<s>[INST] I am experiencing a very stiff right index finger middle knuckle. It feels like it gets stuck in either straight or bent mode. When I try to either straighten or to bend it feels like it gets stuck and the two parts of the knuckle are rubbing against each other. When it finally moves it is very uncomfortable. [/INST] Trigger finger. Tendon usually glides easily through the tissue that covers it (called a sheath) because of a lubricating membrane surrounding the joint called the synovial. Sometimes a tendon may become inflamed and swollen. When this happens, bending the finger can pull the inflamed tendon through a narrowed tendon sheath, making it snap or pop and it some feels like locking. This can be caused by a repeated movement or forceful use of finger. Rheumatoid arthritis, gout, and diabetes also can cause trigger finger. So can grasp something, such as a power tool, with a firm grip for a long time. </s>