Knee Ligament Injuries
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Direct answer
The knee is stabilised by four ligament complexes — anterior and posterior cruciate, medial and lateral collateral — and each fails by its own mechanism. The anterior cruciate ligament tears during a pivoting or hyperextension moment with an audible pop, an immediate tense haemarthrosis and a positive Lachman test, and symptomatic instability is treated by reconstruction. The posterior cruciate ligament is the dashboard ligament, the medial collateral ligament fails in valgus stress, and the lateral-posterolateral corner is the least forgiving.
What you must remember
- ACL injury: pivoting, deceleration or hyperextension on a planted foot; the patient often hears a pop, cannot continue playing, and the knee swells with blood within hours.
- Tests: the Lachman test — anterior translation at 20 to 30 degrees of flexion — is the most sensitive; the anterior drawer and pivot-shift tests support the diagnosis.
- Acute traumatic haemarthrosis has the ACL as its commonest cause; the differential includes patellar dislocation, meniscal bleeding and osteochondral fracture.
- PCL injury: a posteriorly directed force on the flexed knee — the dashboard pattern — or a fall on the tibial tuberosity; posterior drawer and posterior sag sign are positive; isolated injuries often do well conservatively in a brace.
- MCL injury: valgus stress opens the medial side; graded I to III by the gap in 30 degrees of flexion, and most are treated conservatively in a hinged brace.
- Lateral and posterolateral corner injuries: varus force, often with peroneal nerve injury and foot drop, and usually surgical because chronic instability is disabling.
- The unhappy triad of O'Donoghue combines ACL, MCL and (classically) medial meniscal tears, though lateral meniscal tears accompany many acute ACL injuries; suspected knee dislocation — multiple ligaments torn — demands popliteal artery assessment.
Common confusion
The cruciates are mixed up by test, so anchor them: Lachman and the anterior drawer pull the tibia forwards and implicate the ACL; the posterior drawer and sag sign push it back onto the PCL. The anterior drawer is then over-trusted acutely — pain and guarding hide it, which is why the Lachman test is the bedside winner and MRI the confirmatory test. Every swollen knee is not a haemarthrosis, a child's injured knee needs X-rays first to exclude a physeal fracture, and the spontaneously reduced knee dislocation is a popliteal-artery emergency until proved otherwise.
Exam-focused takeaway
NEET-PG constructs mechanism-to-structure vignettes: a footballer's pivot with a pop and a tense swollen knee asks for the Lachman test and then ACL reconstruction for instability; a dashboard strike with a sagging tibia points to the PCL; a valgus blow opening the medial side grades the MCL. One-liners test the commonest cause of acute haemarthrosis, the most sensitive test and the unhappy triad, while any multi-ligament stem should end with a popliteal artery question.
Frequently asked questions
How does an ACL injury typically happen?
A non-contact pivot, deceleration or hyperextension moment on a planted foot, with a pop, rapid haemarthrosis and inability to continue activity.
Which test is most sensitive for an ACL tear?
The Lachman test — anterior translation of the tibia with the knee flexed 20 to 30 degrees — supported by the anterior drawer and pivot-shift tests.
What is the dashboard injury of the knee?
Posterior cruciate ligament injury from a force driving the tibia backwards on the flexed knee, showing a positive posterior drawer and sag sign.
How are MCL injuries managed?
Graded I to III by valgus laxity, most are treated conservatively with a hinged brace and graded rehabilitation rather than surgery.
What is the unhappy triad of the knee?
The O'Donoghue triad of ACL tear, MCL tear and a meniscal tear — classically medial, with lateral meniscal injury common in acute ACL ruptures.