Commentary & Perspective by
Peter J. Millett, MD, MSc*,
Department of Orthopaedic Surgery, Brigham & Women's Hospital, Boston, MA
Revised October 7, 2004
Protecting the axillary nerve and understanding its anatomical course are among the basic tenets of shoulder surgery. With more conditions being treated successfully with arthroscopic surgery, the axillary nerve may be seen less commonly, but it still remains essential to know where it is and to know where it is vulnerable to injury 1-3.
The axillary nerve originates from the posterior cord of the brachial plexus, principally the C5 and C6 nerve roots. In the plexus, it is lateral to the radial nerve and posterior to the axillary artery. As it exits, it initially runs anterior to the subscapularis and medial to the coracoid process, where it is most frequently encountered in open shoulder surgery. From there it courses inferiorly, adjacent to the inferomedial glenohumeral joint capsule, and then passes through the quadrilateral space with the posterior humeral circumflex artery4. The quadrilateral space is defined by the long head of the triceps medially, the humeral shaft laterally, the teres minor superiorly, and the teres major inferiorly. The nerve exits this space, branches into anterior and posterior branches, and moves anterolaterally on the deep surface of the deltoid to innervate that muscle and to provide sensation to the upper lateral portion of the arm as the superolateral brachial cutaneous branch5. The nerve is vulnerable in several locations but is most vulnerable during arthroscopic surgery at its inferior position.
In a cadaveric anatomical study, Price et al. simulated the lateral decubitus position that is commonly used during shoulder arthroscopy to determine the course of the axillary nerve, with particular focus on the segment of nerve that runs inferior to the inferior glenohumeral ligament complex. Because the relationship between the axillary nerve and the joint capsule is dependent on arm position and because the relationship may be distorted by an open anatomical dissection, the authors used two novel approaches to examine the course of the nerve and to measure its distance from the joint and the capsule.
In the first technique, they created a rectangular window in the capsule so that the course of the nerve could be visualized; in the second technique, they stained the nerve, placed the arm in 45° of abduction and 20° of flexion, and then froze the specimens. The specimens were then sectioned, and anatomical measurements were made. In the windowed specimens, they found that the branch of the axillary nerve that innervates the teres minor was closest to the glenoid rim. It arose from the posterior deltoid branch at the 6 o'clock position together with the superior lateral brachial cutaneous branch. This is consistent with the work of Ball et al.5, who recently demonstrated similar findings in an open anatomical study of the posterior branch of the axillary nerve. In the frozen sections, they found that the nerve was closest to the glenoid and to the joint capsule at the 6 o'clock position, at a mean of 12.4 mm (range, 12.2 to 14 mm) from the glenoid, and a mean of 2.5 mm from the inferior glenohumeral ligament complex.
The nerve-staining technique and cross-sectional analysis used in the study by Price et al. seemed to provide less distortion and provide a more accurate representation of the anatomy encountered in a simulated, lateral decubitus position. The technique could certainly be applied to the study of other anatomical relationships. It is unfortunate that only the lateral decubitus position was studied, as this allows us only to speculate on the location of the axillary nerve when the patient is placed in the beach-chair position, the other position most commonly used for shoulder arthroscopy. The model was static and did not permit movement of the arm, which also would affect the position of the nerve. Furthermore, the joints were not distended with fluid as they are during arthroscopic surgery, although it would be expected that distention would result in an added margin of safety. While the specimens in this study were normal shoulders, the most common reasons for surgery in the axillary pouch are instability or stiffness, where a patulous axillary capsule or a scarred shortened pouch would be expected. How these conditions affect the course of the axillary nerve remains open to interpretation.
What is clear from this study and from others, however, is that the axillary nerve runs very close to the inferior glenoid, just inferior to the inferior glenohumeral ligament5,6. The proximity of the axillary nerve to the joint may explain the increased prevalence of nerve injury associated with arthroscopic thermal capsulorrhaphy. Thermal damage to the axillary nerve is the presumed cause of the axillary neuritis that has been seen in several series1,3,7. Laboratory studies have also demonstrated that the temperature along the axillary nerve can reach very high levels6,8. Anatomical studies have shown that posterior skin portals for arthroscopic shoulder surgery are usually safely out of harm's way9,10, although care still must be taken to avoid damaging the nerve with the trocar or with other arthroscopic instruments. Once the joint has been entered, caution and judgment should be used when performing surgery in the axillary pouch. Plication sutures, sharp dissection, or thermal energy could all conceivably injure the nerve. Studies such as this one clearly highlight the importance of understanding the relevant anatomy of the axillary nerve during arthroscopic surgery in the lateral decubitus position. The axillary nerve is in close proximity to the axillary pouch and is most vulnerable to injury at the 6 o'clock position.
*The author did not receive grants or outside funding in support of his research or preparation of this manuscript. He did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.
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