Assessment of Glenohumeral Bone Loss

1. Clinical Context

• Some degree of glenoid bone loss is present in 40% of first-time dislocations and 85% of recurrent dislocations.
• A Hill-Sachs lesion is seen in 40% of patients with recurrent subluxation, 90% after first dislocation, and approaching 100% with recurrent dislocations.
• Bipolar lesions (combined glenoid + Hill-Sachs) occur in 33% of primary instability and 62% of recurrent instability cases.

2. Imaging Modalities

Plain Radiography

CT Scan (Primary Modality)

• Identifies smallest osseous fragments and glenoid asymmetry
• 3D CT with humeral subtraction: gold standard for glenoid bone loss quantification
• Also detects Hill-Sachs lesions with accuracy comparable to arthroscopy
• Essential when bipolar lesions are suspected

MRI / MR Arthrogram (MRA)

• MRA preferred over plain MRI for greater sensitivity for labral and ligamentous lesions
• ABER position (abduction-external rotation) increases sensitivity for anteroinferior labral injuries
• Circle method on MRI is accurate to within 1.3% of 3D CT for measuring glenoid bone loss
• Demonstrates capsular volume, glenoid version, and associated soft-tissue pathology

3. Quantifying Glenoid Bone Loss

A. Circle Method (most widely used)

B. Burkhart Bare-Spot Arthroscopic Method

• Distance from bare spot to posterior glenoid margin = Dp
• Distance from bare spot to anterior glenoid margin = Da
• % Bone loss = (Dp − Da) / (2 × Dp) × 100

C. Width-Based Formula (Owens et al.)

• Males: Normal glenoid width = ⅓ height + 15 mm
• Females: Normal glenoid width = ⅓ height + 13 mm

4. Critical Thresholds

5. Hill-Sachs Lesion Assessment and the Glenoid Track Concept

Glenoid Track Calculation

Hill-Sachs Interval (HSI)

Classification

• On-track (safe): HSI < GT → no engagement risk → arthroscopic Bankart repair alone is appropriate
• Off-track (high risk): HSI ≥ GT → engagement occurs → requires remplissage or Latarjet in addition

6. Summary Algorithm

Plain XR (Stryker Notch, West Point, Bernageau)
        ↓
   Suspected bone loss?
        ↓
3D CT with humeral subtraction  ←  primary quantification
+/- MRI/MRA (soft tissue, labrum, version)
        ↓
  Measure glenoid bone loss (circle method)
        ↓
< 20%          20–25%          > 25%
  ↓               ↓               ↓
Arthroscopic   Borderline:     Bony augmentation
Bankart repair  glenoid track  (Latarjet / bone graft)
                analysis
                  ↓
            On-track → ABR
            Off-track → ABR + remplissage or Latarjet

• Rockwood and Green's Fractures in Adults, 10th ed., 2025 — Chapter 35
• Miller's Review of Orthopaedics, 9th ed. — Chapter 4, Anterior Instability

Forces Acting on the Shoulder During Anterior Glenohumeral Dislocation

1. Overview of Mechanism

2. Primary Force Vectors

A. Indirect Force — Abduction + External Rotation (ABER)

B. Direct Force — Fall onto Forward-Flexed, Externally Rotated Arm

• The axial load transmits proximally up the shaft of the humerus
• The force vector is directed posteriorly relative to the humeral head — but because the arm is forward-flexed, this translates into an anterior displacement of the humeral head relative to the glenoid
• The subscapularis, anterior capsule, and IGHLC fail under the combined compressive + shear load

C. Non-contact / Muscular Imbalance Mechanisms

• Seizures and electrocution most commonly cause posterior dislocation (the stronger internal rotators — subscapularis, pectoralis major, anterior deltoid — overwhelm the weaker external rotators)
• Missed punches, sudden eccentric loads, or muscular imbalance can precipitate anterior subluxation/dislocation in hyperlax individuals at lower force thresholds

3. Forces That Must Be Overcome for Dislocation to Occur

Static Stabilizers (Passive Restraints)

Dynamic Stabilizers (Active Restraints)

4. Resultant Tissue Injuries from Force Transmission

1. Bankart lesion — avulsion of the anteroinferior capsulolabral complex from the glenoid rim (occurs in ~90% of recurrent instability)
2. Bony Bankart — if force is sufficient, the attached glenoid rim fractures with the labrum (present in ~40% of first dislocations)
3. Hill-Sachs lesion — impaction fracture of the posterosuperior humeral head against the hard anteroinferior glenoid rim as the head dislocates; occurs in ~90% after first dislocation and ~100% with recurrent dislocations
4. Capsular distension/tear — stretching or frank tearing of the anterior capsule; HAGL (humeral avulsion of the glenohumeral ligament) occurs in 1–9% when the IGHLC avulses from the humerus rather than the glenoid
5. ALPSA lesion — anterior labroligamentous periosteal sleeve avulsion; the capsulolabral complex strips medially with the intact periosteum (as opposed to the frank rupture of a Bankart lesion)
6. Associated injuries (more common in older patients, up to 40%):
   • Greater tuberosity fracture (most common, ~15% of older patients)
   • Rotator cuff tear
   • Axillary nerve palsy (susceptible due to its tethered position in the axilla)
   • Brachial plexus injury
   • Axillary artery injury (rare, high-energy)

5. Force Summary Diagram

ARM IN ABDUCTION + EXTERNAL ROTATION
              ↓
Posterior / axial force applied to arm or shoulder
              ↓
IGHLC anterior band → maximally tensioned
Subscapularis → overcome at extremes of ER
Labrum-glenoid interface → shear force exceeds attachment strength
              ↓
Humeral head levers over anteroinferior glenoid rim
              ↓
ANTERIOR DISLOCATION
  ├─ Bankart / bony Bankart (glenoid side)
  ├─ Hill-Sachs impaction (humeral side)
  ├─ Capsular tear / HAGL
  └─ Axillary nerve / rotator cuff injury

• Rockwood and Green's Fractures in Adults, 10th ed., 2025 — Mechanisms of Injury & Injuries Associated with Glenohumeral Instability
• Miller's Review of Orthopaedics, 9th ed. — Anterior Instability

Hill-Sachs Index (Hill-Sachs Lesion Quantification)

1. Why Quantify the Hill-Sachs Lesion?

• Present in ~40% of first dislocations, ~90% after first true dislocation, approaching 100% with recurrent dislocations
• Average lesion size: 22 mm width × 5 mm depth
• Size directly correlates with number of dislocation events
• Determines whether the lesion is engaging (off-track) or non-engaging (on-track)
• Guides surgical decision-making: observation, remplissage, bone grafting, or arthroplasty

2. Imaging Modalities for Measurement

3. Hill-Sachs Quotient (Kralinger Classification)

Measurement

• True AP radiograph with the arm in 60° internal rotation → measure:
  • x = width of the lesion
  • y = depth of the lesion
• Bernageau (glenoid profile) view → measure:
  • z = length of the lesion

Formula

Hill-Sachs Quotient (HSQ) = x × y × z (in cm³)

Grading

4. Percentage of Articular Surface Involved

5. Rowe Classification (Descriptive — Axillary Projection)

6. Calandra / Franceschi Classification (Arthroscopic)

7. Glenoid Track / Hill-Sachs Interval (Most Clinically Important)

Decision Rule

1. Large, wide defect engaging in functional positions
2. Narrow but medially located defect — engages due to proximity to glenoid rim

8. Summary: Which Index to Use?

• Rockwood & Green's Fractures in Adults, 10th ed., 2025 — Evaluation of the Hill-Sachs Lesion
• Provencher MT et al. "The Hill-Sachs lesion: diagnosis, classification, and management." J Am Acad Orthop Surg 2012 [PMID: 22474094]
• Kralinger et al. — Hill-Sachs Quotient grading system
• Open Orthopaedics Journal — Management of Humeral Defects in Anterior Shoulder Instability, 2017