Tendon injuries may occur on the flexor or extensor side of the fingers, hand, wrist or forearm. Overwhelmingly they occur as a consequence of penetrating trauma, but in some cases a forceful contraction of the tendon will pull it off its bony insertion, without an external injury. An inability to move the finger or hand following penetrating trauma should be considered a tendon injury until proven otherwise.
There are two flexor tendons to each digit and one for the thumb. These tendons are thick strong structures that exert incredible force (think climbers), yet they have a delicate surrounding bloody supply and run through an intricate array of pulleys in the fingers.