bursa and the large subdeltoid portion. The subscapularis bursa frequently pierces the joint capsule to communicate with the joint.
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Figure 1-55 The subacromial bursa directly protects the supraspinatus tendon from the bone and ligamentous complex that covers it.
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Figure 1-56 Rupture of the supraspinatus tendon allows direct communication between the joint and the subacromial bursa. An arthrogram of the shoulder will reveal this communication, helping to establish the diagnosis of a torn rotator cuff.
Deep Surgical Dissection and Its Dangers
Supraspinatus Muscle
The supraspinatus, which is a multipennate muscle, passes laterally beneath the coracoacromial ligament. This muscle is the most frequent site of degenerative changes and frank tears within the rotator cuff. Within the rotator cuff. Degeneration in its tendon invokes an inflammatory response in the overlying subacromial bursa, and most cases of subacromial bursitis probably reflect pathology in the muscle.41 The close relationship of the supraspinatus to the coracoacromial ligament may result in mechanical abrasion between the two structures during abduction of the arm, causing degeneration of the tendon. The subacromial bursa minimizes this tendency (see Figs. 1-55 and 1-59).
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Figure 1-57 Abduction of the arm can impinge the subacromial bursa between the greater tuberosity and the undersurface of the acromion and coracoacromial ligament.
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Figure 1-58 Superior view of the shoulder, showing the rotator cuff and the acromioclavicular joint. The suprascapular nerve supplies the supraspinatus and infraspinatus muscles after passing through the suprascapular notch and ligament. Supraspinatus. Origin. Medial three-fourths of supraspinous fossa of scapula. Insertion. Upper facet of greater tuberosity of humerus. Action. Initiates abduction of shoulder. Nerve supply. Suprascapular nerve.
When the arm is by the patient’s side, the supraspinatus tendon takes a 90-degree turn over the humeral head before its insertion, putting the blood supply to the tendon on a stretch. Vascular insufficiency may result, which is another possible cause of degenerative change.42
Regardless of the mechanism of degeneration, about one-fourth of all individuals who reach 65 years of age rupture their supraspinatus tendon.43 Patients with complete ruptures of the supraspinatus are unable to abduct their arms without adopting such trick movements as a shrug mechanism because the supraspinatus muscle is the initiator of shoulder abduction. If patients with a ruptured supraspinatus lower the affected arm slowly from the vertical, they lose control of it at about 30 degrees and it drops suddenly to their side.
The suprascapular nerve is a branch of the upper trunk of the brachial plexus; it enters the muscle on its deep surface. Some methods of repairing
tears of the supraspinatus tendon involve mobilizing the entire muscle belly and advancing it laterally to take tension off the suture line of the repair.33 Take great care in mobilizing the supraspinatus muscle from its fossa, to avoid damaging its nerve (Fig. 1-58).
Impingement Syndrome
Abduction of the arm may pinch the supraspinatus muscle between the head of the humerus and the arch created by the acromion and the coracoacromial ligament. The anatomy of the acromion varies considerably from individual to individual, and certain acromial shapes have been associated with an impingement syndrome. Performing an acromioplasty and cutting the coracoacromial ligament may provide relief in some patients with impingement syndrome. This procedure can be carried out by an open operation (see Anterolateral Approach to the Acromioclavicular Joint and Subacromial Space) or by arthroscopic techniques.
Special Anatomic Points
The acromioclavicular joint is a synovial joint between the lateral end of the clavicle and the medial border of the acromion. The lateral end of the clavicle is higher than the acromion; the joint can be palpated by pushing medially against the thickness at the end of the clavicle.
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Figure 1-59 A: Superior view of the shoulder joint, revealing the bone structure and acromioclavicular joint capsule. B: Cross section of anterior view of the shoulder, revealing the acromioclavicular joint and meniscus, as well as the supraspinatus tendon and its relationship to the coracoacromial ligament.
The two most common disease processes affecting the acromioclavicular joint are acromioclavicular dislocations and acromioclavicular arthritis. In cases of acromioclavicular dislocation, it is important to remember that the major accessory ligaments of the joint from the coracoid process to the undersurface of the clavicle are some
distance from it. They cannot be repaired directly to restore joint stability; however, if the joint is reduced and stabilized by another technique, they will heal. Acromioclavicular arthritis commonly is associated with the development of inferior osteophytes, which are a contributing factor to cases of impingement syndromes.
The joint is exposed easily by way of a superior approach because it is essentially subcutaneous. The insertions of the trapezius and deltoid muscles to the superior surface of the clavicle are confluent; however, the two muscles are separated easily by subperiosteal dissection (Fig. 1-60). In cases of acromioclavicular dislocation, however, this dissection will have been done for you and the distal end of the clavicle often lies in a subcutaneous position.
The joint may also be approached from its anterior surface (see Anterolateral Approach to the Shoulder).
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Figure 1-60 Superior approach to the acromioclavicular joint.