Fractures and Sprains about the Knee in Children

Knee Fractures

Fractures in and around the knee are common in children.  They are often the result of sports activities or falls from bicycles or other playground equipment.  It is extremely important to distinguish fractures from “sprains” in children.  In children, the ligaments around the knee are often stronger than the growth plates adjacent to the knee or the cartilage of the knee joint itself.  What may seem like a ligament sprain by exam could in fact be a fracture through the growth plate itself.  The consequences of missing this are significant.

Knee sprains

Knee sprains can be difficult to diagnose in kids.  It is extremely important to distinguish fractures from “sprains” in children.  In the absence of a documented fracture, ligamentous injuries most commonly involve the Anterior Cruciate Ligament, or dislocation of the knee cap (patella)

Anterior Cruciate Ligament (ACL) Injuries

ACL injuries are being found more commonly than was thought in the past.  This is especially true in adolescent females, and with participation in soccer.  The incidence of ACL injuries is four times greater in teenage girls playing soccer than their male counterparts.  The reasons for this may be related to a greater degree of ligamentous laxity in adolescent females.

Diagnosis.   The diagnosis of an ACL injury in children may be more difficult than is typical for adults.  Common signs of ACL injury in children include a “pop” in the knee, followed by the knee filling with a tense collection of fluid shortly after the injury.  This swelling is usually actually blood, known as a hemarthrosis.  It is a painful injury, and many kids will have difficulty walking afterwards.

If you suspect a significant injury has occurred to your knee, or your child’s knee, then it is important that you have an x-ray of the knee.  In skeletally immature athletes, it is possible to injure the growth plate above or below the knee, or to pull off a large fragment of bone called the tibial eminence where the ACL attaches.  It is important to recognize these fractures because they would change the type and urgency of surgical treatment for the injury.

There are several ways to confirm if the ACL has been torn.  For many complete tears, a standard examination of the knee for instability will often reveal laxity of the ACL.  However, due to the strength of the secondary restraints of the knee (other ligaments, muscles, tendons, etc) the exam is not always reliable.  Mechanical testing with the KT 1000 will sometimes differentiate an ACL injury in one knee when comparing it to the opposite side.  If uncertain, an MRI of the knee will usually show whether the ACL is torn, and offers the added benefit of looking at the associated structures such as the meniscus, supporting ligaments of the knee, and the weight bearing surfaces of the knee joint.  The gold standard for confirming the diagnosis is arthroscopy of the knee.

Treatment.  Having an ACL injury as a adolescent is clearly different than having this injury as an adult.  Most kids that sustain ACL injuries are actively participating in sports and wish to continue to do so.  It is clearly established that the risk of developing additional injuries to the knee such as meniscal injuries is greater without a functional ACL.  Additionally, it is difficult to expect teenagers to modify or quit sports that they enjoy at such a young age.

Most ACL reconstruction requires the creation of a “tunnel” that crossed the growth plates adjacent to the knee.  There have been concerns that this would result in significant disturbance in the growth of the leg.  This was used as the rationale to not reconstruct the ACL prior to the end of growth.  However, there is now a growing track record for ACL reconstruction prior to skeletal maturity that indicates that the risk of growth disturbance is not as great as previously thought.   Other than children < 10 years old (in whom fractures are more common anyway…) ACL reconstruction is well tolerated.   The benefits in the active teenage athlete seem to outweigh the risks

Tibial Eminence fractures

Tibial eminence fractures are similar to ACL injuries, with the exception that  the ACL remains intact and pulls off a large fragment of bone off of the top surface of the tibia.  The most common mechanism for this type of fracture is a fall of a bicycle.  The fragment of bone that is pulled off is much larger than it appears on x-ray, and may involve a large weight-bearing portion of the tibial surface.

Treatment of these injuries almost always involves surgical repair of the bone fragment..  The fragment along with its’ attached ACL are pulled back into their anatomic insertion on the top of the tibia and held there with sutures pulled through the bone.  It is very important that this injury is not missed or your child may have prolonged instability of the knee.

Proximal Tibia Fractures

Proximal tibial fractures are common injuries.  These are usually fairly innocent appearing breaks in the upper part of the tibia below the growth plate.  The most common way that this fracture happens is by falling on a trampoline.  Often, the crack will look very mild, but it is usually enough to keep the child from walking.  Overall, the leg looks reasonably straight.  Only in severe cases is a reduction needed.  The typical treatment is placement in a weight-bearing cylinder cast for about 4 weeks with the knee bent about 40 degrees.

There is one unique feature about this fracture that needs mentioning.  During the year or two after the fracture has healed, there may be a tendency for the leg to grow crooked, into a knock-kneed position (doctors call this genu valgum).  The reason that this happens is not known;  some theories point towards uneven growth stimulation, others towards the effect of having some of the periosteum (tissue surrounding the bone) trapped in the fracture site.  This growth difference is a late phenomenon, and not generally not related to how your doctor treated the fracture.  It will sometimes correct on its own over time without treatment, or will occasionally need to be fixed with a staple.