You know the joint isn’t dislocated because you can straighten it with your other hand. Instead, since the joint can’t extend on its own, you must have cut the extensor tendon—the one on top that extends your finger. (Similarly, if the finger wouldn’t bend itself, you’d know you’d cut the flexor tendon, which is underneath—palm side. Remember, tendons help muscles move bones.)
Now that you know the problem, you can figure out the proper way to treat it, assuming you can’t get expert help.
After than, you’ll want to splint the finger straight and hope like the dickens you can get to a doctor within a few days to surgically attach the tendon back together. (If you don’t splint your finger, the flexor tendon will permanently shorten a bit, and the surgeon will have a harder time stretching it out enough so the two ends of the extensor tendon can meet and can be sewn together.)
The same tendon injury can happen without so much as a scratch on your skin. If you fall or bump it just right one of the finger joints can extend backwards with such sudden force that it tears the tendon. The treatment is the same—splint and get to a doctor. There is a chance it might heal just by splinting it if the tendon has only partially torn.
“The ability to move a body part or joint isn’t an indication that the affected bone isn’t broken. Even if a bone is broken, if the tendon is intact, you can move the joint unless the joint has been severely injured or is dislocated.”
If you injure a joint and can fully straighten and flex it, then, no matter the pain and swelling, you’ve probably either bruised the joint or injured the ligaments on the side. Chances are it will heal with time. Just protect it for a few days to weeks, while it’s sore, with a splint or buddy splint.
And then, there’s always a break. With or without any of these injuries, you could have broken your finger. Signs and treatment for that are for another post. (Unless the bone is distorted and the break is obvious, you’re going to need an X-ray. If the bone is tender and you’re not sure and can’t get to a doctor, you’ll need to splint it for four to six weeks.) But remember this: Being able to move or not move the finger the finger does not tell you if it’s broken or not.
What about you? Have you ever injured a tendon or a ligament? Finger, toe, wrist, ankle, knee, hip? How did you know? What did you do?