Pain at the base of the thumb usually starts small — a jar lid, a car key, a firm handshake. For a lot of patients it's the most common form of hand arthritis, and it's also one of the most treatable if it's caught before it limits daily function.

A fall onto the thumb usually gets waved off as a sprain. Enough of the time it's actually a torn ligament — and the two look identical for the first 48 hours. Here's the exam that tells them apart, and why the window to fix it matters.