The sudden failure to straighten the thumb is commonly caused by the following two conditions: i. The most common one in life is stenosing tenosynovitis of the thumb. When stenosing tenosynovitis occurs in the thumb the tendon sheath will compress the flexor tendon and prevent its flexion and extension movements, which eventually leads to the failure to straighten the thumb. The flexor tendon of the thumb may also become edematous due to inflammation, thus affecting straightening. Second, the sudden failure to straighten the thumb may also be caused by various closed and open injuries. When a closed injury is formed, it can lead to a rupture of the extensor tendon stop of the thumb, thus making the thumb unable to straighten. Alternatively, an open injury can also damage or rupture the extensor tendon of the thumb and cause the thumb to become unstraight.