With the knee in extension, the lateral collateral ligament (LCL) is approximately 6 cm long and 3–5 mm thick [2–7]. The ligament is superficially located and is a static stabilizer during varus angulation. Lateral collateral ligament extends from the lateral femoral condyle, posterior to the lateral epicondyle and 2 cm above the joint line to the fibular head (Fig. 4.1) [8, 9]. At the fibular insertion, the lateral collateral ligament and the biceps tendon form a conjoined tendon (Fig. 4.1) [10]. Between the two structures, the lateral collateral ligament-biceps femoris bursa is constantly described [8].

The anterior oblique band (AOB) is a band of fibrous tissue that extends from the lateral collateral ligament to the lateral portion of tibia [11]. Some fibers of the anterior oblique band (AOB) blend with posterior fibers of the iliotibial tract [11].

On MR images, the lateral collateral ligament appears as a straight homogeneous hypointense structure on all sequences (Fig. 4.2). The anterior oblique band (AOB) is seen on axial MR images as a thin hypointense band extending from the lateral collateral ligament to the lateral tibia and the iliotibial tract (Fig. 4.3).

The popliteal tendon (PT) has its proximal attachment on the lateral femoral condyle, anteroinferiorly to the lateral collateral ligament. Two separate bundles are described at the proximal attachment: the posterior superficial bundle and the anterior deep bundle [12]. The popliteal tendon (PT) is intra-articular and extrasynovial at the level of the femorotibial joint and is surrounded by the popliteal bursa [13]. Distally, the tendon is extra-articular, deep to the fabellofibular ligament and the arcuate ligament [14]. It extends to the popliteal muscle. An extension of the synovial membrane between the posterior horn of the lateral meniscus and the popliteus tendon, known as popliteus or subpopliteus bursa, surrounds the tendon and may communicate with the superior tibiofibular joint (Fig. 4.4) [13]. On MR images, the tendon is hypointense on all sequences (Fig. 4.4). However, magic angle artifacts may occur due to its curved and oblique course especially at the proximal part. Thus, signal intensity irregularities should not be mistaken as tendinopathy or rupture per se but need to be verified on other sequences.

The anterolateral ligament (ALL) is a relatively consistent structure that is found during knee arthroplasty [15]. It was first described as a reinforcement of the lateral capsule. The anterolateral ligament (ALL) takes origin from the lateral femoral condyle just anterior to and blending with the popliteus tendon, and it inserts distally to the lateral meniscus and lateral tibial plateau, typically about 5 mm distal to the joint line [15]. It can be seen on coronal MR images as a thin hypointense linear structure anterior to the popliteal tendon (PT) (Fig. 4.5).

The popliteal tendon (PT) is strongly attached to the lateral meniscus through the two popliteomeniscal fascicles (PMF). The posterosuperior popliteomeniscal fascicle extends from the popliteal tendon to the posterolateral aspect of the lateral meniscus. The anteroinferior popliteomeniscal fascicle is stronger and shorter and extends from the popliteal tendon to the middle third of the lateral meniscus [12, 16]. On MR images, the popliteomeniscal fascicles are inconsistently seen as hypointense structures on sagittal planes (Fig. 4.6).

The popliteofibular ligament (PFL) attaches the popliteal tendon (PT) to the fibular head. It originates proximal to the myotendinous junction of the popliteus muscle and attaches to the medial fibular styloid [17]. The popliteofibular ligament (PFL) is approximately 10 mm long having a thickness similar to the lateral collateral ligament [12, 18]. The ligament is inconsistently identified on coronal MR images as a hypointense band (Fig. 4.7).

The arcuate ligament (AL) is a Y-shaped thickening of the capsule with a medial and a lateral limb. The body of the arcuate ligament (AL) originates from the lateral edge of the styloid process of the fibula [19]. The lateral limb blends with the capsule and inserts to the lateral femoral condyle near the lateral gastrocnemius muscle (Fig. 4.8). The medial limb attaches to the posterior capsule.

The fabellofibular ligament (FFL) attaches proximally to the fabella and extends inferiorly and vertically to the fibular styloid process (Fig. 4.9). However, the fabellofibular ligament (FFL) may be present even in the absence of fabella [8, 20].

The arcuate ligament (AL) and the fabellofibular ligament (FFL) are not always present in anatomical studies. When present, the normal arcuate ligament (AL) and fabellofibular ligament (FFL) are depicted on MR images on coronal (arcuate ligament and fabellofibular ligament) and sagittal planes (fabellofibular ligament) as hypointense homogeneous bands (Figs. 4.8 and 4.9). However, in the authors’ experience, it remains difficult to identify both structures in the clinical routine imaging. One has to look specifically for these structures, and high-resolution images are mandatory for that.

The sprain with intact fibers of the lateral collateral ligament (LCL) implies the presence of edema around the ligament with intact fibers (Fig. 4.10). A partial tear of the lateral collateral ligament (LCL) is seen on MR images as inhomogeneous signal intensity within the ligament. The ligament may be thinned or thickened without complete interruption of the fibers, and high-signal-intensity edema around the ligament is typically present (Figs. 4.11 and 4.12).

Most lesions of the popliteal tendon (PT) are extra-articular at the myotendinous junction. In the case of a partial tear of the popliteal tendon (PT), the injury is associated with edema and hemorrhage within the tendon and the musculotendinous junction. On MR images, the partial tear is seen as an amorphous or feathery signal intensity changes that may extend also into the muscle belly (Fig. 4.13).

In complete tears, there is a discontinuity of the involved anatomical structures that can be associated with waviness of the remaining ligament. In complete LCL tears, there is a discontinuity of the fibers, and edema or hemorrhage of high signal intensity on fluid-sensitive MR images is detected at the tendon defect (Figs. 4.14 and 4.15). A complete discontinuity of the popliteus musculotendinous junction with tendon retraction indicates a complete tear. When a hematoma is present, enlarged muscle volume and peri-fascial fluid collections are frequently seen [1]. In some cases, avulsion of the femoral insertion of popliteal tendon (PT) may be present.

Individual assessment of the integrity of the anterolateral ligament (ALL), arcuate ligament (AL), popliteofibular ligament (PFL), and fabellofibular ligament (FFL) may not be possible [17]. MR signal abnormalities around the capsule, surrounding soft tissue edema or hemorrhage, and the lack of visualization of these structures are suggestive for popliteomeniscal fascicles (PMF) (Fig. 4.16), anterolateral ligament (ALL), arcuate ligament (AL), and fabellofibular ligament (FFL) tears. Normally, there should be fat tissue in this region which presents as homogeneous low signal intensity on fat-suppressed T2-weighted images (Fig. 4.17). Hyperintense signal on fat-suppressed T2-weighted MR images located posterior to the popliteal tendon (PT) suggests capsular tearing.