Part 1: How Children differ from Adults when it comes to ACL injury

A brief introduction to the topic....main content below...

How children differ from adults when it comes to ACL surgery

The Anterior Cruciate Ligament (ACL) is one of two central ligament stays in the centre of the knee joint, helping to secure the relationship of the femur to the tibia. The two ligaments can be seen within the notch between the rounded ends of the femur when the knee is bent. In small children the ACL is 'weedy' and not the same strong structure that you see in the adult.

The ACL prevents the tibia sliding forwards in relation to the femur when a person stops suddenly, and helps to contain rotational movement between the two bones.

ACL injuries in children are increasingly becoming the remit of the orthopaedic ligament surgeon. The paediatric ACL injury has traditionally been thought of as relatively uncommon but with the increase in competitive sports, better awareness, improved imaging and greater demands on athletes at an earlier age, we are seeing a significant rise in the number of children coming through - particularly in their adolescent years - with ruptured anterior cruciate ligaments. 

Australian Medicare statistics in patients under the age of 16 show the numbers of reported ACL ruptures increasing year on year through to 2010, and this is certainly a trend that is being seen throughout the western world.​

When it comes to how these injuries should be managed in this group, the 'growth plates' or 'physes' of children dominate the debate. The discussions focus upon whether or not you should do nothing at all surgical - just go down the conservative route - or whether you should operate and risk damaging the growth plates. Parents and doctors have to make difficult decisions on behalf of these young people, and some are only small children not yet in their teens who cannot fully participate in the decision-making process.

If a choice is made to NOT operate, there are some real issues with compliance with bracing with small children, and these young patients may go on to develop chronic knee instability. Instability can interfere not only with sport but also day to day activities, and leave patients coping poorly. Episodes of the knee 'giving way' may result in further secondary injury of the meniscus or joint surface, and this can be disastrous.

It is now well appreciated that such conservative management gives poor results, but the concern continues that surgical efforts at re-stabilising the knee could jeopardise the growth plates, resulting eventually in a shortened or deformed limb.

Showing the percentage contribution of the various growth plates to the eventual length of the long bone

Long bones in the growing child get longer, not from the ends of the bones but from an active cartilage 'growth plate'. This is properly called a physis, and is sandwiched between the end of the bone and the main body of the bone. On X-ray the physes look like fractures, but that is because the X-rays pass easily through cartilage and less easily through the bone. The end of the bone to the one side of the physis is called the 'epiphysis' and the shaft of the bone to the other side is called the 'metaphysis'. So the physis is like a cartilage 'sandwich' within the bone. As new cartilage is formed it adds to the length of the metaphysis and this subsequently matures into new bone.

The knee bones from the side, showing the growth plates or physes

The various growth plates contribute different amounts to the final length of the adult bone. At the knee the growth plates of the thighbone or femur contribute to 70% of the eventual length of the thigh, while the growth plate of the shin bone or tibia contributes 55% of the eventual length of the shin. You can appreciate that, although both are important, it is particularly critical to try not to damage the growth p​late of the femur.

Injury to the physis may result in a slowing down or even arrest of growth in the limb, or an angular deformity.

Children who injure their ACL tend to play contact sports like basketball, or 'cutting' sports like soccer, where there are sudden abrupt turns and stops, and sudden changes of direction. The injury might also occur on jumping and landing on the feet on a hard surface, when the immature quadriceps muscle of the lap is not strong enough to contain the action.

Teenage girls are especially at risk, with ACL injuries several times more common than in boys of the same age, because they have increased risk factors to do with their body shape, their limb alignment, their muscle bulk and hormonal influences.

It may be difficult to obtain the history from the child, but the parent may have been an observer or spoken to witnesses, so it is always important to ask the parent, too. The child may have felt a 'pop', and been unable to bear weight immediately afterwards. Sudden onset of marked swelling may be reported. Typically things may then seem to settle down, but the child may have an unusual gait or be reluctant to participate in activities.

The ACL damage may be proximal (at the attachment to the femur), mid-substance within the ligament, or distal (at the attachment to the tibia).

Proximal (at femur attachment)

​Although not so frequently reported, proximal detachment is an injury that we have seen commonly in younger children, and this pattern seems to be associated with high speed injuries, such as skiing. The ligament tears off from its bony origin in the notch of the femur, leaving an 'empty notch' when this area is examined during arthroscopy.


A mid-substance tear of one or both bundles of the ACL is a more common pattern in the older child or adolescent.

Distal (at tibia attachment)

The growth plate in young children can be weaker than the ligaments, and sudden strong traction on the tibial attachment may lead to an 'avulsion fracture', where a fragment of cartilage or cartilage-&-bone breaks off, still attached to the intact ligament. The site of avulsion is commonly at the anterior tibial spine in the middle of the top of the tibia. Such avulsions are more likely to occur in children before adolescence, while after adolescence, as mentioned above, it is more common for the ligament substance to tear rather than for an avulsion to occur.

Generally the orthopaedic surgeon sees the child after the swelling has reduced and the parents are wondering if there is actually anything wrong with the knee. Examination, however, typically shows a lax knee, with positive signs on performing Lachman, anterior drawer and pivot shift  tests.

Note that in children, the degree of translation for such laxity tests - for both operated and normal knees - is generally higher than that documented for adults. This may be due to constitutional increased joint mobility present in children.

On an MRI scan the normally 'weedy' ACL in a child may look thickened when torn because of oedema (or fluid in the tissues), and this may confuse the observer. Signs of bone oedema on tibia and femur may hint at the damage.

Bernard Moyen and his team [1] looked at rupture of the anterior cruciate ligament in children and they took 2 groups - those that they operated on quickly or those upon whom surgery was delayed until skeletal maturity. In 56 patients where the mean times to surgery was less than 5 months in the one group and more than 30 months in the other, the number of meniscal tears went up from 14% to 41% with the delayed group.

Another paper by Justin Roe's team [2] was a meta-analysis of 5,086 patients. They found that there was two times the risk of secondary damage if there was a five month delay to surgery, and six times the risk if there was a one year delay, and double the risk in anyone under the age of 17.

A further paper by Todd Lawrence [3], looking at 14 year data in patients where there was a delay of more than 12 months, again showed a massive increase; a four-fold increase in new meniscal tears, 14% of which were non-repairable bucket handle tears, and a significant number of lateral condyle injuries.

So should we wait? Probably no, and that is the feeling of many surgeons who undertake the treatment of this injury in children.


1. Rupture of the anterior cruciate ligament in children: early reconstruction with open physes or delayed reconstruction to skeletal maturity? Henry J et al. Knee Surg Sports Traumatol Arthrosc. (2009)

2. K. Sri-Ram, L. J. Salmon, L. A. Pinczewski, J. P. Roe. The incidence of secondary pathology after anterior cruciate ligament rupture in 5086 patients requiring ligament reconstruction. Journal of Bone and Joint Surgery January 2013

3. 28. Lawrence JT, Argawal N, Ganley TJ (2011) Degeneration of the knee joint in skeletally immature patients with a diagnosis of an anterior cruciate ligament tear: is there harm in delay of treatment? Am J Sports Med 39:2582–258729.

Additional Resources (Click to view)

Classification of ACL avulsion fractures

Associated Injuries in Pediatric and Adolescent Anterior Cruciate Ligament Tears: Does a Delay in Treatment Increase the Risk of Meniscal Tear?


Here are answers to some frequently asked questions:

What is the Lachman test?

What is the Pivot Shift test?

What is the Anterior Drawer test?

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