Hill-Sachs lesion

What it means

A Hill-Sachs lesion is a small dent, or compression fracture, in the back-outer surface of the humeral head, the round ball at the top of the upper arm bone that fits into the shoulder socket. It forms when the shoulder dislocates and the humeral head is forced hard against the front rim of the socket (the glenoid), leaving a permanent impression in the softer bone of the humeral head, similar to a dent left in a car door after a hard impact against a curb.

Why it appears on a CT or MRI report

Because it forms at the moment of dislocation, a Hill-Sachs lesion is one of the classic signs radiologists look for when evaluating someone with a history of shoulder instability, even if the shoulder has since been put back in place. Reports describe its size, depth, and location, and often comment on whether it's an "engaging" lesion, meaning it catches on the socket's rim during certain arm movements, which matters for how likely the shoulder is to dislocate again. It's frequently reported alongside a related injury to the socket's cartilage rim, called a Bankart lesion, since the two often occur together in the same dislocation event.

What it usually means

A Hill-Sachs lesion confirms that a true dislocation happened, even years earlier, and its size gives a clue about how unstable the shoulder may be going forward. Small, shallow lesions found after a single dislocation often cause no ongoing problems and may need no specific treatment of their own. Larger or engaging lesions, especially when paired with damage to the socket rim, are associated with a higher chance of the shoulder dislocating again, sometimes with less and less force each time, and can make the joint feel like it's going to "give way" during certain overhead or reaching motions.

When to follow up

Bring this finding to an orthopedic or sports medicine specialist, particularly if you're having recurring episodes of shoulder instability, a feeling of the joint slipping, or pain with overhead activity. Many first-time dislocations with a small lesion are managed with physical therapy focused on strengthening the muscles around the shoulder. Larger lesions, repeat dislocations, or a shoulder that keeps giving way during sports or daily activities are often discussed for surgical options, which can include repairing the socket rim or, for larger defects, procedures aimed at filling in or bypassing the dent itself.

A plain-language way to picture it

Think of the humeral head as a smooth billiard ball meant to roll freely inside a shallow cup. A Hill-Sachs lesion is like a chip knocked into the edge of that ball where it once struck the rim of the cup hard enough to leave a mark. Most of the time the ball still rolls smoothly, but if the chip lines up with the cup's edge at certain angles, it can catch and slip out again, which is why doctors pay close attention to where exactly that chip sits.