Yo-yo on a string sign - Stener lesion
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The sign occurs as a consequence of the tear of the ulnar collateral ligament of the thumb (gamekeeper thumb) whereby the ulnar collateral ligament has displaced superficially to the adductor pollicis muscle 1.
Diagnosis of the yo-yo on a string can be made with either MRI or ultrasound.
In MRI it is best observed in the coronal plane, where the "yo-yo" represents the torn ulnar collateral ligament (UCL) which has curled back and the "string" is the aponeurosis of the adductor pollicis muscle. The UCL and aponeurosis are both typically hypointense and can be further outlined by an adjacent joint effusion, which is hyperintense on T2 weighted sequences 2.
On ultrasound, the detection can be made with a high frequency (12-15 MHz) linear array probe having a good contrast resolution in the near field, which can be further improved by using a gel pad. A smaller "hockey stick" probe is particularly well-suited for the evaluation of the ulnar collateral ligament (UCL) 3. The UCL is usually best seen by placing the probe to the medial aspect of the first metacarpophalangeal (MCP) joint, longitudinally to the axis of the thumb. The normal UCL and the more superficial and thinner aponeurosis are both hyperechoic. In the presence of a Stener lesion the retracted, torn UCL typically appears as an inhomogeneous hypoechogenic nodular structure proximally distorting the normally smooth outline of the aponeurosis 4.
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