Remember to check and document the ulnar collateral ligament (UCL) and radial collateral ligament (RCL) stability at the thumb MCP joint. Check them with the thumb fully extended or you may get a false positive finding. Compare with the other thumb as well!
I x-ray these as it can help with diagnosis. If abnormal movement/laxity discuss with Plastics/Ortho as there is a 2-3 week window for surgical repair (Stener repair)
If you want to learn what a Stener lesion is have a look at this YT video – Stener Lesion