By Dr. Eric W. Edmonds
The anterior cruciate ligament (ACL) is a major ligament in the center of the knee that limits forward motion of the leg bone relative to the thigh bone, as well as providing rotational control of the knee. Tears of the ACL appear to be occurring with increasing frequency in our young population.
Traditional thinking suggests that children do not tear the ACL, but rather sustain a specific fracture related to the same mechanism of injury called a tibial spine fracture.
Some orthopedic surgeons believe that tibia spine fractures are the ACL equivalent of tears in the growing child and that children “never” tear their ACL. This is simply not true; however, it is much more common for children to have an injury to the bone than the ligament.
Tibial spine fractures historically occurred while riding a bicycle. When the child’s leg extends to the ground to stop but the forward momentum forces the knee to bend backward, or hyperextend, (usually by striking the peddle) it places significant strain on the ACL complex resulting in a tibial spine fracture. We now know that this fracture may also occur with other mechanisms consistent with tearing an ACL, such as pivoting on the knee. We also know that the ACL will stretch a little prior to the fracture.
If your child sustains an injury to the knee and there is immediate swelling, then this is likely blood in the joint (hemarthrosis). Blood in the joint implies that something was torn during the injury, but this finding is not very specific since the bleeding could be related to knee cap dislocation, ACL tears, ligament sprains, cartilage injury or fractures. The tibial spine fractures could involve the cartilage and therefore may be treated with surgery to restore the joint surface as well as to restore a taut ACL complex.
Prior to any surgery, your child should have X-rays to look for this or other fractures in the knee. The second diagnostic test would be either a CT scan or MRI, depending on the X-ray findings. If a fracture is seen, then CT is appropriate, but if the X-rays look normal then an MRI would be best since many things that tear in the knee require surgery to fix.
If the fracture is close to its normal location, then your child may be treated in a long leg cast for a few weeks to a month. These fractures are prone to stiff joints; therefore, early motion is important. If the fracture is displaced, or if it involves the joint surface, then surgery can restore normal anatomy. There is no way to directly prevent this type of injury, but it is important to recognize that children are more likely to sustain a fracture than a torn ligament.
If your child does sustain a knee injury, contact or see your doctor if the injury involves obvious deformity, knee joint swelling or does not improve with two weeks of rest.
The next Playing Safe: True ACL injuries in children.
Dr. Eric W. Edmonds is a pediatric and adolescent orthopedic sports medicine specialist at Rady Children’s Hospital-San Diego and an assistant professor of orthopedics at UC San Diego. He can be reached at firstname.lastname@example.org.