Is the unhappy triad more than ACL, medial collateral ligament, and meniscus injury?

2018 Brighton’s Aaron Connolly receives treatment after injury. Action Images via Reuters/Andrew Couldridge

If one knee injury wasn’t bad enough, imagine the triple whammy of the unhappy triad. Athletes often refer to this type of injury as ‘blowing out’ a knee because of the damage done to the important knee stabilizers. As we continue our series on running injuries, we turn our focus from knee injuries of endurance runners to those who run as part of their sport. Athletes who play field and court sports such as football, rugby, and handball additionally must stop and change direction. This ‘cutting’maneuver is often implicated as the cause of the unhappy triad, but how does one apparent motion ‘take out’ so many structures?

Researchers from the Norwegian School of Sports Sciences analyzed the injury videotapes of 10 female handball and basketball players to evaluate the injury process(1). They concluded that when the knee experiences valgus stress, the MCL tightens and the lateral structures in the joint compress. As the lateral joint space reduces, the quadriceps contract, causing the femur to move posteriorly while the tibia internally rotates and moves anteriorly. This forced anterior translation of the tibia stretches the ACL to its limits and causes it to tear. They noticed on the injury films that after the extreme tibial internal rotation, the tibia, without the control of the ACL, moved into external rotation while the medial femoral condyle moved posteriorly.

While this study only analyzed the mechanism of ACL injuries, one can conclude that further valgus stress, along with anterior tibial translation and external rotation, could lead to rupture of the MCL. The shearing forces within the joint as the femur translates posteriorly and the tibia rotates both internally and externally, may provide enough stress to tear the menisci as well.

While assumed since it’s initial description of a triad of injuries in the 1950s to include the medial meniscus, due to its attachment to the MCL, some studies show a greater incidence of lateral meniscus tears in an unhappy triad( 2). Physiotherapist Chris Mallac recently described the anatomy and biomechanics of the menisci and the implications of tears at the root in his recent two-part series (see figures 1 and 2). Rehabilitation of the meniscus after root tear requires special attention to the application of forces at the knee, therefore when rehabilitating an athlete after an unhappy triad injury, inquire as to the extent of the damage to the meniscus.

Figure 1: Anatomy of the menisci

Figure 2: Meniscal root tear

Just released research calls into question the limitation of this injury to only these three structures. An Italian study evaluated 125 patients with ACL tears to identify 11 of those with MCL involvement. All 11 subjects underwent ACL repair and exploration of the anterolateral complex. The researchers found damage to the anterolateral complex in all 11 cases and all received repair to the anterolateral ligament. These authors concluded that the unhappy triad is really a tetrad – including the anterolateral complex. As I point out in a recent two-part series on the anterolateral complex and ACL injuries, damage to the anterolateral complex may hold the key to solving the mystery of repeated ACL injury (see figure 3).

A knee with triple or quadruple injuries presents as a complex rehabilitation challenge, especially if the athlete desires to return to sport. As ever, Sports Injury Bulletin reviews the latest research so that you can bring science to your treatment of complex injuries such as this. For more information on knee injury and rehabilitation, review the articles on the knee under the anatomy tab.

Figure 3: ACL and anterolateral complex anatomy


  1. Am J Sports Med. 2010 Nov;38(11):2218-25
  2. BMJ Case Reports. 2013 Jan 23
  3. Int Orthop. 2018 Oct 1;[Epub ahead of print]


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