The flexor tendons which allow you to bend your fingers and thumb pass through a tight sheath on the front of the finger. This can become tight causing pain, clicking or the finger to become stuck in the bent position. In severe cases you may find that you need to use your other hand to straighten the finger or that it becomes impossible to straighten out.  

This is known as a trigger finger or thumb and can usually be treated with a steroid injection. If this fails to resolve the problem or it comes back again an operation will be recommended to release the tendon

Trigger finger or thumb