Hill-Sachs Lesion: Formation, Anatomy, and Clinical Impact

How Does a Hill-Sachs Lesion Occur?

A Hill-Sachs lesion is an impaction fracture of the posterior-superior humeral head, most commonly occurring during an anterior shoulder dislocation when the humeral head forcefully impacts the anterior glenoid rim.

Understanding Shoulder Anatomy and Stability

To grasp the mechanism of a Hill-Sachs lesion, it's crucial to first understand the unique anatomy of the shoulder joint, or glenohumeral joint. This is a ball-and-socket joint, where the head of the humerus (the "ball") articulates with the glenoid fossa of the scapula (the "socket").

Key Anatomical Features:

• Humeral Head: The rounded, proximal end of the humerus.
• Glenoid Fossa: A shallow, pear-shaped depression on the lateral aspect of the scapula.
• Glenoid Labrum: A fibrocartilaginous rim that deepens the glenoid fossa, enhancing stability.
• Joint Capsule and Ligaments: A complex network of tissues that enclose the joint and provide passive stability.
• Rotator Cuff Muscles: A group of four muscles (supraspinatus, infraspinatus, teres minor, subscapularis) that dynamically stabilize the shoulder and enable movement.

While the shoulder is the most mobile joint in the body, its inherent mobility comes at the expense of bony stability, making it prone to dislocation.

The Mechanism of Injury: Anterior Shoulder Dislocation

The vast majority (over 95%) of shoulder dislocations are anterior, meaning the humeral head displaces forward and often slightly downward relative to the glenoid fossa. It is during this specific type of dislocation that a Hill-Sachs lesion typically forms.

Steps in Hill-Sachs Lesion Formation:

1. Forced Abduction and External Rotation: The primary mechanism for anterior shoulder dislocation involves a force that drives the arm into excessive abduction (lifting the arm away from the body) and external rotation (rotating the arm outward). This can occur during falls onto an outstretched arm, sports injuries (e.g., throwing, tackling), or direct trauma to the posterior shoulder.
2. Humeral Head Dislocation: As the humerus is forced beyond its anatomical limits, the humeral head levers out of the glenoid fossa, typically moving anteriorly.
3. Impaction Against the Glenoid Rim: Once dislocated, the posterior-superior aspect of the humeral head forcefully impacts the relatively sharp, unyielding anterior-inferior rim of the glenoid. This impact occurs as the humeral head is driven forward and then often "catches" or "dents" against the bony edge of the glenoid.
4. Bone Impaction Fracture: The high-energy impact, combined with the compressive and shearing forces, results in a localized impaction fracture or defect on the posterolateral aspect of the humeral head. This indentation is what defines a Hill-Sachs lesion.

Essentially, the Hill-Sachs lesion is a "signature" injury of an anterior shoulder dislocation, representing the point of contact where the humeral head was compressed against the glenoid rim during the dislocation event.

What is a Hill-Sachs Lesion?

A Hill-Sachs lesion is specifically an impaction fracture of the posterior-superior (or posterolateral) aspect of the humeral head. It is characterized by a cortical depression or defect.

Key Characteristics:

• Location: Always on the humeral head, typically posterolateral.
• Type: An impaction fracture, not a break clean through the bone. It's a "dent" or "divot."
• Size and Depth: Can vary significantly from small, superficial indentations to large, deep defects that involve a substantial portion of the humeral head's articular surface.

Factors Influencing Hill-Sachs Lesion Formation

While an anterior shoulder dislocation is the prerequisite, several factors influence the likelihood and severity of a Hill-Sachs lesion:

• Force and Velocity of Dislocation: Higher energy trauma generally leads to larger and deeper lesions.
• Position of the Arm at Impact: The exact position of the arm (degree of abduction and rotation) at the moment of dislocation influences where the humeral head makes contact with the glenoid rim.
• Number of Dislocations: Recurrent anterior shoulder dislocations significantly increase the risk of developing or enlarging a Hill-Sachs lesion. Each subsequent dislocation can deepen the existing defect.
• Bone Quality: Individuals with osteopenia or osteoporosis may be more susceptible to larger lesions even with less forceful dislocations.
• Associated Injuries: The presence of a Bankart lesion (an injury to the anterior-inferior glenoid labrum), which often occurs concurrently with anterior dislocations, can also influence the mechanism of impaction.

Clinical Significance and Management Implications

A Hill-Sachs lesion is not merely an anatomical finding; it has significant clinical implications for shoulder stability.

Impact on Shoulder Stability:

• Engaging Lesion: A larger or more medially located Hill-Sachs lesion can "engage" or "catch" on the anterior glenoid rim when the arm is placed in positions of abduction and external rotation. This engagement can lead to a sensation of apprehension, pain, and significantly increases the risk of recurrent anterior shoulder dislocations.
• Recurrent Instability: The presence of a Hill-Sachs lesion, especially an engaging one, is a major predictor of recurrent shoulder instability after an initial dislocation.

Management Considerations:

• Diagnosis: Hill-Sachs lesions are typically diagnosed via MRI, CT scan, or sometimes plain X-rays (especially specific views like the West Point axillary view).
• Treatment: Small, non-engaging lesions may not require specific treatment beyond rehabilitation for the dislocated shoulder. However, larger or engaging lesions often necessitate surgical intervention to restore shoulder stability. Surgical options may include procedures to augment the humeral head (e.g., remplissage, where the posterior capsule and infraspinatus are sutured into the defect) or to address the glenoid side (e.g., Latarjet procedure).

Understanding the precise mechanism of how a Hill-Sachs lesion occurs is fundamental for accurate diagnosis, effective treatment planning, and ultimately, for restoring optimal shoulder function and preventing recurrent instability in athletes and active individuals.