Cartilage

Cartilage is a greatly specialized type of connective tissue, mainly composed of water (70-80% by wet weight). It is avascular and aneural. The solid component of cartilage is formed of cells (chondrocytes) that are scattered in a firm gel-like substance (extracellular matrix) consisting of collagen and proteoglycans. Collagen forms a network of fibrils, which resists the swelling pressure generated by the proteoglycans. In the musculoskeletal system there are two types of cartilage: hyaline and fibro-cartilage. Compared to hyaline, fibrocartilage contains more collagen and is more resistant at tensile strength. Fibrocartilage is found in intervertebral disks, symphyses, glenoid labra, menisci, the round ligament of the femur, and at sites connecting tendons or ligaments to bones. Hyaline cartilage is the most common variety of cartilage. It is found in costal cartilage, epiphyseal plates and covering bones in joints (articular cartilage). The free surfaces of most hyaline cartilage (but not articular cartilage) are covered by a layer of fibrous connective tissue (perichondrium). Hyaline cartilage structure is not uniform (Fig. 3.1). Instead, it is stratified and divided into four zones: superficial, middle, deep, and calcified. The superficial zone, also called tangential zone, is considered the articular surface and is characterized by flattened chondrocytes, relatively low quantities of proteoglycan, and numerous thicker fibrils arranged parallel to the articular surface in order to resist tension. In articular cartilage this layer acts as a barrier because there is no perichondrium. The middle zone, or transitional zone, in contrast, has round chondrocytes, the highest level of proteoglycan among the four zones, and a random arrangement of collagen. The deep (radiate zone) is the thickest zone, characterized by collagen fibrils that are perpendicular to the underlying bone, acting as an anchor to prohibit separation of zones and in order to resist at torsional and compressive mechanical strength. Columns of chondrocytes are

Anatomical diagram of hyaline cartilage structure

Calcified Cartilage Diagram

arrayed along the axis of fibril orientation. The zone of calcified cartilage is partly mineralized, and acts as the transition between cartilage and the underlying sub-chondral bone. A boundary point (tidemark) represents a change in cartilage stiffness from radiate to calcified. The orientation of collagen fibers varies through the four zones of articular cartilage in order to give better tensile strength. The fibrillar framework seems to have an arcade-like arrangement, as hypothesized by Benninghoff. Nevertheless, the arcade model of Benninghoff has not been confirmed at electron microscopy evaluation.

Hyaline cartilage is easily detectable by ultrasonography as a homogeneously hypo-anechoic layer delimited by thin, sharp and hyperechoic margins.

Normal articular cartilage appears as a well-defined layer with the following distinguishing features [1-3]:

1. high degree of homogeneous transparency due to its high water content;

2. sharp and continuous synovial space-cartilage interface (superficial margin);

3. sharp hyperechoic profile of the bone-cartilage interface (deep margin).

The synovial space-cartilage interface is slightly thinner than the bone-cartilage interface. Both margins are best visualized when the direction of the ultrasound (US) beam is perpendicular to the cartilage surface.

The pronounced difference in chemical structure between articular cartilage and subchondral bone allows easy detection of the deep margin, whilst the superficial margin requires careful examination techniques for clear identification.

Optimization of the visualization of the cartilage margins is essential for measuring the cartilage thickness [4].

Cartilage thickness ranges from 0.1 mm on the articular surface of the head of the proximal phalanx to 2.6 mm on the lateral femoral condyle of the knee joint [5]. Measurement of cartilage thickness is rapid (several seconds), painless, noninvasive and reproducible (inter-observer repro-ducibility of measurements of cartilage thickness seems to be relatively good) [6-8].

Sharp margins and homogeneity of the echotex-ture are hallmarks of normal cartilage (Figs. 3.2,3.3).

Healthy subject.Longitudinal dorsal US scan of the second metacar-po-phalangeal joint obtained with a 5-13 MHz broadband linear transducer.The articular cartilage of the metacarpal head appears as a homogeneous anechoic layer with clearly defined hyperechoic contours. m = metacarpal head; p = proximal phalanx g.3.2

Healthy subject. Knee. Suprapatellar longitudinal scan of the articular cartilage of the lateral femoral condyle obtained with a 510 MHz broadband linear transducer.a Normal features of the articular cartilage obtained with the ultrasound beam directly perpendicular to the cartilage surface. b Apparent loss of sharpness of the cartilage margins due to imperfect insonation angle

Healthy subject. Knee. Suprapatellar longitudinal scan of the articular cartilage of the lateral femoral condyle obtained with an 8-16 MHz broadband linear transducer. Both images show the characteristic homogeneous echotexture of the cartilage layer.a Anechoic,obtained with low levels of gain. b Hypoe-choic, obtained with relatively higher levels of gain

Healthy subject. Knee. Suprapatellar longitudinal scan of the articular cartilage of the lateral femoral condyle obtained with an 8-16 MHz broadband linear transducer. Both images show the characteristic homogeneous echotexture of the cartilage layer.a Anechoic,obtained with low levels of gain. b Hypoe-choic, obtained with relatively higher levels of gain

These sonographic features are remarkably similar at different anatomic sites and largely dependent upon the equipment settings.

The typical anechoic pattern is obtained at lower levels of gain (Fig. 3.4 a, b).

Many different factors contribute to the final sonographic visualization of the hyaline cartilage, including size of the acoustic window, operator experience, transducer frequency and patient position. In order to reduce misinterpretation, multi-planar examination and comparison with the contra-lateral side must be carried out [2,9].

The complex anatomical structure of the knee joint poses particular acoustic barriers to accurate evaluation of the cartilage, meaning that only femoral condylar cartilage can be assessed.

The weight-bearing surfaces of the femoral condyles can be assessed by transverse suprap-atellar scanning with the knee in maximal flexion or with an infrapatellar transverse scan with the leg fully extended.

Suprapatellar scanning of weight-bearing areas can be difficult in patients with limited degrees of flexion due to pain.

Further assessment of the weight-bearing cartilage of the medial femoral condyle can also be obtained by the medial parapatellar view with the knee in maximal flexion.

The transverse suprapatellar scan of the knee demonstrates that, in healthy subjects, the femoral cartilage typically appears as a clear-cut, wavy hypo-anechoic layer, with upper concavity, which is thicker at the level of the intercondyloid fossa (Fig. 3.5).

This particular scan should be carried out with the knee flexed to an angle of at least 90°.A panoramic view of the entire cartilaginous profile can best

Fig.3.5

Healthy subject. Suprapatellar transverse view of the knee.Artic-ular cartilage appears as a curved anechoic band.The image was obtained with an Aplio,Toshiba, equipped with a 7-14 MHz broadband linear transducer. f = femur

Fig.3.5

Healthy subject. Suprapatellar transverse view of the knee.Artic-ular cartilage appears as a curved anechoic band.The image was obtained with an Aplio,Toshiba, equipped with a 7-14 MHz broadband linear transducer. f = femur be obtained with wide footprint and medium frequency probes (not higher than 10 MHz). Linear probes do not allow the ultrasound beam to reach the cartilaginous layer with the same angle of incidence, leading to apparent inhomogeneity in the cartilaginous echotexture and profile of the margins.

In addition, the transverse scan demonstrates the femoral cartilage most clearly at the level of the peripheral portions of the femoral condyles.

Conversely, longitudinal scans carried out on contiguous planes allow for accurate evaluation of the profile of the condylar cartilage, from its most proximal portions that articulate with the patella, to the more distal portions that relate to the tibial plateau (Fig. 3.3).

Articular cartilage of the metacarpal head can be evaluated by longitudinal and transverse dorsal scans with the metacarpophalangeal joint held in maximal flexion. Standard longitudinal dorsal and volar scans also may be useful.

Higher frequency probes (> 10 MHz), must be used in order to study the articular cartilage of the metacarpal head. Particular attention must be paid to the identification of the superficial margin that, in healthy subjects, appears as a thin hyperechoic line (of about a tenth of a millimeter thick), visible in tracts perpendicular to the direction of the ultrasound beam. This must be identified in order to obtain a correct measurement of the cartilaginous thickness. In a healthy subject, the thickness of the cartilage of the metacarpal head can vary between 0.2 and 0.5 mm [10].

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