The Anatomy and Biomechanics of The Posterior Cruciate Ligament

What is the PCL? Where is the PCL located in the knee?

PCL stands for the posterior cruciate ligament. There are two cruciate ligaments present in the knee joint – The ACL & the PCL. The ACL crosses across the PCL to make an ‘X’ shape within your knee, representing ACL lies in front and PCL lies backward.

The posterior cruciate ligament is located in the backside of the knee joint. Like the Anterior cruciate ligament (ACL), the PCL is intracapsular but ‘extra synovial,’ which means it is not surrounded by synovial fluid, but it has an adequate blood supply.

The PCL is shorter and less oblique than the ACL. The cross-sectional area of the PCL ligament is greater than the ACL. Therefore it can increase up to 120% to 150% greater than that of ACL due to this reason the posterior cruciate ligament is the strongest ligament of the knee joint. Also, the structural orientation of the PCL (size and shape) enables it to prevent greater load than the ACL.

Posterior Cruciate Ligament Anatomy

The posterior cruciate ligament originates from the lateral side of the medial femoral condyle and finally inserts distally on the posterior surface of the tibia (approx 1 cm below the joint line).

PCL Attachment

PCL origin

The posterior cruciate ligament originates from the lateral side of the medial femoral condyle.

PCL Insertion 

The PCL inserts distally on the posterior surface of the tibia (approx 1 cm below the joint line).

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PCL bundles

Just like the ACL, the PCL structure is made up of two functional bundles: The Anterolateral bundle (AMB) and the Posteromedial bundle (PMB). Let’s talk about there functions

Function of the bundles

The primary function of the posterior cruciate ligament (PCL) is to resist posterior translation of the tibia. Depending upon the knee flexion angle, the functions is, however attributes to either the Anteriolateral bundle (AMB) and the Posteriomedial bundle (PMB).

In knee extension – When the knee is in extension, the posteriormedial bundle (PMB) is taut, while the ALB is lax.

In knee flexion – At 80 to 90 degrees of flexion, the ALB is maximally taut, and the PMB is lax. But at full flexion, the function is again taken up by the PMB (tight), and the ALB role becomes less significant (loose).

posterior-cruciate-ligament-(PCL)-bundles-anatomy

the ALB role becomes less significant because, at full or deep flexion, the femoral attachment of the PMB translates a bit anteriorly. This anterior translation makes the PMB tighter, therefore increasing its capability to prevent the tibia’s posterior translation.

Simultaneously (at the same time), the orientation of the anterolateral bundle (ALB) decreases its capacity to prevent the posterior translation of the tibia in full or deep flexion.

Some more functions of the PCL

The posterior cruciate ligament’s primary function is to prevent the posterior translation of the tibia or posterior shear of the tibia.

At full knee extension, the PCL prevents 93% of the posteriorly directed load applied to the tibia. And at most of the knee flexion angles, the PCL prevents approx 90% of the posterior load applied to the tibia.

At 75° to 90° of knee flexion, there is the maximum chance of posterior displacement of the tibia. As all the secondary structures that help in preventing the posterior displacement of the tibia becomes inactive at full or deep flexion, therefore, only PCL have to play it’s role in preventing posterior translation of the tibia.

The PCL prevents medial rotation of the tibia at 90° of the flexion but performs less in full extension. Well talking about lateral rotation, the PCL does not able to prevent lateral rotation of the tibia perfectly.

Role of Muscles on the PCL

If the PCL is absent, damaged or not working, muscles are recruited to prevent the posterior translation of the tibia. The muscles that are mainly involved are popliteus, hamstrings, quadriceps, and gastrocnemius. Lets discuss muscles one by one.

Popliteus –  When there is a rupture, damaged or in the absence of the PCL, the popliteus muscle shares or takes up the function of the PCL to prevent the posterior translation of the tibia.

Hamstrings – The hamstrings muscles are the knee flexors, Isolated hamstrings contraction (knee flexion) is capable of producing posterior translation of the tibia. Thus, this posterior tibial translation will gonna increase the strain on the PCL.

in conclusion, an isolated hamstrings contraction will gonna increase the strain on the PCL in the absence, rupture or damage of the PCL.

Quadriceps – The Quadriceps muscle works as a knee extension. Knee extension (Quadriceps contraction) will gonna relieve or reduce the strain on the PCL. 

So, if someone has suffered an PCL injury, one should strengthen or stabilze the quadriceps muscles.

Gastrocnemius – Gastrocnemius first relieve or decrease the strain on the PCL. But after 40° of knee flexion, the gastrocnemius again gonna strain the PCL.

Related post 

The Anatomy and Biomechanics of the Anterior Cruciate Ligament

Anatomy and Biomechanics of the Lateral Collateral Ligament 

The Anatomy and Biomechanics of the Medial Collateral Ligament 

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