What causes the Hill-Sachs deformity on shoulder radiography, CT and MRI?
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Answer:
A Hill-Sachs lesion is an impaction fracture of the posterosuperolateral humeral head, created when that surface is driven against the anterior glenoid rim during an anterior shoulder dislocation. On radiographs it is best seen on the AP view in internal rotation, which brings the defect into profile, and on the Stryker notch view. CT with three-dimensional reconstruction is the standard for quantifying bone loss on both sides of the joint, and MRI adds the labral, capsular and rotator cuff assessment. Modern management turns on the glenoid track concept: the humeral head contacts a defined zone of glenoid during motion, approximately 83% of the glenoid width minus any anterior glenoid bone loss. If the Hill-Sachs interval, the defect width plus the intact bone bridge medial to it, exceeds that track, the lesion is off-track and will engage the glenoid rim, predicting recurrent instability and requiring a procedure that addresses the bone rather than soft tissue alone. The mirror-image lesion of posterior dislocation is the reverse Hill-Sachs, seen as the trough sign.
Why is it called so?
It is named for Harold Hill and Maurice Sachs, who described the defect in 1940.
Pathophysiology
In an anterior dislocation the humeral head leaves the glenoid fossa and comes to rest anteriorly and inferiorly. As it does so, the relatively soft cancellous bone of the posterolateral head is compressed against the dense cortical anterior glenoid rim, and the head is further loaded against that rim by muscular contraction while dislocated. The result is a wedge-shaped impaction defect whose size and depth increase with the duration of dislocation and the number of recurrences. Because the rim is fixed and the head rotates, the defect always lies posterosuperolaterally, above the level of the coracoid, which is what distinguishes it from the normal bare area of the humeral head.
Alternative names: Hill-Sachs lesion, hatchet deformity (imprecise; hatchet head is the ankylosing spondylitis erosion)
Other associated named signs: Bankart lesion and its bony variant, trough sign or reverse Hill-Sachs, ALPSA, Perthes and GLAD lesions
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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