Arthroscopic Posterior Shoulder Instability Repair: A Masterclass

Introduction and Epidemiology

Posterior shoulder instability results in pathologic glenohumeral translation ranging from mild subluxation to traumatic dislocation. Most patients with this pathologic entity report pain in provocative positions of the glenohumeral joint, a condition referred to as recurrent posterior subluxation.

Posterior shoulder instability is much less common than anterior instability, representing approximately 5% to 10% of all patients with pathologic shoulder instability. A decision must be made regarding surgical treatment of this condition when an extended trial of conservative measures, such as targeted physical therapy focusing on periscapular and dynamic stabilizers, has failed.

Unlike anterior instability, which frequently presents as an acute, traumatic dislocation requiring closed reduction in an emergency setting, posterior instability often presents more insidiously. The microtraumatic etiology is highly prevalent in overhead athletes, weightlifters (particularly those performing heavy bench presses), and offensive linemen in American football. These repetitive axial loads applied to a forward-flexed, internally rotated arm gradually attenuate the posterior capsulolabral complex. Acute traumatic dislocations do occur but are typically the result of high-energy trauma or massive unbalanced muscle contractions.

Patients with a history of a chronically locked posterior dislocation are at increased risk for the development of severe chondral injury and subsequent rapid-onset degenerative arthritis. Static posterior subluxations of the humeral head have been directly correlated with the presence of advanced osteoarthritis in young adults whose instability was left untreated. Currently, no long-term studies on the arthroscopic treatment of shoulder instability have definitively documented a reduction in the development of osteoarthritis, though restoring concentric joint kinematics remains the primary biomechanical goal of surgical intervention.

Surgical Anatomy and Biomechanics

The important stabilizing structures of the glenohumeral joint are the articular surfaces and congruity of the humerus and glenoid of the scapula, the capsular structures, the glenoid labrum, the intra-articular portion of the biceps tendon, and the rotator cuff muscles. Pathologies of the posterior capsule and labral complex are believed to be the main contributors to posterior instability.

With the arm forward-flexed to 90 degrees, the subscapularis provides significant stability against posterior translation, and as the arm is placed in neutral, the coracohumeral ligament resists this force. With internal rotation of the shoulder, particularly during the follow-through phase of throwing, the inferior glenohumeral ligament complex is the main restraint to posterior translation.

Histologic evaluation of the posterior capsule shows it to be relatively thin and composed of only radial and circular fibers, with minimal cross-linking. This distinct lack of robust collagenous cross-linking makes the posterior capsule highly susceptible to plastic deformation and elongation under repetitive stress, leading to capsular redundancy.

Posterior instability can be the result of trauma in the form of a direct blow to the anterior shoulder or may occur as the result of indirect forces acting on the shoulder, causing the combined movements of shoulder flexion, adduction, and internal rotation. Electrocution and convulsive seizures are the most common causes of an indirect mechanism resulting in posterior dislocation. In these scenarios, the significantly greater muscle mass of the internal rotators (latissimus dorsi, pectoralis major, and subscapularis) dramatically overpowers the external rotators (infraspinatus and teres minor), violently levering the humeral head posteriorly over the glenoid rim.

Osseous anatomy also plays a critical role. The native glenoid typically exhibits 1 to 2 degrees of retroversion. However, patients with posterior instability frequently demonstrate increased glenoid retroversion (often exceeding 10 to 15 degrees) or posterior glenoid hypoplasia. This osseous dysplasia severely compromises the static stabilizing effect of the glenoid concavity, predisposing the joint to recurrent subluxation.

Clinical Evaluation and Diagnostic Imaging

A thorough history is obtained, documenting whether a frank dislocation has occurred, including the need for closed reduction, or if the primary symptoms are pain and mechanical catching. The circumstances regarding pain are documented, namely onset, specific provocations, severity, ability to participate in sports, and whether symptoms are present at rest.

Athletes may report that velocity with throwing is diminished, and a sharp pain may accompany the follow-through phase of throwing. Other associated injuries such as superior labrum anterior posterior (SLAP) lesions, rotator cuff tears, reverse Hill-Sachs defects, and chondral injuries may be present and contribute to the pathology. Any response to conservative treatment, including physical therapy, rest, and anti-inflammatory medication, should be strictly noted.

As with the examination of any joint, the shoulder is palpated to elicit tenderness, and range of motion is documented. Any restriction in motion should be compared to the contralateral extremity, and differences between active and passive motion may indicate pain or capsular contracture. Impingement signs are tested to determine whether any associated rotator cuff tendinitis is present.

Strength testing must be meticulously performed. Weakness in the external rotators (infraspinatus and teres minor) may indicate underlying tendinopathy, tearing, or secondary suprascapular nerve stretch neuropathy caused by the recurrent posterior subluxation of the humeral head.

Provocative Physical Examination Maneuvers

Specific tests for posterior instability are essential for an accurate diagnosis:
* Jerk Test: The patient's arm is placed in 90 degrees of forward flexion and internal rotation. An axial load is applied through the elbow while moving the arm horizontally into adduction. A sudden "jerk" or clunk as the humeral head subluxates posteriorly indicates a positive test. A second clunk may be felt as the arm is brought back to the starting position and the head reduces.
* Kim Test: The patient's arm is elevated to 90 degrees of abduction. The examiner applies an axial load while simultaneously elevating the arm diagonally upward and forward, applying a downward and posterior force to the proximal humerus. Pain or a clunk indicates a posteroinferior labral tear (Kim lesion).
* Load and Shift Test: Assesses the degree of static translation. The humeral head is grasped and loaded into the glenoid, then translated posteriorly. Translation is graded from 1 (to the rim) to 3 (dislocates and remains dislocated).

Diagnostic Imaging Protocols

Standard radiographs must include a true anteroposterior (Grashey) view, an axillary lateral, and a scapular Y view. The axillary view is paramount for identifying static posterior subluxation, posterior glenoid rim fractures, and reverse Hill-Sachs lesions (impaction fractures of the anteromedial humeral head).

Magnetic Resonance Imaging (MRI), preferably an MR arthrogram with intra-articular gadolinium, is the gold standard for evaluating the capsulolabral complex. It allows for the identification of reverse Bankart lesions, posterior humeral avulsions of the glenohumeral ligament (pHAGL), and concealed interstitial tears of the posteroinferior labrum. Computed Tomography (CT) with 3D reconstruction is mandatory if significant bone loss or severe glenoid retroversion is suspected, as accurate quantification dictates the surgical approach.

Indications and Contraindications

Surgical intervention is highly dependent on the chronicity of the instability, the presence of structural lesions, and the patient's functional demands. An exhaustive trial of physical therapy is the first line of treatment for atraumatic or microtraumatic posterior subluxation. Surgery is considered when patients experience persistent pain and mechanical symptoms that preclude them from their desired level of activity despite a minimum of 3 to 6 months of targeted rehabilitation.

Operative Versus Non Operative Management

Pre Operative Planning and Patient Positioning

Thorough preoperative planning hinges on the accurate interpretation of advanced imaging to anticipate the need for capsular plication, labral repair, or bone grafting. The surgeon must evaluate the volume of the posterior capsule and the integrity of the posterior band of the inferior glenohumeral ligament.

Anesthesia typically consists of a regional interscalene nerve block combined with general endotracheal anesthesia. The regional block provides excellent postoperative analgesia and reduces the required intraoperative volatile anesthetic concentration, thereby minimizing postoperative nausea and vomiting.

Patient Positioning Considerations

The procedure can be performed in either the lateral decubitus or the beach chair position. However, the lateral decubitus position is heavily favored by many orthopedic surgeons for posterior instability.

In the lateral decubitus setup, the patient is placed on a beanbag with all bony prominences meticulously padded. The operative arm is placed in a pneumatic traction sleeve. Approximately 10 to 15 pounds of traction is applied in 45 degrees of abduction and 15 to 20 degrees of forward flexion. This specific vector distracts the glenohumeral joint, exceptionally widening the posterior and inferior joint spaces, which allows for unparalleled visualization of the posteroinferior recess and the posterior labrum.

If the beach chair position is utilized, the patient must be positioned laterally enough on the table to allow unrestricted access to the posterior shoulder. An articulated arm positioner is highly recommended to control rotation and apply manual distraction when accessing the inferior capsule.

Detailed Surgical Approach and Technique

The goal of arthroscopic posterior stabilization is to restore the native bumper effect of the posterior labrum and eliminate capsular redundancy through precise plication.

Portal Placement and Diagnostic Arthroscopy

Standard portal placement is critical for the appropriate angle of approach.
1. Standard Posterior Portal: Established approximately 2 cm inferior and 1 cm medial to the posterolateral corner of the acromion. This serves as the initial viewing portal.
2. Anterosuperior Portal: Established high in the rotator interval, just anterior to the biceps tendon. This serves as a working portal and later as a viewing portal to assess the posterior structures looking from anterior to posterior.
3. Anteroinferior Portal: Established just superior to the subscapularis tendon.
4. Accessory Posterolateral Portal (7-Point Portal): Located approximately 1 to 2 cm lateral to the standard posterior portal. This is the most critical portal for anchor insertion, as it provides the correct trajectory (typically 45 degrees to the articular surface) to address the posteroinferior glenoid (the 7 o'clock to 9 o'clock positions in a right shoulder) without penetrating the articular cartilage.

A comprehensive diagnostic sweep is performed. The surgeon evaluates the anterior labrum, the superior labrum, the biceps anchor, the rotator cuff, and the articular surfaces. The arthroscope is then driven posteriorly to evaluate the posterior labrum and capsule. A probe is introduced to assess the competency of the labral attachment. A "Kim lesion" may present as an intact superficial labrum but an avulsed deep labral attachment, which must be mobilized.

Preparation of the Glenoid Footprint

Once the pathology is identified, the arthroscope is moved to the anterosuperior portal to view the posterior compartment. Using an elevator or a rasp through the posterior working portal, the posterior labrum and capsule are mobilized from the glenoid neck. It is imperative to release any adhesions down to the 6 o'clock position to allow for superior shift of the capsulolabral complex.

Following mobilization, a motorized burr or rasp is used to decorticate the posterior glenoid neck. The bone must be prepared to a bleeding bed to optimize the biological healing of the soft tissue to the osseous footprint. Care must be taken not to aggressively burr the glenoid rim, which could inadvertently increase glenoid retroversion and exacerbate instability.

Anchor Placement and Capsulolabral Plication

Suture anchors (typically 3.0 mm or smaller, biocomposite or all-suture anchors) are placed starting at the most inferior aspect of the tear, usually at the 6:30 or 7 o'clock position. The accessory posterolateral portal is utilized to ensure the drill trajectory is safe. The anchor is deployed, and the sutures are retrieved.

To address the characteristically thin and redundant posterior capsule, a "pinch-tuck" capsular plication technique is employed. A suture-passing instrument (e.g., a curved suture lasso) is passed through the posterior capsule, taking a robust bite of tissue approximately 1 cm lateral to the labrum and slightly inferior to the anchor location. This effectively shifts the capsule superiorly and medially. The instrument is then passed through the labrum, and the suture is shuttled.

Knots are tied using a sliding-locking knot backed up by alternating half-hitches, or knotless anchors can be utilized to minimize intra-articular knot prominence, which can lead to chondral abrasion. This process is repeated sequentially moving superiorly, placing anchors at the 8 o'clock and 9 o'clock positions until a robust, tensioned posterior bumper is restored.

Management of Bony Defects

If a reverse Hill-Sachs lesion engages the posterior glenoid rim during internal rotation, a modified McLaughlin procedure (transfer of the subscapularis into the defect) or an arthroscopic remplissage-equivalent (filling the defect with the subscapularis and anterior capsule) may be required. If posterior glenoid bone loss exceeds 15% to 20%, arthroscopic soft tissue stabilization is contraindicated, and an open or arthroscopically assisted posterior bone block using iliac crest autograft or distal tibia allograft is mandated.

Complications and Management

While arthroscopic posterior stabilization yields excellent outcomes in appropriately selected patients, complications can significantly impact functional recovery. The surgeon must be adept at recognizing and managing these adverse events.

![Image](\media\upload\29b4df4f-b762

Clinical & Radiographic Imaging