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Year : 2018  |  Volume : 11  |  Issue : 3  |  Page : 203-209  

Reviewing the magnetic resonance imaging anatomy of the posterolateral corner of the knee and its applied importance

Department of Radiodiagnosis, Dr. D. Y. Patil Medical College, Hospital and Research Centre, Dr. D. Y. Patil Vidyapeeth, Pune, Maharashtra, India

Date of Web Publication29-Jun-2018

Correspondence Address:
Vipul Sehrawat
H. No - 42, Sector-10, Dwarka, New Delhi - 110 075
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/mjdrdypu.MJDRDYPU_201_17

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The purpose of this article is to review the anatomy and magnetic resonance imaging appearances of all the structures which are noted in the posterolateral corner of the knee. These structures may not be commonly heard of but play a crucial role in providing the required stability and force to the knee joint. We hope that by putting light on this less explored part of the knee this article will be useful to many practicing clinicians and operating orthopedicians.

Keywords: Knee, lateral knee joint, posterolateral corner

How to cite this article:
Kharat A, Sehrawat V. Reviewing the magnetic resonance imaging anatomy of the posterolateral corner of the knee and its applied importance. Med J DY Patil Vidyapeeth 2018;11:203-9

How to cite this URL:
Kharat A, Sehrawat V. Reviewing the magnetic resonance imaging anatomy of the posterolateral corner of the knee and its applied importance. Med J DY Patil Vidyapeeth [serial online] 2018 [cited 2021 Jun 12];11:203-9. Available from: https://www.mjdrdypv.org/text.asp?2018/11/3/203/235559

  Introduction Top

The posterolateral corner (PLC) was once considered as the most poorly understood side due to the complex and variable anatomy superimposed on the inconsistent terminology used in the literature to describe the structures in this region. Although infrequent, injuries to the PLC can lead to devastating consequences, including chronic knee instability, cartilage damage, and failed cruciate ligament reconstruction, if they are not detected and reported in cross-sectional imaging studies like the magnetic resonance imaging (MRI). Timely diagnosis and surgical intervention are imperative for improving long-term outcomes.

The increased awareness and recognition of these previously elusive injuries can be attributed to recent studies in which the anatomic detail of this region was defined, as well as advancements in imaging. In this article, we review the anatomy, biomechanics, and imaging appearances of the PLC in the hope of demystifying this region of the knee.

In this review article, we tend to review the biomechanics, anatomy as well as some common injuries with the help of example cases which may prove useful in reporting the modern-day PLC images on MRI.

  Materials and Methods Top

A group of fifty patients with or without any pathological or traumatic history were scanned on a 3T Siemens scanner. The following sequences were taken in all the patients: proton density-weighted sequences (PDFS), T2W, T1W, and short tau inversion recovery in coronal, axial, and sagittal planes. Since the anatomy of the knee and its surrounding stabilizing structures is best depicted on PDFS, images from PDFS were stressed on with repetition time (TR) =2700–3000 and echo time (TE) =40–45, giving the optimal image resolution.

  Anatomy and Biomechanics Top

The structures of the PLC are primarily responsible for resisting varus angulation—sometimes referred to as varus rotation and external tibial rotation. They act as secondary stabilizers, along with the cruciate ligaments, to prevent anterior and posterior translation during the early phase of flexion (0°–30°).[1],[2]

Initially, the lateral supporting structures of the PLC were divided into three layers.[3] The superficial layer was described as consisting of the iliotibial band (ITB), its anterior expansion, and the biceps femoris. The middle layer was described as comprising the lateral patellar retinaculum, the two patellofemoral ligaments, and the patellomeniscal ligament. Finally, the deep layer was described as being composed of the lateral capsule, lateral collateral ligament (LCL), coronary ligament (also called the lateral meniscotibial ligament), arcuate ligament, popliteus muscle-tendon unit, popliteofibular ligament (PFL), and fabellofibular ligament [Flowchart 1].

Our understanding of the PLC has since evolved such that the focus is now on individual structures that work both in isolation and in concert to provide static and dynamic stability to the joint, rather than on structures that are grouped into anatomic compartments. Therefore, to simplify this complex anatomy, we herein describe the most critical structures of the PLC, with an emphasis on the three main stabilizers: the LCL, popliteus tendon, and PFL.[4],[5] The interplay and complementary roles of the tendons and ligaments of the PLC are largely due to their anatomic relationships and the proximity of their insertions onto the fibular head;[6] this anatomy is best illustrated in [Figure 1] and [Figure 2]. An injury to any one structure alters the synergistic interactions of the components of the PLC and diminishes the ability of the knee to resist varus and external rotational forces.
Figure 1: An artist's view of the structure in the posterolateral corner of the knee joint, illustrating the lateral collateral ligament (LCL), the medial (mAR) and lateral (lAR) head of the arcuate ligament, popliteus muscle (P), and tendinous part of the popliteofibular ligament (*). (Adapted from “unraveling the posterolateral corner of the knee” by Humberto G Roses)

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Figure 2: Another view of the structures in the posterolateral corner of the knee joint, illustrating biceps femoris (BF), in addition to the structures shown in Figure 1. (Adapted from “unraveling the posterolateral corner of the knee” by Humberto G Roses)

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Lateral collateral ligament

The LCL interchangeably known as fibular collateral ligament arises from the lateral femoral condyle, 2 cm above the joint line. It then traverses distally and toward the medial side on the fibular head. The popliteus tendon courses beneath it. It is tubular and is no more than 3 mm or 4 mm in diameter. The LCL is superficial to and separate from the lateral capsule. This ligament is found in all knees. On MRI, it is visualized on axial, sagittal, and coronal planes as a low signal intensity structure extending from the lateral aspect of the distal femur to the proximal fibula. Only before its insertion, the LCL joins the distal biceps femoris tendon to form a conjoined structure [7] [Figure 3].
Figure 3: Coronal proton density-weighted sequences image with arrow showing lateral collateral ligament represented by a hypointense linear structure

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The LCL abnormalities are commonly seen in PCL injuries and well depicted on MRI.[8] Injuries to the LCL are seen on T2-weighted images and include soft-tissue avulsion of the femoral attachment, peri-ligament–edema, complete or partial thickness tears, and soft tissue or bone avulsion of the fibular head.[7],[8]

Popliteus musculotendinous complex

The popliteus muscle has its own series of interactions with multiple structures of the knee. In addition to the popliteus tendon attachment on the femur, here are three popliteomeniscus fascicles, the PFL, a thick aponeurotic attachment to the posterior capsule and posterior horn of lateral meniscus, and popliteus muscle belly attachment on tibia itself.[9] Both the popliteus tendon and the popliteofemoral ligament are two of the three main structures essential to provide static stability to the PLC.

The popliteus tendon attachment on the femur is always anterior to the LCL attachment.[9] It attaches at the most proximal and anterior fifth of the popliteus sulcus. Quantitative studies showed that the average distance between the midpoints of the femoral attachment sites of the popliteus tendon and the LCL is 18.5 mm.[9] Recognizing the large distance between these two attachment sites is important during either a primary repair or a reconstruction procedure of these two structures, as one can see that one repair site or reconstructive bone plug would create more of a sling procedure than an actual reconstruction for these two structures [9] [Figure 4].
Figure 4: (a and b) Coronal proton density-weighted sequences images showing muscular and tendinous part of the popliteus muscle

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The popliteus tendon and muscle are best seen on axial and coronal images as low- and intermediate-signal intensity structures, respectively. Although avulsions at the femoral insertion may occur, injuries of the popliteus muscle and tendon usually involve the muscle belly or musculotendinous junction.[10] Since this area is a challenge for the arthroscopist to view, the radiologist plays a key role in making this diagnosis. Partial tears of the musculotendinous junction appear as amorphous increased signal intensity within the tendon and muscle. Disruption of fibers, enlargement of the muscle belly, or both may be present.[11]

Arcuate and fabellofibular ligaments

This is a Y-shaped ligament which arises from the posterior part of the capsule around the distal surface of the femur and condenses down onto its insertion on the posterior aspect of the fibular head. In its course, it runs over the popliteus muscle, deep to the lateral inferior geniculate vessels. It was found to be present in 80% of knees by Seebacher et al.,[3] in 55 of 115 knees by Watanabe et al.[4] but only in 24% of knees by Sudasna and Harnsiriwattanagit.[12]

The arcuate ligament, in general, is difficult to visualize on MRI.[13] However, it can be thought of as a thickening of the posterolateral capsule, a portion of which forms the bowed roof of the popliteal hiatus,[13] and can be seen as a low-signal structure on axial images [Figure 5]. Inspection of the posterolateral joint capsule on axial MR images at the level of the joint line may reveal gross disruption, implying injury to or a tear of the arcuate ligament.
Figure 5: Axial proton density-weighted sequences image showing arcuate ligament

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The fabellofibular ligament is occasionally visualized on MRI and is best seen on coronal T2-weighted images as a low-signal structure located posteriorly with respect to the fibular collateral ligament.[14] MR evidence of injury includes distal avulsion from the fibular styloid process, which can be seen concurrently with avulsion of the direct arm of the short head of the biceps femoris tendon, thickening, and increased signal intensity.[5] Due to the infrequency with which this ligament is well visualized in even noninjured knees, it is not as useful as some of the other structures in the evaluation for PLC injuries with MRI.

Biceps femoris tendon

The long and short heads of the biceps femoris tendon generally join above the knee and course distally to insert predominantly onto the fibular head. Although both the long and short heads of the biceps femoris tendon have multiple tendinous and fascial components, not all of these components are consistently visible as separate structures on MRI.[8],[13] The direct and anterior tendinous arms of the long head of the biceps femoris attach to the anterior and posterolateral aspects of the fibular head, and the direct arm of the short head of the biceps femoris tendon attaches to the more anteromedial aspect of the fibular head [Figure 6], with the anterior arm of the short head attaching along the superolateral edge of the lateral tibia.[13]
Figure 6: (a and b) Coronal and axial proton density-weighted sequences images showing fibular attachment and path of the biceps femoris tendon

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On MRI, the insertions of the direct arms of the short and long heads are often seen as a single low-signal intensity structure on coronal T2-weighted images,[8] and as mentioned earlier, the biceps femoris tendon is often joined by the fibular collateral ligament only above their insertions to form a conjoined insertion.[13] Injuries to the biceps femoris tendon are often seen in conjunction with PLC injuries and include myotendinous junction tears above the level of the knee and soft-tissue or bone avulsion from the fibular head [8] and are best shown on coronal and axial MR images.

Popliteofibular ligament

The existence of a consistent attachment of the popliteal tendon to the fibular head is well established and is called the PFL. This structure originates near the popliteus musculotendinous junction and courses distally and laterally to attach to the medial aspect of the fibular styloid process and is thought to be present in most knees as an important static stabilizer of the PLC.[15],[16] The PFL originates from the popliteus tendon just distal to the popliteomeniscus fascicles and proximal to the popliteus musculotendinous junction and extends distally to insert on the anterior part of the medial aspect of the fibular styloid process, near to the tibiofibular joint [Figure 7].[5] This ligament is a short, strong tendinous band that is as wide as or even wider than the popliteus tendon.[17] Despite this fact and the fact that this ligament is present in most, if not all, knees, the PFL is only variably visualized on MRI. The ligament can sometimes be seen as a small low-signal intensity structure on coronal and sagittal images and can occasionally be followed over several images in the axial imaging plane.
Figure 7: Sagittal section of proton density-weighted sequences image showing popliteofibular ligament represented by a hypointense linear structure just posterior to the posterior horn of lateral meniscus

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Iliotibial band

The ITB is a thickened fascia that forms from the fascial investments of the tensor fasciae latae, gluteus maximus, and the gluteus medius muscles. Proximal to the knee, the ITB inserts on the supracondylar tubercle of the lateral femoral condyle and blends with the intermuscular septum. Distally, its main attachment is the Gerdy tubercle [Figure 8].[18] The ITB is located anterior to or adjacent to the lateral femoral epicondyle with the knee extended. The width of the ITB varies among individuals.
Figure 8: Coronal section of proton density-weighted sequences image showing complete extension of the ileotibial band on the lateral aspect of the knee

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Usually, ITB is visualized as a structure showing hypointense signals on T2-weighted images which is its normal appearance, and it becomes of significance in cases of ITB syndrome where there is an increased ill-defined T2 signal intensity within the fatty soft tissues between the ITB and lateral femoral epicondyle.[19]

Coronary ligament (meniscofemoral ligament)

The meniscofemoral ligament arises from the posterior horn of the lateral meniscus and passes to attach to the lateral aspect of the medial femoral condyle. It splits into two bands at the posterior cruciate ligament, and the parts are named in accordance to the relation with the PCL: anterior meniscofemoral ligament (ligament of Humphrey) and posterior meniscofemoral ligament (ligament of Wrisberg) [Figure 9].
Figure 9: (a and b) Ligament of Humphery (right image) and ligament of Wrisberg as seen on sagittal proton density-weighted sequences and short tau inversion recovery images on magnetic resonance imaging

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The meniscofemoral ligament is variably described as either possessing one band (35%) or as described above possessing two bands (65%).[20]

Approximately 80% of people will have at least one meniscofemoral ligament with posterior ligament more commonly present as compared to the anterior and 20%–30% of the people will have both of them.[21]

On MRI, the meniscofemoral ligament appears as thin low-signal intensity structure on T2W and PDFS images.

Lateral retinaculum

The anatomy of the lateral retinaculum is less emphasized in the literature as compared to the medial retinaculum. Furthermore, its contribution to the stability of the lateral knee joint and its functioning are less precisely defined as compared to the medial retinaculum.[22] The lateral retinaculum is composed of a superficial and a deep layer. The superficial fibers originate from the ITB and vastus lateralis fascia. The fibers pass anteriorly and insert into the patella and patellar tendon.[22],[23] The deep structures consist of several discrete structures: the transverse ligament, patellotibial bands, and epicondylopatellar bands [23] [Figure 10].
Figure 10: Lateral patellar retinaculum visualized as a thin hypointense structure on axial proton density-weighted sequences image

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On T1W images, the lateral retinaculum appears as a low-signal intensity structure, with a distinctive bilaminar appearance at the level of patella corresponding to the superficial lateral retinaculum and the transverse ligament. Its thickest part is at the patellar margin and measures approximately around 8–9 mm where a condensation of the superficial and deep fibers occurs.[22]

  Role of Imaging and How to Report Findings Top

Diagnosis of PLC injuries is based on the clinical presentation. In practice, however, the physical examination findings are often dominated by associated injuries, particularly those involving the cruciate ligaments. Imaging, especially MRI, has an important role in uncovering unsuspected PLC injuries, as their early detection (i.e., <3 weeks before they incite injury) and treatment can result in improved outcomes and potentially obviate additional procedures.[24],[25] While reporting, we should keep in mind the above-mentioned structures and a brief mention about the following should be made:

  1. LCL
  2. Popliteus musculotendinous complex
  3. Arcuate and fabellofibular ligaments
  4. Biceps femoris tendon
  5. PFL
  6. ITB
  7. Coronary ligament (meniscofemoral ligament)
  8. Lateral retinaculum.

At MRI, injuries can be classified as strains, which primarily involve an intact ligament or tendon with the surrounding edema; partial tears, with increased intrasubstance signal intensity and disruption of portions of the muscle, ligament, or tendon; or complete rupture—with these often corresponding to the clinical findings of Grade I, II, and III injuries, respectively, as described by Fanelli et al.[26] Although there are no imaging criteria to distinguish clinically unstable Grade III PLC injuries, complete tears of the major stabilizers (i.e., popliteus tendon, PFL, and LCL) should be reported as findings suspicious for posterolateral instability, particularly when two or more components are involved or there is a concomitant cruciate ligament injury. Edema around the LCL, popliteus muscle-tendon complex, and the biceps femoris tendon [Figure 11] are the common findings we see in modern-day posttraumatic knee joint. In complex knee injuries, these findings may direct the orthopedic surgeon to closely interrogate the PLC complex and, if necessary, perform an examination with use of anesthesia. The statuses of the menisci, cruciate ligaments, medial collateral ligament, hyaline cartilage, and structures of the posteromedial corner of the knee also should be included in the report, as these will affect the overall treatment-related decision-making. In the setting of a knee dislocation, computed tomography or MR angiography should be performed to assess the popliteal vasculature and nerves.
Figure 11: (a-c) A posttraumatic knee joint of a 30-year-old male, scanned on a 3T scanner showing coronal, axial, and sagittal proton density-weighted sequences images which depict edema around the lateral collateral ligament, biceps femoris, and popliteus muscle tendon complex, respectively

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  Conclusion Top

PLC injuries are not common, but if present should be reported in detail as the PLC of knee joint forms a stabilizing force and if neglected can act as a major factor in the morbidity of the patient. Although commonly associated with anterior cruciate ligament and PCL injuries, all diagnostic reports should comment on the normal/abnormal structures which we visualize in this part of the knee irrespective of the pathologies present.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]


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