Residency · Residency · Orthopedic Surgery
Shoulder Instability: Anterior, Posterior, and Multidirectional
Overview
Shoulder instability represents a spectrum from subluxation to frank dislocation. The glenohumeral joint sacrifices bony congruency for range of motion, with the glenoid covering only approximately 25% of the humeral head surface. Static stabilizers include the glenoid labrum, glenohumeral ligaments (superior, middle, and inferior), joint capsule, and negative intra-articular pressure. Dynamic stabilizers include the rotator cuff, periscapular muscles, and long head of the biceps.
Anterior Instability
Epidemiology and Mechanism
Anterior instability is the most common direction, accounting for approximately 95% of traumatic dislocations. The classic mechanism involves abduction, external rotation, and extension (the ABER position). Recurrence rate is inversely related to age at first dislocation: patients under 20 have up to 90% recurrence, while those over 40 have approximately 10-15% recurrence but a higher rate of associated rotator cuff tears.
Pathoanatomy
The Bankart lesion is a detachment of the anteroinferior labrum from the glenoid rim. A bony Bankart adds a fracture of the anteroinferior glenoid with labral detachment. The Hill-Sachs lesion is a compression fracture of the posterosuperior humeral head from impaction on the glenoid rim. The HAGL lesion is a humeral avulsion of the glenohumeral ligament and is often missed. The ALPSA lesion is an anterior labral periosteal sleeve avulsion where the medially displaced labrum heals in a non-anatomic position. The GLAD lesion is a glenolabral articular disruption involving a cartilage injury without frank instability.
Glenoid Bone Loss Quantification
The critical bone loss threshold is traditionally 20-25% of glenoid width, producing an inverted-pear glenoid configuration. The best-fit circle method on 3D CT (en face view) is the gold standard for measurement. The glenoid track concept (Yamamoto/Di Giacomo) calculates whether a Hill-Sachs lesion engages the glenoid rim based on combined bone loss. An "on-track" lesion does not engage, meaning soft tissue repair is sufficient. An "off-track" lesion engages the rim, requiring bony augmentation such as remplissage or a Latarjet procedure.
Instability Severity Index Score (ISIS/IRIS)
This validated scoring system predicts failure of arthroscopic Bankart repair. Factors include age under 20, competitive sport, contact or overhead sport, shoulder hyperlaxity, Hill-Sachs visible on AP radiograph, and glenoid bone loss on AP radiograph. A score greater than 6 favors bony augmentation (Latarjet) over isolated soft tissue repair.
<image>Anterior Bankart lesion and Hill-Sachs lesion on arthroscopic view and MRI</image>
| Procedure | Indication | Mechanism | Key Considerations |
|---|---|---|---|
| Arthroscopic Bankart repair | Soft tissue instability; minimal bone loss | Labral restoration to glenoid face | Failure rate 5-15%; higher with bone loss |
| Latarjet | Glenoid bone loss >20%; off-track Hill-Sachs; ISIS >6 | Triple effect: bone block + sling + capsular reinforcement | Risk: nerve injury, nonunion, hardware |
| Remplissage | Engaging Hill-Sachs without glenoid bone loss | Infraspinatus tenodesis into Hill-Sachs defect | Minimal ER loss; combined with Bankart |
| Open Bankart | Select revision cases | Direct capsular shift | Historical gold standard |
| Posterior bone block | Posterior glenoid bone loss | Iliac crest or distal tibia allograft augmentation | Less common; emerging evidence |
Surgical Management
Arthroscopic Bankart repair is the standard for soft tissue anterior instability with minimal bone loss. Suture anchors restore the labrum to its anatomic position on the glenoid face. Key technical points include adequate capsular shift, anchor placement on the glenoid face (not the rim), and use of at least 3-4 anchors. The failure rate is 5-15% overall but higher with significant bone loss, contact athletes, and revision cases.
Open Bankart repair was the historical gold standard and is now reserved for select revision cases, allowing direct capsular shift.
The Latarjet procedure transfers the coracoid process with its attached conjoint tendon to the anteroinferior glenoid. It achieves a triple effect: bone augmentation, a sling effect from the conjoint tendon, and capsular reinforcement from the coracoacromial ligament stump. Indications include significant glenoid bone loss (greater than 20%), off-track Hill-Sachs lesions, high-risk recurrence (ISIS greater than 6), and failed arthroscopic repair. Complications include nerve injury (musculocutaneous, axillary), hardware issues, nonunion of the bone block, and recurrence.
Remplissage is an arthroscopic infraspinatus tenodesis into the Hill-Sachs defect to prevent engagement, often combined with Bankart repair. The concern about limiting external rotation has proven clinically marginal in most studies.
<image>3D CT reconstruction showing glenoid bone loss measurement with best-fit circle method</image>
Posterior Instability
Epidemiology and Mechanism
Posterior dislocations account for 2-10% of shoulder dislocations. Mechanisms include seizures, electrocution, and axial loading with the shoulder in forward flexion, internal rotation, and adduction. Posterior dislocations are frequently missed on initial evaluation, with up to 50% having a delayed diagnosis.
Diagnosis
Radiographic clues include the "lightbulb sign" on AP view (internally rotated humeral head) and loss of the half-moon overlap sign. The axillary lateral view is critical and must be included in the trauma series. CT identifies a reverse Hill-Sachs (McLaughlin) lesion, which is an impaction of the anteromedial humeral head. The jerk test and Kim test assess for recurrent posterior instability.
Treatment
Acute posterior dislocation is managed with closed reduction under sedation. If the dislocation is locked with a reverse Hill-Sachs involving more than 20% of the articular surface, operative management is required. Posterior Bankart repair (arthroscopic) is indicated for recurrent posterior subluxation or instability with a labral tear. A posterior bone block using iliac crest autograft or distal tibia allograft addresses posterior glenoid bone loss. The McLaughlin procedure or its modification (lesser tuberosity transfer) addresses engaging reverse Hill-Sachs lesions.
Multidirectional Instability (MDI)
Definition and Pathophysiology
Multidirectional instability is defined as symptomatic instability in two or more directions, most commonly inferior combined with anterior or posterior. The underlying pathology is capsular redundancy and ligamentous laxity. Onset is often atraumatic, and the condition may be bilateral. The Beighton score assesses generalized ligamentous laxity. The sulcus sign reveals a sulcus below the acromion with inferior traction on the arm; a positive sulcus sign that does not reduce with external rotation suggests an incompetent rotator interval.
Treatment
Nonoperative management with a prolonged rehabilitation program (minimum 6 months) focusing on rotator cuff and periscapular muscle strengthening is successful in up to 80% of patients. When conservative treatment fails, operative options include arthroscopic capsular plication (360-degree capsular shift) or the open inferior capsular shift (Neer procedure), which is historical but still effective. Rotator interval closure may be addressed if incompetent, though this remains controversial due to the risk of overtightening.
<image>Clinical photograph demonstrating positive sulcus sign in multidirectional instability</image>
First-Time Dislocation Management
Decision-Making in Young Athletes
Immobilization in external rotation versus internal rotation (Itoi concept) has shown inconsistent evidence on reducing recurrence. Early surgical stabilization in young (under 25 years), active patients reduces recurrence compared to nonoperative management. Shared decision-making should consider sport demands, season timing, and patient preferences.
<image>Latarjet procedure illustration showing coracoid transfer to the anteroinferior glenoid</image>
Clinical Pearls
Always obtain a true AP, scapular Y, and axillary lateral view in the trauma series; the axillary view is the most important for confirming dislocation direction. After reduction, axillary nerve function must be assessed and documented by testing regimental badge area sensation and deltoid contraction. Concomitant rotator cuff tears must be evaluated, especially in patients over age 40 with first-time dislocation. A bony Bankart on radiographs changes the surgical plan significantly, and CT should always be obtained to quantify glenoid bone loss before planning arthroscopic repair. The glenoid track concept represents a paradigm shift: Hill-Sachs and glenoid bone loss should be evaluated as coupled lesions, not in isolation. Recurrent instability after arthroscopic Bankart repair should prompt evaluation for unrecognized bone loss, a HAGL lesion, or capsular insufficiency.
References
- Bankart ASB. The pathology and treatment of recurrent dislocation of the shoulder-joint. Br J Surg. 1938;26(23):23-29.
- Balg F, Boileau P. The instability severity index score: a simple pre-operative score to select patients for arthroscopic or open shoulder stabilisation. J Bone Joint Surg Br. 2007;89(11):1470-1477.
- Di Giacomo G, Itoi E, Burkhart SS. Evolving concept of bipolar bone loss and the Hill-Sachs on-track/off-track lesion. Arthroscopy. 2014;30(1):90-98.
- Latarjet M. Treatment of recurrent dislocation of the shoulder. Lyon Chir. 1954;49:994-997.
- Provencher MT, et al. Glenoid bone loss in anterior shoulder instability: evaluation and management. JAAOS. 2017;25(4):e91-e101.
- Kuhn JE. A new classification of shoulder instability. Br J Sports Med. 2010;44(5):341-346.
