Tenosynovitis refers to inflammation of the protective sheath that surrounds a tendon. Stenosing tenosynovitis, or trigger finger, is diagnosed when a patient presents with a symptomatic locking or clicking of a finger or the thumb. It is caused by a mismatch between the volume of the flexor tendon sheath and its contents. As the flexor tendon attempts to glide through a relatively stenotic sheath, it catches, producing an inability to flex or extend the digit smoothly.