ACL vs PCL vs Meniscus Tear: How to Tell from Physical Exam
Joint Biomechanics · 8 min read · 2026-02-14
Introduction
Knee injuries are among the most common presentations in sports medicine. The anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and menisci each produce characteristic injury patterns and examination findings. Understanding these patterns allows confident clinical differentiation before MRI.
ACL Tear
Mechanism: Non-contact pivoting, cutting, or landing from a jump. The patient often hears a "pop."
Presentation: Rapid effusion (within 2–4 hours — hemarthrosis). Immediate inability to continue activity. Feeling of the knee "giving way."
Key tests:
- Lachman's test: Gold standard. Anterior translation of the tibia on the femur at 20–30° flexion. A soft or absent endpoint indicates a tear.
- Anterior drawer: Less sensitive than Lachman's but still useful. At 90° flexion.
- Pivot shift: Reproduces the subluxation event. Highly specific but difficult to perform in an acute, guarded knee.
Why Lachman's is better than anterior drawer: At 90° flexion, the hamstrings and posterior horn of the meniscus can mask anterior laxity. At 20–30°, these restraints are relaxed.
PCL Tear
Mechanism: Dashboard injury (tibia driven posteriorly on a flexed knee), or a fall onto a flexed knee with the foot plantarflexed.
Presentation: Less dramatic than ACL tears. The knee may feel "not quite right" rather than unstable. Effusion develops more slowly.
Key tests:
- Posterior drawer test: The tibial plateau sags posteriorly. Apply a posterior force at 90° flexion — increased posterior translation indicates PCL injury.
- Posterior sag sign: With both hips and knees at 90°, observe from the side. The affected tibia sags posteriorly compared to the other side.
- Quadriceps active test: With the knee at 90°, ask the patient to contract the quadriceps. The tibia translates anteriorly (reduces from its posteriorly subluxed position).
Critical pitfall: A PCL tear can make the anterior drawer test falsely positive. The tibia starts posteriorly subluxed, so pulling it forward to neutral mimics anterior translation.
Meniscal Tear
Mechanism: Twisting injury on a loaded, flexed knee. Can be traumatic (young athletes) or degenerative (older adults).
Presentation: Delayed effusion (6–24 hours). Joint line tenderness. Mechanical symptoms — locking, catching, clicking. Pain with deep squatting.
Key tests:
- McMurray's test: Rotation of the tibia with valgus/varus stress during knee flexion and extension. A palpable click with pain at the joint line is positive.
- Apley's compression test: Prone, knee at 90°. Compression with rotation reproduces pain (meniscal). Distraction with rotation reproduces pain (ligamentous).
- Thessaly test: Standing on the affected leg at 20° flexion, rotating the body. Reproduces mechanical symptoms.
Quick Comparison
Clinical Pearl
Always examine the uninjured knee first to establish a baseline for laxity. Some patients have naturally lax ligaments, and what feels like a "positive" Lachman's might be their normal. Side-to-side comparison is essential.
Summary
Pop + rapid swelling + giving way = ACL. Dashboard mechanism + posterior sag = PCL. Delayed swelling + locking + joint line tenderness = meniscus. Three injury patterns, three examination sequences.