Книги по МРТ КТ на английском языке / MRI for Orthopaedic Surgeons Khanna ed 2010

■ Overview of Articular Cartilage

There have been considerable advances in the MRI of articular cartilage in recent years. Cartilage-sensitive pulse sequences should be included as a part of all joint-imaging protocols to provide a reproducible, noninvasive means of monitoring disease progression in inflammatory and degenerative arthritides, detecting traumatic cartilage injury, and evaluating surgically manipulated cartilage. Cartilage repair techniques such as autologous chondrocyte implantation, microfracture, and osteochondral autografting are being performed with increased frequency, and MRI o ers a noninvasive method for evaluating the results of these procedures.

To di erentiate the MR appearance of normal and abnormal cartilage morphology, it is important to understand the structure of articular cartilage, which is the basis for the development of new imaging techniques. Articular cartilage, a metabolically active tissue, has viscoelastic properties and is composed of chondrocytes (approximately 1%) and an extracellular matrix that consists mainly of water (65% to 80%), proteoglycan, and collagen.1 Type II collagen is the most common type (95%), but other types have been identified (IV, VI, IX, X, and XI).1 Collagen provides the tensile strength of articular cartilage. The proteoglycans consist mainly of chondroitin and keratin sulfates, which provide compressive strength to the cartilage.

Articular cartilage ranges in thickness from 2 to 5 mm, depending on the contact pressures that occur across a joint. Because of high peak pressures, the patellofemoral joint has the thickest articular cartilage in the body. Articular cartilage can be divided structurally and functionally into four zones (Fig. 14.1):

•Superficial

•Transitional or middle

•Deep or radial

•Calcified

The superficial zone accounts for 10% to 20% of the total thickness of cartilage, has the highest collagen content of all of the zones, and has highly organized collagen fibers that are oriented parallel to the articular surface.2 This zone resists shear stress and has a low proteoglycan content. At

clinically relevant field strengths, this zone is typically not distinguishable from the transitional zone. The middle or transitional zone accounts for 40% to 60% of the cartilage thickness, has collagen fibers that are randomly oriented, and has a higher compressive modulus than the superficial zone because the inhomogeneity of fiber orientation distributes the stress more uniformly across the loaded tissue.3 The deep or radial zone accounts for 30% of cartilage thickness and has highly organized collagen fibers that are oriented perpendicular to the cartilage surface. This zone also has the lowest water content and highest proteoglycan content. The collagen fibers have a radial orientation that crosses the tidemark, the interface between the articular cartilage and the calcified cartilage beneath it, which anchors the cartilage to the underlying bone.4 The calcified cartilage layer is the final zone. This zone is separated from the radial zone by the tidemark, which also represents a potential shear plane for articular cartilage defects.5 At clinically relevant field strengths, the tidemark cannot be di erentiated from the subchondral plate.

The water, proteoglycan, and collagen content all account for the MR signal characteristics. However, the bulk of the signal comes from the free water present and not from the water that is electrostatically bound to proteoglycan or from the water that is associated with collagen. Depending on the pulse sequence used, cartilage will have a bilaminar or trilaminar appearance secondary to the highly ordered structure of the collagen of the deeper radial zone, which yields a shorter T2 relaxation time and corresponding lower signal intensity. Understanding the normal signal characteristics and anatomy of articular cartilage is imperative for detecting abnormal cartilage morphology.

■Specialized Pulse Sequences and Imaging Protocols

Although MRI provides soft-tissue contrast that is superior to that of traditional imaging techniques, standardized conventional radiographs are also a valuable part of cartilage assessment. In particular, when planning for cartilage repair,

Fig. 14.3 Knee images. (A) A sagittal FSE sequence shows a focal partial-thickness cartilage defect overlying the medial femoral condyle (arrowhead). (B) A sagittal fat-suppressed T1–weighted gradientecho sequence, in which the lesion is not as well appreciated. Obtain-

more mobile, corresponding to longer T2 values. In contrast, the deep zone has highly ordered collagen, and therefore T2 values are short because water is relatively immobilized (Fig. 14.5). Clinically, T2 mapping is important because of its ability to detect changes in cartilage structure before the

ing this image took twice as long as the FSE image. (From Potter HG, Foo LF. Magnetic resonance imaging of articular cartilage: trauma, degeneration, and repair. Am J Sports Med 2006;34:661–677. Reprinted by permission.)

substantial loss of cartilage thickness or the development of gross signal alterations in the cartilage gray scale. This early detection can help surgical decision-making by potentially optimizing the timing of operative procedures, such as meniscal transplantation or patellofemoral realignment.

Fig. 14.4 Sagittal FSE image of the knee in a 50-year-old patient that shows gray-scale stratification of the tibial plateau articular cartilage (arrow). Note the di erential contrast for the high signal intensity joint fluid within the meniscosynovial recess (arrowhead), the intermediate signal intensity of hyaline cartilage, and the low signal intensity of meniscal fibrocartilage. (From Potter HG, Foo LF. Articular cartilage. In: Stoller D, ed. Magnetic Resonance Imaging in Orthopaedics and Sports Medicine. Baltimore: Lippincott Williams & Wilkins; 2007:1099–1130. Reprinted by permission.)

of cartilage injury and repair, and it may be applied to MRI (Table 14.2).19,24

■ Clinical Cartilage Imaging

Knee

MRI after injury allows for noninvasive evaluation of cartilage and can detect clinically relevant lesions such as a cartilage shear injury or a displaced cartilage flap, which can mimic other injuries such as a displaced meniscal tear25 (Fig. 14.6). In the knee, articular cartilage injuries have been

A

■ Classification

Many scoring systems can be used to classify articular cartilage lesions.19–22 The most common is the Outerbridge system, which is an arthroscopic system that divides lesions into five grades22:

•Grade 0: normal cartilage

•Grade I: cartilage with softening and swelling

•Grade II: a partial-thickness defect with fissures on the surface that do not reach subchondral bone or that are <1.5 cm in diameter

•Grade III: fissuring to the level of subchondral bone in an area with a diameter of more than 1.5 cm

•Grade IV: exposed subchondral bone

This classification system was originally used to classify chondromalacia patellae but has been extrapolated to classify chondral lesions throughout the body with moderate accuracy.23 However, the limitation of this system is that it does not include a description of lesion depth for grade II or grade III lesions. The International Cartilage Repair Society has a validated standardization system for the evaluation

B

Fig. 14.5 Images of the knee in an avid marathon runner with anterior knee pain. (A) An axial FSE image shows focal increased signal (arrowhead) a ecting normal-thickness cartilage in the lateral patella facet with subchondral sclerosis. (B) A corresponding quantitative T2 relaxation time map shows geographic loss of stratification and prolongation in T2 values (arrowhead) throughout the thickness of the cartilage at this site. (From Potter HG, Foo LF. Articular cartilage. In: Stoller D, ed. Magnetic Resonance Imaging in Orthopaedics and Sports Medicine. Baltimore: Lippincott Williams & Wilkins; 2007:1099–1130. Reprinted by permission.)

Table 14.2 Modified International Cartilage Repair Society Classification: MRI, Arthroscopic, and Anatomy Correlations

Fissure ≤50%

(continued on next page)

Cartilage Articular 14

Table 14.2 (continued)

Source: Adapted from Shindle MK, Foo LF, Kelly BT, et al. Magnetic resonance imaging of cartilage in the athlete: current techniques and spectrum of disease. J Bone Joint Surg Am 2006;88:27–46. Adapted by permission.

*Modified Outerbridge classification.19,24

Considerations Special V 358

Fig. 14.6 A coronal cartilage-sensitive image of the knee in a 31-year-old man with a clinically suspected meniscal tear shows a focal chondral flap (arrowhead) over the right medial femoral condyle. (From Potter HG, Foo LF. Magnetic resonance imaging of articular cartilage: trauma, degeneration, and repair. Am J Sports Med 2006;34:661–677. Reprinted by permission.)

associated with poor clinical outcomes.26–28 Thus, MRI before surgical intervention can aid in diagnosis, help predict prognosis, and identify patients who may benefit from cartilage repair techniques.27

Chondral delamination can occur when shear stresses cause separation of the articular cartilage from the underlying subchondral bone at the tidemark29,30 (Fig. 14.7). Careful scrutiny of traumatic cartilage injury is necessary to distinguish an osteochondral fracture from an isolated cartilage shearing injury. Osteochondral injuries are recognized by the absence of the thin, low signal intensity subchondral plate and tidemark between the bone and cartilage or the presence of a hyperintense fatty marrow signal attached to the cartilage fragment7 (Fig. 14.8).

OCD is the result of the separation of a portion of subchondral bone along the articular surface secondary to an acute shear injury or repetitive trauma. It most commonly occurs at the lateral aspect of the medial femoral condyle. In this setting, MRI is useful for assessing the stability of the lesion (Fig. 14.9). Signs of an unstable fragment include the following31:

•High signal intensity surrounding the fragment on water-sensitive pulse sequences (e.g., proton density) and T2-weighted images

•Size >5 mm

Fig. 14.7 Chondral delamination. This sagittal FSE image of the knee shows articular cartilage delaminating (arrowhead), with fluid signal intensity between the cartilage flap and the underlying subchondral bone. (From Shindle MK, Foo LF, Kelly BT, et al. Magnetic resonance imaging of cartilage in the athlete: current techniques and spectrum of disease. J Bone Joint Surg Am. 2006;88:27–46. Reprinted by permission.)

•High signal intensity defect in the overlying cartilage

•Prominent cystic changes of ≥5 mm between the fragment and host bone

Hip

Articular cartilage injuries of the hip are di cult to evaluate because of the deep ball-and-socket configuration. Traditionally, chondral damage to the hip has been associated with progressive generalized joint deterioration from such conditions as osteoarthritis or inflammatory arthritis. However, there are several other mechanisms that result in more focal chondral lesions, such as trauma, osteonecrosis, femoroacetabular impingement, and dysplastic conditions. Focal chondral injuries on the femoral side are relatively uncommon, but they may result from axial loading or a shear injury of the head within the socket. Cartilage injuries on the acetabular side are more common and typically present as localized cartilage delamination in the anterosuperior weight-bearing zone of the acetabular rim. Femoroacetabular impingement is the most common underlying condition resulting in these types of cartilage defects. The articular surfaces of the acetabulum and femoral head should be evaluated with the use of all three imaging planes. Although some authors advocate the use of MR arthrography to improve contrast between the articular cartilage and synovial fluid, this addition converts