How ACL and PCL Injuries Differ

Differences in Symptoms, Causes, Diagnosis, and Treatment

Mature woman in gym holding painful knee

Barry Austin / Digital Vision / Getty Images

In This Article
Table of Contents

The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are two major ligaments in the knee that work together to provide stability. They are also common sites of serious knee injuries, particularly in athletes.

Although ACL and PCL injuries have similar symptoms, including knee instability and pain, they vary considerably in who they affect, the extent to which the ligament is likely to tear, and how treatments are delivered.

Anatomy

The knee is one of the most complex joints in the human body, connecting the femur (thigh bone) to the larger tibia and smaller fibula bones of the lower leg. These bones are connected by four bands of fibrous connective tissue known as ligaments.

The ACL and PCL are two major ligaments that crisscross around the joint, allowing the knee to flex and extend without side-to-side motion. The ACL prevents the tibia from sliding forward along the femur, while the PCL prevents the tibia and femur from sliding towards each other.

The other two ligaments are known as the medial collateral ligament (MCL) and lateral collateral ligament (LCL). These prevent side-to-side motion of the femur across the tibia and are less prone to injury than the ACL and PCL. When an injury does occur, it is most often the result of an ACL or PCL injury.

Symptoms

Both ACL and PCL symptoms have similar symptoms, although ACL injuries tend to be more severe. This is because the PCL is a much stronger ligament and less vulnerable to tears. As a result, fewer than 20% of knee ligament injuries involve the PCL.

Symptoms characteristic of both ACL and PCL injuries include pain, swelling, and knee instability. Where they tend to differ is the extent of the injury and the severity of symptoms.

The ACL is far more likely to experience a complete tear given its smaller size. When this occurs, it can damage adjacent structures, including joint cartilage, other ligaments, and the cushion of the knee joint known as the menisci. As a result, the pain from an ACL tear will usually be more severe with a significant (or total) loss in the range of motion of the knee.

By comparison, PCL tears are more often partial rather than complete. The pain will correspondingly be less severe or not at all. Moreover, with a PCL injury, the swelling of the knee will usually be immediate, while the swelling of an ACL tear will tend to develop over the course of 24 hours.

Another differentiating sign is an audible "pop" that can occur at the moment of an ACL rupture. While it is possible that a PCL tear can cause a popping sound, it is uncommon.

Causes

Both ACL and PCL injuries can occur during sports, albeit for considerably different reasons. Injury can also occur during a motor vehicle accident, a fall, or a blunt force impact to the knee.

With an ACL injury, the damage will usually be caused when there is a sudden stop or rapid change in direction, such as occurs with soccer, basketball, football, or downhill skiing. Landing awkwardly from a jump can also cause injury.

With a PCL injury, the damage will typically occur when the knee is bent, such as when you fall on a bent knee with your knee pointed downward or when your shinbone is hit hard just below the knee.

These factors also influence your risk of injury. With an ACL injury, for example, you are more more likely to be harmed if you are in poor physical condition, have ill-fitting footwear (or ski binding), and play on slippery artificial turf.

By contrast, PCL injuries are more likely to occur as a result of a car accident (especially "dashboard accidents") and participation in sports where blunt force injuries to the knee are common (such a football or soccer). A misstep on an uneven surface can also cause a PCL injury.

Another differentiating factor is sex. Because women's knees are turned inward more than men, they tend to be less bent when jumping or landing. As a result, ACL tears are more common in females than males.

Diagnosis

An ACL and PCL injury can both be diagnosed with a physical examination and imaging studies such as an X-ray (which is able to show complete ruptures) or magnetic resonance imaging (which is better able to visualize ligaments and other soft tissues).

One tell-tale sign of a PCL injury is the backward, sagging position of the knee as it is bent. The kneecap can slide even further back when the knee is bent more than 90 degrees.

With the ACL, there will typically be tenderness with palpation (touch) along the joint line. Flexion of the knee will be difficult, if not impossible. There may also be spasms and the guarding of the hamstring muscles at the back of the thigh.

Treatment

The treatment of ACL and PCL injuries are pretty much the same. The recommended treatment will be largely based on the grade of the injury:

  • Grade 1 involves a slightly stretched ligament, although the knee itself will be technically stable.
  • Grade 2 indicates a ligament that has become loose or is partially torn.
  • Grade 3 is indicative of a complete ligament rupture.

Depending on the grade, the injury may be treated with rest, ice application, compression, and elevation (referred to as the RICE protocol). Physical therapy is often recommended to regain joint strength and range of motion. Complete tears may require arthroscopic surgery and ligament reconstruction.

The only true difference between the treatment of ACL and PCL injuries is the likelihood of surgical intervention. Since ACL tears are more likely than not to be complete, the course of treatment tends to be far more extensive.

With that said, not everyone with a complete ACL rupture needs surgery. People who are largely inactive or elderly can often manage just fine with a knee brace or assisted mobility device after an ACL tear.

On the other hand, most PCL injuries can heal on their own without surgery and may only require crutches and a knee immobilizer to prevent knee movement during recovery.

Was this page helpful?

Article Sources

  1. Canale ST, Beaty JH. (2012) Campbell's Operative Orthopaedics (12th Ed). Maryland Heights, Missouri: Mosby/Elsevier.

  2. Marieswaran M, Jain I, Garg B, et al. A Review on Biomechanics of Anterior Cruciate Ligament and Materials for Reconstruction. Appl Bionics Biomech. 2018;2018:4657824. doi:10.1155/2018/4657824

  3. Lee BK, Nam SW. Rupture of Posterior Cruciate Ligament: Diagnosis and Treatment Principles. Knee Surg Relat Res. 2011 Sep;23(3):135-41. doi:10.5792/ksrr.2011.23.3.135

  4. Cimino F, Volk BS, Setter D. Anterior Cruciate Ligament Injury: Diagnosis, Management, and Prevention. Am Fam Physician. 2010 Oct 15;82(8):917-922.

  5. Salzler MJ, Chang J, Richmond J. Management of Anterior Cruciate Ligament Injuries in Adults Aged >40 Years. J Am Acad Ortho Surgeons. 2018 Aug;26(16):553-61. doi:10.5435/JAAOS-D-16-00730

Additional Reading