The Unhappy Triad Knee Injury

The “unhappy triad” refers to a sprain injury that involves three structures of the knee. These structures are the medial collateral ligament, anterior cruciate ligament, and the medial meniscus. The unhappy triad is a severe injury, and it is named such because the athlete who suffers it typically becomes really unhappy. The severity of this injury typically leads to surgery to fully correct.

Picture of a man holding injured knee.
PhotoAlto / Sandro DiCarlo Darsa / Getty Images

What Causes the Unhappy Triad? 

The mechanism for the unhappy triad injury occurs when a lateral force to the knee is received while the foot is fixed on the ground. This places an increased valgus, or abduction, and rotational stress on the knee that results in the overstretching of these three structures. When the ligaments in the knee fail, increased stress may be suddenly placed on the medial meniscus, causing injury or a tear there.

A good example of the unhappy triad occurring is during a tackle in football when the athlete is hit on the outside of his knee while in the standing position with the foot planted on the ground.

Each Structure in the Unhappy Triad

The three structures injured in the unhappy triad include the medial collateral ligament, the anterior cruciate ligament, and the medial meniscus. Understanding the function of each of these ligaments is important to understanding the rehab after suffering the unhappy triad.

  • The anterior cruciate ligament: The anterior cruciate ligament, or ACL, is a strong ligament in the knee that helps to keep your shin bone situated directly under your thigh bone. The ACL resist shear forces in your knee, preventing rotation and anterior, or forward, moving of your shin bone under your knee. It is an important player in maintaining knee stability while a person is jumping and landing or running and cutting or stopping suddenly.
  • The medial collateral ligament: The medial collateral ligament, or MCL, resists forces from the outer side of your knee. It prevents gapping on the inside aspect of your knee joint. The MCL is a big, thick ligament, and it often heals without surgery when it is sprained.
  • The medial meniscus: The medial meniscus is a cartilage structure in your knee joint. It, along with the lateral meniscus, is responsible for shock absorption through your knee joint. Injury to the medial meniscus can make basic functions like walking, running, or stair climbing very painful.

Physical Therapy After Experiencing the Unhappy Triad

If you have the unhappy triad, you may benefit from physical therapy. Your therapy should focus on regaining normal motion and function in your knee. Your physical therapist may use therapeutic modalities like ice, ultrasound, or electrical stimulation to help control your pain. Research into the use of modalities shows varied results; some studies show improvements with things like ultrasound or e-stim, while other shows no extra benefit from using these modalities.

Typically, swelling around your knee may limit its range of motion (ROM) when you suffer the unhappy triad. Physical therapy knee ROM exercises may be performed to help restore normal mobility to your joint.

With any knee injury, the quads tend to shut down while healing is occurring, a condition known as quadriceps inhibition. Getting your quads working properly again is of utmost importance. This quad inhibition occurs due to pain and swelling in your knee joint. A form of electrical stimulation called NMES may be used to help improve the way your quadriceps muscles contract after you suffer the unhappy triad.

Typically, swelling around your knee may limit its range of motion (ROM) when you suffer the unhappy triad. Physical therapy knee ROM exercises may be performed to help restore normal mobility to your joint.

Since the unhappy triad involves the tearing of two ligaments, your knee will likely feel very unstable. Exercises to improve balance and proprioception may be incorporated into your physical therapy program. This may include:

Surgery is often performed to repair the ligaments and meniscus in your knee if you have the unhappy triad. After surgery, you will likely require physical therapy to help you restore normal mobility and strength in your knee. Your physical therapist can help you regain your functional mobility and can help you return to your previous level of function and activity.

A Word From Verywell

The unhappy triad is a serious injury, and if you suspect you have it, you need to see your healthcare provider right away. Working with your healthcare provider and your physical therapist can help you quickly and safely recover after suffering the unhappy triad.

7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. American Academy of Orthopaedic Surgeons. Anterior cruciate ligaments (ACL) injuries.

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  4. American Academy of Orthopaedic Surgeons. Meniscus tears overview.

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  7. Glaviano NR, Saliba S. Can the use of neuromuscular electrical stimulation be improved to optimize quadriceps strengthening?. Sports Health. 8(1):79–85. doi:10.1177/1941738115618174

By Laura Inverarity, DO
 Laura Inverarity, PT, DO, is a current board-certified anesthesiologist and former physical therapist.