What is the Sail Sign?

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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 14 August 2019
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The sail sign is a radiological finding in the lung or elbow, potentially indicative of a problem. The name of this clinical sign derives from its distinctive triangular shape, resembling a spinnaker sail, and some people may refer to it as the spinnaker or spinnaker sail sign. A skilled radiologist can identify it on an X-ray and provide more information to help a clinician decide whether the patient's X-rays are normal, and what steps to take in order to provide treatment.

In the elbow, the sail takes the form of a distinctive triangle around the head of the elbow. It usually appears when a patient's elbow is broken and the fat and soft tissue around the elbow experience displacement as a result. Even if the joint does not immediately appear to be broken or dislocated, the sail sign can be a warning that something is occurring, and the patient may need more evaluation to find out what is causing the problem. Usually, a radiologist will order images from several angles to make sure a strange finding is not just a shadow or glitch in the X-ray.

When a patient has a lung X-ray, the sail sign consists of a triangular section at the base of the lung. It may indicate a partial lung collapse. In young children, however, it can be a normal anatomical structure, and is sometimes caused by the thymus, a part of the patient's anatomy. Likewise with many animals, like dogs.


Learning to read X-rays and other medical imaging studies requires patience and training. Some findings may be obvious, like a significant fracture, but others can be very faint. Someone without skills may not immediately notice a problem that would be apparent to a trained radiologist or technician. Something like the sail sign can sometimes be very subtle and unobtrusive. When doctors request X-rays, they usually ask for someone to review them to make sure there are no clinically significant findings if they have concerns about an apparently normal X-ray.

When the sail sign is visible, the doctor may request follow up testing to find out more about it, and will eventually develop a treatment plan to address the issue. In the case of something like an elbow fracture, this can include reducing the fracture and casting it to immobilize the arm during healing, along with administering analgesia for comfort and antiinflammatory drugs to reduce swelling around the site of the fracture.


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