What Is the Posterior Drawer Knee Examination?
A guide to the Posterior Drawer Test, a clinical examination that assesses the integrity of the Posterior Cruciate Ligament and overall knee stability.
When knee stability is in question, clinicians use physical examinations to assess the joint’s internal structures. One such assessment is the posterior drawer test, a manual procedure used to evaluate the integrity of the posterior cruciate ligament (PCL). This test helps a healthcare provider determine if the PCL has been stretched or torn, which can cause pain and instability.
The Role of the Posterior Cruciate Ligament
The posterior cruciate ligament (PCL) is one of four ligaments that stabilize the knee, connecting the thigh bone (femur) to the shin bone (tibia). The PCL is the strongest ligament in the knee and is considerably stronger than its more commonly known counterpart, the anterior cruciate ligament (ACL). Its primary function is to prevent the tibia from moving too far backward in relation to the femur. This restraint is important during activities that involve knee flexion while bearing weight, such as walking downhill, as it protects the knee from excessive motion.
Understanding PCL Injuries
PCL injuries result from a powerful force applied to the front of a bent knee. A common example is a “dashboard injury” during a car accident, where the knee is forcefully pushed against the dashboard. In sports, these injuries can happen when an athlete falls hard on a bent knee or from a direct blow during contact sports like football or soccer.
The symptoms often include immediate pain and swelling in the knee. A person might also experience stiffness, a feeling that the knee is unstable, and difficulty walking. In some cases, a distinct popping sound may be heard at the moment of injury.
Performing the Posterior Drawer Test
During the posterior drawer test, the patient lies on their back with their hip flexed to 45 degrees and the affected knee bent to 90 degrees. The patient’s foot is kept flat on the examination table, and the examiner often sits on the foot to keep the leg stable. For an accurate assessment, the patient’s hamstring muscles must be relaxed.
The examiner grasps the upper part of the shinbone with both hands, placing their thumbs on the bony prominence just below the kneecap. From this position, the examiner applies firm, steady pressure, pushing the tibia straight back. The clinician feels for the amount of backward movement and notes the quality of the “end feel,” which is the sensation felt when the joint reaches its motion limit.
Interpreting Posterior Drawer Test Results
A “negative” test means there is no excessive backward movement, indicating the ligament is likely intact. A “positive” test is shown by increased posterior movement of the tibia compared to the uninjured knee, suggesting a PCL tear. The results are graded to classify the injury’s severity.
A Grade 1 injury involves minimal movement (less than 5mm). A Grade 2 injury occurs when the tibia moves backward to become flush with the femoral condyles (5-10mm). A Grade 3 injury signifies a complete tear, where the tibia can be pushed backward beyond the femoral condyles (more than 10mm).
Clinical Importance of the Posterior Drawer Test
The posterior drawer test is an accurate tool for diagnosing PCL tears, with studies reporting high rates of success. The test is effective at both correctly identifying PCL tears when present and ruling them out when they are not, especially for more severe (Grade 2 and 3) tears. It is used in combination with other assessments and the patient’s history.
Observing a “posterior sag sign,” where the tibia visibly droops backward due to gravity, can also aid the diagnosis. If the physical exam suggests a PCL injury, a clinician may order an MRI to confirm the diagnosis and check for other injuries.