Knee



Knee


Thomas H. Berquist



INTRODUCTION

The value of magnetic resonance imaging (MRI) for imaging the knee was apparent almost immediately after the introduction of this modality in the early 1980s.1,2,3,4,5

With the introduction of special closely coupled extremity coils, high-field systems (1.5 to 7.0 T), open systems, extremity units (see Chapter 3), and other technical advances, the utility of MRI in the knee has expanded dramatically.2,5,6,7,8,9,10,11,12 These capabilities have made knee imaging one of the most widely accepted applications of MRI. Studies have demonstrated that MRI is equally effective when requested by orthopedic surgeons and primary care physicians.13 MRI has also been demonstrated as a cost-effective technique by reducing unnecessary surgical or arthroscopic interventions.14,15 Improved diagnostic accuracy has been clearly demonstrated and MRI resulted in changes in patient management in 41% of patients.16 MR examinations obtained prior to more expensive arthroscopic studies can reduce the need for arthroscopy in up to 42% of patients.17,18

More recent literature by Alioto et al.19 revealed that MR examinations beneficially altered the treatment plans of the orthopedic surgeon in 18% of patients. Furthermore, they found MRI to be more useful in the decision-making process when pathology involved the meniscus or chondral surfaces. MRI was not as useful for evaluating anterior cruciate ligament (ACL) insufficiency.19


TECHNIQUES

The techniques used for evaluating the knee should be tailored to the clinical indication and the imaging system that is employed. Although many techniques and image planes may be used, the following discussion is oriented toward the routine screening examination that is commonly used to evaluate most articular and periarticular disorders of the knee. More specific techniques are discussed later in the applications section as they apply to specific clinical disorders.


Positioning and Coil Selection

Typically, the patient is placed in the supine position with the knee placed in a closely coupled extremity coil (Fig. 7.1). The knee may be externally rotated 15° to 20° to facilitate visualization of the ACL on sagittal images.2,20,21,22 This practice is not routinely performed at all MRI centers. The knee should be flexed slightly (5° to 10°) to increase the accuracy of assessing the patellofemoral compartment and patellar alignment.22 Excessive flexion or hyperextension does not permit accurate evaluation of patellar alignment.2,22 Different coil systems and positions may be required if motion studies are required for patellofemoral evaluation. In most cases we use a volume coil (Fig. 7.2) for 1.5- and 3.0-T imaging.2 Antonio et al.23 added targeted imaging using a 4-cm loop receive coil to obtain more detail in the area of interest. A 10-cm field of view (normal 14 cm in our practice) with 256 × 512 (displayed at 512 × 512) matrix was utilized.

Most high-field imagers allow more limited motion than the open gantries of lower field units. New extremity units also limit positioning options. Positioning techniques will be discussed more completely later with patellofemoral disorders.







Figure 7.1 Knee positioned in a volume coil. The knee is flexed 5° to 10° and can be externally rotated 15°.


Pulse Sequences/Image Planes

When selecting pulse sequences, physicians have many options: spin-echo sequences, fast spin-echo (FSE) sequences, various gradient-echo (GRE) sequences, diffusion-weighted imaging, and three-dimensional imaging (Table 7.1).10,11,12,24,25,26,27,28,29,30,31,32,33,34,35,36,37 The contrast requirements in knee imaging demand some variation in sequences. Meniscal tears are best imaged with MR sequences that are not purely T1 or T2 weighted. Other structures such as ligaments are best evaluated with T2-weighted images.1,2

A technical requirement that should not be underestimated is the need to use adequate image geometry. The menisci and the cruciate ligaments are complex structures, and it is unreasonable to expect to be able to evaluate them reliably using a single-slice orientation.

The pulse sequences that are now widely available for knee imaging include spin-echo and FSE techniques, GRE techniques, both slice-selective and three-dimensional Fourier transform (3DFT) versions. Short inversion time recovery (STIR) and FSE STIR sequences may also be selected.38

In the spin-echo category, we can consider short repetition time/echo time (TR/TE) sequences and long TR multiecho sequences. At this time, there seems to be little to recommend the sole use of short TR/TE spin-echo sequences for knee imaging. Although they are technically undemanding, rapidly acquired, and sensitive for medullary bone lesions, they only provide low contrast for meniscal lesions, they are not well suited for demonstrating acute ligamentous injuries or the interface between joint fluid and articular cartilage, and they require special windowing during photography (Fig. 7.3).






Figure 7.2 Volume knee coils (A and B). (Courtesy of Siemens Medical Systems, Erlangen, Germany.)

Long TR, multiecho spin-echo sequences are very effective for knee imaging. A short first echo provides intermediate contrast that is excellent for identifying meniscal lesions, and a second, long echo provides T2-weighted contrast that is critical for evaluation of the cruciate ligaments and other structures.4,38

A commonly used approach for knee imaging is to perform sagittal and coronal spin-echo acquisitions (Fig. 7.4).39,40 We also obtain axial images though we no longer pursue the utility of radial image planes. The exact technical approach will depend on the imaging hardware utilized. We have found the parameters summarized in Table 7.1 to be efficient and reliable using a 1.5-T imager.

Long TR multiecho sequences have the advantages of high-slice throughput (in terms of images per unit time) and favorable contrast characteristics. They have the
disadvantage of requiring longer acquisition times, and the long TE T2-weighted images are technically demanding in terms of imager performance.








Table 7.1 MR Screening Examination of the Knee at 1.5 T




























































































































Image Plane


Pulse Sequence


Slice Thickness


Field of View (cm)


Matrix


Acquisitions (NEX)


Axial scout localizer


FLASH 15/5


8 mm/skip 8 mm


14


256× 128


1


Coronal/sagittal scout localizer


FLASH 15/5


8 mm/skip 8 mm


14


256 × 128


1


Axial







PD FSE fat suppression


2,480/28 ET 3


4 mm/skip 0.5


14


512 × 256


1


Coronal







T1WI


TSE 800/11 ET 3


4 mm/skip 0.5 mm


14


512 × 256


2


DESS


DESS 19.6/5.47 ET 3


1 mm/20%


14


256×192 with interpolation


2


Sagittal







PD FSE


TSE 2850/42 ET 9


4 mm/skip 0.5 mm


14


512× 256


2


PD FSE with fat suppression


TSE 2850/42 E T 9


4 mm/skip 0.5 mm


14


512 × 256


2


Axial







PD FSE with fat sat


TSE 4,000/26, ET 7


4 mm/skip 0.5 mm


14


256 × 192


2


Additional sequences







Fat-suppressed fast spin-echo T2WI (axial, coronal, or sagittal)


3,500/20-30 ET 2-8


4 mm/skip 0.5 mm


14


512 × 256


2


STIR FSE (axial, coronal, or sagittal)


4,230/86, TI 160


5 mm/skip 1 mm


14


256 × 192


2


Conventional T2 (axial)


2,230/20


6 mm/skip 1.5 mm


14


256 × 192


1


T1WI, T1-weighted image; T2W, T2-weighted image; FSE, fast spin echo; TSE, turbo spin echo; PD, proton density; DESS, dual-echo steady state; STIR, short inversion time recovery; DE, dual echo; SE, spin echo; FLASH, fast low-angle single-shot; SAT, saturation; ET, echo train.


In recent years, new FSE sequences have essentially replaced conventional spin-echo sequences.15,38,41,52,42 These sequences permit, in theory, faster data acquisition with repeated spin echoes following a 180° pulse. The echoes have different degrees of phase encoding and all contribute to a single image. We have used this technique sparingly in the knee as both short- and long-effect TE FSE sequences appear to have less contrast and fat suppression than conventional spin-echo sequences even though the sequences can be performed in half the time (Fig. 7.5). When we select FSE sequences, we generally add fat suppression. Rubin et al.41 reported that FSE sequences were less useful than conventional spin-echo sequences for detection of meniscal tears. Escobedo et al.38 reported that FSE sequences with a short echo train were comparable to proton density (SE 2,000/20) images and could be performed in 5 minutes and 20 seconds compared with 7 minutes and 38 seconds for spin-echo sequences.

GRE techniques (Fig. 7.5) have seen increasing use for musculoskeletal imaging in the last several years. They provide interesting capabilities in terms of contrast and speed. These techniques can be broadly divided into “steady state” sequences such as gradient-recalled steady state (GRASS) and fast imaging with steady-state free precession (FISP), and “spoiled” sequences such as fast low-angle single shot (FLASH) and spoiled GRASS (see Chapter 2).36,43 We commonly use dual-echo steady state (DESS) images for our routine knee examination in the coronal plane (Fig. 7.6).

Multislice GRE acquisitions can be performed in two ways: sequentially acquiring each slice individually, or acquiring the slices in an interleaved fashion similar to multislice spin-echo imaging. In the first alternative, the TR must be very short so that the total imaging time to acquire the entire set of slices will not be excessively long. It turns out that for knee imaging with GRE, the short-TR, sequential, single-slice approach is less favorable than a long TR, multislice approach. In the specific application of knee imaging, the longer TR of the interleaved approach has the effect of improving the contrast and signal-to-noise characteristics of the images, compared with short TR gradient echoes (Table 7.1).

Long-TR, medium-TE GREs provide excellent contrast for delineating meniscal tears. An advantage of this technique is that special windowing of the images is not required at photography. Long-TR GRE sequences can provide very pronounced T2-weighted contrast for depicting ligamentous lesions. Long-TR GRE images seem to have
special capabilities for depicting chondral and osteochondral lesions as shown in these images, but these have not been fully explored as yet.






Figure 7.3 A: Coronal SE 450/15 image of the knee demonstrates normal signal intensity in the marrow. The menisci are low signal intensity and the articular cartilage is intermediate signal intensity. The cruciate ligaments (a, anterior; p, posterior) are seen in the intercondylar notch. The collateral ligaments (arrowheads) are also low signal intensity. B: Sagittal proton density (2,300/26) image of the posterior medial knee demonstrates the posterior horn of the medial meniscus with intrasubstance increased signal intensity (arrowhead), but not communication with the articular surface. C: Sagittal fast spin-echo T2-weighted sequence (3,300/80) image of the knee demonstrates abnormal signal and truncation of the anterior horn and body of the lateral meniscus (arrowhead). D: Coronal dual-echo steady-state image (three-dimensional, 23.87/6.73) demonstrates normal meniscal signal intensity and excellent cartilage detail.

Most authors now advocate 3DFT acquisition for knee imaging.29,31,32,33,34,35 The great attraction of this technique is that a high-resolution volume data set can be processed retrospectively to generate any arbitrarily oriented plane of section (Table 7.1). Three-dimensional imaging may be particularly useful for evaluating articular cartilage (Fig. 7.6).7,11,25,31,32,33,36,43,44,45,46,47,48,49,50,51,52,53,54,55

Following are the advantages of three-dimensional imaging. The ability to acquire thin sections without gaps and the potential for three-dimensional renderingandreformatting. The disadvantage of 3DFT imaging is that the costs include a significantly larger requirement for resources such as computing power, memory, display, and storage. Other, less well-established concerns are that the examinations may take longer to interpret, given that more sections must be viewed, and that there may be a penalty in signal-to-noise and contrast, which accompanies the requirement
for isotropic resolution in three-dimensional imaging. This is imposed by the short repetition time that must be used.






Figure 7.4 Axial anatomy of the knee and selected coronal and sagittal sections.

Additional sequences commonly employed or used as alternatives include FSE STIR sequences, FSE T2-weighted imaging, as well as occasionally performing conventional T2-weighted axial images for evaluation of mass lesions. Fast STIR sequences are useful for detection of subtle soft tissue and marrow abnormalities27,46,47 (Table 7.1).

Gadolinium-diethylenetriamine penta-acetic acid (Gd-DTPA) has also been used to evaluate the knee. However, the need for intra-articular contrast medium for conventional examinations of the knee has not been clearly established.48,49,50,51,52,53,54 Intravenous gadolinium may be useful in certain arthropathies for enhancing synovium (Fig. 7.7) and joint fluid and for evaluation of cartilage and meniscal repairs.55,56,57

There are many good approaches for imaging the knee. Techniques should be tailored to the specific clinical situation. We perform MR knee examinations on 1.5- and 3.0-T imagers with slightly different parameters. Our current screening examination on 1.5-T magnets (Fig. 7.8) (Table 7.1) includes axial proton density turbo-spin echo with fat suppression, sagittal turbo-spin echo proton density (BLADE), sagittal turbo-spin echo proton density with fat suppression, coronal turbo-spin echo T1-weighted, and coronal DESS sequences. We use similar sequences on 3.0-T magnets with slightly different parameters (Fig. 7.9). Additional technical considerations will be discussed in more detail with specific knee disorders.


ANATOMY

The multiple image planes that are used to evaluate the knee increase the need to completely understand the articular and periarticular anatomy in all commonly used image planes (Figs. 7.10, 7.11, 7.12).42,58,59 In addition, a thorough knowledge of this anatomy is essential to properly select special image planes to demonstrate properly certain anatomic structures.60,61,62,63,64,65


Bone and Articular Anatomy

The knee is formed by the femoral and tibial condylar articulations. The tibiofibular articulation (Fig. 7.11B), though often considered a part of the knee, is in fact not a portion of the true knee joint.64,65 The knee is primarily a hinge joint that is protected anteriorly and posteriorly by muscles with special ligamentous attachments to the capsule. The articular surfaces of both the femoral condyles and tibial condyles are covered with hyaline cartilage. Hyaline cartilage has four zones with variations in chondrocytes, collagen fiber orientation, and proteoglycans. MR image features described in normal and abnormal articular cartilage are due to variation in normal content of hyaline cartilage. Normal cartilage is
60% to 80% water. Collagen makes up 50% of the weight of cartilage and proteoglycans contribute 30% to 35%.52,66






Figure 7.5 Coronal images of the knee with the same plane of section in a patient with meniscal tears and articular cartilage loss. A: Fast spin-echo (4,000/108) is similar in appearance to a T1-weighted spin-echo sequence (marrow and fat have high signal intensity) except for high signal intensity of joint fluid and vessels (arrows). Other intra-articular structures are not clearly defined. Gradient echo (700/31, flip angle 25°) (B) and fat-suppressed spin-echo (SE 2,000/80) (C) images demonstrate the meniscal tears (arrows) and loss of articular cartilage (arrowheads) more clearly.

The femoral condyles are oval anteriorly and rounded posteriorly to provide increased stability in extension and increased motion and rotation in flexion (Fig. 7.10).64 The medial femoral condyle is larger and important in load transmission across the knee. Medial and lateral tibial condyles form the expanded articular portion of the tibia. The condyles are separated by the intracondylar eminence that serves for cruciate attachment. The intercondylar eminence has medial and lateral tubercles (Fig. 7.12). The weight-bearing surfaces of the tibial and femoral condyles are separated by fibrocartilaginous menisci. The menisci are triangular when viewed tangentially and thicker laterally than medially (Fig. 7.13).26,64,65,66,67,68

The patella is the largest sesamoid bone in the body and develops in the tendon of the quadriceps (extensor mechanism) (Figs. 7.10 and 7.11).65,69 The patellar retinacula are formed by expansions in the quadriceps tendon and fascia that extend from the sides of the patella to the femoral
and tibial condyles (Fig. 7.20). The patella is divided into several Wiberg types. The medial and lateral facets are of equal size in type I. Type II, the most common configuration, has a smaller medial than lateral facet (Fig. 7.10). Type III has a very small medial facet that is convex and a large, concave lateral facet. Both facets are covered with hyaline cartilage and most easily seen on axial MR images (Fig. 7.10).64,70,71






Figure 7.6 Three-dimensional gradient-echo images (A-C) of the knee from posterior to anterior demonstrating superior cartilage detail.

The capsule of the knee is lined by synovial membrane that is subdivided into several communicating compartments.65 Anteriorly, the synovial membrane is attached to the articular margins of the patella (Fig. 7.14A). From the medial and lateral sides, the synovium extends circumferentially (Fig. 7.14C), in contact with the retinacula (Fig. 7.10). From the inferior aspect of the patella, the synovial membrane extends downward and backward and is separated from the patellar ligament by the infrapatellar fat pad (Fig. 7.114A and B). At the lower margin of the patella there is a central fold, the infrapatellar synovial fold that is also sometimes referred to as the ligamentum mucosum (Fig. 7.14B). This structure is joined by two lesser alar folds or plicae that extend down from the sides of the patella. As the synovial fold extends into the femoral notch, it attaches to the intracondylar fossa of the femur anteriorly (Fig. 7.14A and B). The membrane fans out at its sides medially and laterally so that it covers the front and sides of the femoral attachment of the posterior cruciate ligament (PCL) (Fig. 7.14D and E). Inferiorly, the synovial membrane continues down to the intracondylar area of the tibia covering the attachment of the ACL (Fig. 7.14B and E). Due to the fact that the fold attaches to both the femur and the tibia, it in fact divides the knee into medial and lateral synovial cavities separated by the extrasynovial space that houses the cruciate ligaments (Fig. 7.14E).19,64,65,72

The synovial membrane extends superiorly from the upper margin of the patella for a variable distance and is closely applied to the quadriceps muscle (Fig. 7.14A and B). It then reflects onto the anterior aspect of the femur. This forms the suprapatellar bursa that lies between the quadriceps and the anterior margin of the femur (Fig. 7.14A and B).
Along the medial, lateral, and posterior aspects of the capsule, the synovial membrane attaches to the femur at the edges of the articular surfaces posteriorly. Medially and laterally, it passes from the articular margins inferiorly to attach to the articular margins of the tibial condyles (Fig. 7.14D). The intrasynovial space that extends from the intracondylar fossa superiorly to the intracondylar area of the tibia inferiorly houses the cruciate ligaments. The cruciate ligaments are, therefore, covered superiorly, medially, laterally, and anteriorly by synovial membrane but not posteriorly (Fig. 7.14A, B, and E). Posterolaterally, the synovial membrane is separated from the fibrous capsule by the popliteus tendon. It is not unusual to identify a bursa along the popliteus tendon that communicates with the joint space posterolaterally. The other common bursae about the knee are listed in Table 7.2 (Fig. 7.15).19,65 There is also a lateral synovial extension that may be implicated in iliotibial band syndrome.73






Figure 7.7 Sagittal (A) and anterior coronal (B) intravenous post-contrast fat-suppressed T1-weighted images demonstrating synovial enhancement (arrows). Due to the early imaging the synovial fluid has not enhanced on the sagittal image (A). There is early enhancement of synovial fluid on the coronal image performed two sequences later in the examination.








Table 7.2 Bursae About the Knee




























































Anterior




Prepatellar


Between patella and skin



Retropatellar


Between patellar ligament and upper tibia



Pretibial


Between tibial tuberosity and skin



Suprapatellara


Between quadriceps and femur (communicates with joint)


Lateral




Gastrocnemius


Between large gastrocnemius and capsule



Fibular


Between fibular collateral ligament and biceps tendon



Fibulopopliteal


Between fibular collateral ligament and popliteus tendon



Popliteala


Between popliteus tendon and lateral femoral condyle (communicates with joint)


Medial




Gastrocnemiusa


Between medial head of gastrocnemius and capsule (often communicates with joint)



Pes anserine


Between tibial collateral ligament and gracilis, sartorius, and semitendinosus tendons



Semimembranoustibial collateral ligament


Between semimembranosus tendon and medial collateral ligament


a Communicating bursae.


From references 65, 67, 74, 75.








Figure 7.8 Screening examination of the knee performed at 1.5 T. A: Axial images are obtained using a turbo spin-echo proton density fat-suppressed sequence (2,480/28, ET 3). Coronal images are obtained using turbo spin-echo T1-weighted (800/11, ET 3) (B) and the second coronal sequence using DESS (19.6/5.47, ET 1) (C). Sagittal sequences include turbo spin-echo proton density (BLADE, 2,850/42, ET 9) with fat saturation (D) and the same sequence without fat saturation (E).

The fibrous capsule and periarticular ligaments of the knee are important for support and must be understood if one is to completely evaluate MR images of the knee.74,75,76,77,78,79,80,81 Anteriorly, the knee capsule is essentially replaced by the quadriceps and its tendon, the patella, and the patellar ligament and retinacula (Figs. 7.10 and 7.15).65,82 Medially and laterally, the capsule is attached to the femur just outside the synovial membrane and extends from the articular margin of the femoral condyles to the articular margin of the tibial condyles (Figs. 7.14 and 7.15). Laterally, the primary ligamentous support is provided by the lateral or fibular collateral ligament. The lateral collateral ligament is clearly separated from the capsule (Fig. 7.15). The posterolateral support structures are restraints to prevent varus angulation and external rotation of the tibia.77,83 The main restraints are the fabellofibular and arcuate ligaments and
the popliteal muscle and tendon.84 The coronary ligament, ligament of Winslow, and lateral collateral ligament are secondary restraints.65






Figure 7.8 (continued)

The medial capsule is supported by the medial or tibial collateral ligament. Unique medial features include the fact that the medial collateral ligament (MCL) blends with the capsule and the medial meniscus is attached to the capsule (Fig. 7.15A). The posteromedial supporting structures are classically divided into three layers.64,76 The first or superficial layer is composed of fascial extensions from the sartorius and vastus medialis.65,76 The intermediate layer consists of the MCL and posterior oblique ligament. The MCL (Figs. 7.12D and 7.15A and B) extends from the femoral condyle to attach to the tibia 5 to 7 cm below the joint line and deep to the gracilis and semitendinosis tendons. The posterior oblique ligament lies posterior to the MCL and extends from the adductor tubercle to the posteromedial meniscus.76 The third or deep layer is the joint capsule.64,76,85

The coronary ligament is the portion of the capsule to which the meniscus is attached to the tibia (Fig. 7.15A). This ligament has some laxity that allows slight motion of the menisci on the tibia.64,65

The cruciate ligaments are intra-articular but lie outside the synovial compartment of the knee (Fig. 7.14E) and are covered by synovial membrane anteriorly, medially, and laterally, but not posteriorly. The ACL arises from the anterior nonarticular surface of the intracondylar area of the tibia adjacent to the medial condyle. It extends obliquely, superiorly, and posteriorly to attach the medial side of the lateral femoral condyle. The ACL has significant variability in its appearance and is typically more slender and longer than the posterior cruciate. This and its oblique course account for some of the difficulty encountered in evaluating this structure with MRI (Fig. 7.15A).2,64,65,86,87,88 The ACL is about 32 mm in length and contains two functional sections termed the anteromedial and posterolateral bands.86,87,88 The terms used for each band or bundle are related to their respective insertions on the tibia.87 The bundles are parallel in extension and twisted 90° during flexion of the knee.86 The anteromedial bundle reaches maximum tension at 60° to 90° of flexion. The posterolateral bundle is lax during flexion and becomes tense during extension. The ACL bundles resist anterior translation of the tibia on the femur. The posterolateral band maintains rotational stability in the knee.86,87,88

The PCL is the primary restraint for posterior translation of the knee. The PCL is also a secondary support for varus, valgus, and external rotation of the knee. Like the ACL, it is composed of two distinct bundles.78 The anterolateral bundle is taut at about 90° of flexion and the posteromedial bundle is taut near full extension.78 The PCL arises from the posterior intercondylar area and passes obliquely upward and forward in a nearly sagittal plane to attach to the anterior intercondylar fossa of the lateral surface of the medial femoral condyle (Figs. 7.12 and 7.15).65,78 The posterior cruciate, because of its larger transverse diameter and straight sagittal course, is consistently identified on sagittal MR images (Fig. 7.12).2

The menisci of the knee are composed of three collagen layers. These collagen bundles are oriented longitudinally with radial fibers interposed among these layers from the periphery to the apex of the meniscus.89 It has been
suggested that the orientation of the layers explains the origin of most meniscal tears as they appear to align with the axis of the collagen fibers.89 The fibrocartilaginous menisci have a differing shape, with the medial meniscus being larger and thicker in transverse diameter posteriorly than anteriorly (Figs. 7.11, 7.13, and 7.15A). The lateral meniscus is more C-shaped and uniform in width (Fig. 7.15A). Despite the difference in shape, the medial meniscus covers less articular surface (50%) compared with 70% coverage by the lateral meniscus.89 There are several ligamentous attachments that may cause confusion on MR images. For example, the posterior horn of the lateral meniscus is closely applied to the PCL and may give off a band of fibers, termed the meniscofemoral ligament, that follows the PCL to its attachment on the femur. Between the anterior horns of the medial and lateral meniscus there is a transverse band of fibers termed the transverse ligament of the knee. This can easily be confused with an anterior meniscal tear, especially on the medial side (Fig. 7.16).64,65 Another variant, the meniscomeniscal ligament, may also be confused with meniscal pathology. The medial meniscomeniscal ligament extends from the anterior horn of the medial meniscus to the posterior horn of the lateral meniscus. The lateral meniscomeniscal ligament extends from the anterolateral to posteromedial meniscus.90 The central attachments of both menisci are defined as the anterior and posterior root ligaments.89






Figure 7.9 Screening examination of the knee performed at 3.0 T (A). Axial images are obtained using a turbo spin-echo proton density sequence (4,614/61, ET 7). Coronal images are obtained using a turbo spin-echo T1-weighted (800/11, ET 2) (B) and DESS sequence (14.59/5.03, ET 2) (C). The first saggital sequence is a turbo spin-echo proton density sequence (3,370/40, ET 7) (D), followed by a similar sequence with fat suppression (E) sagittal STIR (F).







Figure 7.10 Axial SE 500/15 MR images through the knee with level of section demonstrated. A: Axial image through the distal femoral shaft above the patella. B: Axial image through the upper patella. C: Axial image through the upper femoral condyles and patella. D: Axial image through the femoral condyles and lower patella. E: Axial image through the lower femoral condyles below the patella. F: Axial image and illustration through the upper tibia. G: Axial image and illustration through the tibia and fibular head. H: Axial image through the upper tibia and fibula.







Figure 7.10 (continued)







Figure 7.10 (continued)

The inner portion of the triangular meniscus is avascular and receives nutrition from the synovium. The peripheral meniscus has a rich vascular supply, which is why peripheral tears can heal.64,89

There are multiple fat pads about the knee located between the joint capsule and synovial lining of the knee.91 In the anterior aspect of the knee there are three fat pads including Hoffa, anterior suprapatellar (quadriceps), and a
posterior suprapatellar fat pad anterior to the femur (Fig. 7.16).91,92 Hoffa fat pad is bordered by the inferior pole of the patella superiorly, the patellar tendon anteriorly, the joint capsule posteriorly, and the deep infrapatellar bursa inferiorly.91 The transverse ligament can be seen in the posterior aspect of Hoffa fat pad (Fig. 7.17).91,92






Figure 7.11 Sagittal SE 500/15 images of the knee from lateral to medial with plane of section demonstrated. A: Sagittal image through the lateral margin of the fibular head. B: Sagittal image through the fibular head. C: Sagittal image through the anterior cruciate ligament. D: Sagittal image through the posterior cruciate ligament. E: Sagittal image through the medial compartment. F: Sagittal image through the medial soft tissues and margin of the femoral condyle.







Figure 7.11 (continued)







Figure 7.12 Coronal SE 500/15 images of the knee from posterior to anterior with level of section demonstrated. A: Coronal image through the soft tissues and fibular head. B: Coronal image through the posterior tibia and femoral condyles. C: Coronal image through the mid-joint. D: Coronal image through the anterior joint.







Figure 7.12 (continued)


Muscles About the Knee

The muscles of the thigh, calf, foot, and ankle are discussed in Chapters 6 and 8. Therefore, a thorough review of their origins and insertions would be redundant. However, it is important to review the muscles about the knee and their neurovascular and biomechanical function. Chief movements at the knee are those of flexion and extension. Mild rotation, however, can occur. If one starts from the flexed position (Fig. 7.18), the posterior condyles of the femur are in contact with the posterior horns of both menisci. The medial and lateral collateral ligaments are also relaxed in this position. In full flexion, both anterior and PCLs are taut. In this flexed position more rotary motion is allowed. As one goes from the flexed to the extended position, the femoral condyles shift such that the more anterior parts of the menisci and tibial condyles are now in contact (Fig. 7.18).64,65

The primary flexors of the knee are the hamstring muscle group (semimembranosus, semitendinosus, and biceps femoris) along with the gracilis and sartorius (Figs. 7.10, 7.11, 7.12). The popliteus muscle has some significance in the early phases of flexion due to its rotary action upon the femur or tibia (Table 7.3).84 In the nonweight-bearing state, the gastrocnemius muscle is also utilized in flexion of the knee. The quadriceps group is the chief extensor of the knee (Figs. 7.10, 7.11, 7.12) (Table 7.3).64,65


Neurovascular Supply of the Knee

The arterial supply about the knee is primarily via branch vessels of the distal superficial femoral and popliteal arteries (Fig. 7.19). Superiorly there are medial and lateral genicular arteries as well as muscular branches at the level of the knee joint. Inferiorly, medial and lateral inferior genicular arteries supply the knee. There are numerous anastomoses that interconnect this vascular supply.65

Innervation of the knee (Table 7.3) is primarily by branches of the femoral, obturator, and sciatic nerves. Posterolaterally, recurrent branches of the peroneal nerve also supply the knee.64,65


PITFALLS

The majority of the pitfalls in evaluating the knee are related to normal anatomy or variants and artifacts created by flow, motion, and software problems.93,94,95,96,97,98 Partial volume effects must also be considered when evaluating the knee. As with any anatomic region, failure to compare MR images with routine radiographs or other available imaging studies can result in significant errors in interpretation.

Problems with flow artifacts occur less frequently during examination of the knee than in the more peripheral extremities, where the number of vascular structures per unit area is more numerous (see Chapters 3 and 11). The largest problem with flow artifact occurs in the sagittal plane, where the midline anatomy can be distorted by flow artifact from the popliteal artery (Fig. 7.20). This can be corrected by swapping the phase to the superior inferior direction instead of the anteroposterior (AP) direction.2,99 The artifact can also be reduced to some degree by placing the patient prone. In the latter situation, the pulsatile effect of the popliteal artery
does not create as much motion artifact as when the patient is supine.2






Figure 7.13 Coronal DESS three-dimensional (23.87/6.73, field of view 14 cm, matrix 256 × 192, two acquisitions) images (A-F) of the knee demonstrating both medial and lateral menisci and superior cartilage detail.







Figure 7.14 MR images of the knee demonstrating the synovial and capsular attachments (broken lines) of the knee. A: Sagittal image in the plane of the posterior cruciate ligament. B: Sagittal image in the plane of the anterior cruciate ligament. C: Axial image at the patellar level demonstrating the synovial reflection (broken lines). D: Posterior coronal image demonstrating the posterior synovial and capsular margins (broken lines). E: Axial image of the tibial articular surface demonstrating meniscal and cruciate attachments and synovial reflections. ACL, anterior cruciate ligament; PCL, posterior cruciate ligament.

Flow artifact can create false-positive interpretations if the phase direction is not considered. For example, when patellofemoral disease is being evaluated in the axial plane, phase encoding should be in the “X” or transverse direction (Fig. 7.21). This prevents the artifact from entering the area of interest.

Marrow changes have also been reported in highly trained athletes and marathon runners (Fig. 7.22).
Shellock et al.100 reported an increase in hematopoietic marrow in 43% of marathon runners, compared with only 15% of patients with knee disorders and 3% of normal healthy patients. This marrow conversion pattern may be due to “sports anemia,” which has been attributed to numerous problems such as hemolysis, hematuria, increased plasma volume, and gastrointestinal blood loss.100






Figure 7.14 (continued)

A significant number of interpretation errors are related to lack of familiarity with normal anatomy and anatomic variants (Table 7.4).22,60,93,96,98,101,102,103 The majority of these pitfalls are related to normal variations in the appearance of the menisci and/or variation in lesser-known ligaments associated with the menisci.60,103 There is vascular tissue and variable amounts of fat and synovial tissue near the attachment of the menisci, especially the posterior medial meniscus.104,105 Signal intensity created by this tissue is a more difficult problem on nontangential sagittal and coronal images as tissue is projected between the meniscus and the margin of the capsule. This should not be confused with a peripheral tear in the meniscus (Fig. 7.23). Typically, this tissue has the appearance of fat or similar signal intensity to fat on T1- and T2-weighted images. Meniscal tears are seen as an area of high signal intensity, similar to fluid. Fat-suppression techniques can reduce confusion in this region.








Table 7.3 Knee Flexors and Extensors










































































Muscle


Origin


Insertion


Innervation


Flexors






Semitendinosus


Posteromedial ischial tuberosity


Medial tibia posterior to gracilis and sartorius


Tibial nerve (L5-S1)



Biceps femoris


2 head: long head with semitendinosus, short head—mid-linea aspera


Fibular head


Peroneal (L5-S1)



Semimembranosus


Posterolateral ischial tuberosity, medial meniscus


Proximal medial tibia


Tibial (L5-S1)



Gracilis


Inferior pubic ramus near symphysis


Medial upper tibia between sartorius and semitendinosus


Obturator (L3-L4)



Sartorius


Anterior superior iliac spine


Upper medial tibia above gradilis and semitendinosus


Femoral (L3-L4)


Extensors






Rectus femoris


Anterior inferior iliac spine


Quadriceps tendon to patella


Femoral (L3-L4)



Vastus lateralis


Below greater trochanter


Quadriceps tendon to patella


Femoral (L3-L4)



Vastus medialis


Below lesser trochanter


Quadriceps tendon to patella


Femoral (L3-L4)



Vastus intermedius


Mid-femur


Quadriceps tendon to patella


Femoral (L3-L4)


From Rosse C, Rosse PG. Hollinshead’s Textbook of Anatomy. Philadelphia, PA: Lippincott-Raven; 1997.








Figure 7.15 Axial (A), coronal (B), sagittal (C), and posterior (D) illustrations of the ligaments, menisci, and bursae of the knee.








Table 7.4 Common Variants and Artifacts in MR Imaging of the Knee



























Popliteal tendon sheath near posterior horn of lateral meniscus



Accessory popliteus muscle


Meniscofemoral ligament variations



Transverse ligament



Meniscomeniscal ligaments



Increased signal near central attachment in anterior horns of lateral meniscus



Truncation artifact



Magic angle effect


From Campos JC, Chung CB, Lektrakul N, et al. Pathogenesis of the Segond fracture: anatomic and MR imaging evidence of an iliotibial tractor anterior oblique band avulsion. Radiology. 2001;219:381-386.


As with arthrography, evaluation of the posterior lateral meniscus can be difficult due to the popliteus tendon and its sheath (Table 7.4) that pass between the capsule and meniscus posteriorly. This should not be mistaken for a meniscal tear (Fig. 7.24).103,104 Additional anatomic structures that can cause confusion are the inferior lateral geniculate vessels that course along the margin of the lateral meniscus anteriorly. The signal intensity of these vessels can give the appearance of a peripheral detachment (Figs. 7.19 and 7.24).104 Irregular increased signal intensity may also be seen in normal anterior lateral menisci near the central attachment. The transverse meniscal ligament is seen anteriorly and can be confused with a horizontal tear in the anterior horn of the meniscus (Fig. 7.17).73,109,122







Figure 7.16 Fat pads of the anterior knee. Saggital T1-weighted 3.0-T MR image demonstrating the three anterior fat pads of the knee.

Discoid menisci can cause significant problems if one does not consider this variant. Discoid menisci are more common laterally and occur in 1% to 2% of arthrographic and 2% to 5% of surgical series. The meniscus has a waferlike appearance on the coronal and axial images (Fig. 7.25) and projects farther into the joint space on sagittal images. Discoid menisci are more prone to tears and are important to recognize.60,106






Figure 7.17 Sagittal T1-weighted image of the knee demonstrating a normal anterior horn of the medial meniscus (m, arrow) and the transverse ligament (tl, arrowhead). This should not be confused with a meniscal tear.






Figure 7.18 Sagittal illustrations of the knee and cruciate ligaments in the extended (A) and flexed (B) positions.

Other meniscal or perimeniscal features have also been described that can be confused with pathology. These include vacuum phenomena, truncation artifacts, the “magic angle” effect, and artifacts created by recent orthopedic interventions.46,60,107,108,109

Turner et al.109 described truncation artifacts in the meniscus on sagittal images when using 128 × 256 matrix and phase encoding in the superior-inferior direction. This subtle linear area of high signal intensity (Fig. 7.26) is commonly identified two pixels from the meniscal joint fluid interface. This defect can be seen extending beyond the meniscal margins when images are optimally windowed. This artifact can be removed using 256 × 256 or 192 × 256 matrix and/or by switching the phase direction to the AP direction.109






Figure 7.19 Neurovascular anatomy of the knee.







Figure 7.20 A: Sagittal proton density image of the knee demonstrating pulsatile motion artifact from the popliteal artery. The phase encoding is in the anteroposterior direction (arrow). Artifact is decreased (B) by switching the phase encoding to the superior-inferior (arrows) direction.






Figure 7.21 A: Axial proton density image in a patient referred for patellofemoral pain. The focal defect in the lateral femoral condyle (large arrowhead) is due to flow artifact (small arrowheads). This should not be confused with an articular defect. This problem can be avoided by swapping phase direction to the transverse (dotted line) plane. B: Axial T2-weighted image through the patellofemoral compartment. There is an effusion, a medial plica (arrowhead), and grade IV chondromalacia on the medial patellar facet (small arrows). Flow artifact (white arrows) with phase encoding in the anteroposterior direction creates a false lesion (increased signal) in the lateral facet.







Figure 7.22 Normal male athlete. A: Anteroposterior radiographs of the knees demonstrate normal marrow and medial compartment narrowing. T1-weighted coronal (B) and sagittal (C) images show decreased signal intensity in the femoral metaphysis and diaphysis and upper tibia due to red marrow reconversion.

The vacuum phenomenon (Fig. 7.27) can simulate discoid menisci or articular abnormalities in the joint.108 Careful review of multiple image planes should prevent interpretation errors in this setting.

The magic angle phenomenon has been described with tendons and menisci when collagen fibers are oriented approximately 55° to the static magnetic field. At this angle, the interactions that contribute to T2 relaxation among water protons are nulled. This can result in increased signal intensity on the upper medial portion of the posterior horn of the lateral meniscus.107 This is particularly common with short TE sequences. Peterfy et al.107 reported this finding in 74% of 42 patients. The meniscal segment with increased signal intensity was oriented 55° to 60° in 80% of these patients.107

Variations in the cruciate ligaments and collateral ligaments may also cause confusion. The main problem with the ACL is its oblique course (Fig. 7.15A). This can result in incomplete visualization of the ligament. Further oblique views should be obtained when the ligament is not
completely identified. Normal variations in the posterior ligaments can be confused with partial tears in the PCL, a meniscal tear, or osteochondral fragment.110,111,112 Themeniscal femoral ligament extends from near the posterior capsular attachment of the lateral meniscus to the medial femoral condyle and may have two branches. The most common segment is the ligament of Wrisberg that is seen just posterior to the PCL (Fig. 7.28). This is evident in 23% to 32.5% of sagittal MR images and should not be confused with a partial tear.93,96,103,104,112 The anterior bundle of this ligament or the ligament of Humphrey is seen in 34% of patients (Fig. 7.29).96,103 This lies just anterior to the PCL.






Figure 7.23 Sagittal gradient-echo image of the posterior medial meniscus demonstrating increased signal intensity at the meniscosynovial junction (arrow), which can be confused with a tear.






Figure 7.24 Sagittal gradient-echo image of the knee. The popliteus tendon (large arrowhead) and tendon sheath pass between the lateral meniscus and capsule. This should not be mistaken for a meniscal tear. The small areas of high signal intensity (small arrowhead) are due to the inferior genicular vessels.

Cho et al.93 described several variations in the meniscofemoral ligaments. Familiarity with these variants will assist in proper interpretation of coronal and sagittal MR images. Previous reports describe how meniscofemoral ligaments can be identified in 33% to 59% of patients. This report noted an incidence of 93%.93 The ligament of Wrisberg was identified in 90% and Humphrey in 17% of cases (Fig. 7.30).93 The meniscofemoral ligament can be classified into three types on the basis of the proximal insertion. Type I ligaments (Fig. 7.31) insert on the medial femoral condyle and were completely separated from the PCL. Type II ligaments blend with the PCL and are less vertically oriented (Fig. 7.32). Type III ligaments blend with the inferior PCL forming a distal thickening on the sagittal images. Type I ligaments are most common (45%, Fig. 7.31). Type II ligaments were noted in 31% and type III in 21% of 90 cases.93

There is frequently a linear collection of fat (Fig. 7.33) between the fibers of the MCL and between the lateral collateral ligament and capsule.93,103 These areas can be confused with tears. However, the fat signal is suppressed on T2-weighted sequences. Fluid and blood from a tear should have high signal intensity. Using double-echo or fat-suppressed sequences is useful, as fat signal is suppressed on the second echo and the signal intensity of fluid increases.2,46

Deformity or buckling of the patellar tendon is usually due to position and should not be confused with pathology, especially if signal intensity is normal (Fig. 7.34).46 Subtle increase in signal intensity at the junction of the lower pole of the patella and tibial insertion is noted in 74% and 32% of normal patients. Increased signal intensity that does not increase on the second echo using T2 or T2*. sequences should be insignificant.46 Familiarity with the anatomic variations and normal ligamentous attachments in the knee is critical in evaluating MR images so that false-positive interpretations do not occur.

There are also accessory muscles about the knee that can cause confusion and also in some cases cause clinical symptoms.95,98 The medial and lateral heads of the gastrocnemius arise from the posterior femur just proximal to the femoral condyles.65 Anomalous origins and slips have been noted proximally (Fig. 7.35) in both the medial and lateral heads of the gastrocnemius muscles. Also, anomalous orientation of the gastrocnemius and popliteal artery may result in popliteal artery entrapment syndrome.98 This disorder will be discussed more completely at the end of this chapter.

The tensor fasciae suralis is a rare accessory muscle that may take its origin from the distal aspect of any of the hamstring muscle group. The origin is most often from the semitendinosus. Distally this accessory muscle inserts on the posterior fascia of the leg, the medial head of the gastrocnemius, or via a slender tendon on to the Achilles tendon.98







Figure 7.25 Discoid medial meniscus. A: Axial fat-suppressed proton density image demonstrating the size of the discoid meniscus (short arrows) with a tear (long arrow) anteriorly. Coronal T1-weighted (B) and DESS (C) images demonstrating the extension of the meniscus into the joint (short arrows).

The accessory popliteus muscle arises from the lateral cortex of the femoral condyle along with the lateral head of the gastrocnemius. It extends inferomedially anterior to the popliteal vessels to insert on the posterior capsule of the knee.65,95,98

Artifacts can also be created by previous percutaneous (arthroscopy, injection) or open orthopedic procedures (Figs. 7.36, 7.37, 7.38). The patient’s history should be carefully reviewed or the case reviewed with the referring physician so errors in interpretation related to these procedures can be avoided. Technique can also be varied to minimize metal artifact. Titanium causes less artifact than stainless steel implants.46 Suh et al.113 found that metal oriented perpendicular to the magnet bore caused more significant artifact compared with metal parallel to the magnet bore. Therefore, modifying the patient position to change the orientation of the metal should be considered before imaging. Variations in pulse sequence parameters and decreasing voxel size may also reduce metal artifacts.113

Additional potential pitfalls will be reviewed with specific disorders in upcoming sections.


APPLICATIONS

MRI provides excellent soft tissue contrast and is capable of evaluating the soft tissue and bony structures of the knee in multiple image planes, which provides significant advantages over conventional arthrography, computed tomography (CT), and other imaging techniques. The major application for MRI of the knee has been evaluation of
patients with trauma or suspected internal derangement of the knee.3,21,81,89,114,115,116,117,118






Figure 7.26 Sagittal proton density image of the knee using a 128 × 256 matrix and 16-cm field of view. The phase encoding is in the superior-inferior direction. The truncation artifact creates a faint linear area of increased signal intensity approximately 2 pixels from the meniscal margin (arrows).

Most patients are referred for MRI of the knee to exclude tears in the menisci or ligamentous structures along with other articular, osseous, and soft tissue injuries. Arthroscopy usually is considered the gold standard for evaluating the accuracy of imaging techniques. However, keep in mind the accuracy of arthroscopy varies from 69% to 98% depending on the experience of the examiner.9,19






Figure 7.27 Oblique radial gradient-echo image showing an irregular low-intensity structure (open arrows) extending into the joint from the meniscal margin due to vacuum phenomenon.






Figure 7.28 Sagittal proton density-weighted image demonstrating a normal posterior cruciate ligament. The ligament of Wrisberg (arrowhead) should not be confused with a posterior cruciate defect.






Figure 7.29 Sagittal T1-weighted image of the posterior cruciate ligament. Note the ligament of Humphrey (arrow) anteriorly that should not be confused with a posterior cruciate abnormality. There is also a prominent transverse ligament (arrowhead) anteriorly at the posterior margin of Hoffa fat pad.







Figure 7.30 Sagittal proton density-weighted MR image demonstrating the usual locations of the meniscofemoral ligaments of Humphrey (H) and Wrisberg (W).






Figure 7.31 Coronal T1-weighted image demonstrating a type I meniscofemoral ligament inserting on the medial femoral condyle (arrow).






Figure 7.32 Coronal T2-weighted image demonstrating a type II meniscofemoral ligament (arrows) that is less vertically oriented than the type I in Fig. 7.31. There is high signal intensity in the notch (open arrows) due to an ACL tear.






Figure 7.33 Focused coronal T1-weighted image of the medial collateral ligament demonstrating a linear fat plane (arrow) between the deep and superficial segments.







Figure 7.34 Sagittal proton density-weighted image demonstrating buckling of the patellar tendon (arrow) due to the extended position of the knee.


MENISCAL LESIONS


Meniscal Tears

The most common causes of knee pain and disability are tears in the medial and/or lateral menisci. Pain due to meniscal tears can be mediated via the neurovascular bundle that is in the outer third of the meniscus or can occur when innervated synovium invaginates into a tear. Patients may also present with locking, which is usually related to a bucket-handle tear, or giving way, which is more often related to pain.64,117,119






Figure 7.35 Accessory gastrocnemius. Axial fat-suppressed proton density (A) and sagittal T1-weighted (B) images demonstrating an accessory lateral head of the gastrocnemius (arrow).






Figure 7.36 Axial T2-weighted image of the knee in a patient with knee pain. There is an effusion with a low-signal-intensity structure (arrow) medially. This could be mistaken for a loose body, medial patellar fragment, or thickened soft tissue structure. The signal abnormality was created by an air collection from arthroscopy 2 days earlier.







Figure 7.37 Patient with advanced osteoarthritis and a low-signal-intensity defect medially (arrows). The artifact is subtle on the coronal T1-weighted image (A) and more obvious on the gradient-echo image (B). This artifact was created by a small metal remnant from previous arthroscopy.






Figure 7.38 Coronal T1-weighted image, demonstrating artifacts from a previous screw tract (arrows) created by residual microscopic metal fragments.

Before discussing the MR features of meniscal tears, it is essential to review certain anatomic and pathophysiology aspects of meniscal injury. The lateral meniscus is C-shaped and thicker than the medial meniscus. The transverse diameter is similar in the body, posterior, and anterior horns (Fig. 7.39). The lateral meniscus is also less firmly attached to the capsule and is in fact separated posteriorly by the popliteus tendon and tendon sheath (Figs. 7.24, 7.26, and 7.39). The horns of the meniscus attach to the tibia in the intercondylar region (Figs. 7.14E and 7.15). The medial meniscus is more firmly attached to the capsule. The anterior horn attaches to the intercondylar eminence anterior to the ACL (Figs. 7.14E and 7.15). The transverse diameter of the anterior horn is smaller (Figs. 7.39, 7.40, 7.41) than the posterior horn. The posterior horn attaches to the intercondylar eminence anterior to the PCL.19,64,65

The menisci perform an important function in load bearing and knee function. Up to 50% of load bearing is transmitted through the menisci when the knee is in extension and 85% in flexion.120 The contact area can be reduced significantly after partial menisectomy that can increase contact pressures by 350%.120

Tears in the menisci may result from acute trauma or repetitive trauma and progressive degeneration.37,61,64,121,122,123,124 Acute tears are usually due to athletic injuries, with crushing of the meniscus between the tibia and femoral condyles. Most tears extend from the
posterior to the anterior.61 Chronic repetitive trauma is common both in athletes and nonathletes with aging.75,125 Chrondrocyte necrosis and increase in mucoid ground substance can lead to meniscal tears.126






Figure 7.39 A: Tangential sections of the medial and lateral meniscus. (From Rand JA, Berquist TH. The knee. In: Berquist TH, ed. Imaging of Orthopedic Trauma. 2nd ed. New York, NY: Raven Press; 1992:333-432.) B: Sagittal turbo spin-echo fat-suppressed proton density-weighted image of the knee through the lateral meniscus demonstrating the similar size of the anterior and posterior horns (arrows). C: Sagittal turbo spin-echo fat-suppressed proton density-weighted image through the medial meniscus demonstrating the posterior horn is significantly larger than the anterior horn (arrows).

Examination techniques for meniscal pathology (see Techniques section) may vary depending on the software and preferences of the examiner.2,89,127,128 Patient throughput and ease of lesion detection are both important (Table 7.1). Over the years, many pulse sequences have been studied to determine the optimal imaging sequence for detection of meniscal tears.2,29,41,48,54,60,89,129,130 In the past, we preferred proton density and three-dimensional gradient-echo sequences for meniscal imaging. The contrast of lesions (high signal intensity) compared with the normal low signal intensity (black) of the menisci allows defects tobe more easily appreciated. Lesions are often less conspicuous
on the second echo of spin-echo sequences. Today, turbo or FSE proton density sequences are commonly used with or without fat suppression.89,129,130 The increased signal intensity seen on proton density MR images is felt to be related to hydrogen protons attached to macromolecules in the region of the tear.89 There is still some controversy regarding the accuracy of conventional spin-echo and FSE proton density sequences. Rubin et al.41 found similar sensitivities and specificities with both conventional and FSE techniques. If using FSE techniques, Rosas and De Smet89 recommend lower echo train lengths (<4) and larger bandwidths (>30 mHz) to reduce blurring. We currently obtain FSE proton density sagittal images with and without fat suppression. False negatives can be reduced using fat-suppression techniques (Fig. 7.42).129,130 Statistics for fat-suppressed proton density FSE sequences result in 92% to 95% sensitivity, 92% to 93% specificity, and 92% to 93% accuracy.130






Figure 7.40 Lateral (A) and posterior (B) illustrations of the knee demonstrating the joint space and associated ligament and meniscal anatomy.






Figure 7.41 Menisci and their attachments and associated ligament and tendon anatomy.

Additional parameters include slice thicknesses of 3 to 4 mm and 14 cm field of view with matrix of greater than or equal to 256 × 256.2 Field strength does not seem to result in significant differences in accuracy whether comparing 0.1 to 1.5 T or 1.5 to 3.0 T.89,131,132

Some authors use 0.7-mm thick sections (28 contiguous), a 50/15 TR/TE, 20° flip angle, and 128 × 256 matrix with 3DFT to evaluate meniscal lesions. This technique yielded a sensitivity of 97% and specificity of 96%.2,46 Disler et al.29 found three-dimensional techniques especially useful for posterior horn lesions.

The appearance of normal menisci and meniscal tears has been well documented in MRI literature (Fig. 7.42). Increased signal intensity has been noted on T1-weighted, T2-weighted, proton density, and GRE sequences.4,22,89,129,130,131,132,133,134,135,136 These changes can be seen with mucoid degeneration as well as meniscal tears. Differentiating degeneration from true meniscal tears can be difficult. Communication with the articular surface or meniscal distortion must be demonstrated to feel confident that a tear is present.89,137 When these criteria are documented on two or more contiguous sections, the positive predictive value for a medial meniscal tear is 94% and for the lateral meniscus 96%. When comparing data to finding the above abnormalities on only one section the positive predictive value drops to 43% for the medial and 18% for the lateral meniscus.138







Figure 7.42 Sagittal proton density fast spin-echo without (2,000/25, ET 5, 4-mm thick sections) (A) and with fat suppression (3,000/37, ET 5, 4-mm thick sections) (B) demonstrating the increased conspicuity of the peripheral medial meniscal tear communicating with the inferior articular surface (arrow) on the fat-suppressed image (B).

Grading systems for meniscal tears have been described by Stoller,81 Crues,3,4 and Mesgarzadeh et al.60 on the basis of pathologic findings in cadaver specimens and operative features (Figs. 7.43 and 7.44). A grade 1 meniscal lesion is globular in nature and does not communicate articular surface. Histologically, this stage correlates with early mucoid degeneration. It is felt that these changes are not symptomatic but represent a response to mechanical stress and loading that result in increased production of mucoid polysaccharide ground substance.81






Figure 7.43 Normal and abnormal menisci. A: Normal 3.0-T sagittal turbo spin-echo fat-suppressed proton density-weighted image of the lateral meniscus. There is no signal in the meniscus. Note the popliteus tendon and sheath posteriorly. B: Grade 2 globular increased signal intensity in the medial meniscus on a sagittal 3.0-T proton density-weighted image. C: Grade 3 increased signal intensity in the posterior horn of the medial meniscus communicating with the inferior articular surface on 1.5-T proton density image (small arrowhead). D: Sagittal 1.5-T gradient-echo image demonstrating a grade 3A tear (small arrowhead) with an associated meniscal cyst (large arrowhead). E: Sagittal 1.5-T gradient-echo image demonstrating a more complex linear tear in the posterior horn of the medial meniscus. F: Grade 3B meniscal tear with a broad area of articular involvement (arrowheads) on gradient-echo 1.5-T image.







Figure 7.43 (continued)

Grade 2 signal intensity is linear (Fig. 7.44) in nature and remains within the substance of the meniscus. Once again, there is no evidence of communication with the articular surface of the meniscus. Histologically, grade 2 menisci are characterized by more extensive bands of mucoid degeneration. Most feel that grade 2 changes represent progression of grade 1. Some authors feel that grade 2 lesions are precursors to complete tears.40,81,139 However, Dillon et al.140 found that most were stable when followed for 3 years. Reinig et al.126 found progression when evaluating football players over a period of one season.

With grade 3 tears (Fig. 7.44) there is increased signal intensity within the meniscus that extends to the articular surface (Fig. 7.43C). Demonstrating communication with the articular surface is important, as the tear is not likely to be confirmed arthroscopically. Some authors have used narrow “meniscal windows” to assist in confirming communication with the articular surface. In our experience and that of Buckwalter et al.,141 there is little value in this technique. In fact, if done routinely without conventional windows, bone lesions and other abnormalities can be overlooked. We prefer the “two-slice touch” rule described by De Smet and Tuite138 to confirm articular surface involvement. Grade 3 tears can be further divided into subcategories. Grade 3A signal intensity is a linear intrameniscal signal that abuts the articular margin (Figs. 7.43D and 7.44). Grade 3B is a more irregular area of signal intensity that abuts the articular margin (Figs. 7.43F and 7.44). The grade 3B lesions are most often associated with more extensive degenerative change in the adjacent areas of the meniscus associated with the tear. It is not unusual to have difficulty in differentiating grade 2 from grade 3 tears. Careful windowing and evaluating adjacent sections is useful in these cases.

Grade 4 menisci (Fig. 7.44) are distorted (Fig. 7.45) in addition to changes described with grade 3. With more severe meniscal tears, meniscal extrusion and associated articular cartilage loss are common (Fig. 7.46).142

These categories do not include all possible meniscal injuries such as truncated menisci, bucket-handle tears, and so on.60,61 The grading system is most useful in describing
significant signal-intensity changes that communicate with the articular surface.






Figure 7.44 Meniscal tear grading system.

The MR appearance of different types of meniscal tears is similar to those that have been described with arthrography (Fig. 7.47). Vertical tears are usually traumatic compared with horizontal cleavage tears that are more often degenerative. Degenerative fraying of the surface of the meniscus may also be evident on MR images and is demonstrated as areas of irregular increased signal intensity on the meniscal surface compared with the normal dark or low intensity of the body of the meniscus.






Figure 7.45 Sagittal 1.5-T proton density-weighted (A) and coronal three-dimensional DESS (B) images demonstrate complex degenerative tearing of the anterior horn and body of the lateral meniscus. Additionally, the coronal image B shows extrusion of the fragments (white arrowhead).

Radial tears may be partial or complete and involve the free edge of the meniscus.89 Radial tears separate the longitudinal fibers resulting in loss of function and distortion of the meniscus with axial loading.89 Radial tears may be somewhat difficult to diagnose but are typically seen as areas of increased signal in the inner margin of the menisci (Figs. 7.47 and 7.48). Multiple MRI signs have been described to increase the accuracy for detection of radial tears. The meniscus may appear truncated, or a missing segment may be seen when reviewing contiguous images (Fig. 7.48).143,144 Full-thickness radial tears are demonstrated on coronal and sagittal (Fig. 7.44) images as areas of increased signal involving the entire meniscus with normal meniscal signal on adjacent coronal sections.145

Displaced menical tears require surgical intervention.89 The most common type of tear is the bucket handle tear that accounts for up to 10% of all meniscal tears. The tear may involve the entire meniscus or only the anterior or posterior portion.89 A bucket-handle tear is a tear with displacement of an attached inner fragment for variable distances (Figs. 7.47, 7.48, 7.49).48,54,89 Up to 82% of bucket-handle tears involve the medial meniscus.40 When truncated menisci are identified, one must search carefully for the displaced (bucket-handle) fragment (Fig. 7.49). Several signs (Table 7.5) have been described to assist in detection of bucket-handle tears. Signs of a bucket-handle tear include the double-PCL sign, flipped meniscus sign or double anterior horn sign, the absent bow-tie sign, and a fragment in the intercondylar notch.89 The double-PCL sign (Fig. 7.50) is seen on coronal and sagittal images when the displaced fragment lies below the PCL, giving the appearance of two ligaments. This feature is more common with medial tears (53%) than lateral (14%) bucket-handle tears (Table 7.5).48,54,89 The
flipped-fragment sign (Fig. 7.51) is seen with 44% of medial and 29% of lateral meniscal bucket-handle tears (Table 7.5).54 Another sign is a fragment in the intercondylar notch, which is not the same as the double-PCL sign.89 Defining a fragment in the notch may be difficult, especially when they are small and the configuration of the meniscus is not significantly truncated (Fig. 7.48; Table 7.5). Larger fragments are identified with 66% of medial and 43% of lateral meniscal tears.54 Detection may be improved by using coronal STIR images. Magee and Hinson48 reported detection of 93% of fragments using STIR sequences. Defining the fragments is important, as they need to be removed arthroscopically.48 The final sign is the absent “bow-tie sign,” which is seen as absence of the normal meniscal configuration on sagittal images through the meniscus (Fig. 7.52).89






Figure 7.46 1.5-T proton density (A) and proton density with fat suppression (B) images of a complex medial meniscal tear with associated cartilage loss (arrow) and a large popliteal cyst.






Figure 7.47 A: Types of meniscal tears. B: (1) Radial tear with cross-sectional appearance. (2) Horizontal tear that is only seen on tangential view. (3) Flap tear, oriented oblique to the long axis of the meniscus. Note the distance from the apex increases (a to b) as the tear extends into the meniscus. (4) Vertical tear. Meniscal tears seen tangentially.

Parrot-beak tears are radial tears that progress to a peripheral component giving the appearance of a parrot beak (Figs. 7.47A and 7.53).89 These lesions are most common at the junction of the body and posterior horn of the lateral meniscus. The meniscal flap tear (Fig. 7.47A) has a similar appearance, and though described as a different entity it is essentially the same as a parrot beak on MR images.146 These two tears are often lumped with bucket-handle tears. Flap tears are less common than bucket-handle tears but account for 19% of symptomatic meniscal injuries. Proper description of the tear and fragment location are more important than the eponym used.89

Horizontal meniscal tears extend from the free edge parallel to the articular surface (Fig. 7.54). When the tear extends to the periphery of the meniscus it is not uncommon
to find an associated meniscal cyst. The etiology of meniscal cysts is felt to be related to extension of joint fluid through the tear.89 Horizontal meniscal tears are more common in elderly patients with degenerative disease.89,147






Figure 7.48 Bucket-handle tear with a displaced fragment in the intercondylar notch (arrow).

Meniscal root tears have been lumped into the radial tear category (Fig. 7.55). However, these tears are often more complex resulting in management issues for the orthopedic surgeon.89,148,149 Recent data indicate that lateral meniscal root tears may be difficult to detect on MR images and arthroscopy.148,149 Detection on MRI may be affected by the magic angle phenomenon, and pulsation artifact from the popliteal artery.89 In addition, the incidence of ACL tears is significant as 8% to 9.8% of patients with ACL tears have associated lateral meniscal root tears.148,149 Medial mensical root tears are seen with ACL tears in only 3% of cases. Meniscal extrusion is seen with both medial and lateral posterior root tears.149 The incidence of meniscal extrusion with medial root tears is 88% compared with 23% with lateral root tears.149 As reported by others, we find that coronal water-sensitive sequences are most useful for diagnosis (Figs. 7.55 and 7.56).89,149








Table 7.5 Bucket Handle Tears: MRI Features

























Incidence (%)


Feature


Medial Meniscus


Lateral Meniscus


Double-posterior cruciate ligament sign (Fig. 7.50)


53


14


Flipped-fragment sign (Fig. 7.51)


44


29


Fragment in notch (Fig. 7.49)


66


43


From references 48, 54, 119.


Peripheral tears or separations in the meniscus can also be identified. These injuries are usually easily diagnosed with MR arthrography but can be subtle on conventional MR images due to vascular tissue and the normally increased signal intensity at synoviomeniscal junction along the margins of the meniscus (Figs. 7.23 and 7.57). It is important to clearly define the location (Fig. 7.57) (medial, lateral, inner margin, peripheral), type of tear, and associated bone and/or ligament injury.3,135,150 Grade 1 and 2 lesions will not be identified arthroscopically.150,151 Certain lesions are stable and can heal with conservative therapy, so arthroscopy may be avoided.13,16 Peripheral lesions are particularly suited for meniscal repair (Fig. 7.57).3,151

Patients with suspected meniscal tear can be accurately diagnosed with history alone in 75% of cases. Clinical examination with stress testing, specifically the Thessaly test is94% accurate for medial meniscal tears and 96% accurate for lateral meniscus tears. The Thessaly test is performed with the patient weight bearing and the knee flexed at 5° and 20° with internal and external rotation maneuvers.152 Clinical evaluation along with MRI prepare the treating physician for the most appropriate approach in a given clinical setting. Several reports in the MR literature have described the accuracy of MRI for identification of meniscal tears (Table 7.6).48,143,156,157,158 Sensitivity for MRI in detecting meniscal tears seen at arthroscopy ranges from 75% to 100%. Review of our data demonstrated a sensitivity of 99%, specificity of 90%, accuracy of 90%, positive predicted value of 96%, and negative predicted value of 98% for the medial meniscus. MRI of the lateral meniscus showed 97% sensitivity, 97% specificity, and 91% accuracy.

Management of meniscal tears varies significantly depending upon the patient status and location and type of tear.151,152,153,154 The type and location of meniscal tear are important in determining if the lesion is responsible for the symptoms so the appropriate therapy can be selected.3,16,150,151,152,153,154

The orthopedic surgeons and arthroscopists classify meniscal tears by location and tissue integrity.154 The meniscus is divided into anterior, middle, and posterior as well as inner, middle, and outer thirds (Fig. 7.58). The vascular supply peripherally has lead to calling outer third tears red tears.154 Inner third tears have no blood supply and are referred to as white tears.

Recent articles have described the indications, contraindications and available treatment options for meniscal injuries.151,152,153,154,155 Noyes and Barber-Westin154 described indications and contraindications for meniscal repair.







Figure 7.49 Radial tear in the posterior horn of the lateral meniscus. Axial fat-suppressed proton density 1.5 T (A), coronal DESS (B), and sagittal proton density-weighted (C) images demonstrate a focus of free edge increased signal intensity (arrows in A and B) and a truncated appearance at the level of the tear on the saggital image (arrow in C).






Figure 7.50 Coronal fat-suppressed T2-weighted image (A) demonstrating a medial tear (curved arrow) with a large displaced fragment (black arrow) that gives the appearance of two posterior cruciate ligaments (PCL). There is also a complex tear of the lateral meniscus (white arrow) and loss of articular cartilage. Sagittal proton density-weighted image (B) demonstrating a medial meniscal tear with a large displaced fragment (small arrow), resulting in a double-PCL sign.







Figure 7.51 Flipped-fragment (large or double anterior horn) sign. A: Illustration of the flipped fragment sign seen in the axial and sagittal planes. B: Sagittal proton density-weighted MR image with a large anterior horn (arrow). A portion of the posterior horn is still evident.






Figure 7.52 The absent bow-tie sign. A: Normal fat-suppressed turbo spin-echo proton density sagittal image demonstrating the normal bow-tie configuration of the meniscus. B: Sagittal proton density-weighted image demonstrating absence of the posterior portion of the bow tie (arrow). Note the double appearance of the anterior horn of the meniscus.

Arthroscopic intervention is most optimal in active patients in the second through fourth decades. Unstable fragments greater than 10 to 12 mm in length involving the middle third (Fig. 7.58A) of the meniscus are good candidates for repair. Meniscal tissue should appear near-normal arthroscopically and there should not be secondary tears. Patients must also agree to the postoperative rehabilitation program. Older patients (>60 years of age) or those unwilling to comply with rehabilitation programs can be treated with partial meniscectomy. Tears of the inner third (Fig. 7.58B) are not recommended for repair. Degenerative tears, partial tears, and longitudinal tears less than 10 mm in length are also not usually repaired.154

Arthroscopic repair may be partial meniscectomy or reattachment of the torn fragment using suture material or bioabsorbable arrows.153,154 Partial resection of the meniscus often results in increasing damage to the articular cartilage over time. Therefore, repair is preferred when possible.153 Mensical transplantation is also performed on younger patients with prior meniscectomy to preserve meniscal function and preserve joint stability.154


Postoperative Meniscus

Detection and characterization of meniscal tears can establish the need for surgery and the type of procedure. Other injuries, such as occult bone injury and injury to the capsule or supporting structures, must be evaluated along with meniscal tears as explanations of patients’ symptoms. Conservative therapy or watchful waiting has little downside risk in patients without locking of the knee.159

The meniscus serves multiple important functions. It is a shock absorber and assists in joint lubrication and
chondrocyte nutrition. Stress to the articular cartilage is also reduced. The meniscus also restricts anterior displacement of the tibia on the femur, reducing stressonthe ACL.64,160,161






Figure 7.53 Parrot-beak tear. Coronal DESS (A), sagittal proton density (B), and axial fat-suppressed proton density (C) images demonstrate a radial tear extending peripherally (arrow).

Decisions regarding surgical repair depend upon the type and location (peripheral or central) of the tear as well as what other injuries may be associated with the meniscal tear. Partial meniscectomy, complete removal, or replacement with cadaver allograft or prosthesis may be considered.160,161 Replacement procedures are relatively new and suggested for patients with previous total meniscectomy or partial meniscectomy with continued symptoms but good joint alignment.161

As noted earlier, partial menisectomy is usually performed for flap tears or tears in the inner or avascular zones of the meniscus (Fig. 7.57).120,151,154,155,159 Peripheral tears are frequently repaired (sutured), so the normal meniscal configuration is maintained but with potentially confusing signal intensity changes on MR images.2,62,158

MRI is also of value, though potentially more difficult to interpret, for studying patients who have had either partial or complete menisectomy or primary arthroscopic repairs of the meniscus. Postoperatively, patients are generally referred to exclude residual fragments, remnants of a tear that was not completely resected, or new tears (Fig. 7.59).2,89,135,161,162,163

MR features noted following meniscal repair are similar to preoperative findings. Increased signal intensity that communicates with the articular surface (primary repair) or a truncated margin (partial menisectomy) (Figs. 7.60 and 7.61) are common findings.160,162 Although meniscal tears may fill in with fibrous tissue (dark on MR images; Fig. 7.59), the tear can also fill in with chondrocytes or granulation tissue that will have increased intensity similar to a tear. Increased signal intensity in the meniscus can persist even though the tear is healed.164 Differentiation of postoperative changes versus residual or new tear can be difficult with short TE sequences. T2-weighted or water-sensitive
sequences are more useful in this setting as fluid signal intensity extending to the articular surface are more specific. Specificities have been reported as high as 90%.165 More recent studies suggest that the specificity ranges from 73% to 88% using the earlier criteria.89,162

Although the MR features at the operative site may be confusing, additional causes of pain such as bone or ligament lesions and residual or new meniscal fragments (Fig. 7.60) can be identified.2,3,162






Figure 7.54 Horizontal tear of the lateral meniscus with an associated meniscal cyst. Sagittal proton density-weighted (A) and coronal DESS (B) images demonstrate a horizontal tear in the lateral meniscus (arrow) with an associated menical cyst (arrowhead).






Figure 7.55 Meniscal root tear. Axial fat-suppressed proton density-weighted (A) and coronal DESS (B) images demonstrate a radial appearing posterior medial meniscal root tear (arrow). There is also a popliteal cyst with a loose body (arrowhead).

Improved accuracy using MR arthrography for the postoperative knee has been reported.166,167 Sciulli et al.163 compared conventional arthrography with MR arthrography using iodinated contrast and gadolinium. MR arthrogram studies were most accurate (92%) and conventional
arthrograms least accurate (58%) for evaluating menisci after surgical procedures.163 The use of arthrography remains somewhat controversial.89 In a recent study, De Smet et al.168 found that similar to the nonoperative knee, any signal intensity contacting the meniscal surface on two or more contiguous sections is most likely a new tear. This group reserves MR arthrography for patients with greater than 25% of the meniscus removed.168 We have not used MR arthrography except in selected cases or when specifically requested by the orthopedic surgeons. In our experience, MRI equivocal cases are most often reexamined arthroscopically.






Figure 7.56 Medial meniscal root tear. Axial fat-suppressed proton density (A), coronal DESS (B), and sagittal proton density-weighted (C) images demonstrate a complete posterior medial mensical root tear (arrows). There is extrusion of the anterior meniscus (arrowhead in C).








Table 7.6 MRI of Meniscal Tears



















































































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May 25, 2016 | Posted by in RHEUMATOLOGY | Comments Off on Knee
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Mayo Series


Glashow et al.d


Crues et al.c


Munk et al.e


Dorsay et al.a,b



MM


LM


Both Menisci


MM


LM


MM


LM


MM


LM


No. of cases


129


129


50


144


144


242


242


43


43


Accuracy


90%


91%



89%


94%


94%


92%




Sensitivity


99%


97%


83%


87%


88%


97%


92%


86%-96%


86%-96%


Specificity


90%


97%


84%


91%


98%


89%


91%


89%-98%


89%-98%


Positive predictive value 96%


76%


75%


93%


96%






Negative predictive value


98%


99%


90%


84%


92%