What causes skier's thumb?
A fall on an outstretched hand with a ski pole in the palm of your hand creates the force necessary to stress the thumb and stretch or tear the ligament. A simple fall on an outstretched hand with an empty palm usually does not create this same force. It can also happen if your thumb jams into packed snow at high speed.
A less common cause is an automobile crash when the driver has the thumb alone draped over the steering wheel.
From: Skier's Thumb WebMD Medical Reference
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