This is a break of the patella. That's the small bone in the front of your knee often called the "kneecap." The kneecap protects the joint and helps link your thigh muscles to your lower leg. A fracture can cause pain and other problems.
You can fracture a patella in a few ways. It can break if you fall hard on your knee. It can break if the front of your knee is hit very hard, such as during a road accident. Your patella can also fracture because of a quick, forceful contraction of your thigh muscles. That can happen if you suddenly lose your balance and your leg tenses and straightens to try to stop you from falling.
A patellar fracture causes pain and swelling in the front of your knee. Your skin may bruise. You may have trouble straightening your leg, and you may have trouble walking.
Treatment options depend on how badly you've broken the bone, and whether or not its pieces have shifted out of position. A cast or a brace helps a minor fracture to heal. A severe fracture can be repaired with surgery. Your healthcare provider can create a plan that's right for you.