To understand SCFE, it helps to know a little about the hip joint. The hip is a ball-and-socket joint, which means that the rounded end of one bone (in this case, the "ball" of the thighbone) fits into the hollow of another bone (the acetabulum, or cup-shaped "socket" of the pelvis). Ball-and-socket joints offer the greatest range of movement of all types of joints, which explains why we can move our legs forward, backward, and all around.
Kids and teens who are still growing also have a growth plate at the top of the thighbone, just under the "ball" portion (also known as the femoral head) of the joint. This growth plate is called the physis and it's made of cartilage, which is weaker than bone. The job of the physis is to connect the femoral head to the thighbone while allowing the bone to lengthen and grow.
In SCFE, the femoral head of the thighbone slips through the physis, almost the way a scoop of ice cream might slip off a cone. Sometimes this happens suddenly — after a fall or sports injury, for example — but often it happens gradually with no prior injury.
Usually, SCFE is classified as:
- Stable SCFE. A stable SCFE causes some stiffness or pain in the knee or groin area, and possibly a limp that causes a child to walk with a foot outward. The pain and the limp usually tend to come and go, worsening with activity and getting better with rest. With stable SCFE, a child still can walk, even if crutches are needed.
- Unstable SCFE. An unstable SCFE is a more severe slip that usually happens suddenly, and is usually much more painful. A child will not be able to bear weight on the affected side. An unstable SCFE is also more serious because it can restrict blood flow to the hip joint, leading to tissue death in the head of the femur.
Sometimes SCFE can irritate the nerves that run down the leg, causing referred pain (pain that starts in one part of the body but is felt in another). In this case, pain begins in the abnormal hip joint but is felt in the normal knee joint.
Some cases of SCFE affect only one hip, but many are eventually found to affect both hips (be bilateral). When SCFE affects one hip, doctors may closely watch the other to see if it develops SCFE; or, if that's considered very likely, they might treat both hips at the same time. Catching SCFE early makes a big difference in how easily doctors can trust it.