Hill-Sachs Lesions: Classification, Clinical Importance, and Diagnostic Imaging

How are Hill-Sachs lesions classified?

Hill-Sachs lesions are primarily classified based on their size (depth, width, and percentage of humeral head involvement) and, most critically, whether they are "engaging" or "non-engaging" with the glenoid rim during shoulder movement, which directly impacts shoulder stability and treatment strategies.

Understanding Hill-Sachs Lesions

A Hill-Sachs lesion is a posterolateral compression fracture of the humeral head, specifically the articular cartilage and underlying bone, resulting from the forceful impaction of the humeral head against the anterior glenoid rim during an anterior shoulder dislocation. This divot or defect is a common sequela of shoulder instability, particularly recurrent anterior dislocations, as the humerus repeatedly "dings" against the stable glenoid. Understanding its classification is crucial for assessing shoulder stability, predicting recurrence risk, and guiding appropriate management.

Primary Classification Methods

The classification of Hill-Sachs lesions is multifaceted, considering various characteristics to provide a comprehensive understanding of their clinical significance.

• By Size and Depth:

  • Small: Typically refers to lesions that are shallow and involve a minor percentage of the articular surface, often less than 20%.
  • Medium: Lesions with moderate depth and width, commonly affecting 20-40% of the articular surface.
  • Large: Deep and wide lesions involving a significant portion of the humeral head, often exceeding 40% of the articular surface.
  • Quantitative Measurement: Radiographic and MRI imaging allow for precise measurement of the lesion's depth (distance from the intact articular surface) and width (craniocaudal and anteroposterior dimensions). The percentage of the humeral head circumference involved is also a key metric. Larger lesions are generally associated with greater instability.

• By Engagement (The Critical Classification):

  • Engaging Hill-Sachs Lesion: This is the most clinically significant classification. An engaging lesion occurs when the defect on the humeral head makes contact with, or "engages" against, the anterior glenoid rim when the arm is placed in positions of abduction and external rotation (the "at-risk" position for anterior dislocation). This engagement can cause the humeral head to "lock" or "catch," preventing it from reducing back into the glenoid and contributing significantly to recurrent instability.
  • Non-Engaging Hill-Sachs Lesion: In contrast, a non-engaging lesion does not contact the anterior glenoid rim in provocative positions. While still a defect, it does not directly contribute to the mechanical instability or "locking" phenomenon during normal or extreme ranges of motion. These lesions are generally less problematic in terms of recurrent dislocation risk compared to engaging lesions.

• By Location and Orientation:

  • While inherently posterolateral, the exact orientation of the Hill-Sachs lesion relative to the glenoid can influence its engagement. For instance, a lesion that is more medially or anteriorly oriented on the posterolateral humeral head might have a higher propensity to engage with the glenoid rim. Advanced imaging helps determine this precise anatomical relationship.

• By Bone Loss:

  • Some classification systems also consider the amount of associated bone loss from the humeral head. Significant bone loss can further compromise shoulder stability and often necessitates more complex surgical interventions, such as bone grafting or resurfacing procedures.

Clinical Significance of Classification

The meticulous classification of a Hill-Sachs lesion is paramount for several reasons:

• Prognosis and Recurrence Risk: Engaging lesions significantly increase the risk of recurrent shoulder dislocations, even after successful initial reduction.
• Treatment Planning: The size and engagement status of a Hill-Sachs lesion heavily influence surgical decision-making. Non-engaging lesions may be managed conservatively or with less aggressive surgical techniques. Engaging or large lesions often necessitate specific surgical procedures, such as remplissage (filling the defect by tenodesing the infraspinatus and posterior capsule into the lesion) or bone grafting, in addition to standard Bankart repair (repair of the torn labrum and capsule).
• Rehabilitation Strategy: The presence and type of Hill-Sachs lesion can impact post-operative rehabilitation protocols, particularly regarding range of motion limitations and strengthening exercises.

Diagnostic Imaging for Classification

Accurate classification relies on advanced imaging techniques:

• X-rays: While a basic X-ray can sometimes show a large Hill-Sachs lesion, it often underestimates its size and cannot assess engagement. Specific views (e.g., Stryker notch view) may improve visualization.
• Magnetic Resonance Imaging (MRI): MRI is excellent for visualizing soft tissues and can show the defect's size, depth, and associated cartilage damage. It provides good anatomical detail.
• Computed Tomography (CT) Scan: CT is superior for precise bone detail and accurately measuring the size and depth of the lesion, especially for surgical planning. It can also be used with 3D reconstructions to assess the engagement potential by simulating arm positions.

Conclusion

The classification of Hill-Sachs lesions moves beyond simply identifying their presence. By meticulously categorizing them based on size, depth, and crucially, their potential to engage with the glenoid rim, clinicians can accurately assess the degree of shoulder instability, predict the likelihood of recurrent dislocations, and formulate the most effective and individualized treatment strategies. For fitness professionals and enthusiasts, understanding these classifications underscores the complexity of shoulder instability and the importance of appropriate rehabilitation and injury prevention strategies.