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ACL (Anterior Cruciate Ligament) — Clinical Guide

Anatomy

The ACL consists of two fiber bundles named by their tibial insertions:

Bundle	Length	Taut in
Anteromedial (AM)	~38 mm	Flexion
Posterolateral (PL)	~20 mm	Extension

The femoral origin lies in the posterior lateral intercondylar fossa. The AM bundle's origin borders superiorly on the intercondylar line; the PL bundle courses along the bone-cartilage border. Together, the two bundles provide anterior stability and rotational stability through the full arc of knee motion.

ACL bundle anatomy — AM and PL bundles at medial tibial plateau

ACL anatomy showing AM and PL bundles and their femoral/tibial footprints — General Anatomy and Musculoskeletal System (THIEME Atlas)

Mechanism of Injury

Usually noncontact: deceleration, hyperextension, or marked internal rotation of the tibia on the femur
Classic presentation: audible "pop" (considered pathognomonic), rapid hemarthrosis (swelling within hours), and a sense of instability
The combination of this mechanism + traumatic effusion is highly suggestive of ACL disruption

Diagnosis

Clinical Tests

Test	Sensitivity	Specificity	Technique
Lachman test (most sensitive)	81%	81%	Knee at 30° flexion; stabilize femur, anteriorly translate tibia at the tibial tubercle level. Positive = anterior displacement or soft end point
Anterior drawer test	38%	81%	Hip 45° flexion, knee 90° flexion; pull tibia anteriorly. Positive = >6 mm displacement vs. contralateral knee
Pivot shift test	28%	81%	Knee in full extension; apply valgus + internal rotation, then flex. Positive = sudden audible/palpable clunk of tibial reduction at ~20–30° flexion

Lachman test — 30-degree flexion, anterior tibial translation

Lachman test — Tintinalli's Emergency Medicine

Anterior drawer test — 90-degree knee flexion

Anterior drawer test — Tintinalli's Emergency Medicine

Pivot shift test — valgus and internal rotation, clunk at 20–30°

Pivot shift test — Tintinalli's Emergency Medicine

Pearls:

Anterior drawer can give a false negative (hamstring guarding) or false positive (PCL tear → tibia starts posteriorly subluxed, giving pseudo-anterior displacement)
Pivot shift is best assessed under anesthesia in the OR
Lachman is preferred in the ED; it can identify partial tears

Imaging

Plain radiographs: usually normal; may show a Segond fracture (avulsion of the lateral tibial plateau — a specific marker for ACL tear) or a tibial spine avulsion (pediatric equivalent)
MRI: gold standard for confirmation; demonstrates ACL fiber discontinuity, altered signal, bone bruising pattern (lateral femoral condyle + posterolateral tibial plateau)

Management

Acute / ED Management

Ice, compression, elevation; non-weight-bearing with crutches
Analgesics / NSAIDs
Knee immobilizer for comfort
Urgent orthopedic referral — especially if combined ligament injury, locked knee, or neurovascular compromise

Definitive Treatment

Patient Group	Approach
Active/young patients, pivoting sports	ACL reconstruction (patellar tendon or hamstring autograft preferred)
Sedentary/elderly or isolated partial tears	Conservative: physiotherapy, bracing, quadriceps strengthening
Combined instability (PCL, MCL, posterolateral corner)	Surgical reconstruction, timing individualized

Surgical note: Replacement grafts face a major challenge — no human-made material fully replicates the ACL's mechanical properties. Reconstruction options include bone-patellar tendon-bone (BTB), hamstring tendon, and allograft.

Associated Injuries (the "Unhappy Triad")

Valgus force + ACL injury commonly involves:

Medial collateral ligament (MCL) tear
Medial meniscus tear

Also watch for peroneal nerve injury with lateral-sided instability.

Key Differentials

PCL tear: posterior drawer positive, sag sign; mechanism is anterior-to-posterior force on tibia (e.g., dashboard injury)
Meniscal tear: may coexist; McMurray/Thessaly tests positive
Tibial spine avulsion: radiographic equivalent of ACL rupture in children/adolescents

Sources: Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 9e | General Anatomy and Musculoskeletal System, THIEME Atlas of Anatomy

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