The Posterior Drawer and Sag in PCL Tears
A PCL tear is easy to miss in a swollen knee, and a missed corner injury that comes with it can leave the patient with a knee that sags backward and wears out early.
Key terminology
- Posterior cruciate ligament (PCL)
- This ligament runs inside the knee from the medial femoral condyle to the back of the tibia. It is the main restraint to backward tibial translation.
- Posterior drawer
- With the knee bent to 90°, the examiner pushes the upper tibia backward. More backward glide than on the other knee points to a PCL tear.
- Step-off
- This ledge is the front of the medial tibial plateau sitting ahead of the femoral condyles in a bent knee. The examiner feels it with the thumbs during the drawer.
- Posterolateral corner (PLC)
- This group of structures sits at the back outer corner of the knee and includes the lateral collateral ligament and popliteus tendon. It resists varus and outward rotation of the tibia.
- Dial test
- With the patient prone and both knees bent, the examiner turns both feet outward and compares the angles. Extra outward rotation on the injured side points to a posterolateral corner injury.
- Bone bruise
- This MRI finding is marrow edema from bone striking bone, with no fracture line. Its location shows which bones collided.
- Stress radiograph
- This lateral knee film is taken while the tibia is pushed backward, for example with the patient kneeling. It measures in millimeters how far the tibia sags.
Takeaways
- The posterior cruciate ligament (PCL) is the main restraint to the tibia sliding backward on the femur. It is the largest and strongest ligament in the knee, made of a larger anterolateral bundle and a smaller posteromedial bundle. Both bundles do more work in flexion, and the PCL also resists rotation beyond 90° of flexion (Pache et al, Arch Bone Jt Surg 2018;6:8-18, PMID 29430489; Winkler et al, Knee Surg Sports Traumatol Arthrosc 2021;29:672-681, PMID 33201271).
- The PCL tears from a backward blow to the front of the tibia with the knee bent. The classic story is a dashboard strike or a fall onto a flexed knee, vs the non-contact pivot of an ACL tear. A PCL injury was found in 38% of 222 acute knee hemarthroses at a trauma center, and 96.5% of those came with another ligament injury (Fanelli and Edson, Arthroscopy 1995;11:526-529, PMID 8534292; Logterman et al, Curr Rev Musculoskelet Med 2018;11:510-514, PMID 29855794).
- The posterior drawer at 90° of flexion is the best single exam for a PCL tear. The examiner pushes the upper tibia backward and feels the step-off at the front of the knee. In a blinded study of chronic isolated tears, sports surgeons were 96% accurate, with 90% sensitivity and 99% specificity. A 2013 systematic review called the accuracy of most PCL tests largely unknown (Rubinstein et al, Am J Sports Med 1994;22:550-557, PMID 7943523; Kopkow et al, J Orthop Sports Phys Ther 2013;43:804-813, PMID 24175598).
- The tibia normally sits in front of the femoral condyles, and a torn PCL lets it sag backward. With hips and knees flexed to 90°, gravity drops the unsupported tibia back and the step-off flattens (posterior sag sign). Tears are graded by backward translation of <5 mm, 5-10 mm and >10 mm. In grade I the tibia still sits in front of the condyles (Winkler et al, Knee Surg Sports Traumatol Arthrosc 2021;29:672-681, PMID 33201271; StatPearls NBK430726).
- A grade III PCL tear should prompt a search for a posterolateral corner (PLC) injury. The PLC works with the PCL to resist backward glide, varus and outward rotation. Combined PCL and PLC injury occurs in 15-42% of PCL tears. On the dial test a side-to-side difference of ≥10° counts as positive, and agreement between examiners is only fair (Winkler et al, Knee Surg Sports Traumatol Arthrosc 2021;29:672-681, PMID 33201271; Slichter et al, J Exp Orthop 2018;5:18, PMID 29900519; StatPearls NBK430726).
- An isolated PCL tear usually presents with pain rather than giving way. Pain sits at the front, inner side or back of the knee and is worse walking uphill or downhill. Instability is rare in an isolated tear and becomes more common in chronic and combined injuries. Isolated tears are uncommon, with an estimated incidence of 1-6% (Winkler et al, Knee Surg Sports Traumatol Arthrosc 2021;29:672-681, PMID 33201271).
- MRI is close to perfect for an acute PCL tear and can miss a chronic one. In 203 knees MRI had no false positives or false negatives, with only 13 torn PCLs. A chronic tear can heal stretched and still look intact. Bone bruises appear in more than 80% of acute PCL injuries, most often in the anteromedial knee, vs the lateral bruises of an ACL tear (Gross et al, Am J Sports Med 1992;20:732-737, PMID 1456368; Winkler et al, Knee Surg Sports Traumatol Arthrosc 2021;29:672-681, PMID 33201271).
- An isolated low-grade PCL tear is treated without surgery, and long-term results are mixed. In 133 athletes treated nonoperatively, half returned to the same sport at the same level, and laxity grade did not predict function. Other series found occasional pain in 81% and more arthritis than after reconstruction (44.1% vs 21.5%) (Shelbourne et al, Am J Sports Med 1999;27:276-283, PMID 10352760; Boynton and Tietjens, Am J Sports Med 1996;24:306-310, PMID 8734880; Schroven et al, Knee Surg Sports Traumatol Arthrosc 2022;30:1733-1743, PMID 34505176).