Unhappy Triad Knee Rehab

So what is the unhappy triad?

The unhappy triad (also known as the terrible triad) was first described in 1964 by O’Donoghue. Back then, it was described as an injury to the ACL, MCL, and medial meniscus.  It was estimated to occur in this combination in 25% of acute athletic knee injuries. O’Donoghue explained that this combination of injuries occurs due to the motion of the knee caving and rotating inward also known as the valgus moment that occurs during this injury.

Let’s look at these structures one by one:

• The anterior cruciate ligament runs through the middle of the knee joint limiting the shin bone from:
  • Moving forward on the thigh bone

• • Rotating on the thigh bone

• • Caving inward relative to the thigh bone

• The medial collateral ligament runs along the inner or medial side of the knee joint from the thigh bone to the shin bone. It primarily limits the thigh and shin bone from opening up or being spread apart on that medial side.
• The medial meniscus does not serve as a primary limit to motion at the knee but rather as a cushion between the thigh and shin bone which protects the bone and the cartilage surrounding the bone from damage.

Which Meniscus Is More Often The Culprit In The Unhappy Triad?

Thus, when the knee caves inward excessively to the point of injury, it makes sense that the ACL & MCL would both be injured. As injury to the above structures creates a level of instability in the knee joint, logically the menisci which are anchored in between the 2 surfaces of the knee joint are also at risk of injury. However, as technology has improved and the invention of the arthroscope has increased the surgeon’s ability to view the knee joint, subsequent research has shown that the more common combination is injuries to the lateral meniscus.

• The lateral meniscus, much like the medial meniscus, does not serve as a primary limit to motion at the knee but rather as a cushion between the thigh and shin bone on the lateral (outside) rather than medial (inside) of the knee.

Thus, it appears that the more common version of the unhappy triad is more accurately described as a combination injury to the ACL, MCL, and lateral meniscus. At this point, the sports medicine community defines the unhappy triad as an injury to the ACL, MCL, and either meniscus.

Is There Really an Unhappy Tetrad?

While our focus has often been on the structures inside of the knee joint (intra-articular) more recently structures outside the knee joint have been receiving more attention. One of these, in particular, is the anterolateral ligament (ALL) which was only recently rediscovered. This ligament is part of the anterolateral complex, which are the structures stretching across the outside & front of the knee and which limit the shin bone from rotating inward.  When this inward rotation persists after an injury it is called anterolateral rotational instability, often described as a “pivot-shift”, where the shin bone rotates forward and inward on the thigh bone. A recent study found that in 11 patients who underwent surgery for an unhappy triad injury, all 11 had injuries to their anterolateral complex as well.

The Unhappy "Tetrad" Rather Than The "Triad"

Thus, although this is an area about which even the medical community knows little, there is good reason to suspect that there are actually FOUR structures involved in this mechanism of injury, thus making this an “unhappy tetrad”. These include the:

• ACL
• MCL
• Meniscus (medial or lateral)
• Anterolateral complex (most of the anterolateral ligament)

Why does this matter?

While the ACL is still the primary structure involved and the most important ligament for knee stability, there are some patients who have successful ACL reconstruction surgery but continue to deal with instability in their knees. In many cases, this is anterolateral rotational instability as described above. This continued instability can lead to less successful outcomes for the patient, as well as increased stress and risk to the ACL graft.

For this reason, surgeons are increasingly seeking to address this anterolateral rotational instability at the time of the initial injury. Currently, there are two surgical procedures gaining popularity.

• The Lateral Extra-articular Tenodesis (LET) procedure: This utilizes a portion of the iliotibial band (ITB) to reinforce the anterolateral complex and limit the internal rotation of the shin bone.
• The ALL Reconstruction: This often involves fashioning a graft similar to an ACL graft (often from the hamstring tendon) to replace the anterolateral ligament.

In many cases, the rehab from these additional procedures is very similar, just with an increased focus on protecting the integrity of these additional structures.

Unhappy Triad Knee Rehab

In most cases, if you are young, and looking to return to a high-level activity, surgical reconstruction of the ACL, with the potential of meniscal surgery will occur sooner rather than later. What is interesting is that more recently in regards to the timing of surgery, individuals have had more success if they allow the knee to "calm down" for a couple of weeks rather than immediately operating on the knee. This has a couple of benefits. Swelling can reduce, pain may be better modulated, and the MCL can begin to scar down. During this time, you can work on regaining adequate knee range of motion with light exercises prior to surgery.

Want to learn more ways how to improve knee flexion? Watch this video below!

Post-Surgical Latter Phase Unhappy Triad Knee Rehab

Once you have had your surgery, tissue healing has commenced, your range of motion has been restored, and your quad muscle has demonstrated good activation, you can start to return to more closed chain exercises, and shift a focus on joint proprioception. Depending on which structures were operated on, weight-bearing may be protected in the early phase, especially if the meniscus was repaired.

Closing Thoughts

The unhappy triad is a well-described phenomenon although its definition has continued to grow and expand over the last 50 years. Currently, the unhappy triad is defined as a combination of injury to the ACL, MCL, and either meniscus. The recent focus on the importance of rotational instability in the knee after ACL injury and surgery has led to consideration for expanding this definition to the unhappy tetrad. This would include the anterolateral complex in addition to the 3 structures described above. Awareness of this phenomenon is good for patients as newer surgical procedures can more readily address instances of anterolateral stability potentially leading to improved outcomes after ACL surgery. These procedures include the ALL reconstruction and LET procedure, both of which seek to limit the internal rotation of the shin bone about the thigh.