Open repair of posterior shoulder instability

OVERVIEW

Chapter synopsis

This chapter describes the background, etiology, diagnosis, treatment, rehabilitation, and outcomes for posterior instability of the glenohumeral joint treated with open surgical repair.

Important points

Differentiate from other shoulder pathology using exam and imaging
Open procedure best suited for revisions, poor capsular tissue, and significant osseous insufficiency

Clinical and surgical pearls

Can be performed in lateral or beach chair positions
Perform any supplemental arthroscopic procedures prior to opening
Utilize landmarks to identify infraspinatus for capsular approach
Choose side for longitudinal limb of T capsular incision based on planned procedure

Clinical and surgical pitfalls

Stay less than 15 mm medial to glenoid to avoid suprascapular nerve
If performing arthroscopy prior to opening, do not perform biceps tenodesis to avoid rehabilitative complications
Avoid axillary nerve and posterior humeral circumflex artery inferior to teres minor
Be careful not to overtighten the capsular plication to avoid loss of internal rotation and abduction
If bone block augmentation is performed, ensure bone and screw fixation remain extra-articular and don’t impinge on glenohumeral mobility

Background

The glenohumeral joint has the most capacity for movement of any joint in the body. The shoulder relies on an intricate balance of static (e.g., osseous, capsular, and labral tissues) and dynamic stabilizers (e.g., rotator cuff and muscular tissues) that keep the humeral head centered within the glenoid during shoulder motion by “concavity compression.” This balance can become disrupted, resulting in shoulder instability—most commonly anterior. Posterior instability in the shoulder arises most commonly from posterior capsular laxity secondary to repetitive posterior subluxation (e.g., bench press, blocking by football linemen), or from acute injury including trauma and muscular overpull (i.e., seizure or electrocution).

Posterior shoulder instability is less common than anterior or multidirectional instability, representing only 2% to 10% of shoulder instability events and occurring with a prevalence of 1.1 per 100,000 persons per year. However, the true rate is likely higher due to (1) missed auto-reduction and (2) missed or delayed diagnosis by clinicians. Accurate diagnosis relies on a detailed history, comprehensive physical examination including special tests for posterior instability, appropriate imaging, examination under anesthesia, and intraoperative confirmation of pathology.

Anatomy of posterior instability

Osseous

Shoulder stability relies in part on the congruence of the glenoid and humeral head. In a normal shoulder, the scapula is anteriorly orientated at 10 to 20 degrees relative to the spine and tilts posterior with humeral elevation, and the glenoid concavity is retroverted with respect to the scapular plane. Normal glenoid version is approximately 0 degrees but may vary by 10 degrees in either direction, and the humeral head is typically retroverted 15 to 37 degrees. Glenoid hypoplasia and increased humeral or glenoid retroversion beyond normal have been associated with recurrent posterior instability and increased likelihood of operative intervention. Osseous lesions may also increase risk of recurrence just as with anterior shoulder instability, either fracture/erosion at the posterior-inferior glenoid rim or impaction of the anteromedial humeral head (reverse Hill-Sachs lesion).

Capsulolabral

The primary soft tissue stabilizers preventing posterior instability are the posterior-inferior glenohumeral ligament (PIGHL) and the posterior labrum. The PIGHL is a capsular thickening that is under tension with shoulder flexion and internal rotation and typically fails in this position during posterior instability. ^, In about 90% of posterior dislocations, this will be disrupted as this is also the thinnest and weakest region of the capsule and labrum. ^, The posterior labrum, which acts to centralize the humeral head within the glenoid cavity and increase its functional depth, is also disrupted (reverse Bankart or Kim lesion) in up to 66% of traumatic posterior instability. While the posterior capsule itself also contributes significantly to stability, anterior capsular injury is necessary for complete posterior dislocation.

Other

To a lesser degree, posterior translational forces are resisted dynamically by the rotator cuff musculature, primarily the subscapularis, with others contributing in different portions of the arc of motion. In contradistinction to anterior instability, many cases of posterior dislocation are associated with at least partial tear of the rotator cuff. There may also be some involvement of the rotator interval, though the closure of the rotator interval does not appear to affect posterior instability.

Patient history

Patients with posterior instability uncommonly report a discrete posterior dislocation event or even the sensation of instability. Rather, as posterior subluxation secondary to repetitive microtrauma is far more common, the primary complaint is pain. Other complaints include subjective weakness, early fatigue, or less often mechanical clicking or popping. ^, Findings may overlap with those of other shoulder pathology; symptoms may include those of biceps tendonitis, with pain, often located deep and posterior, at the biceps tendon, or the superior rotator cuff. Pain is also elicited by the position of instability—forward flexion, adduction, and internal rotation—and may be mistaken for subacromial impingement. As bipolar and multidirectional instability occur more frequently than isolated posterior instability, clinicians should consider posterior instability when assessing patients with anterior dislocation events. Patient history may also indicate participation in activities that should clue surgeons in to a possibility of posterior instability, including weightlifting, football, rowing, and swimming. Decreasing strength and/or performance may also be a presenting complaint. Pain may intensify over the course of the activity as muscles fatigue.

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