Before starting on this section, it is vital that you can answer the following questions:

  1. Where are the 5 main ligaments of the knee joint?
  2. What is the action of both the cruciate ligaments and where do they attach to on the femur?
  3. Which three structures (hint: 2 ligaments, 1 meniscus) are firmly attached together?

Not quite sure what the answers are? Do not worry, click on the button below!

After completing the “Unhappy Triad” section, you will be able:

  • To apply knowledge of the knee joint to explain the pathophysiology
  • To explain the clinical presentation and immediate management
  • To describe the diagnosis
  • To discuss the management
  • To validate knowledge by completing the quiz


The Unhappy Triad: Tears of the ACL, MCL + Medial Meniscus

Sport Risk Factors

The 15-45-year-old age group is at the highest risk. Injuries occur primarily among active people especially in sport involving sudden twisting, bending and decelerating (e.g. football, basketball, skiing) or contact (e.g. American football, rugby).


Tom Brady, winner of 5 Super Bowls and the greatest NFL quarterback of all time, receives the ball at the start of the play and looks up to see his teammates around him. Just before he throws the ball he is tackled hard at knee level, forcing his knee to buckle inwards. A popping noise could be heard on impact, and Brady is in severe pain around his knee and is unable to fully move the joint.


The Unhappy Triad of the Knee consists of tears to the ACL, MCL and medial meniscus. These structures are commonly injured together due to the firm attachment to each other, as we discussed earlier.

  • Typically, a blow from the lateral side (valgus stress) while the foot is fixed on the ground (such as a tackle with force towards the midline in contact sports e.g. football, rugby or American football).
  • Or sudden twisting (change of direction) of the knee with a fixed foot (such as in skiing).
  • Force of the blow/twist > force that the structures can withstand = ACL, MCL and medial meniscus tear

TIP: * Mnemonic Alert * As The Unhappy Triad is associated with American Footballers who earn a lot of money. ATM for money-maker.

  • A: ACL
  • T: Tibial (Medial) collateral ligament
  • M: Medial meniscus
  • Unhappy Triad pathophys 1 1
  • Unhappy Triad pathophys 2 1
  • Unhappy Triad pathophys 3
  • Screen Shot 2017 04 19 at 22.37.43

Clinical Presentation

  • Mechanism of injury: typically, direct blow to the lateral aspect of the knee, or a twisting injury.
  • Audible pop noise at the time of injury
  • Immediate severe knee pain, tenderness and swelling (haemarthrosis) within a few hours
  • Knee joint instability – knee “giving way”/buckling or inability to fully bend or extend knee
  • Patient typically presents with inability to return to activity
  • Unhappy Triad pathophys 4 posssibly 1
  • Brady 2

Immediate Management

  • Immediately stop the activity and PRICE + analgesia
    • TIP: *Mnemonic Alert* PRICE
    • P: Protection – using crutches
    • R: Rest – from activity
    • I: Ice – reduces swelling
    • C: Compression
    • E: Elevation
  • Full knee examination to assess the injury (see video)
    • Focussing on the Anterior drawer test, assessment of the medial collateral ligament and the joint line tenderness
  • Urgent referral to an orthopaedic surgeon


 1. Clinical history and examination:

Clinical History:

Reveals clinical presentation and typical pathophysiology

On Examination:

  • Joint line tenderness:
    • Especially around the MCL
  • Anterior Drawer Test +ve:
    • Tibia can be pulled forwards (anteriorly) on the femur due to the tear of the ACL
  • Valgus stress test +ve:
    • Applying valgus (towards the midline) pressure to the femur, whilst applying varus (away from the midline) pressure to the ankle produces significant movements (abduction/adduction of the bones) due to the tear of the MCL

 2. MRI:

  • Highly sensitive and specific for ligament and especially meniscus tears


  • Ligament and meniscus reconstruction surgery to fix the tears
  • ACL tear is the most significant injury of the triad and needs to be reconstructed surgically, normally using part of the patient’s own hamstring or patellar tendon (see video)
  • The medial meniscus has no blood supply and therefore is slow to heal, hence reconstruction surgery is performed at the same time as the ACL reconstruction
  • The MCL normally heals naturally on its own and does not usually require reconstruction surgery
  • Significant rehabilitation through cautious physiotherapy to strengthen and mobilise the structures within and surrounding the joint including the muscles

Click here for the NICE Guidance.


Baldwin A, Hjelde N, Goumalatsou C, Myers G. (2016). Oxford Handbook of Clinical Specialities. 10th ed. Oxford: Oxford University Press.

British Medical Journal: Best Practice. (2016). Anterior cruciate ligament injury. [online]. Available at: [accessed 1 March 2017].

British Medical Journal: Best Practice. (2016). Meniscal tear[online]. Available at: [accessed 1 March 2017].

Kumar P, & Clark M. (2016). Kumar & Clark’s Clinical Medicine. 9th Ed. Edinburgh: Elsevier.

National Institute for Health and Care Excellence: Clinical Knowledge Summaries. (2011). Knee pain – assessment[online]. Available at:!topicsummary [accessed 1 March 2017].

Tortora G, Derrickson B. (2011). Principles of Anatomy and Physiology. 13th ed. Chichester: John Wiley & Sons.


Image and Video References

Tom Brady.   Pathophysiology 1.   Pathophysiology 2.   Pathophysiology 3.   Pathophysiology 4.   ATM.   Clinical Presentation 1.   Clinical Presentation 2.   Knee Examination Video.   Diagnosis 1.   Diagnosis 2.   ACL Surgery Video.