Trigger thumb occurs because of a size difference between the flexor tendon (which bends your thumb) and the tendon sheath (the tunnel it passes through). This mismatch leads to a snapping or popping sensation when you try to straighten your thumb.
Here's a breakdown of the mechanism:
- Flexor Tendon Swelling: The flexor tendon in your thumb can become swollen or develop a nodule.
- Tendon Sheath Constriction: The tendon sheath, particularly at the level of the metacarpal head (base of the thumb), can become narrowed.
- Size Mismatch: The root of the problem is a "size mismatch of the diameter between the swollen flexor tendon and its sheath at the level of the metacarpal head".
- Catching and Clicking: When you bend your thumb, the swollen tendon may pass through the constricted sheath with difficulty. When you attempt to straighten your thumb, the tendon can get stuck, then suddenly pop through the narrowed sheath, causing the characteristic clicking or popping sensation. This is often painful.
In simpler terms, imagine a rope (the tendon) with a knot in it trying to pass through a tunnel (the sheath) that's too small. The knot gets stuck, and then suddenly pops through, causing a jerk.
