Incidence

There is very little information in the literature on the exact incidence of nerve injuries following instability surgery. A review article by Boardman and Cofield reported a rate of nerve injury of 1% to 2% in patients undergoing rotator cuff surgery, 1% to 4% in patients undergoing shoulder arthroplasty, and 1% to 8% in patients undergoing anterior shoulder stabilization procedures.^1,2 The actual incidence of nerve injuries is likely unknown and underreported.

One of the largest series in the literature specifically on nerve injuries following shoulder instability surgery was by Ho et al.³ They reviewed the records of 282 patients who underwent an anterior reconstruction for recurrent shoulder instability. The authors found an 8.2% incidence of a neurologic deficit following surgery. There was complete resolution of the symptoms in 18 of the 23 patients. There were no patients in the study who underwent surgical exploration. In regard to risk factors, older age at the time of surgery and a Bankart lesion were associated with developing a neurologic deficit. At a mean follow-up of 8.7 years, the authors did not find a significant difference in Rowe scores comparing those with and without a nerve injury. The authors believed that the injuries were primarily due to traction at the time of surgery.

Commonly injured nerves

The most commonly damaged nerves associated with both open and arthroscopic surgery are the axillary and musculocutaneous. The axillary and musculocutaneous nerves are both located close to the glenohumeral joint. The musculocutaneous nerve pierces the coracobrachialis muscle a minimum of 5 cm inferior to the tip of the coracoid process.⁴ However, it is the axillary nerve that is usually at greatest risk during anterior shoulder stabilization procedures. The axillary nerve may be found adjacent to the capsule with the sensory branch being located very close to the rim of the glenoid.⁵

Loomer et al examined the anatomy of the axillary nerve and its relation to an inferior capsular shift.⁵ The authors examined 12 cadaveric specimens. The authors noted that the axillary nerve originates immediately posterior to the coracoid and conjoint tendon. It then crosses the inferior-lateral border of the subscapularis 3 to 5 mm medial to the musculotendinous junction. The nerve is in close contact with the inferior capsule as it passes through the quadrilateral space.

McFarland et al performed a study evaluating the anatomic relationship of the brachial plexus and axillary artery to the glenoid.⁶ The authors used eight fresh-frozen cadaveric specimens. They measured the distance from the glenoid rim to the brachial plexus with the arm in 0, 60, and 90 degrees of abduction. They found that the axillary artery and brachial plexus were within 2 cm of the glenoid rim. In some cases, the brachial plexus was within 5 mm of the glenoid rim. There was not a significant change in the distance from the rim of the glenoid to the medial cord, posterior cord, axillary artery or musculocutaneous nerve with the various degrees of abduction. It also was noted that retractors placed superficial to the subscapularis or placed on the scapular neck were in contact with the brachial plexus in all the positions evaluated.

Although less frequently recognized as a potentially injured nerve, the upper and lower subscapular nerves may be injured during the course of open shoulder stabilization. A study by Kasper et al evaluated the innervation of the subscapularis muscle and developed guidelines to prevent denervation.⁷ The authors found that the subscapularis muscle has a highly variable innervation pattern. A common variation found by the authors was that the lower subscapular nerve frequently (25%) derived as a branch of the axillary nerve. They noted that the subscapular nerves innervate the subscapularis close to the glenohumeral joint and that unintentional damage at the time of surgery may result in dysfunction.

Mechanism of injury

There are multiple potential causes of injury. One of the most common mechanisms of injury in open surgery is related to retractors that may be placed around the glenohumeral joint. In arthroscopic surgery, inadvertent portal placement around the brachial plexus region may result in direct trauma to nerves. One of the most commonly used portals placed during instability is the anterior-inferior portal. Great care must be taken in placing this cannula and avoiding brachial plexus. A thorough understanding of the anatomy is critical to ensure proper placement. Nerve injury also may result from compression secondary to fluid extravasation at the time of arthroscopy.⁸

Thermal capsulorrhaphy became a very popular procedure to treat shoulder instability in the earlier part of the decade.⁹ The popularity of this procedure has decreased dramatically since that time because of concerns of a high rate of recurrent instability. This procedure was used to treat a variety of instability types, including unidirectional stability, multidirectional instability, and microinstability in overhead throwing athletes. In addition to a high rate of recurrent instability, one of the complications found with thermal capsulorrhaphy was nerve injury. The most commonly injured nerve was the axillary nerve and, specifically, the sensory branch of the axillary nerve.

A frequently overlooked potential cause of nerve dysfunction may be related to patient positioning. It is very important to ensure that all bony prominences are carefully padded. Additionally, proper positioning of the cervical spine is essential. One needs to ensure that the head and neck are placed in a neutral position avoiding excessive flexion or extension. Unnecessary tilting of the head in the beach chair position may result in inadvertent cervical spine injury. When the lateral position is used for arthroscopy, it is imperative to properly protect the opposite upper extremity. One must be vigilant in carefully monitoring the traction placed on the operative arm when using the lateral position. Too tight wrapping of the operative upper extremity can result in a tourniquet-like effect on the operative arm. It is also important that the lower extremities are carefully positioned and padded. It may be possible to induce a neurapraxia from pressure placed by posts or other positioning devices.

Neurologic injuries may result from regional anesthesia. There are reports of nerve injury from interscalene blocks.¹⁰ Various injuries have been reported, including phrenic nerve palsy, laryngeal nerve palsy, pneumothorax, vocal cord paralysis, and Horner’s syndrome.

An additional cause of nerve injury may be the incarceration of nerves in suture at the time of surgery. A lack of clear identification of the nerves before tying sutures may result in accidental incarceration of the nerve. Additionally, nerves may be cut with cautery, scalpels, or scissors. A clean, dry field is always imperative to perform safe surgery.