A tear of a cruciate ligament will represent a major injury to any patient. This multi-page Primer highlights the key issues.

Page updated July 2023 by Dr Sheila Strover (Clinical Editor)


Knee ligament injuries of anterior or posterior cruciate ligament are often combined with injury to other knee ligaments or to the menisci.


What is complex or multiligament instability?

'Complex' or 'multi-ligament' instability of the knee implies damage to more than one stabilising structure, often giving considerable difficulty in diagnosis and management. It is very much super-specialist territory. The damage may include tears of both cruciates, or one or more cruciates and a collateral ligament - plus or minus one or more structures of the posterolateral or anterolateral corners.

Surgical Management of the Multiple-Ligament Knee Injury. Buyukdoga K, Laidlaw MS and Miller MD. Arthrosc Tech. 2018 Feb; 7(2): e147–e164.

Back to top


The Unhappy Triad of O'Donoghue

This particular multi-ligament knee injury -

has earned itself the infamous name of 'The unhappy triad of O'Donoghue'!

In fact much more common is -

  • torn anterior cruciate ligament (ACL)
  • torn medial collateral ligament (MCL)
  • torn lateral meniscus (if the medial meniscus is torn, the lateral one is usually torn also)

The unhappy triad is also sometimes called the 'terrible triad' or a 'blown knee'.

What is the terrible triad? Barber FA. Arthroscopy. 1992;8(1):19-22.

Back to top


Posterolateral rotatory instability

Posterolateral rotatory instability is a particular multi-ligament injury involving damage to the structures of the posterolateral corner -

Posterolateral corner ligament injuries often go unrecognised or not fully diagnosed, leaving the patient with long term pain and instability.

Current Concepts of Posterolateral Corner Injuries of the Knee Shon O-J, Park J-W and Beum-Jung Kim B-J. Knee Surg Relat Res. 2017 Dec; 29(4): 256–268.

Back to top


Anterolateral rotatory instability

Appreciation of rotatory instability involving the anterior cruciate ligament deficient knee has grown since the full recognition in 2013 of the anatomical structure known as the anterolateral ligament (ALL). New studies since then have drawn attention to several previously poorly-recognised contributors to anterolateral stability - the integrity of the ALL, the integrity of the capsule and its junctures with the menisci, and the contribution of the slope of the tibia to stability. Appreciation and repair of ramp lesions has become more commonplace. Slope-changing osteotomy is being embraced by the ostotomy surgeons.

The Anterolateral Ligament is Not the Whole Story: Reconsidering the Form and Function of the Anterolateral Knee and its Contribution to Rotatory Knee Instability. Sheean AJ, Shin J, Patel NK, Lian J, Guenther D and Musahl V. Tech Orthop. 2018 Dec; 33(4): 219–224.

Back to top


Posteromedial corner problems

The posteromedial corner is the neglected corner of the knee. Problems here may be combined with either ACL or PCL tears and lead to anteromedial rotatory instability of the knee.

Back to top

PREVIOUS PART: The torn posterior cruciate ligament (PCL)

NEXT PART: Cruciate ligament repair