Dog-walking accident causes knee injury

Dear Doctors: I was walking our dog at night and didn't see a big hole in the sidewalk. I stepped right into it, full force, and I think my knee actually bent backwards. I can walk OK, but now my knee hurts. Could I have damaged something? How do I know if I need to see my doctor?

Dear Reader: From your description of the incident, it sounds as though you hyperextended your knee. That means your knee joint was forced backward, just as you described, opposite of the direction in which it is designed to move. Depending on the force of the impact, and the degree to which the knee over-straightened, this can result in damage to the ligaments that stabilize the joint.

Let's start with a closer look at the knee itself. It's the largest joint in the body, and it is a remarkable feat of engineering. It functions as a hinge, allowing the lower leg to flex and extend. The knee joint links together the thigh bone, or femur, and the tibia, which is the larger bone in your shin. It also includes the kneecap, or patella, which is the small, rounded bone that forms a protective cap at the front of the joint. A series of internal and external ligaments weave their way through the joint capsule. They not only connect the three bones that make up the knee, but also keep the moving parts both flexible and stable. The ligaments limit rotation and sideways motion, which could cause injury to the knee, and yet allow the joint to flex and extend within a precisely calibrated range of motion.

It's when the knee joint is stressed beyond what the ligaments can bear that injuries occur. This includes in a fall, during a jump, from a collision or in the unexpected shift of weight that occurred when you stepped into that hole. In the case of hyperextension, the resulting injury is typically to the anterior cruciate ligament, or ACL, and the posterior cruciate ligament, or PCL, which run through the center of the knee.

When someone hyperextends their knee, they usually feel the joint move out of alignment with the leg, just as you did. It often results in localized pain that ranges from moderate to

severe. You may see swelling or even visible bruising in the area. Straightening the joint may cause pain, and the knee may feel weak or unstable.

When someone experiences a loss of mobility in the knee following hyperextension, it can be a sign of damage to the ligaments, and perhaps to the surrounding tissue. Depending on how severe the injury is, treatment can range from rest, the use of a knee brace for stability and over-the-counter meds for pain and inflammation to surgical repair and physical therapy. In mild injuries, recovery time can take from two to four weeks. When surgery is required, full recovery can take up to six months or more.

Because the knee is such an important and complex joint, we think it's a good idea to have your injury assessed by your doctor.

Eve Glazier, M.D., MBA, is an internist and associate professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and assistant professor of medicine at UCLA Health. Send your questions to askthedoctors@mednet. ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.