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1、Chapter 3Sonographic and power Doppler normal anatomy 913.1 CartilageCartilage 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 (chondro- cytes) that are scattered in a f
2、irm 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 hyalin
3、e,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 ofcarti
4、lage.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)
5、.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 arran
6、ged 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 ofproteoglycan among the four zones,and a ran- dom arrange
7、ment 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 ofzones and in order to resist at torsional and compressive mechanical strength.Columns ofchondrocytes are arrayed
8、along the axis offib- ril 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 ofcollagen f
9、ibers varies through the four zones ofarticular cartilage in order to give better tensile strength. The fibrillar framework seems to have an arcade-like arrangement, as hypothe- sized by Benninghoff.Nev- ertheless,the arcade model ofBenninghoffhas not been confirmed at electron microscopy evaluation
10、.Anatomical diagram of hyaline cartilage structureFig.3.192Musculoskeletal SonographyHyaline cartilage is easily detectable by ultra- sonography as a homogeneously hypo-anechoic layer delimited by thin, sharp and hyperechoic margins. Normal articular cartilage appears as a well- defined layer with t
11、he following distinguishing fea- tures 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-cartil
12、age interface is slight- ly 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 struc- ture between articular cartilage and subchondral bone allows e
13、asy detection of the deep margin, whilst the superficial margin requires careful exam- ination 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
14、 surface of the head of the proximal pha- lanx to 2.6 mm on the lateral femoral condyle of the knee joint 5.Measurement of cartilage thick- ness is rapid (several seconds),painless,non-inva- sive and reproducible (inter-observer repro- ducibility of measurements of cartilage thickness seems to be re
15、latively good) 6-8. Sharp margins and homogeneity of the echotex- ture are hallmarks ofnormal cartilage (Figs.3.2,3.3).Healthy subject.Longitudinal dor- sal US scan of the second metacar- po-phalangeal joint obtained with a 5-13 MHz broadband linear transducer.The articular cartilage of the metacarp
16、al head appears as a homogeneous anechoic layer with clearly defined hyperechoic contours.m = metacarpal head; p= proximal phalanxFig.3.2Healthy subject.Knee.Suprapatellar longitudinal scan of the articular cartilage of the lateral femoral condyle obtained with a 5- 10 MHz broadband linear transducer.a Normal features of the articular cartilage obtained with the ultrasound beam directly perpendicular to the cartilage surface.bApparent loss of sharpness of the cartilage margins due to imp
