The posterior cruciate ligament (PCL) is one of the thickest and strongest ligaments in the knee joint, located in the center of the knee. Its main function is to limit the posterior displacement and external rotation of the tibia relative to the femur. PCL injuries account for approximately 1% to 47% of acute knee ligament injuries, mostly occurring in severe collisions such as car accidents or contact sports. Clinically, PCL injuries are classified into three grades—Grade I, Grade II, and Grade III—based on the posterior drawer test of the knee joint. This grading system is primarily based on the posterior displacement of the tibia in the posterior drawer test, combined with pathological changes in the ligament and joint stability for a comprehensive assessment.
Grade 1 Injury: Mild – Microscopic Tear, Remaining Stable
Grade 1 injuries are mild, characterized by a small tear or microscopic tear of the ligament fibers. Pathologically, the ligament fibers are stretched but the overall structure remains intact, or only a very few fiber bundles are ruptured. In the posterior drawer test, the posterior displacement of the tibia relative to the femur is less than 5 mm. At this stage, the tibial plateau remains anterior to the femoral condyle. Clinically, patients with Grade 1 injuries typically do not experience significant joint instability; some may even be unaware of the injury or have no pain. These injuries are usually treated conservatively, including rest, ice application, compression bandages, and limb elevation.
Grade 2 Injury: Moderate – Partial Tear, Impaired Stability
Grade 2 injuries are moderate, characterized by the rupture of more ligament fibers, representing a partial tear of the ligaments. The integrity of the ligaments is significantly compromised, but not completely severed. In the posterior drawer test, the tibia shifts posteriorly by 5–10 mm. At this stage, the tibial plateau is essentially level with the femoral condyle. Clinically, patients with Grade 2 injuries experience mild to moderate joint instability, and may feel weakness in the knee joint when standing, walking, or undergoing physical examination. Conservative treatment is usually the first consideration for these injuries, and with bracing and systematic rehabilitation training, most patients achieve good results.
Grade 3 Injury: Severe – Complete Tear, Significant Instability
A Grade 3 injury is a severe injury, characterized by a complete rupture of the ligament. In the posterior drawer test, the tibia shifts posteriorly by more than 10 mm, with the tibial plateau regressing posterior to the femoral condyle. Significant knee instability is observed at this stage. It is important to note that Grade 3 injuries are often not isolated and are frequently accompanied by injuries to other important knee structures, such as posterolateral horn injuries, medial collateral ligament injuries, or anterior cruciate ligament injuries. Treatment strategies for this type of injury are somewhat controversial. Conservative treatment may be attempted for elderly patients or those with low activity levels; while for younger patients with high activity levels or those for whom conservative treatment is ineffective, arthroscopic PCL reconstruction surgery should be considered. Surgical indications are more clearly defined in complex cases such as multiple ligament injuries or meniscus root avulsion.
Diagnosis and Assessment
The grading and diagnosis of PCL injuries rely on standardized physical examination and imaging evaluation. The posterior drawer test is the most crucial physical examination method for diagnosis and grading. MRI has a sensitivity of 95%–100% for diagnosing PCL injuries, clearly showing ligament continuity and signal changes, and is an important imaging tool for assessing the degree of injury. Stress X-rays can provide quantitative evidence for grading by objectively measuring the difference in posterior tibial displacement between the two knees.
Prognosis and Rehabilitation
The PCL has a certain self-healing ability, especially the middle and distal third of the solid portion, which has a rich blood supply and strong healing potential after rupture. Therefore, for acute grade 1 and 2 PCL injuries, conservative treatment should be actively pursued. Studies have shown that patients with isolated grade 2 or 3 PCL injuries who receive conservative treatment can return to sports on average after 16 weeks. For patients undergoing surgical treatment, systematic rehabilitation training is required postoperatively, and long-term follow-up is recommended to assess knee joint function recovery and the occurrence of complications.
Overall, the grade 1–3 PCL injury grading system provides clinicians with a systematic framework from pathological changes and the degree of joint instability to the selection of treatment strategies. Accurate injury grading is a prerequisite for developing individualized treatment plans, predicting prognosis, and guiding rehabilitation. For the general public, understanding this classification system helps in seeking timely medical attention after a knee injury, accurately describing symptoms, and communicating effectively with doctors.