Knee Ligaments
The knee is stabilised by four primary ligaments. The cruciate ligaments resist anteroposterior tibial displacement; the collaterals resist varus and valgus stress. Understanding their function, testing, and common injury patterns is essential for every clinician.
✦ The Mnemonic
"All Patients Must Learn Ligamentous Anatomy"
ACL, PCL, MCL, LCL — function, test, injury pattern for each
Clinical Breakdown
The ACL runs from the lateral femoral condyle to the anterior tibial spine — the name refers to its tibial attachment being anterior to the PCL's tibial attachment. The Lachman test is the gold standard for ACL assessment (knee at 20–30° flexion, anterior draw on tibia) — sensitivity ~87%. The anterior drawer test (90° flexion) is less sensitive for acute injuries due to hamstring guarding.
O'Donoghue's unhappy triad: ACL + MCL + medial meniscus injury from a valgus contact injury. In modern practice, lateral meniscus tears are actually more commonly associated with ACL tears than medial. A haemarthrosis developing within 2 hours of injury suggests an ACL tear (or osteochondral fracture) — gradual effusion suggests a meniscal injury.
The MCL has superficial and deep layers; the deep layer is attached to the medial meniscus (explaining why medial meniscus is less mobile and more injury-prone than the lateral). The LCL is a cord-like structure running from lateral femoral epicondyle to the fibular head — it is NOT attached to the lateral meniscus. The posterolateral corner (LCL + popliteus + lateral capsule) must be addressed in complex knee reconstructions.
⭐ Clinical Pearl
Segond fracture: a small avulsion of the lateral tibial condyle seen on AP X-ray of the knee — pathognomonic of an ACL tear. It represents avulsion of the anterolateral ligament or the anterior capsule. When you see a Segond fracture, treat it as a confirmed ACL injury and plan MRI accordingly. The reverse Segond (medial tibial condyle avulsion) is pathognomonic of a PCL tear.
⚠ Exam Trap
PCL injury is frequently missed. The knee can feel stable because the quadriceps can compensate for posterior instability. Look for a posterior sag sign at 90° knee flexion (tibia sags posteriorly compared to the contralateral side). A PCL tear with a haemarthrosis after a 'dashboard injury' is a surgical emergency if associated with popliteal artery injury.