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4.1 Anatomy and Normal MRI Appearance

The posterolateral corner (PLC) of the knee is a complex functional unit consisting of several important ligaments and is responsible for posterolateral stabilization of the joint [1]. There is some variability in the definition of the posterolateral corner (PLC) in the literature, but most descriptions include the lateral collateral ligament (LCL), the anterior oblique band (AOB), the popliteal tendon (PT) with the anterolateral ligament (ALL), the popliteomeniscal fascicles, and the popliteofibular ligament (PFL), as well as the posterolateral capsule including the arcuate ligament (AL) and the fabellofibular ligament (FFL) [1]. These structures are reinforced by the biceps femoris tendon and the iliotibial tract.

The posterolateral corner limits posterior translation, varus angulation, and excessive external rotation. The popliteal tendon (PT) is considered to be a dynamic stabilizer, and the lateral collateral ligament (LCL), the fabellofibular ligament (FFL), the popliteofibular ligament (PFL), and the arcuate ligament (AL) represent static posterolateral stabilizers [1].

4.1.1 Lateral Collateral Ligament (LCL) and the Anterior Oblique Band (AOB)

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].

Fig. 4.1
figure 1

Normal lateral collateral ligament (LCL) in a 21 year old male. Three consecutive coronal proton-density (PD) FSE images from anterior to posterior (a–c) show the normal LCL extending from the lateral femoral condyle to the fibular head (large arrows in a and b). At the fibular insertion LCL (large arrow in c) forms a conjoined tendon with the biceps tendon (small arrow in c)

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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).

Fig. 4.2
figure 2

Normal lateral collateral ligament (LCL) in a 22 year old male. In this case, the LCL is visualized on a single coronal proton-density (PD) FSE image as a continuous hypointense band (large arrows). At the distal insertion, the LCL and the biceps tendon (curved arrow) form a conjoined tendon

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Fig. 4.3
figure 3

Normal anterior oblique band (AOB) in a 17 year old male. Axial proton-density (PD) FSE fat-suppressed image shows the anterior oblique band (AOB) (small arrow) extending from the lateral collateral ligament (LCL) (large arrow) to the iliotibial tract (curved arrow)

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4.1.2 Popliteal Tendon (PT), Anterolateral Ligament (ALL), Popliteomeniscal Fascicles (PMF), and Popliteofibular Ligament (PFL)

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.

Fig. 4.4
figure 4

Normal popliteal tendon (PT) and popliteus or subpopliteus bursa in a 19 year old female. Coronal proton-density (PD) FSE image (a) shows the hypointense PT at its femoral insertion (large arrows). On axial proton-density (PD) FSE fat-suppressed images (b, c), the PT is seen in contact with the lateral femoral condyle (large arrows in b, c). A small subpopliteus bursa (curved arrow in c) is seen between PT and posterior horn of lateral meniscus

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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).

Fig. 4.5
figure 5

The anterolateral ligament (ALL) in a 22 year old male. Coronal proton-density (PD) FSE image shows the ALL (large arrow) with its insertion on the lateral femoral condyle. The ligament blends with the popliteus tendon (small arrow)

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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).

Fig. 4.6
figure 6

The popliteomeniscal fascicles in a 19 year old female. Sagittal proton-density (PD) FSE image shows the posterosuperior popliteomeniscal fascicle (large arrow) and the anteroinferior popliteomeniscal fascicle (small arrow) strongly attached to the lateral meniscus

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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).

Fig. 4.7
figure 7

The popliteofibular ligament (PFL) in a 19 year old female. Sagittal proton-density (PD) FSE image (a) shows the thick hypointense PFL (large arrow) inserting on the fibular head. On coronal proton-density (PD) FSE image (b) through the same level (line in a), the PFL (large arrow) is identified and connects the fibular head with the popliteal tendon (small arrow)

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4.1.3 Arcuate Ligament (AL) and Fabellofibular Ligament (FFL)

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.

Fig. 4.8
figure 8

The arcuate ligament (AL) in a 19 year old female. Coronal proton-density (PD) FSE image shows a thin hypointense band representing the lateral limb of AL (arrows) which originates from the lateral edge of the styloid process of the fibula and inserts to the lateral femoral condyle

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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].

Fig. 4.9
figure 9

The fabellofibular ligament (FFL) in a 35 year old male. Sagittal proton-density (PD) FSE image shows the FFL (arrows) which extends from the fabella (curved arrow) to the fibular styloid process on the fibular head. The fabellofibular ligament (FFL) may be present even in the absence of fabella

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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.

4.2 MRI Pathological Findings

Although posterolateral corner injuries are not as common as injuries to the medial collateral ligament, they are more complex and more difficult to diagnose on physical examination. The most typical injury mechanism is a direct varus force to the anteromedial aspect of the hyperextended knee.

Almost all posterolateral corner lesions are associated with other injuries. The more common associated lesions are anterior and posterior cruciate ligament tears, medial collateral and medial meniscus injuries, anteromedial tibial plateau contusion or fracture, and the Segond fracture [1]. Untreated posterolateral injuries may be associated with chronic instability of the knee, failure of cruciate ligament reconstructions, and osteoarthritis [21, 22].

Appearance on MR images depends upon what structures are injured and the degree of injury. Lesions involving well-defined and well-delineated structures such as the lateral collateral ligament (LCL) and popliteal tendon (PT) can be classified on MR images into partial or complete tear. In the case of all the small structures mentioned above which, in addition, are inconsistently present and part of the capsule, the MR description should only refer to as “probably torn” based mainly on indirect signs.

4.2.1 Sprain and Partial Tears

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).

Fig. 4.10
figure 10

Lateral collateral ligament (LCL) sprain in a 21 year old male. Coronal proton-density (PD) FSE fat-suppressed image shows extensive edema (arrow) around the proximal portion of LCL with intact fibers

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Fig. 4.11
figure 11

Lateral collateral ligament (LCL) partial tear in a 29 year old female. Coronal proton-density (PD) FSE fat-suppressed image (a) shows intrasubstance signal changes (arrow) at the proximal insertion of LCL without complete discontinuity of the ligament. The lesion is confirmed on axial proton-density (PD) FSE fat-suppressed image (b) where the tear is seen as a hyperintense linear signal lesion within the ligament (arrow)

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Fig. 4.12
figure 12

Lateral collateral ligament (LCL) partial tear in a 17 year old male. Coronal proton-density (PD) FSE fat-suppressed image (a) and axial proton-density (PD) FSE fat-suppressed image (b) show intrasubstance signal changes (arrow in a, b) at the proximal insertion of LCL without complete interruption of the fibers and high-signal-intensity edema around the ligament

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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).

Fig. 4.13
figure 13

Partial tear of the popliteal tendon (PT) in a 45 year old male. Coronal proton-density (PD) FSE fat-suppressed image (a) and axial proton-density (PD) FSE fat-suppressed image at the level of the femoral insertion (b) show intrasubstance signal changes (arrow in a, b). The lesion extension is usually better evaluated on serial axial images as in this case in which a more caudally axial (PD) FSE fat-suppressed image (c) shows the intrasubstance partial tear (arrow)

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4.2.2 Complete Tears

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.

Fig. 4.14
figure 14

Complete tear of the lateral collateral ligament (LCL) in a 25 year old soccer player. Coronal proton-density (PD) FSE image (a) shows discontinuity of the ligament (large arrow). Note the normal popliteal tendon (small arrow). Sagittal T2-weighted fat-suppressed image (b) demonstrates the interruption of the fibers (arrow) without a wavy contour of the ligament

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Fig. 4.15
figure 15

Complete tear of the lateral collateral ligament (LCL) in a 22 year old male. Coronal proton-density (PD) FSE fat-suppressed image (a) and axial proton-density (PD) FSE fat-suppressed image (b) show a complete tear of LCL with edema or hemorrhage at the site of the lesion (arrow in a, b)

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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.

Fig. 4.16
figure 16

Sagittal T2-weighted fat-suppressed image (a) and sagittal proton-density (PD) FSE image (b) show signal abnormalities around the capsule (edema) and the lack of visualization of the small posterolateral ligaments (arrows in a, b) suggestive for popliteomeniscal fascicles tears

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Fig. 4.17
figure 17

Normal appearance of the posterolateral capsular region in a 30 year old male. Sagittal T2-weighted fat-suppressed image (a) and sagittal proton-density (PD) FSE image (b) show the presence of homogeneous fat behind the popliteal tendon (PT) hypointense on fat-suppressed image (large arrow in a) and hyperintense on proton-density (PD) FSE image (large arrow in b). Note the presence of a small vessel adjacent to the posterior margin of the popliteal tendon (PT) (small arrow in a, b) which should not be interpreted as edema

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4.2.3 The “Arcuate” Sign and the Segond Fracture

The presence of a small bone fragment detached from the proximal head of fibula on radiography is known as the “arcuate” sign (Fig. 4.18). The sign may indicate avulsion of lateral collateral ligament (LCL), biceps tendon (BT), arcuate ligament (AL) (Fig. 4.19), popliteofibular ligament (PFL), or fabellofibular ligament (FFL) from the fibular insertion, and MR imaging is necessary for complete evaluation [1].

Fig. 4.18
figure 18

The arcuate sign in a 34 year old male. Plain radiography (a) shows a fracture of the fibular head (arrow). Coronal proton-density (PD) FSE image (b), sagittal T2-weighted fat-suppressed image (c), and axial proton-density (PD) FSE fat-suppressed image (d) show partial detachment of the bone fragment (arrow in b–d) at the insertion of the lateral collateral ligament and biceps tendon

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Fig. 4.19
figure 19

The arcuate sign in a 40 year old female. Coronal proton-density (PD) FSE fat-suppressed image shows the fracture of the fibular head (large arrow) with extensive edema in the posterolateral corner. Note the absence of visualization of any band-like structure. In this case, the aspect suggests a tear of the arcuate ligament (AL) (small arrow) with loss of its Y shape

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The Segond fracture is an avulsion of the lateral capsule from the lateral tibial plateau and may be an indicator of an isolated posterolateral corner injury (Fig. 4.20) [23–25]. The lesion may be also associated with anterior cruciate ligament and posterior cruciate ligament tears.

Fig. 4.20
figure 20

Segond fracture. Plain radiography shows a small bone fragment detached from the lateral tibial plateau from the area of the insertion of the lateral capsule (arrow). The MR imaging examination is mandatory in all cases because the Segond fracture may be an indicator of posterolateral corner injuries and always suggest an anterior cruciate ligament involvement

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4.3 Role of Preoperative MRI

The structures that need to be identified on the lateral and posterolateral side of the knee on preoperative MRI are the iliotibial band (ITB), lateral collateral ligament (LCL), biceps femoris tendon, popliteus tendon, and capsular structures. Injury may occur to all or a portion of these structures. It is critical, however, to recognize injury to the key stabilizers, such as the lateral collateral ligament, the conjoined tendon of the biceps femoris, and the popliteofibular ligament.

The iliotibial band, the terminal extension of the tensor fascia lata, inserts distally onto Gerdy’s tubercle. It is uncommon to injure the iliotibial band. However, any discontinuity or significant injury is indicator for a high-energy injury, and one should look for associated injuries of other structures.

It is more common to injure the structures of the posterolateral corner in combination rather than in isolation [26]. The most frequent combination of injury is a lateral collateral ligament-popliteus injury (52 %) followed by a lateral collateral ligament-popliteus-biceps tendon injury (33 %). It is important to identify the location of the lateral collateral ligament rupture, whether it is at the femoral origin, the midsubstance, or at the proximal fibula. It is also important to identify and describe an avulsion of the biceps femoris tendon, particularly if there is retraction. Failure to recognize and repair an avulsed and retracted conjoined tendon early on may have serious consequences because it is often impossible to reattach a retracted biceps femoris in a delayed surgery or in a chronic rupture.

Meniscal injuries are important to recognize in the context of multiligamentous knee injuries. The high energy involved in this injury pattern frequently results in meniscal damage, including root avulsions [27]. The presence of a displaced meniscal tear may dictate that early intervention is warranted. Meniscal root avulsions may also be overlooked given the complexity of the injury; however, failure to recognize these and address them at the time of surgery may have a negative effect on outcome.

4.4 MRI Postoperative Findings

4.4.1 Indications for Posterolateral Corner Repair/Reconstruction

The indications for treatment of posterolateral corner (PLC) injuries are based on a multitude of factors, including the specific anatomical structures of the posterolateral corner (PLC) that are injured, the grade of injury, the presence of concomitant injuries of the knee, and the period of time since the injury. The indications for treatment and methods of treatment are not universally accepted and remain somewhat controversial. The complexity of this issue is further compounded by the difficulty of establishing a clear diagnosis of posterolateral corner (PLC) injury [28–31]. Failure to recognize an injury to the posterolateral corner (PLC) in the context of a multiligament knee injury has been reported to cause a significant loss of function and, e.g., to contribute to cruciate ligament reconstruction graft failure [21, 28, 32–35]. In contrast, early treatment of these injuries has been shown to be associated with improved objective, subjective, and functional outcomes [10, 32]. This further emphasizes the importance of establishing the diagnosis by early MR imaging.

An accurate diagnosis is achieved by a combination of a detailed history of the injury mechanism, a comprehensive physical examination, and a variety of imaging examinations, including radiographs, CT (in case of fractures of the tibia), and MR imaging. It is of critical importance to investigate for the presence of a vascular injury, which takes precedence in terms of treatment. Following this, any associated nerve injury, in particular damage to the common peroneal nerve, should be ruled out. Next, the grade of the injury according to the International Knee Documentation Committee (IKDC) classification, the specific structures that are damaged including any bony injury, and the chronicity of the injury need to be ascertained. An understanding of the true incidence of posterolateral corner injuries in the context of acute knee injuries is also critical to heighten the awareness and suspicion of this serious injury pattern [34].

4.4.2 Operative Versus Nonoperative Management (Table 4.1)

Table 4.1 Posterolateral corner injuries – indications for treatment
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The treatment of knee dislocations in the literature remains controversial. However, the current evidence-based medicine, although limited to a few level III studies, does support operative management. Surgical treatment of knee dislocations showed improved overall knee function, stability, and patient satisfaction compared to the nonoperative treatment [36].

The best surgical treatment of an unstable posterolateral corner (PLC) remains unclear [22, 37]. A number of authors have proposed acute repair of tears of the posterolateral corner if the tissue quality of the torn structures is adequate [22, 38]. If the tissue quality is insufficient or with chronic posterolateral corner (PLC) instability, a wide variety of reconstructive procedures have been advocated to achieve stability. Stannard et al. [39], in a cohort study comparing acute repair versus reconstruction, demonstrated superior results with reconstruction versus repair in the acute setting for high-energy posterolateral corner injuries. The authors recommended that repairs be reserved to treat avulsion fractures with good quality tissue [39]. In reality, a combination of reconstruction and repair is required in most cases of acute high-grade posterolateral corner (PLC) injuries [40].

The timing of surgery may be acute (<3 weeks) or chronic (>6 weeks). In general, operating acutely has been shown by a number of authors to yield better results [41–43]. In patients with bony avulsions or impaction fractures, the earlier the treatment is performed, the easier it is to achieve an anatomical repair or correct the bony deformity. The repair may incorporate reconstruction procedures as well if necessary. The advantages of early, definitive treatment include avoiding the need for multiple procedures, allowing healing of the soft tissue injury, fracture, and reconstruction at the same time, and reducing the potential rehabilitation time. However, it should be considered that early reconstruction is not always possible due to the presence of other significant comorbidities. Injury and retraction of the biceps femoris is a relative indication for early surgical intervention, as delayed treatment may be extremely difficult due to retraction and contracture of the avulsed conjoined tendon of the short and long heads of biceps femoris to the fibular head. Given that multiligamentous injuries are complex and involve a spectrum of injury, some surgeons advocate “staged” surgery. In this setting, the posterolateral corner injuries may be dealt initially, followed by delayed intervention to address the cruciate ligaments [39].

4.4.3 Posterolateral Corner Structures Typically Repaired/Reconstructed

The aim of surgical repair is to reattach any avulsed ligaments or capsular structures to their anatomic location. Avulsion fractures or capsular avulsions are reduced and held in place with a variety of technique, including suture anchors, trans-osseous sutures, and screw fixation. The structures that most frequently require reconstruction are the lateral collateral ligament, the popliteus tendon unit, and the popliteofibular ligament. The choice to proceed with an isolated fibular-based reconstruction or a combined, “two-tailed,” reconstruction involving both the fibular head and the proximal tibia is based on whether the proximal tibiofibular joint has been disrupted and the presence of a hyperextension external rotation recurvatum deformity.

There are a number of different surgical techniques described. Fanelli et al. [44] described a combine PCL and PLC reconstruction involving biceps tenodesis with a posterolateral capsular shift and demonstrated significant improvement in knee stability at 2–10-year follow-up. However, Fanelli et al. [45] has reported that this technique is not as effective at controlling posterolateral instability as a fibular-based free graft. Some authors have described a fibular-based reconstruction technique, which is frequently combined with capsular repair or plication [37]. In the recent past, the combination of tibial and fibular reconstruction with the goal of reconstructing the LCL, the popliteus tendon, and the popliteofibular ligament has been proposed and is being increasingly used [39, 46, 47]. Each technique differs slightly in the position of the femoral tunnels and the orientation of the grafts, but with the mutual aim of restoring the tibiofibular joint stability.

4.4.4 Role of Postoperative MRI

Postoperatively, MR imaging is important to identify the integrity of the reconstruction and the presence of any concomitant intra-articular pathology. In the context of multiligamentous injury, the reconstruction frequently necessitates a number of tunnels in both the femur and tibia. Postoperative graft rupture may occur. Additionally, it is important to take into account other injuries, especially to the menisci, which may have been overlooked acutely because of the extent of soft tissue damage. One should also be aware of the possibility of iatrogenic injury to the meniscal root, which may occur because of aberrant tunnel placement. As with any soft tissue graft reconstruction, tunnel widening may occur over time. Although this may be possible to see on MRI, CT scans may also be required to investigate the location and extent of widening. It is important to be cognizant that in the setting of a failed reconstruction, mechanical malalignment may be a causative factor. Therefore, additional imaging modalities such as three-foot standing films may also be helpful in selected cases.

4.5 MRI Impression

4.5.1 Nonoperative Lateral Collateral Ligament and Posterolateral Corner

Lesions involving well-defined and well-delineated structures:

  1. 1.

    Lateral collateral ligament (LCL) lesions:

    1. (a)

      Sprain with intact fibers

    2. (b)

      Partial tear of lateral collateral ligament with or without posterolateral corner lesions

    3. (c)

      Complete tear of lateral collateral ligament with or without posterolateral corner lesions

  2. 2.

    Popliteus tendon (PT) lesions:

    1. (a)

      Partial tear of PT with or without posterolateral corner lesions involving the musculotendinous junction

    2. (b)

      Complete tear of PT with or without posterolateral corner lesions

    3. (c)

      Avulsion of the femoral insertion of PT

  3. 3.

    Avulsion of the head of fibula (“arcuate” sign) with or without lesions of posterolateral corner (PLC)

  4. 4.

    Segond fracture with or without lesion of posterolateral corner (PLC)

Lesions involving small structures that are inconsistently present and that are part of the capsule

  1. 1.

    Probably torn based on indirect signs (edema, hemorrhage, lack of visualization)

4.5.2 Postoperative Lateral Collateral Ligament and Posterolateral Corner

  1. 1.

    Normal postoperative MRI – normal positioning of the tunnels without any signs of postoperative complications

  2. 2.

    Postoperative graft tears

  3. 3.

    Tunnel or tunnel widening

  4. 4.

    Normal menisci or postoperative meniscal injuries (iatrogenic meniscal root injuries)