ACL injury in children and teenagers

An ACL tear in a child or a teenager is a different problem from the same tear in an adult, for three reasons: the knee is still growing, the years of sport ahead are many, and the re-injury rate in the young is the highest of any group. This page is written for parents, and it is honest about the last of those.

It is becoming more common

Between 2000 and 2015 the fastest growth in ACL reconstruction in Australia was in children aged five to fourteen, at about 8% a year, and paediatric reconstructions under Medicare rose steadily through 2020. The reasons are more organised sport, earlier specialisation in one sport, year-round seasons and better recognition of the injury. Girls aged 15 to 19 had the highest rate of reconstruction of any female age group.

The diagnosis

The same as in an adult: a non-contact twist or landing, a pop, swelling within hours, and a knee that does not feel right. The examination is the same and the MRI confirms it. Two differences matter. The ligament in a child can pull off with a flake of bone from the shin rather than tearing through its substance, which is a different injury with a different operation. And X-rays are taken to judge how much growth is left, which shapes everything that follows.

Surgery or not

The trials of rehabilitation-first treatment were done in adults, and the reasoning in children runs differently. A child or teenager who plays pivoting sport and has an unstable knee is at high risk of tearing the meniscus with each giving-way episode, the years of sport and of life ahead are long, and children are poor at avoiding the activities that make the knee give way. For that child, a reconstruction is usually recommended once the knee has settled. For the child who does not play pivoting sport and whose knee is stable in daily life, rehabilitation first with close review is reasonable. The decision is made with the family.

The growth plate

The growth plates at the lower end of the thigh bone and the upper end of the shin bone are where the tunnels for an adult reconstruction would go. Drilling across an open growth plate, or fixing a graft across it, can in principle disturb growth. The technique is therefore adapted to the child:

Several years of growth left: tunnels and fixation placed entirely away from the growth plates, with the graft routed around them.
A year or two of growth left: small, vertical tunnels crossing the plate, a soft-tissue graft with no bone block, and fixation kept well away from the plate.
Close to skeletal maturity: an adult-style reconstruction.

Across the studies, a growth disturbance, a leg-length difference or an angulation, occurs in about 2% to 5% of cases and is almost always minor. In a 2010 pooled analysis of 55 studies it was 1.8%, and in a 2026 review of 579 children with techniques that avoid the femoral growth plate it was 4.5%, mostly minor overgrowth, with no child needing corrective surgery. The graft is a soft-tissue graft, usually the child’s own hamstring or quadriceps tendon. Donor tendon is avoided in this group: in a registry of skeletally immature patients, revision was 13.2% with allograft against 7.5% with hamstring.

The number parents need to hear

The young athlete has the highest re-injury rate of any group having this operation. In an Australian cohort of 354 patients reconstructed under the age of 20, graft rupture occurred in 18% at an average of five years, and in 28% of boys under 18. Nearly half of the ruptures happened in the first year back. Adding injuries to the other knee, 35% had a further ACL injury. Across studies, about one in four athletes under 25 who return to sport sustain another ACL injury to either knee. Hamstring grafts in patients under 20 had a revision rate of 14% against under 1% in those 20 and over in one series, driven partly by small graft size.

Four things lower it, and none of them is optional in this practice:

The full nine months before pivoting sport. Young athletes who returned to knee-strenuous sport before nine months had about seven times the rate of a second ACL injury. Half of adolescents are back in sport by eight months, and at that point only about three in ten have recovered their muscle function.
Passing a return-to-sport test. Strength, hop tests, movement quality and readiness, at nine months, before any match play, including friendly games.
The right graft, and often a lateral tenodesis. A patellar or quadriceps tendon graft rather than a small hamstring where growth allows, and a lateral extra-articular tenodesis in the high-risk athlete: in a trial of 618 patients aged 14 to 25, it cut graft rupture from 11% to 4% over two years.
A structured warm-up program, every season. Programs of landing, cutting and strength drills, done two or three times a week, halve the rate of ACL injury and reduce non-contact injuries in girls by two thirds. After a reconstruction the program is part of playing, for as long as they play.

For parents

The recovery is nine to twelve months, and the hardest part is the middle, when the knee feels normal and the season is on. The rule to hold is the nine-month rule, and the person who holds it is usually a parent. The ACL reconstruction rehabilitation guide has the week-by-week program; the recovery chapter explains why each step is where it is.

Common questions

Why are ACL injuries in children increasing?

More organised sport, earlier specialisation in one sport, year-round seasons and better recognition. In Australia, the fastest growth in ACL reconstruction between 2000 and 2015 was in children aged five to fourteen, at about 8% a year, and paediatric reconstructions under Medicare rose steadily through 2020.

Does my child need surgery for an ACL tear?

More often than an adult does. A child or teenager who plays pivoting sport and has an unstable knee is at high risk of tearing the meniscus with each giving-way episode, and the years of sport ahead argue for a stable knee. For the child who does not play pivoting sport and whose knee is stable, rehabilitation first is reasonable with close review. The decision is made with the family, not by rule.

Will surgery affect my child''s growth?

The growth plates at the knee are where the tunnels for the graft would normally go, so the technique is adapted to how much growth is left: tunnels placed to avoid the plates, or small tunnels crossing them with a soft-tissue graft and no fixation near the plate. Across studies, a growth disturbance occurs in about 2% to 5% of cases, almost always minor, and in the largest recent review none needed correction.

What are the chances my teenager tears the ACL again?

High, and this is the number parents need to hear. In an Australian cohort reconstructed under 20, graft rupture was 18% at five years and 28% in boys under 18, and 35% had a further ACL injury to one knee or the other. Across studies, about one in four athletes under 25 who return to sport sustain another ACL injury. The young, male, pivoting athlete who returns early is the highest-risk patient in orthopaedics.

What lowers the re-injury risk in a young athlete?

Four things. The full nine months before pivoting sport, because young athletes who returned earlier had about seven times the rate of a second injury. Passing a return-to-sport test, which at eight months only about three in ten young athletes could do. A patellar or quadriceps tendon graft rather than a small hamstring, and often a lateral tenodesis, which cut graft rupture from 11% to 4% in a trial of 14 to 25 year olds. And a structured warm-up program kept up through every season, which halves the rate of ACL injury.