What is the difference between an Anterior Cruciate Ligament (ACL) injury and a Posterior Cruciate Ligament (PCL) injury?

From the Guidelines

To examine for ACL versus PCL injuries, perform a thorough knee assessment including the Lachman test, anterior drawer test, and pivot shift test for ACL injuries, and the posterior drawer test and posterior sag sign for PCL injuries, and consider using CT scans as the next imaging study for evaluation of suspected radiographically occult knee fractures, as it has been shown to be superior to knee radiographs in detection and classification of fractures 1. When evaluating for ACL and PCL injuries, it is essential to begin with observation and palpation of the knee, noting any swelling, deformity, or tenderness.

• The Lachman test is performed by flexing the knee 20-30 degrees and pulling the tibia forward while stabilizing the femur; excessive forward movement indicates ACL injury.
• The anterior drawer test involves flexing the knee 90 degrees and pulling the tibia forward.
• For PCL assessment, the posterior drawer test is performed by pushing the tibia backward at 90 degrees flexion, while the posterior sag sign is observed with the knee flexed at 90 degrees, looking for posterior tibial displacement.
• Compare findings with the uninjured knee, as these tests are crucial because ACL injuries typically result from pivoting or hyperextension mechanisms and cause anterior instability, while PCL injuries often result from dashboard impacts or falls on flexed knees and cause posterior instability. The use of CT scans in the evaluation of knee injuries has been supported by studies, such as the one by Heffernan et al, which showed that 64-channel multi-detector CT had a sensitivity of 87.5% to 100% and specificity of 100% for detection of ACL tears 1. Additionally, dual-energy CT virtual noncalcium technique can subtract calcium from cancellous bone, allowing detection and possibly grading of post-traumatic bone marrow contusions 1. In contrast, US may not be the best imaging modality for evaluating occult knee fractures, but it can be useful in detecting knee joint effusions and meniscal tears, with a pooled sensitivity of 88% and specificity of 90% in diagnosis of meniscal injuries 1. Accurate diagnosis guides appropriate treatment decisions, whether conservative management or surgical intervention, and considering the patient's treatment goals and preferences is crucial in determining the best course of action 1.

From the Research

Diagnostic Tests for ACL and PCL Injuries

• The diagnosis of anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) injuries can be challenging, but various tests and imaging modalities can aid in accurate diagnosis 2, 3.
• For ACL injuries, physical examination tests such as the Lachman test, anterior drawer test, pivot shift test, and lever sign test can be used, with the Lachman test and lever sign test showing high sensitivity and specificity 4, 5.
• The accuracy of magnetic resonance imaging (MRI) in diagnosing ACL injuries has been shown to be high, with a sensitivity of 95.45% and specificity of 91.67% 3.
• For PCL injuries, specific injury mechanisms such as the dashboard direct anterior blow and hyperflexion mechanisms can cause isolated PCL injuries, and physical examination can be difficult due to pain and swelling, but specific functional tests can indicate a PCL tear 2.
• Imaging modalities such as standard x-rays, stress views, and MRI can be useful in diagnosing PCL injuries, with MRI being the gold standard for detecting ligament injuries 2.

Comparison of Diagnostic Tests

• A systematic review and meta-analysis found that the lever sign test had the lowest likelihood ratio (LR-) and the pivot shift test had the highest LR+ for diagnosing ACL injuries 4.
• A blinded, cross-sectional evaluation found that the Lachman test was more sensitive than the anterior drawer test, and the lever test should be interpreted with caution 5.
• A review of physical examination tests for ACL injuries found that the Lachman test and anterior drawer test demonstrated clinical utility, but the results of the lever test should be interpreted with caution 6.

Imaging Modalities

• MRI is the gold standard imaging modality for detecting ligament injuries, including ACL and PCL injuries 2, 3.
• Standard x-rays and stress views can be useful in diagnosing PCL injuries, but MRI is more accurate 2.
• The accuracy of MRI in diagnosing ACL injuries has been shown to be high, with a sensitivity of 95.45% and specificity of 91.67% 3.