Knee Ligaments: ACL, PCL, MCL, LCL Injuries and Examination Tests
A clinical anatomy reference for the four main knee ligaments — anatomy, function, injury mechanisms, and the specific physical examination tests (Lachman, anterior drawer, posterior drawer, valgus/varus stress) used to diagnose each.
Learning Objectives
- ✓Identify the four primary knee ligaments and their attachment sites.
- ✓Recognize the mechanism of injury and clinical presentation for each.
- ✓Apply specific physical examination tests with appropriate technique.
1. Direct Answer: The Four Primary Knee Ligaments
The knee has four primary ligaments stabilizing it: ANTERIOR CRUCIATE LIGAMENT (ACL) — runs from anterior tibial intercondylar area to posteromedial aspect of lateral femoral condyle; prevents anterior translation of the tibia on the femur. POSTERIOR CRUCIATE LIGAMENT (PCL) — runs from posterior tibial intercondylar area to anterolateral aspect of medial femoral condyle; prevents posterior translation of the tibia. MEDIAL COLLATERAL LIGAMENT (MCL) — runs from medial epicondyle of femur to medial tibial condyle; resists valgus (knee inward) stress. LATERAL COLLATERAL LIGAMENT (LCL) — runs from lateral epicondyle of femur to fibular head; resists varus (knee outward) stress. The ACL is the most commonly injured, especially in non-contact pivot injuries during sports.
Key Points
- •Cruciates: ACL (anterior translation), PCL (posterior translation).
- •Collaterals: MCL (valgus), LCL (varus).
- •ACL is the most commonly torn ligament.
- •Cruciates are intra-articular; collaterals are extra-articular.
2. ACL: Anatomy and Injury
The ACL is intra-articular and originates from the anterior intercondylar area of the tibia, running superiorly and posterolaterally to insert on the posteromedial aspect of the lateral femoral condyle. It has two bundles — anteromedial (taut in flexion) and posterolateral (taut in extension). Its primary function is preventing anterior translation of the tibia on the femur. Injury mechanism: non-contact pivot with the foot planted (basketball, soccer, skiing) — sudden deceleration with rotation tears the ligament. Patient hears an audible pop, knee swells within 2-4 hours (hemarthrosis from bleeding), and feels instability with subsequent pivoting. Sport-specific ACL tear rates are 4-6x higher in female athletes due to differences in pelvic anatomy, neuromuscular control, and hormonal factors.
Key Points
- •Origin: anterior intercondylar area of tibia. Insertion: posteromedial lateral femoral condyle.
- •Prevents anterior translation of tibia.
- •Non-contact pivot mechanism most common.
- •Audible pop + immediate swelling (hemarthrosis) is classic.
- •Female athletes have 4-6x higher tear rates than males.
3. ACL Examination Tests
LACHMAN TEST (most sensitive, 80-90% sensitivity): knee at 20-30° flexion, examiner stabilizes femur with one hand and pulls tibia anteriorly with the other. Anterior translation greater than the unaffected side with soft endpoint indicates ACL tear. ANTERIOR DRAWER TEST (less sensitive than Lachman due to hamstring guarding): knee at 90° flexion, examiner pulls tibia anteriorly. Anterior translation indicates ACL tear. PIVOT SHIFT TEST: combines axial load, valgus, and internal rotation while flexing the knee — reproduces the giving-way sensation in ACL-deficient knees. MRI confirms diagnosis and grades severity. The Lachman is performed FIRST because it has the highest sensitivity and is less affected by hamstring guarding than the anterior drawer.
Key Points
- •Lachman test (20-30° flexion) is most sensitive.
- •Anterior drawer (90° flexion) is less sensitive due to hamstring guarding.
- •Pivot shift reproduces the giving-way sensation.
- •MRI for confirmation and severity grading.
4. PCL: Anatomy and Injury
The PCL is also intra-articular, originating from the posterior intercondylar area of the tibia and running superiorly and anteromedially to insert on the anterolateral aspect of the medial femoral condyle. Its primary function is preventing posterior translation of the tibia on the femur. PCL injuries are less common than ACL injuries and typically result from a direct posterior blow to a flexed knee — "dashboard injury" in motor vehicle accidents being the classic example, or falls onto a flexed knee. Examination: POSTERIOR DRAWER TEST (most sensitive): knee at 90° flexion, examiner pushes tibia posteriorly. Posterior translation indicates PCL tear. POSTERIOR SAG SIGN: with patient supine, hips and knees flexed 90°, the tibia sags posteriorly on the affected side compared to the unaffected side. PCL tears often heal with conservative management; surgical reconstruction is reserved for symptomatic Grade III tears or multi-ligament injuries.
Key Points
- •Origin: posterior intercondylar area of tibia. Insertion: anterolateral medial femoral condyle.
- •Prevents posterior translation of tibia.
- •Dashboard injury or fall on flexed knee is classic.
- •Posterior drawer test most sensitive; posterior sag sign characteristic.
- •Often managed conservatively; reconstruction for symptomatic Grade III.
5. MCL and LCL: Collateral Ligament Injuries
MCL: medial epicondyle of femur to medial tibial condyle. Injury from valgus stress (lateral blow to the knee — common in football clipping injuries). Examination: VALGUS STRESS TEST at 0° (full extension) and 30° flexion — laxity at 30° but not at 0° suggests isolated MCL injury; laxity at both positions suggests combined MCL and PCL/ACL injury. MCL injuries usually heal with bracing and rehabilitation; surgery is rare. LCL: lateral epicondyle of femur to fibular head. Injury from varus stress (medial blow) — less common than MCL injuries because of body mechanics. Examination: VARUS STRESS TEST at 0° and 30° flexion. LCL injuries are often associated with posterolateral corner injuries that require complex reconstruction. Grading: Grade I (sprain without laxity), Grade II (partial tear with laxity), Grade III (complete tear with significant laxity).
Key Points
- •MCL: valgus stress injury; valgus stress test at 0° and 30°.
- •LCL: varus stress injury; varus stress test at 0° and 30°.
- •Laxity only at 30° = isolated collateral injury.
- •Laxity at 0° AND 30° = combined collateral + cruciate injury.
- •MCL usually heals conservatively; LCL often requires evaluation for posterolateral corner.
6. Combined Injuries and the Unhappy Triad
Combined ligament injuries are common in high-energy sports trauma. The classic "unhappy triad" (O'Donoghue's triad) consists of ACL, MCL, and medial meniscus injuries from a valgus force applied to a flexed, externally rotated knee — typically a football clipping injury. Modern imaging shows the lateral meniscus is actually more commonly involved than the medial meniscus in many cases, leading some to use "unhappy triad" with the lateral meniscus. Multi-ligament knee injuries are surgical emergencies — they often involve neurovascular injury (popliteal artery, peroneal nerve) and require urgent vascular evaluation. Treatment is staged reconstruction over multiple operations.
Key Points
- •O'Donoghue's "unhappy triad": ACL + MCL + meniscus.
- •Multi-ligament injuries are surgical emergencies due to neurovascular risk.
- •Popliteal artery and peroneal nerve at risk in knee dislocations.
7. Using AnatomyIQ for Knee Ligament Practice
Snap a photo of any clinical vignette or imaging study and AnatomyIQ identifies the suspected ligament, walks through the relevant physical examination tests with technique reminders, and predicts the most likely tear pattern based on the mechanism described. Practice cases come in three difficulty levels with anatomical diagrams. This content is for educational purposes only and does not constitute medical advice.
Key Points
- •Vignette to ligament mapping by mechanism.
- •Physical exam test recommendations.
- •Anatomical diagrams illustrating tear patterns.
High-Yield Facts
- ★ACL: prevents anterior tibial translation; Lachman test most sensitive.
- ★PCL: prevents posterior tibial translation; posterior drawer and sag sign.
- ★MCL: resists valgus; valgus stress test at 0° and 30°.
- ★LCL: resists varus; varus stress test at 0° and 30°.
- ★Unhappy triad: ACL + MCL + meniscus from valgus force on flexed externally rotated knee.
- ★Multi-ligament knee injuries → check popliteal artery and peroneal nerve urgently.
Practice Questions
1. A 22-year-old basketball player pivots, hears a pop, and develops immediate knee swelling. Most likely diagnosis and best test?
2. A driver in an MVA hit their knees on the dashboard. Knee exam shows posterior tibial sag at 90° flexion. Diagnosis?
3. A football player is hit on the lateral side of the knee with the foot planted. Valgus stress test shows laxity at 30° flexion but not at 0°. Diagnosis?
FAQs
Common questions about this topic
The Lachman is performed at 20-30° knee flexion, where the hamstrings are less able to guard against anterior translation. At 90° flexion (anterior drawer position), the hamstrings reflexively contract and limit the anterior pull, masking ACL deficiency. Lachman therefore has higher sensitivity (80-90% vs 50-70% for anterior drawer).
Multiple factors: anatomical (wider pelvis increases Q angle, narrower intercondylar notch), hormonal (estrogen affects collagen properties), and neuromuscular (different landing biomechanics with more valgus alignment). The 4-6x increased rate has driven the development of neuromuscular training programs (FIFA 11+, PEP program) that reduce tear rates by improving landing mechanics.
Surgical reconstruction is considered for active patients who want to return to pivoting sports, patients with continued instability after rehabilitation, patients with associated meniscal or chondral injuries needing repair, and pediatric patients with substantial growth remaining (modified techniques). Non-operative management is reasonable for low-demand patients without instability symptoms.
O'Donoghue's unhappy triad classically describes ACL + MCL + medial meniscus injury from valgus force on a flexed externally rotated knee. Modern arthroscopic studies show the LATERAL meniscus is more commonly injured in many series, so some textbooks now teach the variation. The conceptual point — that valgus force can damage multiple structures — remains valid regardless of which meniscus is most often involved.
Snap a photo of a clinical vignette or MRI and AnatomyIQ identifies the suspected ligament from mechanism, walks through the appropriate physical exam tests with technique reminders, and predicts injury pattern. Practice cases come in three difficulty levels. This content is for educational purposes only.