Acute knee injuries

Acute knee injuries encompass a spectrum of traumatic conditions affecting the ligaments, menisci, articular cartilage, tendons, and bone of the knee joint. They are among the most common musculoskeletal presentations to Australian emergency departments and general practice, particularly in active individuals aged 15–45 years. The mechanism of injury, anatomical structures involved, and stability of the knee determine urgency of management and prognosis. Accurate clinical assessment using structured examination findings enables confident decision-making regarding imaging, weight-bearing, referral, and rehabilitation. The Ottawa Knee Rules and Pittsburgh Decision Rules guide appropriate use of X-ray to exclude fracture without unnecessary imaging.

Acute knee injuries result from mechanical forces exceeding the structural tolerance of specific knee components. The mechanism of injury predicts the pattern of tissue damage and guides clinical examination priorities.

Mechanism-Based Injury Patterns

• ACL injury — non-contact deceleration with valgus collapse and internal tibial rotation (landing from a jump; rapid direction change); contact ACL injury from direct valgus force; the ACL resists anterior tibial translation and internal rotation; complete rupture causes haemarthrosis (80% of acute haemarthroses) and anterolateral instability
• PCL injury — posterior force to the proximal tibia with knee flexed (dashboard injury; fall onto flexed knee); the PCL is the primary posterior stabiliser; isolated PCL injuries are often managed conservatively; combined PCL-PLC (posterolateral corner) injuries require surgical reconstruction
• Meniscal tear — axial load combined with rotation on a flexed knee; medial meniscus tears more commonly than lateral (less mobile, more peripheral attachment); bucket-handle tears produce a mechanical block to extension (locked knee); peripheral tears have a vascular zone capable of healing; central zone tears are avascular and cannot heal
• Collateral ligament sprain — MCL: valgus force to knee in slight flexion (commonest ligament injury; grades I–III); LCL: varus force (less common; often associated with PLC injury); isolated Grade I–II MCL injuries heal reliably with conservative management
• Patellar dislocation — lateral dislocation from quadriceps contraction with knee in valgus and external tibial rotation; trochlear dysplasia and high-riding patella (patella alta) are predisposing factors; osteochondral fracture may occur at time of dislocation or relocation

Clinical examination of the acutely injured knee requires systematic assessment of swelling, stability, meniscal integrity, and neurovascular status. An acute haemarthrosis suggests ACL rupture, tibial plateau fracture, or osteochondral injury until proven otherwise.

History

• Mechanism — contact vs non-contact; direction of force (valgus, varus, hyperextension, rotation); activity at the time (landing, cutting, tackle); audible or palpable pop (ACL rupture; patellar dislocation); immediate swelling within 2 hours suggests haemarthrosis (ACL, fracture); delayed swelling (12–24 hours) suggests synovial effusion (meniscal, minor sprain)
• Function after injury — ability to weight-bear immediately after injury (inability suggests significant structural injury or fracture); locking (inability to fully extend = bucket-handle meniscal tear until proven otherwise); giving way (instability episode)
• Swelling — immediate haemarthrosis (tense, warm joint): ACL rupture (80%), peripheral meniscal tear with vascular supply, osteochondral fracture, tibial plateau fracture; delayed effusion: degenerative meniscal tear, Grade I–II MCL sprain, patellar subluxation

Examination Findings

• Lachman test — knee at 20–30 degrees flexion; stabilise femur with one hand; translate tibia anteriorly with the other; positive = increased anterior translation with soft or absent end-feel; sensitivity 84%, specificity 93% for complete ACL tear; most sensitive ligament test for ACL in acute setting; performed before pivot shift in acute haemarthrosis
• Anterior drawer test — knee at 90 degrees; anterior tibial translation; sensitivity lower than Lachman in acute injury (hamstring guarding); less reliable acutely
• Valgus stress test — applied at 0 and 30 degrees flexion; laxity at 30 degrees = MCL injury; laxity at 0 degrees = MCL + posterior capsule or cruciate involvement; grade by laxity (I: firm end-feel, II: 5–10 mm, III: no end-feel)
• McMurray test — knee flexion–extension with internal/external tibial rotation; click or pain at joint line = meniscal tear; sensitivity 70%, specificity 71%; should be deferred if acute haemarthrosis prevents full flexion
• Joint line tenderness — medial or lateral joint line palpation; most sensitive clinical test for meniscal pathology (sensitivity 83%); also positive in collateral ligament sprain at joint line

The Ottawa Knee Rules guide X-ray use in acute knee injury. MRI is the gold-standard investigation for suspected internal derangement. CT is reserved for complex fracture assessment.

• Essential
  X-ray knee (AP and lateral) — Ottawa Knee Rules

  Indicated if: age ≥55 years; isolated patella tenderness; fibular head tenderness; inability to flex to 90 degrees; inability to weight-bear immediately and in ED (4 steps). Sensitivity 98–100% for fracture; specificity 49%. In general practice: apply Ottawa rules; if Ottawa positive, send for X-ray. Do not X-ray if Ottawa negative with low-energy mechanism.
• Recommended
  MRI knee

  Gold standard for ACL, PCL, meniscal, and osteochondral injury. Indicated for: acute haemarthrosis (high probability of ACL tear or significant structural injury); clinical ACL rupture (Lachman positive) to confirm and assess meniscal/osteochondral associated injury; locked knee (urgent MRI); suspected meniscal tear not responding to conservative management; clinical uncertainty after examination. Request on Medicare: Item 63560 (non-obstetric MRI knee joint). Arrange within 1–2 weeks for ACL injury.
• Specialised
  CT knee

  Indicated for complex tibial plateau fracture assessment (fracture pattern, depression, comminution); osteochondral fracture characterisation; when MRI is contraindicated. Not first-line for soft tissue injury assessment.
• Specialised
  Ultrasound knee

  Limited role in acute knee injury; useful for assessment of quadriceps or patellar tendon rupture (point-of-care); guided aspiration of tense haemarthrosis for patient comfort; not adequate for ACL or meniscal assessment.

Acute knee injuries are stratified by the nature of the structural damage and associated instability to determine the urgency of imaging, referral, and rehabilitation pathway.

Pharmacological management of acute knee injuries focuses on pain control and reduction of inflammation to facilitate early rehabilitation. No pharmacological agent promotes ligament or meniscal healing. Opioids should be avoided except for severe fracture pain in the short term.

Directed therapy for acute knee injuries includes physiotherapy-guided rehabilitation, bracing, aspiration of tense haemarthrosis, and surgical reconstruction for ACL rupture in appropriate candidates.

RICE Protocol and Early Mobilisation

• Rest — relative rest; avoid aggravating activities; complete rest is counterproductive; weight-bear as tolerated for minor sprains; crutches with partial weight-bearing for moderate injuries (ACL, Grade II MCL, meniscal tear)
• Ice — apply for 15–20 minutes every 2–3 hours for first 48–72 hours; reduces haemarthrosis and acute pain; use ice pack wrapped in towel to prevent frostbite; avoid direct ice contact with skin
• Compression — elastic bandage or compression sleeve; reduces effusion; apply from distal to proximal; do not apply over a tight haemarthrosis if causing distal vascular compromise
• Elevation — elevate limb above heart level to reduce swelling; particularly important in first 48 hours

Physiotherapy Rehabilitation

• Phase 1 (acute, 0–2 weeks) — swelling control; range-of-motion restoration; quadriceps activation (straight leg raise, isometric quad sets); protected weight-bearing; cryotherapy
• Phase 2 (subacute, 2–6 weeks) — progressive strengthening (closed-chain exercises: mini-squats, step-ups, leg press); proprioception training; full weight-bearing; return to cycling or swimming
• Phase 3 (rehabilitation, 6–16 weeks) — sports-specific training; running, cutting drills; neuromuscular training; for ACL: 9–12 months before return to pivoting sport (surgery or conservative); for MCL Grade II: 6–8 weeks; for meniscal: 4–6 weeks non-surgical

Bracing

• Hinged knee brace — used for Grade II–III MCL sprain; set at 0–90 degrees flexion; gradually increase range over 4–6 weeks; provides valgus stability during healing; not required for Grade I MCL
• ACL functional brace — proprioceptive and psychological benefit; does not prevent re-injury; used during return to sport in conservatively managed ACL injuries
• Patellar stabilising brace — after patellar dislocation; immobilise in extension for 2–4 weeks then progressive mobilisation; recurrence rate 15–45% after first dislocation

Knee Aspiration

• Indications — tense haemarthrosis causing severe pain and limiting examination or physiotherapy; diagnostic aspiration to distinguish haemarthrosis (fat globules = fracture) from simple effusion
• Technique — suprapatellar approach (medial or lateral); strict aseptic technique; aspirate with 18–21G needle; fat globules in aspirate indicate intra-articular fracture and mandate X-ray review and CT

ACL Reconstruction

• Indications — young, active patients (<40 years, high activity level) with complete ACL tear planning to return to pivoting sports; associated meniscal or chondral injury requiring arthroscopic management; functional instability despite rehabilitation; decision made jointly with patient after MRI and orthopaedic consultation
• Timing — not acute; delay 3–6 weeks for swelling to settle and quadriceps strength to recover (reduces postoperative stiffness risk); prehabilitation before surgery improves outcomes
• Graft choice — hamstring autograft (most common in Australia) vs patellar tendon autograft vs quadriceps tendon; allograft for revision; surgeon and patient preference
• Conservative management — reasonable option for older patients (>40), lower-demand individuals, and those who do not participate in pivoting sports; structured physiotherapy neuromuscular program; 50–60% return to pre-injury level without surgery in appropriately selected patients

Non-pharmacological management is central to acute knee injury recovery. Patient education about the expected course, rehabilitation adherence, and return-to-sport criteria reduces re-injury risk and long-term sequelae.

Patient Education

• Expected course — Grade I MCL: full recovery 1–2 weeks; Grade II MCL: 4–8 weeks; Grade III MCL (isolated): 8–12 weeks; ACL non-operative: 6–12 months rehabilitation; ACL reconstructive surgery: 9–12 months to return to sport; meniscal repair: 3–4 months; meniscal partial meniscectomy: 4–6 weeks; first patellar dislocation: 8–12 weeks
• Return-to-sport criteria — clearance based on functional testing, not time alone; limb symmetry index ≥90% on hop tests and strength testing; neuromuscular control; psychological readiness; ACL: minimum 9 months post-reconstruction and quadriceps strength ≥90% of contralateral side
• Osteoarthritis risk — ACL rupture increases long-term knee OA risk (estimated 5–10 times); meniscectomy further increases risk; counsel patients about weight management, exercise preservation, and monitoring for OA symptoms

Neuromuscular Training and Injury Prevention

• ACL injury prevention programs — FIFA 11+, Sportsmetis, PEP programs; reduce ACL injury rate by 40–60% in high-risk sports (football, basketball, netball); incorporate warm-up, strengthening, plyometrics, balance, and agility; recommend to all athletes returning to pivoting sports after ACL injury
• Proprioception training — single-leg balance exercises; perturbation training; wobble board; critical after meniscal and ligament injury to restore joint mechanoreceptor function

Monitoring after acute knee injury focuses on resolution of swelling and pain, functional recovery milestones, detection of complications (re-tear, OA), and return-to-sport criteria.

Indications for Specialist Referral

• Orthopaedic surgery — confirmed ACL rupture in active patients considering reconstruction; locked knee; tibial plateau fracture; multi-ligament injury; Grade III MCL/LCL; patellar fracture; osteochondral fracture with loose body
• Sports medicine — complex rehabilitation; ACL conservative management; return-to-sport clearance; PRP injection for meniscal or chondral injury consideration
• Emergency — suspected knee dislocation; neurovascular compromise; open injury; tibial plateau fracture with compartment syndrome

Specific considerations apply to paediatric patients, female athletes, and older adults with acute knee injuries.

Paediatric and Adolescent Patients

• ACL injury in skeletally immature patients — ACL rupture in children and adolescents with open physes requires careful surgical planning to avoid physeal damage; conservative management risks chronic instability and meniscal damage; physeal-sparing and physeal-respecting ACL reconstruction techniques used by paediatric orthopaedic specialists; do not delay referral
• Tibial spine avulsion fracture — ACL equivalent injury in skeletally immature patients; avulsion of the ACL tibial attachment rather than midsubstance rupture; presents with haemarthrosis; X-ray may show tibial spine avulsion; MRI or CT confirms; most Type II–III require arthroscopic fixation
• Osgood-Schlatter disease — chronic patellar tendon apophysitis (not acute injury); adolescent males; tibial tuberosity tenderness with activity; managed conservatively; rarely needs imaging if presentation is classic

Female Athletes

• ACL injury risk — females have 2–8 times higher ACL injury rate than males in equivalent sports; contributing factors include trochlear geometry, hormonal influences (oestrogen effect on ligament laxity), neuromuscular recruitment patterns, and higher knee valgus loading during landing; ACL prevention programs particularly important in female athletes in netball, basketball, and soccer
• Return to sport after ACL reconstruction — female athletes have higher re-tear risk post-ACL reconstruction (3–4 times); neuromuscular training and landing mechanics retraining are particularly important; extended rehabilitation periods recommended

Older Adults (>50 years)

• Degenerative meniscal tears — common in older adults; often atraumatic or low-energy mechanism; differentiate from acute traumatic meniscal tear (which requires MRI and orthopaedic referral); degenerative tears often managed conservatively with physiotherapy and activity modification
• ACL rupture — conservative management is appropriate for older, lower-demand patients; physiotherapy neuromuscular program; functional bracing if instability with activity; ACL reconstruction in selected older patients with high functional demands and significant instability

Stewardship in acute knee injury management focuses on appropriate imaging use (applying Ottawa Knee Rules), avoiding unnecessary opioid prescribing, appropriate timing of ACL surgery, and ensuring physiotherapy is the cornerstone of recovery rather than a secondary option.

GP Role

• Apply Ottawa Knee Rules — document Ottawa assessment at each presentation; avoid unnecessary X-ray; use clinical examination findings to guide imaging
• Initiate structured rehabilitation — refer to physiotherapy for all moderate-to-significant knee injuries; provide written home exercise instructions for mild injuries; physiotherapy is the most important intervention for recovery
• Appropriate MRI referral — MRI for acute haemarthrosis, suspected ACL, locked knee, or failure to progress with conservative management; avoid MRI for uncomplicated Grade I MCL sprains
• Timely orthopaedic referral — locked knee = urgent; ACL + active patient = 2–4 weeks; fracture = urgent; isolated Grade I–II MCL = no routine orthopaedic referral needed

Follow-up after acute knee injury should be structured to milestones, not fixed time intervals. Prevention of re-injury and long-term osteoarthritis are key goals.

• Australian Institute of Health and Welfare (AIHW) — Sports injury in Australia
• Therapeutic Guidelines: Musculoskeletal — Acute knee injuries; available via eTG complete
• Stiell IG et al. — Ottawa Knee Rules; Ann Emerg Med 1996
• Frobell RB et al. — Randomised trial of ACL reconstruction vs rehabilitation for acute ACL tears; NEJM 2010
• Ardern CL et al. — Return to sport after ACL reconstruction; Br J Sports Med 2016 consensus statement
• Hewett TE et al. — ACL injury prevention programs; Am J Sports Med 2006
• Royal Australian College of General Practitioners (RACGP) — Musculoskeletal injury management in general practice
• Australian Physiotherapy Association — Clinical guidelines for acute knee injury management
• Sports Medicine Australia — ACL injury prevention and return-to-sport criteria