More often than not, it emanates from a repeated motion or forceful use of the finger or thumb. Tendons are fibrous cords that attach muscle to bone. Every tendon is surrounded by a protective sheath.  Whilst often pain-free, they can cause irritation or interfere with joint movement. De Quervain's Tenosynovitis https://www.theorthopaedicandpainpractice.com/why-cant-i-straighten-the-tip-of-my-finger/