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Anterior cruciate ligament injury, from the pop to the pitch, explained properly.

A pivot, a pop, a swollen knee. What actually happens inside the joint, how it is diagnosed on MRI and how modern UK reconstruction and rehab get you back to sport.

Prefer the shorter version? Read our concise overview at ACL injury.

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A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Editorial team

    Written by our editorial team from the UK sources listed on each page.

  • 02

    Sourced from guidance

    Checked against NICE, BOA, BASK and peer-reviewed orthopaedic sources listed at the end.

  • 03

    Current for 2026

    Reflects modern UK practice on ACL reconstruction, graft choice, LET augmentation and objective return-to-sport testing.

Key facts

ACL injury at a glance.

The essentials, in plain English: what the ACL does, how it tears and what modern UK care actually looks like.

  • What it is

    A tear of the anterior cruciate ligament, one of the two central knee ligaments that stops the tibia sliding forward and rotating on the femur.

  • How it happens

    Around 70% are non-contact pivot, deceleration or awkward landing injuries; roughly 30% follow a direct blow.

  • Sports at highest risk

    Football, netball, skiing, rugby and basketball, where cutting, twisting and jumping loads dominate.

  • Rarely alone

    Meniscal, collateral ligament, posterolateral corner and chondral injuries are common travelling companions.

  • The gold standard

    MRI confirms the tear and maps associated damage; the Lachman test remains the most sensitive clinical sign.

  • Return to sport

    Typically 9 to 12 months after reconstruction, gated by objective strength, hop and psychological readiness testing.

Why this guide matters

A clear plan, not a rushed operation.

ACL care has moved on. Prehab, personalised graft choice, LET augmentation and objective return-to-sport testing all change the odds.

  • Not every ACL needs surgery

    For older or less pivoting patients, a structured non-operative programme with bracing and physio can deliver excellent function.

  • Graft choice is personal

    Hamstring, patellar tendon and quadriceps tendon autografts each have trade-offs based on sport, anatomy and previous injury.

  • Timing and testing beat rushing

    Returning to pivoting sport before nine to twelve months and objective testing sharply raises re-rupture risk.

How the diagnosis is made

From pitch-side pop to a settled plan.

The steps a UK GP, sports physician or knee surgeon will normally follow, in order, so you know what to expect and why.

  1. 01

    Assessing

    Mechanism and history

    A pop, immediate swelling within hours and a feeling of the knee giving way point strongly to an ACL tear.

  2. 02

    Assessing

    Lachman test

    The most sensitive bedside test; the tibia is pulled forward on the femur at 20 to 30 degrees of flexion, looking for a soft end-point.

  3. 03

    Assessing

    Anterior drawer and pivot shift

    Anterior drawer at 90 degrees and the pivot shift confirm rotatory instability, the hallmark of a functional ACL deficit.

  4. 04

    Confirming

    Plain X-ray

    Screens for a Segond fracture (a small avulsion off the lateral tibial plateau) and rules out other bony injury.

  5. 05

    Confirming

    MRI knee

    The gold-standard imaging test; confirms the tear, grades the disruption and shows meniscal, chondral or collateral involvement.

  6. 06

    Confirming

    Instrumented laxity (KT-1000)

    Where available, side-to-side laxity of more than 3 to 5 mm supports a complete tear and helps quantify progress.

  7. 07

    Deciding

    Shared decision on management

    Age, activity level, associated injuries and psychological goals shape whether reconstruction or a structured non-operative trial is right.

Typical timeline: from injury to MRI-confirmed diagnosis in one to three weeks in most UK pathways.

Symptoms

What an ACL tear actually feels like.

The classic story of a pop, rapid swelling and a knee that will not be trusted on twisting or side-stepping, plus the features that mean escalate now.

  • Sudden pop at the moment of injury

    Often audible or felt inside the knee during a pivot, landing or side-step.

  • Rapid swelling

    A tense haemarthrosis usually appears within one to four hours as blood fills the joint.

  • Giving way

    The knee buckles on twisting, side-stepping or descending stairs; a classic rotatory instability sign.

  • Loss of full extension

    A locked or blocked knee suggests a displaced meniscal tear or a bucket-handle fragment alongside the ACL.

  • Bruising and quads inhibition

    Bruising down the calf, wasted quads and inability to straight-leg raise fully are common in the first two weeks.

  • Reduced confidence in the knee

    A sense that the knee cannot be trusted on uneven ground, even when pain has settled.

  • Repeated giving-way episodes

    Recurrent instability episodes cause secondary meniscal and cartilage damage over time.

  • Red flag: locked knee or neurovascular signs

    A truly locked knee, calf swelling or numbness needs urgent orthopaedic and vascular assessment.

Treatment

How ACL injury is treated in the UK.

A staged plan: settle the acute knee, choose reconstruction or a structured non-operative trial, then rebuild strength and control with objective return-to-sport testing.

  • PRICE and early rehab

    Protection, relative rest, ice, compression and elevation for the first days, with early quads activation and full extension work.

  • Prehabilitation (2 to 3 weeks)

    Regain full extension, calm the swelling and rebuild quads and hamstring strength before any surgical decision or operation.

  • Structured non-operative programme

    For older or less pivoting patients: bracing, physiotherapy and activity modification, with reconstruction held in reserve.

  • Hamstring tendon autograft

    Semitendinosus with or without gracilis; the most commonly used graft in the UK, with good outcomes and modest donor-site morbidity.

  • Bone-patellar tendon-bone (BPTB)

    A robust choice for pivoting and contact athletes; slightly lower re-rupture rates balanced against anterior knee pain risk.

  • Quadriceps tendon autograft

    A strong, increasingly popular graft, useful in revision and for patients wanting to avoid hamstring or patellar donor sites.

  • Allograft

    Donor tissue, generally reserved for older, lower-demand or revision cases due to slightly higher re-rupture rates in the young.

  • Lateral extra-articular tenodesis (LET)

    An augmentation to reconstruction that controls rotation; used for high-risk knees such as young pivoting athletes and revisions.

Return-to-sport testing

The MOON criteria: don't just ask when, ask whether.

Nine to twelve months is the earliest sensible window for pivoting sport after ACL reconstruction. Before returning, objective testing should show at least 90% limb-symmetry indices on single, triple and crossover hop tests, symmetrical quadriceps and hamstring strength on isokinetic or handheld dynamometry, and a healthy ACL-RSI score for psychological readiness. Realistically, 85 to 90% of patients return to some form of sport, primary re-rupture rates sit at 3 to 6% (higher in those under 20 returning to pivoting sport), contralateral ACL rupture runs at 3 to 8%, and around 40% of ACL-injured knees show osteoarthritis at 20 years regardless of surgery, which is why long-term strength and load management matter as much as the operation itself.

What this guide is based on

The sources behind every claim on this page.

UK national guidance, specialist society standards and landmark cohort studies, current at the time of last review.

Key references

Guidelines and evidence we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP, knee surgeon or physiotherapist knows your knee and history and can tell you which parts apply to you. If in doubt, get seen.

  • NICE. Anterior cruciate ligament injury assessment and management (NG257).

  • British Association for Surgery of the Knee (BASK). ACL reconstruction consensus statements.

  • British Orthopaedic Association (BOA). Standards for musculoskeletal trauma pathways.

  • MOON Knee Group. Return-to-sport criteria and outcome data after ACL reconstruction.

  • Grindem H et al. Delaying return to sport until 9 months reduces re-injury risk. BJSM.

Red flags

When an ACL injury needs urgent attention.

Most ACL tears follow a subacute pathway. These are the situations that do not, and where a specialist opinion is needed the same day or the same week.

  • Locked knee

    True mechanical block to extension suggests a displaced meniscal fragment and needs urgent orthopaedic review.

  • Neurovascular compromise

    Calf swelling, pallor or altered sensation after knee trauma may indicate vascular injury or compartment syndrome.

  • Multi-ligament injury

    Suspected knee dislocation, gross instability or a "floating knee" needs same-day orthopaedic and vascular assessment.

  • Open wound over the joint

    Any laceration communicating with the knee is a surgical emergency to prevent septic arthritis.

  • Segond fracture on X-ray

    A small avulsion off the lateral tibial plateau is virtually pathognomonic for an ACL tear and warrants MRI.

  • Recurrent giving way

    Repeated instability episodes cause secondary meniscal and cartilage injury and should prompt escalation.

  • Skeletal immaturity

    Children and adolescents with open growth plates need specialist paediatric orthopaedic input for graft choice and technique.

  • Persistent effusion beyond 6 weeks

    Ongoing swelling after apparent recovery suggests missed intra-articular damage and deserves imaging.

  • Sudden re-injury after reconstruction

    A new pop, swelling or instability after ACL reconstruction should be reviewed urgently for graft failure.

Living with it

A recoverable injury, with a long tail.

Four things that make the biggest difference over the year of rehab, and the decade beyond it.

A quiet reminder

The knee you rehabilitate is the knee you keep.

Consistent strength work, weight management and honest self-assessment protect against re-rupture and long-term arthritis alike.

  1. 01 Patience

    Respect the 9 to 12 month timeline

    Returning to pivoting sport before objective criteria are met multiplies re-rupture risk, particularly under the age of 20.

  2. 02 Strength

    Chase symmetry, not just strength

    Aim for at least 90% limb-symmetry indices on quads, hamstring and single-leg hop testing before contact drills.

  3. 03 Mind

    Psychological readiness counts

    The ACL-RSI score captures confidence in the knee; low scores predict re-injury independently of physical measures.

  4. 04 Longterm

    Look after the knee for life

    Around 40% of ACL-injured knees show osteoarthritis at 20 years regardless of surgery; strength, weight and load management help.

Frequently asked

Everything we get asked about ACL injury.

Quick answers on surgery, graft choice, LET, recovery time and re-rupture risk.

  • What is the anterior cruciate ligament and what does it do?

    The ACL is one of two central ligaments inside the knee. It stops the tibia sliding forward on the femur and controls rotation, especially during pivoting, cutting and landing movements.

  • Do I definitely need surgery for an ACL tear?

    Not always. Young, active people who play pivoting sports usually benefit from reconstruction. Older, less active patients often do well with a structured non-operative programme of bracing, physiotherapy and activity modification, with surgery held in reserve if instability persists.

  • Which graft is best for ACL reconstruction?

    There is no single best graft. Hamstring, bone-patellar tendon-bone and quadriceps tendon autografts all give good results in experienced hands. Allograft is generally reserved for older or revision cases. Your surgeon will choose based on your sport, anatomy, previous surgery and preferences.

  • What is LET and who needs it?

    Lateral extra-articular tenodesis is a small additional procedure on the outside of the knee that reduces rotation and lowers re-rupture rates in high-risk patients: those under 25, pivoting-sport athletes, revision cases and people with generalised joint laxity.

  • How long is recovery after ACL reconstruction?

    Most people return to running around three to five months and to pivoting sport at nine to twelve months, provided objective testing (limb-symmetry indices of 90% or more on hop and strength tests and a good ACL-RSI score) is met. Rushing back roughly doubles the re-rupture risk.

  • What are the chances of tearing the ACL again?

    Primary re-rupture sits at around 3 to 6% overall and is higher in athletes under 20 returning to pivoting sport. Rupture of the opposite knee ACL is also more common, at roughly 3 to 8%, so both knees need equal attention in rehabilitation.

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