What Does It Mean When Your Knee Goes Back Too Far? Meet the PCL

By Chris Centeno, MD /

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You might not even notice that your knee goes back too far on one side. Maybe it’s just a little bit that only others will notice or maybe it’s enough to catch your attention. Today I’d like to introduce you to the posterior cruciate ligament (PCL) and how it plays a role in limiting extension of the knee. The good news is that if your PCL is lax and allows too much extension, it can usually be fixed without surgery.

Knee Hyperextension (or When a Knee Goes Back Too Far)

The medical terminology for a knee that goes back too far is hyperextension. If you stand sideways and look into a full-length mirror, if the knee goes back too far, you can see this subtle or obvious hyperextension that takes the knee beyond its normal range. What causes a knee to hyperextend? There are a few possibilities. Simply having poor posture is one reason, which happens when the core muscles are not properly controlled or are weak. It can also be congenital, which means you were born with it, but in that case, it’s usually equal on both sides. Another possibility, and one we see a lot, is a loose or torn PCL.

What Is the PCL?

The posterior cruciate ligament (PCL) might not get the attention its popular cousin the ACL does, but nonetheless, it’s a very important ligament. The PCL, like the ACL, lives in the middle of the knee space. It attaches to the front of the upper leg bone (the femur), stretches diagonally across the knee space, and then attaches to the very back part of the lower leg bone (the tibia). Its job is to keep the knee from going back too far. When the PCL functions properly, it provides stability, control, and proper range of motion. When it becomes loose or torn, this can cause excessive knee hyperextension, which, over time, can beat up the knee joint.

How will you know if the PCL is loose or torn? A knee that goes back too far is certainly one good sign, but you can also perform the self test at this link to check for loose knee ligaments. An MRI would be needed to confirm a torn PCL.

How PCLs Become Loose or Torn

The PCL can become injured or torn during sports—for example, when a player is tackled from the front. Loose PCLs can be the result of a lack of knee coordination or weakness in the leg muscles due to atrophy or even irritated nerves in the low back.  The instability from loose or torn ligaments left untreated can lead to other problems, such as knee arthritis, over time. This is why even small tears that a radiologist might not even read out on an MRI really shouldn’t be ignored if they correlate with an exam showing a loose PCL.

We see evidence of partially torn PCLs on MRI in many patients who have knee hyperextension issues. It’s important to note that many times, these small tears in the PCL are missed or not mentioned by the radiologist producing an MRI report. It’s important to heal the tear and tighten up the ligament so no further damage is done and so patients don’t develop arthritis due to excessive wear and tear from the instability. Treatment can usually be accomplished without invasive surgery.

Treating the PCL Without Surgery

In the video above, I show an example of a small tear in the PCL. The video shows a precise ultrasound-guided injection of platelet rich plasma (PRP) into the PCL insertion at the back of the knee. The injection was also confirmed using fluoroscopy (a real-time X-ray) that allows us to see the contrast as it enters and flows up the PCL. The contrast assures us that the injectate (the material being injected—in this case the PRP) is going where it needs to go. In this case, that it gets to the area of the tear.

Keep in mind that only a handful of providers (typically not an orthopedic surgeon) nationally have the technical skill needed to perform this precise injection into the PCL.

The upshot? If your knee hyperextends, you need to understand the PCL. Precise injections are usually all that’s needed!

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Chris Centeno, MD

Regenexx Founder

Chris Centeno, MD is a specialist in regenerative medicine and the new field of Interventional Orthopedics. Centeno pioneered orthopedic stem cell procedures in 2005 and is responsible for a large amount of the published research on stem cell use for orthopedic applications.
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