Applied Radiology

EDITOR'S NOTE

This issue of Applied Imaging addresses the MR evaluation of the painful knee. Whether the pain is chronic (e.g., osteoarthritis) or acute (e.g., posttraumatic internal derangement), MRI is the preferred imaging modality for further evaluation. MRI very accurately assesses tears of the menisci as well as the cruciate and collateral ligaments. MR is also very sensitive for the presence of bone-marrow edema, which can be seen in the acute setting ("bone bruise") or in the chronic setting from osteoarthritis.

­­William G. Bradley, Jr., MD, PhD, FACR

Knee Pain

Although plain films (x-rays) have traditionally been the first diagnostic imaging study performed in the evaluation of the painful knee, today they are useful only for evaluating joint space narrowing, alignment, and major trauma. Over the past 15 years, magnetic resonance imaging (MRI) has become the premier, first-line imaging study that should be performed in the evaluation of the painful knee. This issue of Applied Imaging describes the MR evaluation of internal derangement (tears of the menisci and the cruciate and collateral ligaments), osteochondral abnormalities (chondromalacia, osteoarthritis, and osteochondral defects), synovial cysts, and bone bruises.

Internal Derangement

In order to evaluate internal derangement adequately, the knee must be scanned in the axial, coronal, and sagittal planes using thin sections (3 to 5 mm thick) with a combination of T ₁ - and T ₂ -weighted techniques, including at least one T ₂ -weighted image with fat suppression. ¹

The anterior and posterior horns of the medial and lateral menisci appear as wedge-shaped, low-intensity structures pointing toward each other on sagittal and coronal views. ² High signal within the meniscus, extending to the superior or inferior articular surface (Figure 1), constitutes a meniscal tear. This is best seen on images with narrowed "meniscal" windows (Figure 1). A caveat: in the setting of prior surgery, it can be difficult to distinguish an old scar from an acute tear without injecting gadolinium into the joint space. Internal meniscal signal not extending to the articular surfaces or extending only to the base of the "wedge" does not constitute a tear. In children, this represents the normal vascular pedicle of the meniscus; in adults it represents myxoid degeneration, ie, normal aging.

Horizontal-oblique tears are considered results of myxoid degeneration and are called "degenerative" tears. Those oriented in the vertical direction, involving both the superior and inferior articular surfaces, are considered "traumatic" tears. ² Tears do not need to be seen in both the sagittal and coronal planes, as they are only visualized when they are perpendicular to the plane of section. In fact, tears running oblique to both the coronal and sagittal planes may well be visualized only by a notch on the articular surface. This "notch sign" ³ has served us very well over the last decade in diagnosing subtle meniscal tears (Figure 2). Truncation of the tip of the meniscus represents a "parrot beak" tear, ie, fraying of the free margin of the meniscus.

Occasionally, a large piece of meniscal fibrocartilage will become detached from its capsular attachment and flip back-to-front or centrally into the intercondylar notch. These "bucket-handle" or "flap" tears ⁴ are recognized on the basis of two findings: an absence of the normal meniscus in its expected position and abnormal meniscal-intensity material in an unexpected location (Figure 3). Bucket-handle tears flipping from posterior to anterior appear as two arrowheads pointing posteriorly in the position of the anterior horn of the meniscus, with nothing at all in the expected position of the posterior horn. This pattern is more common for bucket-handle tears of the lateral meniscus. Tears flipping from the periphery into the intracondylar region of the knee appear as an extra structure adjacent to the cruciate ligaments, with an absence of meniscal signal in the expected position of the posterior horn. Typically, this pattern is seen with bucket-handle tears of the medial meniscus. When the displaced meniscal fragment lies parallel to the posterior cruciate ligament, it may produce the "double PCL sign." ⁴

The anterior cruciate ligament may be seen on sagittal images (Figure 4); however, it is always seen as an upside -down "V" on coronal images (Figure 5). It is most commonly injured with valgus stress to the knee while the leg is in external rotation. ^5-7 MR studies performed shortly after the injury will fail to demonstrate the separate fascicles of the ACL, which are replaced by an ill-defined mass of tissue that is bright on T ₂ -weighted images (reflecting edema). MR imaging performed much later in a chronically "ACL-deficient" knee also fails to demonstrate the well-defined fibers of the ACL, but lacks the T ₂ hyperintensity of an acute tear.

With the medial knee joint distraction resulting from valgus stress, the medial collateral ligament (MCL) as well as the medial meniscus may also be torn (known as O'Donohue's triad). Acute injuries of the MCL are best evaluated in the coronal plane demonstrating discontinuity of the normally black line (Figure 5) running along the medial aspect of the knee. With acute injuries, there is also a linear fluid collection (which is bright on a T ₂ -weighted image) superficial to the torn MCL. This is usually associated with reticulation of the overlying subcutaneous tissues, indicating soft-tissue swelling. Chronic MCL tears result in thickening without associated fluid.

In addition to the medial distraction resulting from valgus stress during an ACL injury, there is lateral impaction. This may lead to bone marrow contusions of both the femoral and tibial surfaces of the lateral compartment (Figure 6). At the time of the MR examination, the leg is in a neutral position (rather than being externally rotated as it was at the time of ACL injury); therefore, the "kissing contusions" are not directly opposite each other. ⁸

Bone marrow contusions (also called "bone bruises" and "trabecular fractures") cannot be seen on plain films and can be a major source of pain. ⁸ They should be suspected in the patient with the acutely injured knee without a meniscal or ligamentous injury in the absence of plain-film findings of fracture. Because their water content is higher than that of adjacent fatty bone marrow, they are best detected on fat-suppressed, T ₂ -weighted images. ⁹ At high field, we use fat-saturated T ₂ -weighted fast spin echo images and at any field strength, STIR images.

In one study of 73 consecutive patients with suspected internal derangement of the knee, fat-saturated T ₂ -weighted fast spin echo imaging detected one or more bone bruises in 30% of patients in whom lesions were not seen on conventional MRI. ⁹

In evaluating bone marrow contusions it is important to determine if they extend to the articular surface, as this is usually an indication to rest the athlete for 30 days. Continued activity in this setting could convert a bone bruise with intact cartilage (type I) (Figure 6) to one in which the cartilage becomes disrupted (type II) (Figure 7A), leading to early osteoarthritis. ⁸

Bone bruises can be seen whenever abnormal forces are applied to normal tissue or essentially normal forces are applied to weakened tissue, eg, osteoporotic bones. Thus, milder versions of the same forces that lead to tibial plateau fractures will lead to bone marrow contusions, resulting in normal plain films and high signal on STIR or fat-saturated T ₂ -weighted fast spin echo images.

An interesting pattern of bone marrow contusion can be seen with traumatic dislocation of the patella. After the patella dislocates laterally, the quadriceps contracts, jamming the medial patellar facet against the lateral femoral condyle, resulting in bone marrow edema on both surfaces (Figure 7B). Usually, this is also associated with disruption of the black line of the medial retinaculum, which, acutely, may have associated bleeding or edema. ¹⁰

Bone marrow contusions or fractures associated with cortical disruption can leak marrow into the joint space, leading to a lipohemarthrosis. Since sagittal images are acquired with the patient supine, fat rises to the top, joint fluid is in the middle, and intact red cells are at the bottom (Figure 8).

Lesions of the posterior cruciate ligament and the lateral collateral ligament are much less common than their anterior counterparts. The PCL is recognized as a dark C-shaped structure on sagittal images and is visualized as a dark rounded structure seen en face on coronal images (Figure 5). ¹ The coronal images are best for visualizing partial PCL tears (which show increased signal in a normally black structure), while the sagittal view may better demonstrate complete disruption.

Osteochondral Abnormalities

The earliest of the osteochondral abnormalities is simple cartilaginous thinning as seen in chondromalacia patellae ¹¹ and osteoarthritis. Although cartilage can be evaluated on our routine sequences, we tend to use a higher resolution (512 * 512 matrix) thin-slice (1.5 mm sections) gradient echo technique when subtle abnormalities of cartilage are being sought.

With advancing osteoarthritis, the articular cartilage is completely denuded and subchrondral sclerosis can be seen as low intensity on all sequences. With continued bone-on-bone irritation, fibrovascular reaction can lead to bone-marrow edema, which appears bright on STIR or fat-saturated T ₂ -weighted fast spin echo images.

Osteochondral defects arise from varying degrees of traumatic and ischemic insults to the articular surface of the knee (Figure 9). Early osteochondral defects appear as localized areas of rounded bone marrow edema without frank fluid collection. ¹² With advancing disease, a ring of frank fluid can be seen surrounding the osteochondral defect, although the overlying articular cartilage remains intact. Subsequently, the articular cartilage also becomes disrupted and, eventually, the fragment separates, becoming "free" within the joint space, leaving a divot behind.

Cystic Disease of the Knee

Synovial cysts are very common in the knee given the large number of synovium-lined surfaces. The most common is a Baker's cyst (Figure 10), which generally results from a knee effusion that forces synovium through the space between the tendons of the medial head of the gastrocnemius and the semimembranosus. ¹³ Baker's cysts may persist following resolution of the knee effusion and are a cause of medial joint line tenderness.

Another, less common, cause of medial joint line tenderness is anserine bursitis, ¹⁴ which is inflammation of the synovium of the pes anserinus (the confluence of tendons from the sartorius, gracilis, and semitendinosus muscles).

Synovial cysts can be seen in conjunction with torn menisci, more commonly involving the lateral than the medial meniscus. These cysts appear to result from a ball valve mechanism that allows fluid to flow from the torn meniscus into the cyst but not in the opposite direction. ¹⁵

Conclusion

MRI is now the preferred imaging technique for the evaluation of the painful knee. Not only can it detect the fractures that are missed on plain films, but it can also detect soft-tissue abnormalities (meniscal and cruciate/collateral ligament tears) that cannot be detected by plain film. These findings are critical for the therapeutic decisions to be made by the orthopedic surgeon.

Clinical Quiz: True or False

1. Areas of high signal intensity within the menisci on MRI scans of the knee represent meniscal tears.

2. A plain film of the knee is a pretty good way to screen for fractures.

3. MRI is recommended for all patients with knee pain.

4. MRI is good for diagnosing tears of the anterior cruciate ligament.

5. MRI can differentiate among different causes of medial joint line tenderness in the knee.

Answers

1. False. Areas of high signal generally represent myxoid degeneration; in order to diagnose a tear, the high signal must extend to the superior or inferior articular surfaces.

2. False. Frank cortical disruptions, which are frequently missed on plain films, can be detected easily using MRI due to its thin slice, tomographic capability. More importantly, bone bruises (which can be a major source of pain) cannot be diagnosed at all by plain films and are seen easily with fat-suppressed T ₂ -weighted MRI studies.

3. False. Patients with cardiac pacemakers, ferromagnetic intracranial aneurysm clips, and neurostimulators cannot have an MR study, even of the knee.

4. True. The normal ACL can be visualized on all coronal images of the knee and on most sagittal images. If it is not seen, it is torn or deficient. Acute tears are associated with abnormal fluid collections in the intercondylar notch, often with "kissing contusions" of the anterior lateral tibial plateau and the posterior lateral femoral condyle.

5. True. MRI can distinguish medial meniscal tears from medial bone marrow contusions (bone bruises) from Baker's cysts (ruptured or not) from anserine bursitis.