The posterior cruciate ligament (PCL) is the strongest ligament of the knee. It is intra-articular and extrasynovial. The PCL arises from the anterolateral surface of the medial femoral condyle and reaches the posterior intercondylar area of the tibia. The femoral origin is more anterior than that of anterior cruciate ligament (ACL), and in contrast to the ACL, the PCL is larger at its femoral origin than at its tibial insertion [1]. The tibial attachment is extra-articular, and it is approximately 1 cm distal to the plane of the articular surface [2]. The PCL is the primary restraint to posterior tibial translation relative to the femur and becomes more important in preventing distraction of the joint as the knee reaches higher degrees of flexion [3, 4].

The PCL consists of two functional bundles: the anterolateral and the posteromedial bundle [5]. The anterolateral bundle (65 %) is usually thicker and stronger than the posteromedial bundle (35 %) [6]. Most PCL fibers are not isometric and the bundles have different functions that enable the PCL to resist posterior translation [5]. The length of the ligament is similar to that of the ACL (mean length, 38 mm) [7].

The PCL is seen as a band of low signal intensity in all MR sequences and is usually visualized in its entire length on one or two consecutive sagittal images (Fig. 2.1). The two-bundle anatomy is often well visualized on axial planes, and all axial planes should thoroughly be evaluated for partial (uni-bundle) rupture. To assess the intact attachments of the PCL, axial, sagittal, and coronal images should be evaluated (Fig. 2.1). The axial images are especially helpful for the femoral attachment, whereas the sagittal images are especially helpful for assessing the tibial attachment.

Since the PCL appears as an angulated structure on sagittal MR images, several indirect signs have been described to detect other ligament abnormalities, one of which is the PCL angle. The mean PCL angle is 123° and is smaller in patients with acute anterior cruciate ligament (ACL) tear but also in younger patients without ACL abnormalities [8]. Due to this overlap, indirect signs for ACL tears, i.e., quantitative and semiquantitative signs, should be used with care, especially in the skeletally immature patient [8].

The posterior meniscofemoral ligament (Wrisberg ligament) attaches proximally separately from the posteromedial bundle of the PCL [9]. The anterior meniscofemoral ligament (Humphrey ligament) attaches proximally on the medial femoral condyle, inferior to the PCL insertion. Both ligaments attach distally to the posterior horn of the lateral meniscus and contribute to posterior drawer stability [10]. At least one meniscofemoral ligament is present in 70–93 % of knees [4, 7, 11].

The meniscofemoral ligaments are best seen on sagittal and coronal images as low-signal-intensity bands anterior and posterior to the PCL (Figs. 2.2 and 2.3).

The orientation and characteristic localization of meniscofemoral ligaments should be taken into account in order to differentiate intra-articular lesions such as cartilage displaced and meniscal fragments or pseudotears of the lateral meniscus.

Retrocruciate fat pad is situated between the PCL and the posterior joint capsule (Fig. 2.4) [12]. The fat pad may be involved in several pathological intra-articular changes (e.g., fluid collections, inflammation).

The PCL avulsion fracture involves the detachment of the tibial plateau that results from the pulling of PCL from its attachment point. The mechanisms include a severe hyperextension or a direct blow to the anterior tibia with the knee flexed [14]. An early diagnosis is usually possible on standard radiographs where a bony fragment may be visible. At MR imaging, bone marrow edema and a small bone fragment attached to PCL is seen in acute trauma (Fig. 2.8). In chronic cases, the bone marrow edema is usually absent (Fig. 2.9).

Diffuse thickening (more than 7 mm on sagittal plane) with low or intermediate signal, often seen as focal changes of the PCL, is suggestive for chronic PCL tear [15]. MR imaging shows focal or diffuse changes of low signal intensity on T1-weighted images and intermediate-high signal intensity on T2-weighted images (Fig. 2.10).

Mucoid degeneration is a rare cause of pain in the middle-aged patients without knee instability [16]. The signal intensity on T2-weighted images is different from that of a joint effusion and enables the differentiation from a ganglion cyst (Fig. 2.11) [16].