Paediatric ACL Injury: Trainee Guide

Epidemiology

Paediatric ACL reconstruction rates roughly tripled over two decades. In New York State, rates rose from 17.6 to 50.9 per 100,000 (ages 3 to 20) between 1990 and 2009, peaking at age 17. [Dodwell 2014, via Dunn 2016]

• Drivers: higher sports participation, year-round training, early specialisation and better recognition.
• Sex: young female athletes have 3 to 7 times the injury rate of males. Reconstruction rates were 15% higher in adolescent girls.
• Haemarthrosis: in pooled series of 186 acute paediatric haemarthroses, 52% had an ACL rupture. [Al-Hadithy 2013]
• Mechanism: mostly non-contact, during pivoting, deceleration or landing.

Injury pattern changes with age

In the immature knee the ACL is relatively stronger than the unossified tibial eminence. So younger children tend to avulse bone, while adolescents tear the ligament.

Assessment

A careful history and examination by an experienced clinician is as accurate as MRI. Use MRI to confirm and to map associated injury.

History and examination

• Acute haemarthrosis (swelling within 24 hours) suggests structural injury.
• A "pop" is reported by about a third of patients.
• Children are poorer historians. Take a collateral history from parents and witnesses.
• Physiological laxity is greater: up to 10 mm of anterior draw can be normal. Always examine the contralateral knee.
• Acute examination is often limited by pain and effusion. Re-examine once settled.
• Different paediatric injuries can mimic ACL rupture: tibial eminence fracture, patellar sleeve fracture, physeal fracture.

Imaging

1. Plain radiographs for every haemarthrosis. Exclude tibial eminence and other fractures before MRI.
2. MRI to confirm and assess associated injury. Include physeal-sensitive and cartilage sequences (for example fat-saturated spoiled gradient echo).
3. Urgent MRI for a locked knee. Look for a displaced bucket-handle meniscal tear or osteochondral fragment.

Diagnostic accuracy

Negative predictive values are high, so a negative examination and MRI make injury unlikely. Positive results are less reliable. [Kocher 2001, adapted in Ardern 2018]

Clinical assessment here means history, examination and X-ray by a paediatric sports medicine specialist. MRI accuracy is lower in children under 12.

MRI pitfalls

• Paediatric menisci are highly vascular. Globular or linear intrameniscal signal can mimic a tear and is usually benign.
• Normal developmental variants make interpretation harder. Ask for a paediatric musculoskeletal radiologist where possible.

Skeletal maturity

The goal is to estimate remaining knee growth. It drives the choice of treatment, timing and technique.

The distal femoral physis provides about 70% of femoral growth. The proximal tibial physis provides about 55% of tibial growth. An insult near the end of growth can still cause premature closure. [Al-Hadithy 2013; Ardern 2018]

How to assess it

No single method is accurate enough. Combine:

• knee imaging: are the distal femoral, proximal tibial and tibial tubercle physes open?
• bone age: PA radiograph of the left hand and wrist against an atlas (Greulich and Pyle, or Gilsanz and Ratib)
• Tanner stage
• whether the adolescent growth spurt has happened
• parental heights

If the knee physes are closed, treat as an adult regardless of chronological age. Take baseline long-leg standing radiographs before surgery. Pre-operative documentation also matters medicolegally if a growth disturbance occurs later.

Tanner stages

Operative vs non-operative management and timing

Practice has moved towards early reconstruction, but non-operative care remains legitimate for selected children with close monitoring. [Ardern 2018]

Goals of treatment (IOC 2018)

1. A stable, well-functioning knee for an active life.
2. Limit existing and future meniscal and chondral damage, degeneration and reoperation.
3. Minimise the risk of growth arrest and deformity.

Indications for reconstruction (IOC 2018)

• Repairable associated injury needing surgery (for example a bucket-handle meniscal tear or osteochondral defect). Treat with early ACLR plus repair.
• Recurrent symptomatic giving way after high-quality rehabilitation.
• Unacceptable participation restriction to avoid giving way.

Non-operative pathway

• Suitable when there is no associated injury and no major instability.
• Supervised rehabilitation for at least 3 to 6 months.
• Can be definitive, or a bridge to reconstruction near maturity.
• Convert to ACLR for recurrent instability or a new intra-articular injury.
• Requires frequent clinical review and repeat MRI when indicated. The number of instability episodes appears to matter more than elapsed time.
• Partial tears: 31% of one mixed cohort needed reconstruction after structured rehab. Under-14s with a stable knee may be managed non-operatively. [Kocher 2002, via Al-Hadithy 2013]

Evidence on timing

Delay mainly threatens the medial meniscus. Lateral tears tend to occur at the index injury.

Practice variation

A 2016 survey of EPOS and POSNA members (n = 182 treating surgeons) found wide transatlantic differences. [Accadbled 2019]

• Prepubescent (age 8): ACLR within 3 months recommended by 70% of POSNA vs 17% of EPOS respondents. Most EPOS respondents preferred physiotherapy with reconstruction if symptomatic.
• Overall, 60% recommended early ACLR at age 8 and 79% at age 13. In 2002 the figures were 16% and 34%.

Technique selection

No technique guarantees physeal safety. Choose by remaining growth, knee size and your own familiarity. Bone age bands below are one published framework, not a rule. [Perkins and Willimon 2020, via De Petrillo 2022]

In a 2016 EPOS/POSNA survey, epiphyseal tunnels were most popular for an 8-year-old (43% tibial, 49% femoral). Transphyseal tunnels dominated at 13 years (85% tibial, 63% femoral). [Accadbled 2019]

Physeal protection principles (IOC 2018)

• Small tunnels. Under 9 mm. Volumetric physeal injury rises about 1.1% per 1 mm of tunnel diameter. [Kercher 2009, via Al-Hadithy 2013]
• Vertical and central. Oblique tunnels remove more physis. Anteromedial portal drilling gives an elliptical physeal footprint, so consider a more vertical orientation than in adults.
• Protect the perichondral ring. Avoid peripheral drilling, femoral rasping in the over-the-top position and aggressive notchplasty.
• Nothing rigid across the physis. No screws, bone plugs or implants. Fill tunnels with soft tissue, never leave them empty.
• Avoid high graft tension across an open femoral physis. It can tether growth.
• Drill slowly, in short bursts, to limit thermal injury.

The ITB technique (Micheli-Kocher)

Used in prepubescent children with substantial growth, especially in small knees where all-epiphyseal tunnels are difficult. [Feroe 2022]

1. Harvest a 1 cm strip of ITB, about 15 cm long, left attached at Gerdy's tubercle.
2. Pass it over-the-top through the notch, protecting the perichondral ring.
3. Route it under the intermeniscal ligament in a small anteromedial epiphyseal groove.
4. Fix to the lateral intermuscular septum at 90 degrees of flexion, then to tibial periosteum distal to the physis.

In 237 patients (aged 5.7 to 15.6) at a mean 6.2 years: Pedi-IKDC 93, revision 5.8%, arthrofibrosis 2.1%, septic arthritis 0.4%. No growth disturbance was reported.

Graft choice

Use soft-tissue autograft in patients with open physes. Avoid allograft. [Ardern 2018]

Graft diameter

• Hamstring grafts under 7 mm fail more often, and patients under 20 are at higher risk. [Alomar 2021]
• Many surgeons use 8 mm as their threshold.
• Pre-operative MRI plus height, weight, age and sex predicted graft diameter within 1 mm in 98% of under-18s. [Partan 2021]
• If the predicted graft is small, plan an alternative (quadriceps tendon, ITB, or adding a lateral extra-articular procedure).

Growth disturbance

Growth disturbance is rare (about 2%) but serious, and it is probably under-reported. [Ardern 2018; Accadbled 2019]

• By technique: a meta-analysis of 55 studies found growth disturbance in 5.9% after physeal-sparing vs 1.9% after transphyseal reconstruction. [Frosch 2010] A later systematic review found similar rates between the two. [Pierce 2017]
• Common causes: hardware or bone plugs across the physis, oversized or peripheral tunnels, and over-tensioned grafts. [Kocher 2002 survey]
• Survey data: 5.5% of EPOS/POSNA respondents reported clinically relevant disturbance (LLD over 1 cm or axial deviation over 5 degrees), more often femoral (33 vs 22 cases). [Accadbled 2019]

Chotel classification

Surveillance (IOC 2018)

• Clinical and radiological review in the first 12 post-operative months.
• With markedly open physes: annual clinical review, knee radiographs and long-leg alignment views until physeal closure.
• Monitor height. If growth exceeds 6 cm in 6 months, bring the review forward.
• Only 54% of surveyed surgeons took pre-operative long-leg films, and 56% followed patients to maturity. Close this gap in your own practice.

Associated injuries

Meniscal preservation is a central goal: repair whenever possible. [Ardern 2018]

Meniscus

• Meniscal tears occur in up to 64% of paediatric ACL injuries. Lateral tears are linked with acute injury, medial tears with chronic instability. [Al-Hadithy 2013]
• Older (over 15 years) and heavier (over 65 kg) children have more associated injury. [Dumont 2012]
• Meniscectomy raises osteoarthritis risk. Prior meniscal repair, unlike meniscectomy, is not linked with later chondral lesions.
• Prioritise repair of bucket-handle, root and radial tears and ramp lesions. If you lack the skills or kit, refer.

Ramp lesions

About 1 in 6 ACL-injured knees have a posteromedial meniscocapsular (ramp) lesion, with similar rates in children. Inspect the posteromedial compartment, using a posteromedial portal if needed. An unrepaired ramp lesion may load the graft.

Cartilage

Chondral injury is less common than meniscal injury. Suspicion should rise with combined ACL and meniscal injury, recurrent instability and longer delay. The medial femoral condyle is most vulnerable.

Multiligament injury

Rare in children, with little evidence. Refer to a specialist centre. Use fluoroscopy before placing anchors or tunnels near the physes for collateral reconstruction.

Lateral extra-articular procedures (LET)

Adding a lateral extra-articular tenodesis to ACLR is increasingly used in high-risk young patients. Level I evidence exists only for ages 14 to 25; paediatric evidence is level III to IV.

Rationale

• The anterolateral complex (ITB deep fibres, Kaplan fibres, anterolateral capsule) resists tibial internal rotation and the pivot shift.
• LET shares load with the intra-articular graft during the ligamentisation period.
• Adolescents combine high laxity, high-risk sport and high re-rupture rates, so the rationale is strongest in this group.
• The Micheli-Kocher ITB reconstruction already includes an extra-articular component.

Evidence

• STABILITY trial (RCT, n = 618, ages 14 to 25): hamstring ACLR with modified Lemaire LET reduced graft rupture compared with ACLR alone (4% vs 11% at 2 years). Patients were selected as high risk. [Getgood 2020]
• Paediatric cohorts: retrospective series in skeletally immature patients report lower graft failure with LET, without excess arthrofibrosis. Comparator groups are not risk-matched. [Monaco 2022; Perelli 2022]
• No paediatric RCT exists. Long-term lateral compartment and growth outcomes in children are unknown.

Physeal considerations

• The LET itself is extra-articular, but femoral fixation sits close to the distal femoral physis in younger patients. Confirm position with fluoroscopy.
• Plan fixation so it does not converge with the femoral ACL tunnel.
• Femoral overgrowth has been reported after combined procedures. Keep the same growth surveillance as for any paediatric ACLR.

Rehabilitation and return to sport

Progress by criteria, not time. Return to full participation no earlier than 9 months after ACLR, and consider 12 months before pivoting sport. [Ardern 2018]

Principles

• Dynamic, multi-joint neuromuscular control is the primary focus.
• Under 12 (wide-open physes): less emphasis on strength and hypertrophy. From puberty: heavier, externally loaded strength work.
• Children are not small adults. Adapt adult protocols, and have a qualified clinician supervise.
• Involve parents in daily rehab. Keep the child training with their team where safe.
• Protect from cutting and pivoting in sport, play and PE through phases I and II.
• Train the uninjured leg too, given contralateral injury risk.

Progression criteria (IOC 2018, Box 2)

With handheld dynamometry or 1RM, raise the LSI cut-off by 10 points. Refer for strength testing if you lack the equipment.

Prepubescent children

• Use home-based, playful, varied programmes.
• Hop and isokinetic tests have larger measurement error. Judge movement quality rather than symmetry indices.
• Return-to-sport criteria were validated in mature athletes. Their validity before puberty is unknown.

Timelines and bracing

• Non-operative: at least 3 to 6 months of rehab.
• Post-ACLR: at least 9 months before full participation.
• Braces are commonly used before surgery and for 2 to 6 weeks after. Their effectiveness in children is unknown.
• Protocols vary. One ITB series assessed return at 6 months with a functional brace for cutting for 2 years. [Feroe 2022] This is earlier than the IOC guidance, which reflects the high early re-rupture risk.

Address fear of reinjury. Psychological readiness affects outcome, and child and parent versions of fear questionnaires exist.

Outcomes and reinjury

Most children return to sport, but second ACL injury is common: about 13% graft rupture and 14% contralateral rupture in patients aged 6 to 19. [Kay 2018, via Ardern 2018]

• Over 90% of children and adolescents return to sport after ACLR, and 79% to their pre-injury level. [Kay 2018]
• In under-25s returning to pivoting sport, about 1 in 4 sustain a new ACL injury (pooled ipsilateral about 10%, contralateral about 12%). [Wiggins 2016]
• Predictors of second injury: young age, return to pivoting sport, allograft.
• Risk is highest in the first 12 months after surgery.

Other complications

Patient-reported outcome measures

Use paediatric-validated measures. Child self-report is typically valid over age 10. Use parent proxy under 10, accepting possible bias. [Ardern 2018]

Consent and shared decision-making

The clinician acts as co-fiduciary with the parents: the child's long-term knee health comes before short-term sporting goals. [Ardern 2018]

• Include the child in every discussion, at a level matched to their understanding.
• Always seek the child's assent, whatever the parents' wishes.
• Know which options are optimal, acceptable and not desirable, and justify each with evidence.
• Watch for pressure from parents or coaches to prioritise early return. Rare exceptions exist, such as a child with exceptional talent who, with their parents, rationally accepts higher risk.
• Where parents choose against advice, consider whether the choice still falls within an ethically defensible standard (for example the harm principle or zone of parental discretion).
• Discuss long-term knee health openly. Long-term outcomes after childhood ACL injury, including osteoarthritis, have not been studied.

Follow your local legal and professional rules on consent in minors (for example, GMC guidance and Gillick competence in the UK).