风湿性疾病的肌肉骨骼超声三

Chapter 3 Sonographic and power Doppler normal anatomy 91 3.1 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 (chondro- cytes) 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 ran- dom 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 arrayed along the axis of fib- 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 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 hypothe- sized by Benninghoff. Nev- ertheless, the arcade model of Benninghoff has not been confirmed at electron microscopy evaluation. Anatomical diagram of hyaline cartilage structure Fig. 3.1 92 Musculoskeletal Sonography Hyaline 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 the 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-cartilage 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 easy 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 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 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 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 metacarpal head appears as a homogeneous anechoic layer with clearly defined hyperechoic contours. m = metacarpal head; p = proximal phalanx Fig. 3.2 Healthy 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. b Apparent loss of sharpness of the cartilage margins due to imperfect insonation angle Fig. 3.3 a, b a b Sonographic and power Doppler normal anatomy 93Chapter 3 These sonographic features are remarkably sim- ilar at different anatomic sites and largely depend- ent 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 posi- tion. In order to reduce misinterpretation, multi- planar examination and comparison with the con- tra-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 car- tilage 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 thick- er 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 panoram- ic view of the entire cartilaginous profile can best 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 Fig. 3.4 a, b 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 broad- band linear transducer. f = femur Fig. 3.5 a b Anatomical diagram of a synovial joint. Insertion and development of the articular capsule with the synovial membrane, articular cartilage and cavity, fat pads and bursae clearly shown Fig. 3.6 94 Musculoskeletal Sonography be obtained with wide footprint and medium fre- quency 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 cartilagi- nous 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 dor- sal 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), visi- ble in tracts perpendicular to the direction of the ultrasound beam. This must be identified in order to obtain a correct measurement of the cartilagi- nous thickness. In a healthy subject, the thickness of the cartilage of the metacarpal head can vary between 0.2 and 0.5 mm [10]. 3.2 Synovial cavity and articular capsule The synovial cavity (Fig. 3.6) is the space found between bone segments and articular capsule; it is delimited by a fibrous wrap internally covered by a synovial membrane and contains a slight film of synovial fluid. The synovial cavity consists, depending on where it is found, of the joint cavity, the bursae and the tendon sheaths [11]. Sonographic and power Doppler normal anatomy 95Chapter 3 The synovial fluid has a variable volume according to the dimension of the articular cavity and it represents, physiologically, a thin veil to protect the cartilage surface; it acts as a lubricant and it has nourishing functions for the cartilage itself. The synovial fluid is filtered from the blood plasma and it contains a maximum of 200 cell/cc. It also contains electrolytes, glucose, enzymes, immunoglobulins and proteins mainly originating from blood, with the addition of mucin - most- ly hyaluronic acid - which is well-represented. The mucin makes the synovial fluid viscous, elas- tic and plastic [12, 13]. The articular capsule consists of intertwisted bundles of connective fibrous tissue, whose insertion onto bone occurs as a continuous line.At some points the capsule is strengthened by the intrinsic capsular ligaments, represented by local thickenings (made of fibrous or fibro-elastic tissue) of the capsule itself, where the fiber bundles become parallel. The articular capsule is inter- nally covered by the synovial membrane. The synovial membrane is a connective tissue of mes- enchymal origin, covering any exposed osseous surface, the synovial bursae in communication with the joint cavity and the intracapsular ligament and tendons; it is not present on meniscal and dis- cal surfaces and it stops right before the edge of joint cartilage, the peripheral area of which, only a few millimeters thick, constitutes a zone of transition from synovial membrane to cartilage [14]. In the synovial cavities of some joints, adipose tissue is stored in specific regions, forming mobile and elastic pads that fill in the spaces of the articular cavity. Such adipose stores, when the joint moves, adapt to the changes of shape and volume of the synovial cavity, supporting the lubrication of the joint surfaces. The synovial membrane is made of a cellular intima lying on a fibrovascular subintimal lam- ina consisting of abundant loose areolar tissue, collagen and elastic fibers. When the synovial membrane covers the intracapsular tendons or ligaments, the subintima is hardly identifiable as a separate layer, being fused together with the capsule, the ligament or the adjacent tendon [12, 14]. The synovial intima is made of cells, called synoviocytes A and B, whose function is to remove the debris found in the joint cavity and to synthesize some molecules for the synovial fluid. The synoviocytes do not actively proliferate under basal conditions, while the speed of cellular divi- sion is considerably increased after trauma and acute hemarthrosis [13]. The bursae are virtual spaces localized in specific regions of the joint where high friction between closely opposing structures occurs. The bursae can be visualized almost solely in patho- logic conditions, because they physiologically contain a slight film of synovial fluid.As above, the bursae are covered by the synovial membrane that continues from the synovial membrane of the articular cavity, so that it constitutes communicating bursae where the synovial fluid is freely cir- culating. The communicating bursae have a further biomechanical function: they decrease the endoarticular pressure when there is a fluid collection in the joint cavity. Normally, the synovial cavities are barely or invisible with US, whereas they can be easily eval- uated when a thickening of the synovial membrane or a joint fluid collection occurs. The size of the synovial cavity of the hip can be depicted and meas- ured by US using a sagittal view passing through the femoral head – the lower limb externally rotated 10-15°; in healthy subjects the interposed distance between the femoral neck outline and the articular capsule has a mean value of 5.1 mm (range 3-7 mm). There is no evident relationship between the sono- graphic size of the synovial cavity and age, gen- der, height or body weight; the maximum differ- ence between one side and the contralateral side is about 1 mm. In the knee, the only synovial cavity accessable to US is the suprapatellar recess. The examination can be performed through supra-patellar longitu- dinal and axial views, with the patient lying supine with the knee in the extended position. The supra- patellar recess appears as a hypoechoic flat struc- ture, with a regular and clear contour, whose antero- 96 Musculoskeletal Sonography posterior diameter does not measure more than 3-4 millimeters. Dynamic assessment, performed during con- traction of the quadriceps femoris muscle, shows a slight increase both in the antero-posterior diam- eter and in the recess length. The increase in fluid collection during this phase, related to the mean sagittal diameter increase of the bursa (1 mm), can be related to the simulta- neous contraction of the suprapatellar recess ten- sor muscle. This small muscle drags the bursa, caus- ing a vacuum effect that causes the bursa to fill with fluid coming from the joint cavity [4]. In people who are fit, compared to those who lead a sedentary life, the suprapatellar recess diameter does not change, but it is well-visualized in 25% and in 66% of patients, respectively, according to the relaxation and the contraction of the quadriceps femoris [15]. The synovial membrane contour can be indi- rectly assessed when the suprapatellar recess is dis- tended by synovial fluid. It appears as a thin echoic band of 1.7 mm (mean value) [4]. The anterior syn- ovial layer of the suprapatellar recess is usually more easily identified than the deep layer. The first can be easily assessed thanks to the different echogenicity of the overlying quadriceps tendon, which appears moderately echoic, while the latter is strictly con- tiguous with the pre-femoral fat pad,which is echoic and has a maximum thickness of about 1 cm. Nor- mally the synovial fluid in the subquadricipital recess is homogeneously hypoanechoic and any change is related to pathology (Fig. 3.7). The articular capsule, is extremely thin and can be barely identified by ultrasound in physiologi- cal situations, whereas acute, inflammatory, or post- traumatic pathology makes it easily visible because of the natural acoustic window provided by the joint fluid collection. In order to identify the capsule, it is necessary to have precise anatomical reference points, joint by joint. The articular capsule appears as a thin hypere- choic layer, hard to differentiate from the adjacent tendons and ligaments that have very similar echogenicity [16]. In the shoulder, the superior edge of the cap- sule corresponds to the inferior echoic edge of the tendons of the rotator cuff muscles (supraspina- tus, infraspinatus and teres minor); it is only when pathology occurs, such as an adhesive capsulitis – causing thickening and retraction – that the capsule can be identified as a marked irregularity of the inferior profile of the rotator cuff tendons. The axil- lary recess of the inferior edge of the capsule is more easily explored. The articular capsule of the knee can be easily assessed in the internal and external compartments, where the collateral ligaments delineate the cap- sule borders. The same procedure can be applied to assess any other joint of the hand and foot [16, 17]. Longitudinal extended field of view (EFV) US scan of anterior compartment of the knee. Small amount of fluid in the joint (*) and distension of suprapatellar capsular recess. T = quadriceps tendon; P = patella Fig. 3.7 Sonographic and power Doppler normal anatomy 97Chapter 3 3.3 Tendons, adnexa, and ligaments Tendons are critical biomechanical units in the musculoskeletal system, the function of which is to transmit the muscular tension to mobile skeletal segments. They are extremely resistant to traction, almost like bone. A tendon with a 10 mm2 transverse section can bear a maximum of 600-1000 kg. On the other hand, tendons are not very elastic, and can only tolerate a maximum elon- gation of 6% before being damaged. Tendons have very slow metabolism, even during action. This can be significantly increased only by inflammatory conditions and traumas. When a reparative process occurs, a proliferation of fibrocytes is observed with deposition of collagen cells [18, 19]. Tendons macroscopically appear as ribbon-like structures, with extremely variable shape and dimensions, characterized by the presence of dense fibrous tissue arranged in parallel bundles. More specifically, they consist of about 70% of type I collagen fibers that form primary bundles. Among the primary bundles are fibrocytes endowed with large laminar protrusions, named teno- cytes or alar cells.Among the collagen fibers of tendons, elastic fibers (about 4%) can also be found; their role is not different from that of a “shock absorber” when muscular contraction begins. The collagen and elastic fibers both have the same direction as the main lines of force and are lying in a gel consisting of proteoglycans and water. The primary bundles are assembled to form second- ary bundles (representing the tendon’s functional unit), which are clustered in tertiary bundles. The endotenon is a thin connective strip surrounding the primary, secondary and tertiary bundles, and separating them.Vessels and nerves run within the endotenon thickness. The epitenon is a stronger connective covering, surrounding the whole tendon (Fig. 3.8) [14, 20]. From a functional and anatomical point of view, tendons can be divided into two types: support- ing tendons (or anchor ten- dons) and sliding tendons. Anchor tendons (such as the Achilles and the patellar tendon) are typically bigger and stronger than sliding tendons, they are not pro- vided with a synovial sheath, but they are surrounded by a connective lamina external to the epitenon, called peri- tenon; the two connective sheaths (epitenon and pe- ritenon) form the paratenon together with highly vascu- larized adipose and areolar tissue [20]. Sliding tendons are wrapped in a covering sheath (tenosynovial sheath) whose function is to guarantee bet- ter sliding and protection to the tendons when they run adjacent to irregular osseous surfaces, sites of potential friction. The tenosynovial sheath consists of two layers: a visceral layer, strictly con-Anatomical drawing of a tendon Fig. 3.8 98 Musculoskelet