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Orthopaedics
Sports Medicine

Anterior Cruciate Ligament (ACL) Rupture

High EvidenceUpdated: 2025-12-26

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Red Flags

  • Locked Knee (Bucket Handle Meniscus)
  • Multi-ligament injury (Knee Dislocation)
  • Neurovascular compromise (popliteal artery)
  • Septic Arthritis (if post-op)
Overview

Anterior Cruciate Ligament (ACL) Rupture

1. Clinical Overview

Summary

Rupture of the Anterior Cruciate Ligament (ACL) is the seminal injury of modern sports medicine. It represents a catastrophic failure of the knee's primary stabilizer against anterior tibial translation and rotational loading. The injury typically occurs via a non-contact pivoting mechanism—a "perfect storm" of deceleration, valgus collapse, and external tibial rotation. Patients report an audible "pop" followed by rapid haemarthrosis. Diagnostically, the Lachman test is the most sensitive clinical tool, while MRI provides confirming visualization of the disruption and associated "kissing contusions". [1,2]

The management of ACL injuries has evolved significantly. It is no longer a binary choice between "surgery or no surgery", but a nuanced decision based on patient goals, instability profile, and concomitant injuries. Modern anatomical reconstruction (ACLR) aims to restore kinematics and protect the menisci, utilizing autografts (Hamstring, Patellar Tendon, or Quadriceps). However, high-quality evidence (KANON trial) supports a role for initial rehabilitation in selected patients. Prevention programs (e.g., FIFA 11+) are now a critical part of the public health approach to this epidemic. [3]

Key Facts

  • Mechanism: Non-contact pivot (70%) > Contact.
  • The "Pop": Experienced by 80% of patients.
  • Haemarthrosis: 70% of acute traumatic haemarthroses are ACL ruptures.
  • Gender Disparity: Females have a 4-6x higher incidence than males in matched sports.
  • Gold Standard Sign: Lachman Test (Sensitivity >95%).
  • Gold Standard Imaging: MRI (95% Sensitivity).
  • Associated Pathology: Meniscal tears (50-70%), MCL injuries (30%), Bone bruises (>90%).
  • Graft Healing: Ligamentization takes >1 year.
  • Recovery Time: Minimum 9 months for return to sport (biology).
  • Long Term: 50% risk of Osteoarthritis at 20 years.
  • Treatment Philosophy: Treat the instability, not the MRI scan.

Clinical Pearls

The "Pop" and "Swell" Rule: If a young athlete describes a non-contact twisting injury with an audible "pop" and is unable to continue playing, followed by rapid swelling (within 4 hours), the diagnosis is an ACL rupture until proven otherwise. Meniscal tears typically swell slowly (overnight).

The Muscle Guarding Pitfall: The Anterior Drawer test is frequently false-negative in the acute setting because purely anterior translation triggers protective hamstring spasm. The Lachman test (at 20-30° flexion) minimizes this hamstring tension and is far superior.

The classic O'Donoghue's triad (ACL + MCL + Medial Meniscus) is actually less common than the pairing of ACL + Lateral Meniscus tears in the acute setting. The lateral meniscus is trapped and torn during the pivot shift subluxation.

Historical Context: A Century of Struggle

The treatment of ACL injuries has a colorful history:

  • The "Cast Era" (Pre-1970s): Ruptures were treated with long-leg plaster casts. Result: Permanent stiffness and persistent instability.
  • Primary Repair (1970s): Surgeons tried suturing the ligament back together. Result: 90% failure rate within 5 years (the ACL healing environment is hostile).
  • Synthetic Era (1980s): Carbon fiber and Gore-Tex grafts. Result: Catastrophic failure due to breakdown products (synovitis).
  • Extra-articular Era: Procedures like the MacIntosh tenodesis tried to tighten the outside of the knee. Result: Reduced pivot shift but did not stop anterior slide. (Ironically, we have returned to this with LET).
  • Modern Era: Intra-articular anatomical reconstruction with autograft.
2. Epidemiology

Incidence & Burden

  • Incidence: Estimated at 200,000 ruptures annually in the USA alone.
  • Trend: Rising incidence in adolescents and young adults, linked to early sports specialization and higher intensity play.
  • Economic Impact: A multibillion-dollar burden considering surgical costs and long-term osteoarthritis risk.

Demographics and Risk Factors

FactorDescriptionMechanism/Note
GenderFemales > Males (4.5x)The primary epidemiological disparity.
AnatomyNarrow Intercondylar Notch"Stenotic" notch impinges on ACL (Notch width index).
HormonalOvulatory PhaseLigament laxity peaks during the pre-ovulatory phase (estrogen/relaxin surge).
NeuromuscularLigament DominanceFemales tend to land with increased knee valgus and less hip flexion, relying on ligaments rather than muscles to absorb ground reaction forces.
SportPivoting/CuttingSoccer, Basketball, Netball, Rugby, Skiing (phantom foot mechanism).
GeneticsCollagen Type I/VPolymorphisms (e.g., COL5A1) associated with intrinsic weakness.

The Female Athlete Risk Profile

The 4-6x increased risk in females is multifactorial:

  • Neuromuscular Control ("Ligament Dominance"):
    • Women tend to land with knees straight (extension) and in valgus (knock-kneed).
    • This position increases strain on the ACL ("The position of no return").
    • Glutes/Hamstrings are often relatively weaker compared to Quads ("Quad Dominance").
  • Anatomical:
    • Wider Pelvis (Increased Q-angle) may increase valgus vector.
    • Smaller Notch Width Index (stenosis): The ACL gets "guillotined" against the lateral wall during twisting.
  • Hormonal:
    • Estrogen receptors are present on the ACL. Ligament laxity increases during the pre-ovulatory phase, potentially reducing the load-to-failure threshold.

Preventive Strategies

The "ACL Epidemic" has led to the development of validated warm-up programs.

  • FIFA 11+ Program: A complete warm-up replacing the standard warm-up.
    • Part 1: Slow running, dynamic stretching.
    • Part 2: Strength, Plyometrics, Balance.
      • Nordic Hamstring Exercise: Eccentric strengthening.
      • Planks: Core control.
      • Single leg stance: Proprioception.
    • Part 3: High speed running, cutting.
  • Effectiveness: Meta-analyses show these programs can reduce ACL injury rates by 50% if adherence is high (at least 2x per week).
  • Target Audience: Adolescent females in high-risk sports (soccer, netball).
3. Pathophysiology

Anatomy and Function

The ACL originates from the posteromedial aspect of the Lateral Femoral Condyle and inserts onto the Anterior Tibial Spine. It consists of two functional bundles:

  1. Anteromedial (AM) Bundle: Tight in flexion. Primary restraint to anterior translation.
  2. Posterolateral (PL) Bundle: Tight in extension. Primary restraint to rotation.

Micro-Anatomy and Biomechanics

  • Collagen: 90% Type I collagen bundles arranged in fascicles.
  • Innervation: Posterior Articular Nerve (Tibial Nerve). Contains mechanoreceptors (Ruffini, Pacinian, Golgi, Free nerve endings) that provide proprioceptive feedback ("position sense"). Rupture leads to de-afferentation (loss of sensory input), contributing to instability beyond just mechanical failure.
  • Blood Supply: Middle Genicular Artery. The ACL is covered by a synovial sheath ("extrasynovial"). Any tear of the sheath leads to hemarthrosis due to the rich vascularity.

The 5-Step Mechanism of Rupture

Step 1: The Loading Event (Valgus Collapse)

  • The athlete plants the foot to cut or land.
  • The knee enters a position of "Dynamic Valgus": Hip internal rotation + Knee Valgus + Tibial External Rotation.
  • The Ground Reaction Force passes lateral to the knee center, creating a massive valgus moment.

Step 2: Ligament Strain and Yield

  • The ACL is loaded well beyond its physiological limit (approx 2000 N).
  • As the tibia translates anteriorly and rotates internally relative to the femur (Pivot Shift), the ACL fibers stretch.
  • The "Yield Point" is reached where plastic deformation occurs.

Step 3: Macroscopic Failure ("The Pop")

  • The ligament snaps (usually mid-substance).
  • Energy is released acoustically (the "Pop") and kinaesthetically (the "give way").
  • The tibia subluxes anteriorly (Pivot Shift).

Step 4: Secondary Collision (Bone Bruise)

  • As the ACL fails, the Lateral Femoral Condyle crashes into the Posterolateral Tibial Plateau.
  • This impact causes the pathognomonic "Kissing Contusion" bone edema pattern seen on MRI.
  • The lateral meniscus is often trapped and torn during this subluxation.

Step 5: Resolution and Instability

  • The tibia reduces (clunks back).
  • Bleeding from the genicular arteries fills the joint (haemarthrosis).
  • The knee is now mechanically unstable (ACL deficient), relying on secondary restraints (menisci, capsule) which are at risk of chronic fatigue failure.

The Biology of Healing ("Ligamentization")

If we perform a reconstruction, the new graft (tendon) must transform into a ligament. This process takes over a year:

  1. Necrosis Phase (0-4 Weeks): The graft loses its original blood supply and cells die. It is essentially a scaffold. Mechanical strength decreases.
  2. Re-vascularization (4-12 Weeks): New blood vessels invade from the fat pad and bone tunnels. Synovial sheath forms.
  3. Re-population (3-6 Months): Host fibroblasts migrate into the scaffold. Collagen is disorganized. Strength is at its nadir (lowest point) here, despite the patient feeling good. High risk period.
  4. Remodeling (>6 Months): Collagen fibers align with lines of stress. Material properties improve but only reach ~50-60% of a native ACL by 12 months.
4. Clinical Presentation

History

Mechanism
Non-contact pivot (70%) or contact blow (30%).
Sensation
"My knee popped" or "My knee shifted out of place".
Immediate Sequence
Severe pain (transient, then deep ache). Inability to continue play. Rapid swelling (0-2 hours).
Chronic Presentation
"Giving way" or "trust issues" with the knee during twisting movements.
5. Clinical Examination

Look:

  • Gross effusion (ballottable patella).
  • Quadriceps avoidance gait (flexed knee).
  • Loss of full extension (suggests incarcerated bucket-handle meniscus or stump impingement).

Feel:

  • Diffuse tenderness.
  • Lateral joint line tenderness (Segond fracture or meniscal pathology).

Move:

  • Range of motion often limited by pain/effusion.

Special Tests:

TestDescriptionPerformance Statistics
Lachman TestKnee at 20-30°. Stabilize femur, translate tibia anteriorly. Positive: Soft end-point >3mm translation.Sensitivity: 95%
Specificity: 94%
(The Gold Standard Clinical Test)
Pivot ShiftValgus + Internal Rotation + Flexion (0 to 30°). Positive: Tibia reduces with a "clunk".Sensitivity: 30% (awake)
Specificity: 98%
(Pathognomonic for rotational instability)
Anterior DrawerKnee at 90°. Anterior pull.Sensitivity: 50-70%
(Often false negative due to hamstring guarding)
MCL Stress TestValgus stress at 30° flexion. Positive: Medial opening.Assess Concomitant MCL injury (Grade I-III).
LCL Stress TestVarus stress at 30° flexion. Positive: Lateral opening.Assess LCL/Posterolateral Corner (PLC).
Dial TestExternal rotation at 30° and 90°. Positive: Increased ER >10°.Assess PLC/PCL injury.
McMurray TestFlexion/Rotation/Extension. Positive: Click/Pain.Assess Meniscal Tears.

Grading of Instability (Lachman)

The degree of anterior translation is graded compared to the normal side:

  • Grade 1: 3-5 mm translation. Solid end point. (Partial tear).
  • Grade 2: 5-10 mm translation. Soft end point. (Complete tear).
  • Grade 3: >10 mm translation. Soft end point. (Complete tear + Secondary restraints failed).

Examination Pitfalls

  • The "Sag Sign" (PCL): If the PCL is torn, the tibia sags posteriorly. If you then pull it forward to neutral, it feels like a lot of anterior movement, mimicking an ACL tear ("Pseudo-Lachman"). Always check PCL first!
  • Hamstring Spasm: In acute pain, hamstrings fire nervously, pulling the tibia back and masking the instability. This is why the Anterior Drawer (90°) is often false negative.
  • Locked Knee: If the meniscus is bucket-handled, the knee cannot extend enough to perform a Lachman properly.

Red Flags (Differential Diagnosis)

[!IMPORTANT] Exclude these emergenices:

  • Knee Dislocation: Gross instability in multiple planes (ACL+PCL+MCL/LCL). Risk of Popliteal Artery injury. Check pulses!
  • Extensor Mechanism Disruption: Inability to straight leg raise (Quad/Patella tendon rupture).
  • Locked Knee: Fixed flexion deformity. Requires urgent arthroscopy to unlock bucket-handle meniscus.

Differential Diagnosis Matrix

Distinguishing the "Big 5" knee injuries:

ConditionMechanismPain SiteSwellingKey Test
ACL RupturePivot/DecelerationDeep/DiffuseImmediate (Haemarthrosis)Lachman
PCL RuptureDashboard impact/Fall on flexed kneePosteriorMinimal/DelayedSag Sign / Post. Drawer
MCL SprainValgus blow (Rugby tackle)Medial Joint LineLocalized (No effusion)Valgus Stress (30°)
Meniscus TearTwist/Deep SquatJoint Line (Med/Lat)Delayed (Overnight)McMurray / Thessaly
Patella DislocationTwist/ValgusMedial RetinaculumImmediatePatellar Apprehension
6. Investigations

Who Needs an X-Ray? (The Ottawa Knee Rules)

To reduce radiation and cost, use these criteria. X-ray is required only if one of the following is present:

  1. Age > 55 years.
  2. Tenderness at the Head of Fibula (lateral).
  3. Isolated tenderness of the Patella (no bony tenderness elsewhere).
  4. Inability to flex to 90 degrees.
  5. Inability to weight bear 4 steps (immediately and in ED). Sensitivity: 98.5% for fracture.

Diagnostic Imaging Matrix

Imaging is essential not just for diagnosis, but for surgical planning.

ModalitySignSensitivitySignificance
X-RaySegond FractureLow (10%)Avulsion of the anterolateral capsule. Pathognomonic (100% Specificity) for ACL rupture.
X-RayDeep Notch SignLowDepression on lateral femoral condyle >2mm. Indicates violent pivot shift impaction.
X-RayTibial Spine AvulsionModerateBony avulsion of ACL insertion. Seen in children. Treated as a fracture.
MRIFiber DiscontinuityHigh (95%)The primary diagnostic sign. Edema and disruption of fibers.
MRIEmpty Notch SignHighFluid fills the space against the lateral condyle where the ACL should be.
MRIKissing ContusionsHigh (>90%)Bone edema on Lateral Femoral Condyle and Posterolateral Tibia. Confirms the pivot shift mechanism.
MRIPCL BucklingModerateBecause the tibia is anteriorly translated, the PCL becomes lax and buckled.
MRIAnterior TranslationModerateTibia sits >7mm anterior to femur on sagittal view ("The Anterior Drawer" on MRI).

MRI Protocols

  • Role: Gold Standard for confirmation and assessment of concomitant injuries (meniscus, cartilage).
  • Sequences:
    • Sagittal T2 FS: Best for visualizing the ACL fibers and fluid.
    • Coronal T1/T2: Best for MCL/LCL and Meniscal roots.
    • Axial: Best for checking ACL footprint and patellofemoral joint.

Associated MRI Findings

  • Meniscal Tears: present in ~60-70%.
    • Medial: Often posterior horn. "Ramp Lesions" (Meniscocapsular separation) are hidden and require arthroscopic probing.
    • Lateral: Often vertical tears. "Root Tears" (avulsion of attachment) are critical to identify as they lead to rapid arthritis.
  • Bone Contusions:
    • Pattern: Lateral femoral condyle (sulcus terminalis) and Posterolateral tibial plateau.
    • Mechanism: The "Pivot Shift" imprint.
    • Prognostic Value: Extensive bruising correlates with higher post-op pain and slower recovery.

Examination Under Anaesthesia (EUA)

  • Often performed immediately prior to surgery to confirm grade of pivot shift, which dictates the need for extra-articular augmentation (LET).
7. Management

Management Algorithm

                 ACUTE KNEE INJURY (Hemarthrosis)
                              ↓
              EXAMINATION + X-RAY + MRI CONFIRMATION
                              ↓
             ┌────────────────┴───────────────────┐
             ↓                                    ↓
       LOCKED KNEE?                         MOBILE KNEE
 (Bucket Handle Meniscus)                         ↓
             ↓                        INITIAL MANAGEMENT
      URGENT SURGERY                  • RICE protocol
      (Repair + ACLR)                 • "Prehab" (Regain ROM)
                                      • Assess Goals/Demands
                                              ↓
                        ┌─────────────────────┴───────────────────────┐
                        ↓                                             ↓
             CONSERVATIVE ("Copers")                      SURGICAL ("Non-Copers")
    (Low demand, No instability, Arthritis)        (Athletes, Instability, Repairable Meniscus)
                        ↓                                             ↓
             STRUCTURED REHABILITATION                    ACL RECONSTRUCTION
             (Strength, Proprioception)                 (Autograft: Hamstring/BPTB/Quad)
                        ↓                                             ↓
             SUCCESS? ───────(NO)─────────→ FAIL          POST-OP REHABILITATION
                ↓                          (Surgery)          (9-12 Months)
         RETURN TO ACTIVITY

Management Decisions

1. Conservative Management ("Rehabilitation First")

  • Rationale: Not all ACLs need surgery. Some patients ("Copers") can stabilize the knee dynamically with neuromuscular adaptation.
  • Evidence: The KANON Trial showed that roughly 50% of patients randomized to rehab alone did not need surgery at 5 years and had outcomes comparable to the early surgery group. [3]
  • Candidates:
    • Older/Sedentary patients.
    • Patients willing to modify activity (avoid pivot sports).
    • Isolated ACL injury (no repairable meniscus tear).
    • Normal joint laxity (no hyperextension).
  • "Prehabilitation" (Pre-Hab):
    • Critical phase prior to final decision or surgery.
    • Goals: Resolve effusion, Regain full extension (critical), Normalize gait.
    • Exercises: Quadriceps sets, straight leg raises, cryotherapy.
    • Success Criteria: If a patient can cope with ADLs and straight-line running after 3 months of rehab, they may choose to remain conservative.

2. Surgical Reconstruction (ACLR)

  • Rationale: To restore mechanical stability, prevent recurrent giving way, and protect the menisci from secondary tearing.
  • Candidates:
    • High-demand athletes (Category 1 pivoting sports).
    • Patients with symptomatic instability ("giving way" with ADLs).
    • Concomitant repairable meniscus tears (to protect the repair).
    • Multi-ligament injuries.
  • Timing: Ideally delayed 4-6 weeks to allow inflammation to settle and full Range of Motion to return. Operating on a stiff, swollen knee ("angry knee") carries a high risk of permanent Arthrofibrosis.

Graft Choices: The Great Debate

GraftProsConsIdeal For
Bone-Patellar Tendon-Bone (BPTB)• Gold standard healing (bone-to-bone)
• Very low failure rate		• Anterior knee pain (kneeling pain)
• Risk of patellar fracture		• High-impact contact athletes (Rugby/Football)
Hamstring (HT)• Lower donor site morbidity
• Strong (quadrupled)		• Slower bone-tendon healing
• Hamstring weakness (flexion)		• Soccer players
• General active population
Quadriceps Tendon• Thicker graft
• Predictable size		• Technically more demanding
• Quad inhibition		• Revision surgery
• Hypermobility
Allograft (Cadaver)• No donor morbidity
• Easier recovery		• Higher failure rate in young patients (<25y)
• Slow incorporation		• Older patients (>40y)
• Multi-ligament cases

Anesthesia and Pain Management

  • Type: General Anesthesia (GA) is preferred to ensure complete muscle relaxation for pivot shift testing.
  • Regional Block:
    • Adductor Canal Block: Gold standard. Blocks sensory (saphenous) nerve but spares motor (quadriceps). Allows early mobilization.
    • Femoral Nerve Block: Avoid! Causes quad paralysis and delays rehab milestones.
  • Tourniquet: Used to visualize the joint, but prolonged time (>90 mins) increases DVT risk and pain.

Technical Aspects: The Surgical Steps

Anatomic Single-Bundle ACL Reconstruction (Hamstring Autograft) usually involves:

  1. Preparation: Examination Under Anaesthesia (EUA) to confirm pivot shift. Tourniquet applied.
  2. Arthroscopy: Diagnostic round to address meniscal pathology (repair/resect).
  3. Harvest: Semitendinosus and Gracilis tendons harvested via anteromedial incision. Whipped with sutures to create a 4-strand loop.
  4. Femoral Tunnel:
    • Drilled through an accessory anteromedial portal (Independent drilling).
    • Target: Center of the anatomical footprint on the lateral femoral condyle.
    • Risk: "High-Noon" vertical tunnels (classic error) fail to control rotation.
  5. Tibial Tunnel:
    • Drilled using a jig set to 55°.
    • Target: Center of tibial footprint (posterior to anterior horn of lateral meniscus).
  6. Fixation:
    • Femur: Suspensory button (flips on cortex).
    • Tibia: Interference screw (squeezes graft) or Staple.
  7. Cycling: Knee cycled to remove creep before final tensioning.

Graft Harvest Morbidity

"No free lunch" - taking a graft has consequences:

  • Hamstring:
    • Weakness: Deep flexion weakness (heel to buttock) lasts 1-2 years.
    • Nerve Injury: The Saphenous Nerve (infrapatellar branch) crosses the harvest site. 30-50% of patients have numbness on the front of the shin.
  • Patellar Tendon (BPTB):
    • Anterior Knee Pain: 20-30% have pain on kneeling ("Gardener's Knee").
    • Fracture: Risk of patellar fracture (rare).
  • Quadriceps:
    • Cosmetic: Scar is visible on thigh.
    • Strength: Quad inhibition acute post-op.

The Role of Bracing (Myth vs Reality)

Patients often ask for a brace.

  1. Functional Bracing (Post-Op):
    • Theory: Protects graft during remodeling.
    • Evidence: High quality RCTs (Wright et al.) show no difference in re-rupture rates, pain, or function.
    • Downside: Can lead to "sensorimotor reliance" (knee feels "naked" without it) and quadriceps atrophy due to cuff pressure.
  2. Prophylactic Bracing:
    • Theory: Wearing a rigid brace to prevent primary injury (common in American Football linemen).
    • Evidence: Conflicting. May reduce MCL injuries (valgus constraint) but does little for rotational ACL forces.
    • Recommendation: Generally not recommended for ACL prevention. Focus on neuromuscular training instead.

Lateral Extra-articular Tenodesis (LET)

  • Concept: Adding a reinforcement on the lateral side (like a seatbelt) to control rotational instability (Pivot shift).
  • Indication: High-grade pivot shift, young patients (<20y), revision surgery, hyperlaxity.
  • Evidence: The STABILITY Study showed LET + ACLR reduces re-rupture rates by 60% in high-risk youth. [5]

Management of Associated Injuries

ACLR is rarely an isolated procedure. ~70% of cases involve other structures:

1. Meniscal Tears

  • Repair: Gold standard for peripheral (Red-Red zone) tears.
    • Technique: "All-Inside" suturing or "Inside-Out" needles.
    • Rehab changes: Restricted flexion beyond 90° for 6 weeks to protect repair.
  • Resection (Meniscectomy): For complex/degenerative tears (White-White zone).
    • Pro: Faster rehab (immediate WB).
    • Con: Guaranteed arthritis later (loss of hoop stresses).

2. MCL Injury

  • Grade I/II: Managed conservatively (brace in extension). ACLR delayed 6 weeks to allow MCL to stiffen.
  • Grade III: May require repair/reconstruction using hamstring/allograft ("internal brace").

3. Chondral Defects

  • Microfracture: Drilling holes to stimulate fibrocartilage. (NWB 6 weeks).
  • OATS: Plug transfer. Requires biological protection.

Special Population: Pediatric ACL

Management in skeletally immature patients (open physes) is complex:

  • Risk: Drilling across the growth plate (physis) can cause growth arrest and deformity (leg length discrepancy or angular deformity).
  • Options:
    • Physeal Sparing: Iliotibial band reconstruction (over the top). No tunnel drilling.
    • Trans-physeal: Standard tunnels but drilled small and central, avoiding screw fixation across the physis.
  • Conservative?: Poor outcomes. Children with ACL deficiency almost invariably tear their menisci if they return to play without stability. Early stabilization is usually favoured.

Rehabilitation Protocol

Rehabilitation is arguably more important than the surgery itself. It is criterion-based, not time-based. The graft is weakest at 6-12 weeks ("Ligamentization" phase).

Phase 1: Acute Recovery (0-2 Weeks)

  • Primary Goal: Full extension (hyperextension to match contralateral side) and resolving effusion.
  • Why Extension Matters: If full extension is not achieved early, the notch fills with scar tissue (cyclops lesion), leading to permanent flexion contracture.
  • Weight Bearing: Weight bear as tolerated (WBAT) with crutches.
  • Exercises:
    • Static Quads: Isometric contraction.
    • Heel Props/Prone Hangs: Letting gravity extend the knee.
    • Patellar Mobilization: Ensuring the knee cap glides freely.
    • Ankle Pumps: DVT prophylaxis.
  • Restrictions: No open-chain knee extension (e.g., leg extension machine) to protect the graft from shear forces.

Phase 2: Strength & Neuromuscular Control (2-12 Weeks)

  • Entry Criteria: No lag on straight leg raise, full extension, minimal effusion.
  • Functional Goals: Normal gait pattern, single-leg stance.
  • Exercises:
    • Closed Kinetic Chain (CKC): Squats, Lunges, Leg Pres (0-60°).
    • Proprioception: Single-leg balance, wobble board.
    • Hamstrings: Curls (avoid if hamstring graft until week 6).
  • Biology: The graft is undergoing necrosis and revascularization. It is mechanically weak. Avoid impact.

Phase 3: Running & Agility (3-6 Months)

  • Entry Criteria: Limb Symmetry Index (LSI) >70% for quad strength.
  • Activities:
    • Straight line jogging (treadmill -> grass).
    • Plyometrics: Double leg jumps -> Single leg lands.
    • Deceleration drills.
  • Focus: Landing mechanics. Avoiding "dynamic valgus" (knee collapsing inwards).

Phase 4: Sport Specificity (6-9 Months)

  • Activities: Cutting, weaving, sport-specific drills (e.g., ball work).
  • Contact: Progressive introduction of perturbation training.

Signature Rehabilitation Exercises

  1. Nordic Hamstring Curl:
    • Execution: Partner holds ankles, patient lowers chest to floor slowly.
    • Mechanism: Eccentric hamstring strengthening.
    • Significance: Hamstrings are the dynamic antagonist to the ACL (preventing anterior translation).
  2. Single Leg Squat:
    • Execution: Squatting on one leg to 45 degrees.
    • Watch for: Dynamic Valgus (knee caving in). This indicates weak hip abductors (Gluteus Medius).
    • Correction: "External Cueing" (e.g., "Keep your knee over your toe").
  3. Drop Jumps:
    • Execution: Step off box, land, and jump immediately.
    • Goal: Train "stiff" landings and shorter ground contact times.

Rehabilitation Prescription Matrix

ExercisePhaseDosageRationale
Quad Sets1 (Acute)10 reps x 10 sets/dayPrevent atrophy. Reset neural drive.
Prone Hangs1 (Acute)5 mins x 3/dayGravity-assisted extension.
Wall Slides2 (Strength)3 x 10 repsSafe closed-chain eccentric control.
Spanish Squats2 (Strength)3 x 45s ISOIsometric load for tendon health.
Drop Landings3 (Agility)3 x 6 repsTrain landing mechanics (valgus control).
Nordic Curls4 (Sport)3 x 5 repsMax eccentric hamstring strength.

Return to Sport Criteria (The "Exit Test")

To clear for unrestricted play (usually >9 months), a battery of tests involves:

  1. Isokinetic Dynamometry:
    • Gold Standard for objective strength measurement.
    • Goal: Limb Symmetry Index (LSI) >90% for Quadriceps and Hamstrings at 60°/sec.
    • H:Q Ratio: Hamstring to Quad ratio should be >60%.
  2. Functional Hop Tests:
    • Single Hop for distance.
    • Triple Hop for distance.
    • Crossover Hop for distance.
    • Goal: LSI >90% on all tests.
  3. Agility Testing:
    • T-Test: Sprint 10m -> Shuffle 5m R -> Shuffle 10m L -> Shuffle 5m R -> Backpedal.
    • Goal: Time within 10% of pre-injury or healthy peers.
  4. Balance:
    • Star Excursion Balance Test (SEBT): Reach distance on Anterior, Posteromedial, Posterolateral axes.
    • Goal: Composite score >94%.
  5. Psychological:
    • ACL-RSI: Score >60 required.
  6. Time:
    • Minimum 9 months. Biology cannot be rushed. Risk reduces by 51% for each month delayed until 9 months.

Psychological Readiness

Fear of re-injury (kinesiophobia) is a major barrier. The ACL-RSI scale helps identify patients who are physically ready but mentally hesitant, as they are at higher risk of second injury due to hesitant mechanics.

8. Complications

Early Complications (0-6 Weeks)

  • Septic Arthritis (<1%):
    • Presentation: Increasing pain, redness, fever >day 3.
    • Urgency: Medical emergency. Requires arthroscopic washout.
    • Graft Retention: Graft can often be saved if treated early with washout and antibiotics.
  • Deep Vein Thrombosis (DVT):
    • Risk: Increased with tourniquet use and immobility.
    • Prophylaxis: Early mobilization and compression stockings. Chemical prophylaxis if high risk.
  • Arthrofibrosis ("Cyclops Lesion"):
    • Pathology: A focal nodule of fibrous tissue forms anterior to the graft in the intercondylar notch.
    • Symptoms: Painful "clunk" at terminal extension; inability to straighten leg.
    • Prevention: Aggressive early extension pre-hab and post-op.

Late Complications (>6 Months)

  • Graft Failure (Re-rupture):
    • Incidence: 5-10% overall; up to 30% in high-risk adolescents.
    • Causes:
      1. Technical: Tunnel malposition (too vertical = rotational failure).
      2. Biological: Failure of ligamentization.
      3. Traumatic: Early return to sport before graft maturity.
  • Contralateral ACL Rupture:
    • 5-10% risk. Driven by same genetic/neuromuscular factors that caused the first.
  • Anterior Knee Pain:
    • Common with BPTB grafts (donor site morbidity).
    • Difficulty kneeling.
  • Osteoarthritis (OA):
    • Glum Statistic: 50% of ACL-injured knees have symptomatic OA at 20 years.
    • Role of Surgery: Reconstruction stabilizes the knee but does not reliably prevent OA. The initial "bone bruise" and chondral impact set the biological clock for degeneration. [1,4]
9. Prognosis & Outcomes

Natural History (Untreated)

  • Instability: Persistent pivoting episodes manifest as a chronic "trick knee".
  • Meniscal Damage: Each instability episode risks tearing the medial meniscus (secondary stabilizer).
  • Osteoarthritis: High risk due to altered kinematics.

The Psychological Toll

An ACL rupture is not just a musculoskeletal injury; it is a neurocognitive and psychological trauma.

  • Loss of Identity: For elite athletes, the injury threatens their self-worth and career.
  • Kinesiophobia: Fear of movement re-injury is the single biggest predictor of failure to return to sport.
  • ACL-RSI Scale: A validated outcome measure checking psychological readiness. Patients who score <60/100 are at high risk of second injury if they return to sport, as they "movement guard" (stiff landings).
  • Post-Traumatic Symptoms: Rates of depression and anxiety are significantly elevated in the first 3 months post-injury.

Surgical Outcomes

  • Return to Sport:
    • 81% return to any sport.
    • Only 65% return to their pre-injury level of competition.
    • 55% return to competitive level sport. [Ardern et al. BJSM]
  • Re-rupture Rate:
    • Overall: 5-10%.
    • Under 20s: Up to 20-30%.
  • Osteoarthritis: Surgery does not eliminate OA risk.

Factors Affecting Prognosis

FactorPositive InfluenceNegative Influence
AgeAdult (>25y)Adolescent (<20y)
GenderMaleFemale
Concomitant InjuryIsolated ACLMeniscal/Chondral damage
GraftAutograftAllograft (in young pts)
RehabCriterion-based RTSAccelerated/Premature RTS
10. Evidence & Guidelines

Landmark Clinical Trials

1. The KANON Trial (Frobell et al. NEJM 2010)

"Rehabilitation vs Early Reconstruction"

  • Question: Does every acute ACL rupture need surgery?
  • Methods: Randomized 121 young active adults to either:
    1. Early ACL Reconstruction (within 10 weeks).
    2. Rehabilitation with optional "Delayed" Reconstruction if unstable.
  • Results: At 2 and 5 years, there was no difference in KOOS scores (Pain, Symptoms, ADLs, Sport, QoL) between the groups.
  • Key Takeaway: 51% of the rehab group never needed surgery. This supports a "Rehab First" strategy for non-elite athletes.
  • Caveat: High-level pivoting athletes were excluded.

2. The MOON Cohort (Multicenter Orthopaedic Outcomes Network)

"The Cost of Delay"

  • Question: What happens if we wait too long in an unstable knee?
  • Findings: Patients who delayed surgery >6 months had a significantly higher rate of medial meniscus tears (which are harder to repair) and cartilage lesions.
  • Clinical Implication: While "Rehab First" is safe, "Watchful Waiting" with an unstable knee is dangerous. If the knee gives way, fix it.

3. The STABILITY Study (Getgood et al. AJSM 2020)

"Adding the Seatbelt"

  • Question: Does adding a Lateral Extra-articular Tenodesis (LET) reduce failure?
  • Population: High-risk youth (<25y) with hyperlaxity or high-grade pivot shift.
  • Results: Adding LET reduced clinical graft failure from 11% to 4%. The Number Needed to Treat (NNT) was 14.
  • Change in Practice: LET is becoming standard of care for high-risk young patients to protect the graft.

Guidelines

  • NICE (UK): Recommend urgent MRI for acute knee with haemarthrosis. Refer to specialist knee surgeon.
  • AAOS (USA): Strong evidence for neuromuscular training (prevention). Moderate evidence for early reconstruction to protect menisci.

Future Directions

The pendulum is swinging back towards preservation.

  • BEAR Implant (Bridge-Enhanced ACL Repair):
    • A bio-engineered sponge injected with the patient's blood is placed between torn stump ends.
    • Goal: Stimulate natural healing (primary repair) without harvesting a graft.
    • Early Results: Promising for mid-substance tears.
  • Anterolateral Ligament (ALL) Reconstruction:
    • The recognition of the ALL has led to more frequent "Lateral Extra-articular Tenodesis" (LET).
    • Trend: We are moving from "Analogue" (single bundle) to "Digital" (customized stability) surgery.
11. Patient/Layperson Explanation

What has happened to my knee?

The ACL is the main "seatbelt" inside your knee. It stops the shin bone from sliding too far forward. During your injury, the force on the knee was too strong for the ligament, and it snapped. The "pop" you heard was the ligament tearing, and the swelling is bleeding inside the joint.

Do I definitely need surgery?

Not necessarily. Many people can live normal, active lives without an ACL. By strengthening the muscles around the knee (hamstrings and quads), you can often compensate for the loose ligament. This is called "conservative management".

  • Yes: If you are young and want to return to pivoting sports (soccer, netball).
  • No: If you are happy to cycle, swim, and run in straight lines.

What are my graft options?

We use your own tissue ("Autograft") to build the new ligament:

  • Hamstrings: Most common. Two tendons from the inner thigh. Small scars.
  • Patellar Tendon: From the front of the knee. Very strong bone-to-bone healing, but kneeling can be painful afterwards.
  • Quadriceps Tendon: From above the kneecap. A newer, strong option.
  • Allograft (Donor): From a tissue bank. No harvest pain, but higher re-rupture risk in young people.

Can I drive?

Usually after 2-4 weeks, once you can perform an emergency stop without pain. Left leg (automatic car) is sooner.

Will I get arthritis?

This is the tough news. About 50% of people with an ACL injury develop arthritis within 15-20 years. This risk is primarily due to the initial impact damage to the cartilage, not whether you have surgery or not. Surgery stabilizes the knee but doesn't erase the initial "bruise".

How long until I play again?

This is the hard part. The surgery takes 1 hour, but the recovery takes 9 to 12 months. The new ligament takes a long time to "bed in" and become strong. If you go back too early, it will snap again. You have to earn the right to play through months of gym work.

Common Myths Busted

  • Myth: "Buying a titanium brace will prevent my ACL tearing."
    • Fact: Braces do not control rotation well enough to prevent rupture in high-force injuries.
  • Myth: "My graft is stronger than my original ACL at 6 months."
    • Fact: At 6 months, the graft is actually in a remodeling phase and vulnerable.
  • Myth: "Running will wear out my new knee."
    • Fact: Straight-line running is healthy for the cartilage. It is the twisting/pivoting that risks the graft.
12. References

Primary Guidelines

  1. NICE CKS. Knee injury - ligament. 2020. [Link]
  2. American Academy of Orthopaedic Surgeons (AAOS). Management of Anterior Cruciate Ligament Injuries: Evidence-Based Clinical Practice Guideline. 2014. [Link]

Landmark Trials

  1. Frobell RB, Roos EM, Roos HP, Ranstam J, Lohmander LS. A randomized trial of treatment for acute anterior cruciate ligament tears (KANON). N Engl J Med. 2010;363(4):331-342. PMID: 20660401
  2. Lohmander LS, Englund M, Dahl LL, Roos EM. The long-term consequence of anterior cruciate ligament and meniscus injuries: osteoarthritis. Am J Sports Med. 2007;35(10):1756-1769. PMID: 17761605
  3. Getgood AMJ, Bryant DM, Litchfield R, et al. Lateral Extra-articular Tenodesis Added to Anterior Cruciate Ligament Reconstruction in High-Risk Patients (STABILITY). Am J Sports Med. 2020;48(2):289-297. PMID: 31835052

Additional References

  1. Griffith JF, et al. Eight-year prospective study of ACL reconstruction: MRI findings and result correlation. AJR. 2015. PMID: 26172012
  2. Shelbourne KD, Nitz P. Accelerated rehabilitation after anterior cruciate ligament reconstruction. Am J Sports Med. 1990;18(3):292-299. PMID: 2372077
  3. Hewett TE, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes. Am J Sports Med. 2005;33(4):492-501. PMID: 15722287
  4. Claes S, Vereecke E, Maes M, et al. Anatomy of the anterolateral ligament of the knee. J Anat. 2013;223(4):321-328. PMID: 23906341
  5. Sanders TL, et al. Incidence of Anterior Cruciate Ligament Tears and Reconstruction: A 21-Year Population-Based Study. Am J Sports Med. 2016;44(6):1502-1507. PMID: 26920430
  6. Paterno MV. Incidence of Second ACL Injuries 2 Years After Primary ACL Reconstruction and Return to Sport. Sports Health. 2014;6(2):116-123. PMID: 24587861
  7. Grindem H, et al. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. Br J Sports Med. 2016;50(13):804-808. PMID: 27162233
  8. WrightRW, et al. Bracing after ACL Reconstruction: A Systematic Review. J Knee Surg. 2007;20(4):270-281. PMID: 17993071
  9. Murray MM, et al. Bridge-Enhanced Anterior Cruciate Ligament Repair: Two-Year Results of a First-in-Human Study. Orthop J Sports Med. 2019;7(3). PMID: 30923690
  10. Ardern CL, et al. Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review. Br J Sports Med. 2014;48(21):1543-1552. PMID: 25157180
  11. Musahl V, et al. The Pivot Shift: A Global User Guide. Springer. 2012. [Book]
  12. Waldén M, et al. Three distinct mechanisms predominate in non-contact anterior cruciate ligament injuries in male professional football players: a systematic video analysis of 39 cases. Br J Sports Med. 2015;49(22):1452-1460. PMID: 25907183
13. Examination Focus

Common Exam Questions

  1. MRCS/Medical Finals: "What is the most sensitive clinical test for an ACL rupture?"

    • Answer: The Lachman Test (at 30 degrees flexion). Specificity is provided by the Pivot Shift test.

  2. Radiology: "Identify the Segond fracture and explain its significance."

    • Answer: Avulsion of the lateral tibial plateau. Indicates avulsion of the Anterolateral Ligament (ALL) and is pathognomonic for ACL rupture.

  3. Pathology: "What is the Unhappy Triad of O'Donoghue?"

    • Answer: Injury to ACL, MCL, and Medial Meniscus. (Note: Modern evidence suggests Lateral meniscus is more common acutely, but the triad is a classic exam answer).

  4. Surgical Principles: "Why do we delay ACL surgery for 4-6 weeks acutely?"

    • Answer: To allow the inflammatory phase to subside and full range of motion to return, reducing the risk of Arthrofibrosis (Cyclops lesion/Stiffness).

Viva Points

  • Anatomy: Two bundles (AM vs PL). Extra-synovial but intra-articular.
  • Blood Supply: Middle Genicular Artery.
  • Mechanism: Dynamic valgus, pivot shift.
  • Complications: Failure (5-10%), Infection, DVT, Stiffness.
  • Copers vs Non-Copers: The dynamic stabilizers (Hamstrings) can compensate in some patients.

History Taking Station (OSCE)

Instruction: "23-year-old footballer twisted knee. Knee is swollen. Take a history."

Candidate Checklist:

  1. Mechanism: Explicitly asks about contact vs non-contact.
  2. Sequence: Asks if knee collapsed then pain, or pain then collapse.
  3. Audible sound: "Did you hear a pop?"
  4. Ability to continue: "Could you play on?" (Crucial differentiator from minor sprains).
  5. Swelling timing: "Immediate (hours) or delayed (next day)?"
  6. Instability: "Does it feel loose or give way now?"
  7. Past History: Previous knee injuries? Hypermobility?
  8. ICE: "What are your goals? What level of sport do you play?"

Examination Station (OSCE)

Instruction: "Examine this knee."

Candidate Checklist:

  1. Look: Comments on effusion and muscle bulk (VMO wasting).
  2. Feel: Joint line tenderness (Meniscus).
  3. Move: Assess active/passive ROM. Detection of Lack of Extension.
  4. Lachman: Performs correctly (30 deg, stabilizing femur).
  5. Anterior Drawer: Performs at 90 deg (commenting on hamstring spasm).
  6. Pivot Shift: Mentions it (but may not perform if conscious).
  7. Collaterals: Tests MCL and LCL at 30 deg flexion.
  8. Synthesis: "This patient has signs consistent with an ACL rupture. I would complete my exam by checking neurovascular status and examining the hips."

Last updated: 2025-12-26

At a Glance

EvidenceHigh
Last Updated2025-12-26

Red Flags

  • Locked Knee (Bucket Handle Meniscus)
  • Multi-ligament injury (Knee Dislocation)
  • Neurovascular compromise (popliteal artery)
  • Septic Arthritis (if post-op)

Clinical Pearls

  • **The "Pop" and "Swell" Rule**:
  • **The Muscle Guarding Pitfall**:
  • Males (4.5x)** | The primary epidemiological disparity. |
  • **Specificity**: 94%<br /
  • *(The Gold Standard Clinical Test)* |

Guidelines

  • NICE Guidelines
  • BTS Guidelines
  • RCUK Guidelines