ACL injury vs PCL, MCL, and LCL injuries
How to tell ACL injury and PCL, MCL, and LCL injuries apart when a board question makes them look alike.
ACL is anterior translation after pivot; PCL is posterior drawer after dashboard; MCL is valgus; LCL is varus.
Side by side
ACL injury follows pivoting or noncontact deceleration, causes pop, rapid hemarthrosis, instability, and positive Lachman or anterior drawer test.
PCL injury follows dashboard blow with posterior drawer; MCL follows valgus stress; LCL follows varus stress and lateral instability.
Anterior cruciate ligament rupture permits anterior tibial translation relative to femur.
PCL resists posterior translation; MCL resists valgus force; LCL resists varus force.
Positive Lachman, anterior drawer, pivot shift, and rapid hemarthrosis support ACL tear.
Posterior sag/drawer supports PCL; valgus laxity supports MCL; varus laxity supports LCL.
The trap
Knee ligament pain overlaps, so mechanism and stress test direction identify the injured ligament.
The repair move
For knee ligaments, convert mechanism into the direction of abnormal tibial or joint-line movement.
Try a free question
CoreA 22-year-old soccer player plants his right foot and pivots, hears an audible 'pop' in the right knee, and develops immediate swelling. Exam 24 hours later shows a large hemarthrosis, positive Lachman test, and positive anterior drawer test. MRI confirms a complete tear of the anterior cruciate ligament. Which diagnosis is most likely?
Show answer and explanation
Answer: A. Anterior cruciate ligament tear. ACL tear classically occurs with non-contact pivoting injury and presents with an audible pop, immediate hemarthrosis, anterior tibial translation (positive Lachman and anterior drawer); treatment for active patients is arthroscopic ACL reconstruction.
Why the other choices are tempting
- B. PCL tear typically occurs from a posteriorly directed force on the proximal tibia (dashboard injury) with positive posterior drawer test.
- C. MCL tear results from a valgus force with medial joint-line tenderness.
- D. LCL tear results from a varus force with lateral instability.
- E. Meniscal tear causes mechanical symptoms (locking, clicking) with joint-line tenderness, often without immediate hemarthrosis.
Flashcards
Tap a card to flip it.
What test supports ACL tear?
Lachman test supports ACL tear.
What mechanism supports PCL tear?
Dashboard injury supports PCL tear.
Quick answers
What is the fastest way to tell ACL injury from PCL, MCL, and LCL injuries?
ACL is anterior translation after pivot; PCL is posterior drawer after dashboard; MCL is valgus; LCL is varus.
What is the key clue for ACL injury?
ACL injury follows pivoting or noncontact deceleration, causes pop, rapid hemarthrosis, instability, and positive Lachman or anterior drawer test.
What is the key clue for PCL, MCL, and LCL injuries?
PCL injury follows dashboard blow with posterior drawer; MCL follows valgus stress; LCL follows varus stress and lateral instability.
What is the common trap with ACL injury vs PCL, MCL, and LCL injuries?
Knee ligament pain overlaps, so mechanism and stress test direction identify the injured ligament.