Q & A: Unhappy Triad of the Knee

This question comes from a Edward Via Virginia College of Osteopathic Medicine student
I need help understanding the unhappy triad of the knee.  Some sources define the triad as a torn ACL, MCL, and medial meniscus… but other sources say ACL, MCL, and lateral meniscus.  Which is it?
Hello Edward Via Virginia College of Osteopathic Medicine student.
You are not alone in your confusion.  The term ‘unhappy triad’ (or ‘terrible triad’, or O’Donoghue triad) has been applied to both clusters of injuries, classically due to a lateral blow to the knee while the foot is planted on the ground.  The original description published by Dr. O’Donoghue in the 1950s was a triad of ruptured ACL, ruptured MCL, and injury of the medial meniscus.  But in the 1990s the validity of this triad was called into question, and studies showed that it was the lateral meniscus that was more commonly involved.  In fact, patients with an ‘unhappy triad’ knee injury only have injury to the medial meniscus in the presence of a concomitant lateral meniscus injury.
So the bottom line is that current use of the term “unhappy triad” or “O’Donoghue triad” should indicated injury of the ACL, MCL, and lateral meniscus.
Unfortunately, old habits die hard, and you may still see the older ‘classic’ definition of the triad used in publications and by older clinicians from time to time.  But for your USMLE and COMLEX exams, we would recommend knowing that the lateral meniscus is most likely to be torn in an acute traumatic knee injury with ACL/MCL rupture.

About Mike McInnis

Dr. McInnis is a chief educator for Doctors In Training’s USMLE Step 1 and Step 2 CK Review Courses. You can also follow Dr McInnis on Twitter (@DrMcInnisDIT).
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