Anterior and posterior cruciate ligament injury

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Ask for exact mechanism, contact, pop, swelling timing, ability to continue and later instability. ACL rupture often follows non-contact pivot or landing and causes haemarthrosis within hours. PCL injury follows a dashboard blow, hyperflexion or fall on the bent knee and may cause posterior or anterior knee discomfort rather than dramatic giving way. High energy, deformity or several directions of laxity means knee dislocation until vascular safety is established, even if it reduced before arrival.

Inspect swelling, alignment, bruising and gait and palpate bone, joint lines, collateral origins and extensor mechanism. Test straight-leg raise. Document pulses, capillary refill, temperature and common peroneal and tibial motor and sensory function. In a possible multiligament injury, measure ankle-brachial pressure and follow the local serial observation and CT angiography pathway. Palpable pulses do not override an abnormal perfusion index or changing examination.

Assess ACL with Lachman and anterior translation while avoiding the error of starting from a posteriorly sagged PCL-deficient tibia. Pivot shift is useful when guarding permits. Assess PCL through posterior sag, posterior drawer and quadriceps active tests, then varus, valgus and rotational corner stability. Compare the opposite side. Acute haemarthrosis and pain limit accuracy, so repeat examination after protection, swelling control or under anaesthesia when surgery is planned.

Obtain AP and lateral trauma radiographs, adding views for suspected patellar or avulsion injury. Look for tibial spine, Segond, plateau, PCL tibial avulsion and osteochondral fracture. MRI is the main soft-tissue map for cruciates, menisci, cartilage and collateral or corner structures, but it follows vascular and fracture priorities. In skeletally immature patients, BOASt expects an acute knee pathway and MRI within two weeks when examination is difficult, with formal growth assessment before reconstruction.

Early treatment aims for a quiet knee: compression, short protection, full extension, progressive flexion, quadriceps activation and normal gait. Avoid prolonged immobilisation and avoid rushing reconstruction into a stiff swollen knee unless an associated emergency dictates timing. Supervised rehabilitation progresses strength, neuromuscular control, landing and running. A functional brace is selective, more often used early for PCL or associated ligament injury, and does not substitute for muscle and movement retraining.

Choose non-operative management when the person has no recurrent functional instability, can modify pivoting demands and progresses through rehabilitation. This is an active programme with reassessment, not discharge after handing out exercises. Persistent giving way, repairable meniscal injury, high pivoting goals, combined ligament damage or inability to perform work despite rehabilitation supports reconstruction discussion. Delayed surgery should not mean allowing repeated instability to damage the meniscus.

ACL reconstruction uses a graft and requires discussion of graft source, tunnel and fixation, donor-site symptoms, infection, stiffness, re-rupture and contralateral injury. PCL reconstruction is less common; isolated lower-grade injury often begins non-operatively, while persistent symptomatic high-grade posterior instability, avulsion and combined injury need specialist surgery. Multiligament operations require staged or combined planning, vascular awareness and experienced rehabilitation. Skeletally immature techniques protect remaining growth and need radiographic surveillance.

Return to sport is criteria based: no effusion, full motion, stable examination, strength and power symmetry, quality landing and psychological readiness, with sport-specific progression. Time alone is insufficient, and early return increases graft risk. Follow instability, extension loss, meniscal symptoms and osteoarthritis. Prevention programmes emphasise neuromuscular landing, trunk and hip control and strength, particularly in pivoting sports. New calf pain, infection signs or sudden loss of extension after surgery requires urgent review.

Key points

ACL injury typically follows pivot, deceleration or landing with a pop, rapid haemarthrosis and later giving way; PCL injury follows posterior tibial force or flexed-knee fall and may be subtler.
A spontaneously reduced knee dislocation can look normal. Document perfusion, ankle-brachial pressure and peroneal and tibial nerve function and use serial or vascular imaging pathways when multiligament injury is possible.
Obtain trauma radiographs first to identify fracture or avulsion. MRI maps ACL, PCL, menisci, cartilage and corners after immediate limb threats are addressed.
Use Lachman and pivot shift for ACL and posterior sag and drawer for PCL, comparing the other knee and recognising that pain and haemarthrosis reduce early accuracy.
Begin swelling control, full extension, quadriceps activation, gait and progressive strength before deciding from a quiet, examinable knee.
Non-operative care is credible for people without symptomatic instability who can meet activity goals through rehabilitation and modification; the MRI rupture alone is not an operative indication.
Reconstruction is considered for persistent functional instability, pivoting goals, repairable associated lesions or multiligament injury after shared discussion of rehabilitation, graft, failure and osteoarthritis risk.
In children, use an acute knee pathway, MRI within two weeks when examination is hard, document remaining growth and do not defer necessary surgery automatically until skeletal maturity.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

ACL pivot injury

Deceleration, landing or change of direction with valgus and rotation commonly ruptures ACL without direct contact and often injures meniscus or cartilage.

PCL dashboard mechanism

A posterior force to the proximal tibia with the knee flexed, classically in a vehicle collision, drives PCL failure.

PCL flexion injury

Falling onto a flexed knee, hyperflexion or hyperextension can tear PCL or avulse its tibial attachment.

Growth-stage injury

Skeletally immature patients may sustain tibial-spine avulsion or ligament rupture and need growth-aware imaging and reconstruction planning.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Anterior restraint loss
ACL rupture permits anterior tibial translation and excessive pivot rotation, producing giving way during cutting and landing.
2
Posterior restraint loss
PCL rupture allows posterior tibial sag and alters patellofemoral and medial compartment loading, especially in flexion.
3
Secondary stabiliser overload
Menisci, collateral ligaments and posterolateral structures carry abnormal rotational loads and may tear or elongate during repeated functional instability episodes.
4
Haemarthrosis and inhibition
Acute intra-articular bleeding suppresses quadriceps activation, restricts motion and limits examination accuracy, making early and repeat assessment complementary.
5
Post-traumatic degeneration
Meniscal and cartilage injury plus repeated instability increase long-term osteoarthritis risk whether treatment is operative or non-operative.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

ACL rupture

Pivot injury, pop, rapid haemarthrosis, positive Lachman and later rotational giving way forms the classic pattern.

PCL rupture

Dashboard or flexed-knee trauma with posterior sag and drawer supports posterior restraint loss, often with less dramatic swelling.

Multiligament dislocation

Gross or multidirectional laxity, bruising and high-energy mechanism requires vascular surveillance even when alignment has returned.

Tibial-spine avulsion

A child with haemarthrosis and extension loss may have bony ACL avulsion visible on radiographs rather than midsubstance rupture.

PCL avulsion

Posterior tibial avulsion on lateral radiograph or CT can be a surgically fixable bony injury distinct from midsubstance rupture.

Failed rehabilitation

Recurrent giving way during required work or sport despite restored strength and motion supports reconstruction discussion.

Red flags requiring action

Knee deformity, gross multidirectional instability, reduced pulse, cool foot or ankle-brachial pressure below the local normal threshold requires immediate vascular and orthopaedic escalation.
Progressive calf pain, tense swelling, pain on passive stretch or neurological loss suggests compartment syndrome and needs emergency assessment.
An acutely locked knee or large haemarthrosis may contain displaced meniscus or osteochondral fracture and requires an acute knee pathway.
Inability to straight-leg raise, patella alta or baja, or palpable tendon gap suggests extensor mechanism rupture rather than isolated cruciate injury.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Immediate neurovascular assessmentFirst step
Why
Detect popliteal artery and peroneal or tibial nerve injury.
Interpretation and limitations
Repeat after reduction and serially; abnormal or equivocal perfusion requires urgent vascular imaging and specialist input.
02
First-line trauma radiographsFirst line
Why
Identify fracture, tibial-spine or PCL avulsion and alignment.
Interpretation and limitations
Normal films do not exclude ligament rupture or a spontaneously reduced dislocation.
03
MRI
Why
Map cruciates, menisci, cartilage, collaterals and corners.
Interpretation and limitations
Use after limb threats and fractures are addressed; imaging supports but does not replace instability and goal assessment.
04
Lachman and pivot assessment
Why
Demonstrate anterior and rotational ACL insufficiency.
Interpretation and limitations
Guarding reduces sensitivity; repeat in a settled knee and avoid false anterior translation when PCL sag is present.
05
Posterior sag and drawer
Why
Demonstrate PCL insufficiency and grade posterior translation.
Interpretation and limitations
Compare sides and assess posterolateral structures because isolated and combined injuries have different treatment.
06
Growth assessment
Why
Plan paediatric tunnel and follow-up safely.
Interpretation and limitations
Document skeletal maturity and monitor operated children radiographically for growth disturbance until maturity.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Patellar dislocation

Transient lateral patellar displacement produces haemarthrosis and apprehension with medial retinacular tenderness and possible patellar or femoral osteochondral injury.

Meniscal tear

Joint-line pain and locking may coexist; fixed extension loss suggests displaced tissue rather than ligament laxity alone.

Collateral injury

Valgus or varus tenderness and laxity localises medial or lateral structures and may form part of multiligament injury.

Extensor mechanism rupture

Loss of active straight-leg raise with tendon defect requires urgent repair assessment and is not explained by ACL or PCL rupture.

Tibial plateau fracture

Marked load pain, focal bony tenderness and lipohaemarthrosis require radiographic and CT-defined fracture management before any ligament rehabilitation programme.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01ThreatTreat as knee dislocationFirst stepDeformity, multiligament laxity or abnormal perfusion suggests dislocation or vascular injury.

1Reduce deformity, immobilise and document pulses, ankle-brachial pressure and nerve function before and after intervention.
2Use urgent CT angiography and vascular consultation for abnormal or changing perfusion.
3Image fractures and all ligament structures and admit for serial examination according to the local pathway.
4DefinitivePlan staged or definitive repair and reconstruction through a multiligament knee team.

02RehabilitationCreate a quiet stable functional kneeIsolated injury is confirmed without a limb threat or urgent repairable block.

1Restore full extension, swelling control, quadriceps activation and normal gait before high-load progression.
2Build strength, neuromuscular control and activity-specific capacity with supervised review.
3Assess whether instability persists during the person's actual work or pivoting goals.
4Continue non-operative care when goals are met without giving way and protect secondary structures.

03ReconstructionOperate for functional instability and combined needGiving way persists, pivoting goals are high or associated repairable injury changes the benefit.

1Discuss graft or PCL technique, rehabilitation, failure, stiffness and long-term osteoarthritis honestly.
2Treat meniscal, collateral and corner injuries as one stability plan rather than isolated MRI findings.
3Use growth-aware MDT planning in children and do not defer failed non-operative care solely until maturity.
4Return through motion, effusion, strength, power and movement-quality criteria rather than a date alone.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

Reduces pain and effusion-related inhibition while movement and quadriceps activation are restored.

Short-term anti-inflammatory analgesia

Use a licensed topical NSAID or one oral NSAID at the lowest effective adult dose for the shortest period that supports motion and rehabilitation, adding gastroprotection when oral treatment and risk assessment indicate it.

Assess ulcer, renal, cardiovascular and liver disease, pregnancy, anticoagulants and other NSAIDs; analgesia must not mask worsening compartment, vascular or fracture signs.

Reduces venous thromboembolism risk in selected people whose surgery, immobilisation or personal factors create sufficient risk.

Post-reconstruction thromboprophylaxis when indicated

Use the exact mechanical and pharmacological regimen from the current local knee-ligament surgery pathway only after individual venous-thrombosis, bleeding, weight-bearing, travel, contraception and previous-event assessment.

Routine medication is not identical for every arthroscopy; coordinate with neuraxial timing, renal function and existing anticoagulants and provide stop and urgent bleeding instructions.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Recurrent instability

Repeated pivot or posterior sag episodes impair sport and work and expose menisci and cartilage to further injury.

Meniscal and chondral damage

Secondary meniscal tearing and focal cartilage injury accumulate when unstable knees repeatedly pivot or give way under meaningful load.

Arthrofibrosis

Operating or immobilising a swollen stiff knee and poorly progressed rehabilitation can produce persistent motion loss.

Growth disturbance

Tunnel placement in skeletally immature reconstruction can injure physes and cause angular or length change requiring monitoring.

Post-traumatic osteoarthritis

Combined cartilage, meniscal and stability damage increases later pain, stiffness and structural degeneration despite technically successful ligament reconstruction.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat perfusion and peroneal and tibial nerve examination through any multiligament admission.
Track effusion, full extension, flexion, quadriceps activation and gait during prehabilitation.
Measure anterior, posterior and rotational stability and record recurrent real-world giving-way episodes.
After reconstruction, monitor wound, infection, thrombosis, motion and graft-protection adherence.
Use objective strength, power and movement-quality testing plus psychological readiness before return to pivoting sport.
In skeletally immature surgery, monitor alignment and limb length radiographically until maturity.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

The knee may have reduced itself

Normal alignment on arrival does not exclude a vascularly dangerous dislocation when several ligaments are lax.

PCL sag can fake ACL laxity

Starting an anterior drawer from a posteriorly displaced tibia exaggerates apparent anterior translation.

MRI does not choose surgery

Functional instability, goals, associated repairable tissue and response to rehabilitation determine benefit.

A quiet knee protects motion

Swelling and extension loss before reconstruction increase arthrofibrosis risk and should be actively corrected.

Children should not simply wait

Failed non-operative ACL care may justify growth-aware reconstruction before maturity rather than years of giving way.

Return tests multiple systems

Time, strength, movement quality, effusion and psychological readiness collectively matter more than graft age alone.

11Common pitfallsFrequent interpretation and management errors.

01
Missing a spontaneously reduced knee dislocation because pulses are palpable once.
02
Testing ACL without recognising posterior tibial sag from PCL injury.
03
Operating on MRI rupture alone without functional instability or goal assessment.
04
Allowing repeated giving way during an undefined trial of non-operative care.
05
Reconstructing a stiff swollen knee without addressing motion and effusion.
06
Clearing return to pivoting sport by elapsed months alone.