Pain in the left thumb when bending may be caused by stenosing tenosynovitis of the thumb, which most commonly occurs in the thumb, when there is pain in the metacarpophalangeal joint on the palmar side of the thumb. This is often due to repeated flexion and extension of the thumb, such as in workers who have been stringing with their thumb for a long time, or in women who have been working with their thumb for a long time, when pain on the palmar side of the thumb can occur, along with symptoms of morning stiffness and, in severe cases, a popping sound in the thumb. If the effect of medication and physical therapy is not good, local closure therapy can be carried out, and most patients can have good results. If the effect of closure therapy is not obvious, and if there are repeated attacks, consider surgical treatment, which can be performed to loosen the tendon and make the painful symptoms of the thumb disappear.