• Posterior shoulder instability is characterized by pathologic glenohumeral translation ranging from a mild subluxation (microinstability) to traumatic dislocation (macroinstability).

• Posterior instability of the shoulder is less common than anterior instability, representing 5%-10% of all patients with shoulder instability.

• Patients are candidates for surgical repair/reconstruction when conservative treatment fails. This may consist of rest, physical therapy, and possibly even a corticosteroid injection for rotator cuff-related symptoms.

• May be from direct trauma (direct blow to the anterior shoulder or landing on an outstretched, adducted arm). This could result in a dislocation. Position of risk is a forward flexed and adducted shoulder.

• Electrocution or seizures are an indirect mechanism that may result in posterior dislocation secondary to the contraction of the subscapularis and pectoralis.

• Recurrent posterior subluxation (RPS) is likely a more common pathologic entity from repetitive microtrauma to the posterior capsule, labrum, and posterior glenoid. This often occurs with repetitive overhead or throwing sports.

• Tears of the posterior labrum or capsule, humeral avulsion of the glenohumeral ligaments (reverse HAGL), and excessive posterior glenoid retroversion can all contribute to the pathologic laxity/instability.

• Age, arm dominance, sport of choice (for athletes), and nature of traumatic event or symptoms are documented.

• Pain is characterized by discomfort in the position of provocation (forward flexed and adducted), vague discomfort during or after sports, or loss of velocity in throwing athletes. Pain often is diffuse and deep-seated in the shoulder.

• Any response to conservative treatment (closed reduction, sling use, immobilization, physical therapy, or corticosteroid injection) is documented.

• Any associated symptoms (periscapular pain, neurologic symptoms radiating into extremity, cervical neck discomfort) are evaluated.

• Inspection of any rotator cuff or periscapular muscle atrophy or asymmetry.

• Palpation to document soreness of the anterior or posterior capsule, coracoid process, acromioclavicular joint, and greater tuberosity.

• Range of motion of the injured shoulder compared to the contralateral extremity.

• Strength assessment to evaluate the rotator cuff muscles and periscapular muscles in forward flexion, abduction, internal/external rotation, and extension.

• Provocative testing, including the Kim test, jerk test, circumduction test, and load and shift testing.

• Standard radiographs often are negative.

• An MRI arthrogram is the gold standard imaging modality to detect injury to the posterior labrum and capsule.

• A CT scan can be obtained if bone injury/deficiency is suspected.

• Viewing is done from both the anterior and posterior portals in an all-arthroscopic fashion.

• Instrumentation is used from both the anterior and posterior portals.

• 30 cc of saline are first used to insufflate the joint from a posterior injection portal.

• The posterior portal typically is created in line with the lateral edge of the acromion (1 cm lateral and 1 cm inferior to a standard posterior arthroscopy portal).

• Placement of this portal more lateral allows easier access to the posterior glenoid and can facilitate anchor placement.

• An anterior portal is established high in the rotator interval with an “inside-out” technique with a switching stick.

• The anterior switching stick is then replaced with a 6- or 7-mm clear cannula.

• A thorough arthroscopic examination is carried out with a 30-degree arthroscope to look for associated pathology (chondral damage, rotator cuff tear, biceps tear, anterosuperior labral pathology). The labral tear is identified along the posterior glenoid margin (Fig. 4-2).

• Debridement can be carried out with a 4.5-mm full radius shaver using the oscillation mode.

• At the conclusion of the diagnostic arthroscopy, the scope is placed in the anterior cannula and the fluid inflow is switched to the side port of the cannula. A switching stick is placed in the posterior portal, this portal is dilated to 8.0 mm, and an 8.25-mm distally threaded clear plastic cannula is placed.

• If necessary, a 70-degree arthroscope can then be placed on the camera, which allows excellent vision of the posterior compartment of the shoulder.

• An elevator can be brought either through the anterior portal before placement of the scope anteriorly or through the posterior portal to elevate the posterior labrum from the posterior glenoid rim/margin (Fig. 4-3).