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The normal joint

image of The normal joint
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Abstract

Detailed knowledge of the normal anatomy and physiology of healthy canine and feline joints is essential to help recognise abnormalities presented by patients. This chapter describes common anatomical features of joints, joint innervation, cartilage, meniscus, synovium and synovial fluid, subchondral bone, ligaments and tendons.

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Figures

Image of 5.1
5.1 Overview of typical synovial joint structure using the example of the stifle. The stifle has intra-articular cruciate ligaments and menisci; these structures are not common to all joints. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.3 Anatomical structures and sensory innervation of a schematic joint. The joint is richly innervated with primary afferent (sensory) nerves detecting movement, position (proprioception) and pain (nociception). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.4 Normal cartilage. (a) Exposed femoral trochlear sulcus and ridges of a young healthy canine stifle joint, showing the normal glossy white appearance of articular hyaline cartilage. (b) Histological section of healthy cartilage, showing cartilage zones, ‘tidemark’, calcified layer and the subchondral bone (Masson tricolour stain). (c) Schematic diagram showing the layers of cartilage. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. (b, Courtesy of Drs Simon Tew and Mandy Peffers)
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5.5 Schematic diagram showing the structure of articular cartilage. A chondrocyte is located in a lacuna in the extracellular matrix (ECM); the ECM contains negatively charged proteoglycan chains and type II collagen fibres. COMP = cartilage oligomeric matrix protein. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.6 Mechanical activity changes the metabolic activity of chondrocytes, by mechanisms that are not yet well understood. (a) Resting chondrocyte. (b) Membrane stretch activates a number of membrane proteins, including integrins and transient receptor potential channels. Together these mobilize intracellular calcium and change the metabolic activity of the chondrocyte. Simultaneously, a sequence of events, including activation of potassium channels and water permeation through aquaporin channels, allows chondrocytes to decrease their physical size and prevent the cell membrane from rupturing. AQP = aquaporin (water) channels; K = calcium-activated potassium channels; TRP = transient receptor potential channels. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. ( )
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5.7 Proteoglycans of cartilage extracellular matrix. (a) Glycosaminoglycans (GAGs) are largely composed of disaccharides, such as chondroitin sulphate or hyaluronate. These disaccharides are linked together to form polysaccharides. These combine with a core protein to form a proteoglycan. (b) Several proteoglycans can combine together, linked by a hyaluronan ‘backbone’. (c) Many GAGs, linked by core proteins, can join together to form aggrecan aggregates, one type of proteoglycan. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.10 Extracellular matrix (ECM) enzymes in cartilage. The correct balance of the enzymes and of their endogenous inhibitors is crucial for healthy cartilage homeostasis. Change in their expression would lead to, and result from, cartilage degeneration. ADAM = a disintegrin and metalloproteinase; ADAMTS = a disintegrin and metalloproteinase with thrombospondin motifs; MMP = matrix metalloproteinase; MT = membrane type; PAs = plasminogen activators; *MMP8 is also known as collagenase 2; **25 ADAM genes, 19 ADAMTS genes.
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5.11 Involvement of chondrocytes and cartilage in endochondral ossification at joints. (a) The chondrocytes in articular cartilage are described as ‘resting’ and are sparsely distributed. Moving towards the subchondral bone, they are organized in a columnar distribution and are called proliferative. Chondrocytes in the next layer are then termed prehypertrophic, while those closest to the subchondral bone are hypertrophic and greatly enlarged. (b) Resting and (c) proliferative chondrocytes from cartilage as indicated by the arrows (stained with haematoxylin and eosin). (b–c, Reproduced with permission from )
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5.12 The meniscus of a stifle joint, located around the edges of the articular cartilage. In the stifle, the peripheral aspect of the medial meniscus is continuous with the joint capsule, while the lateral meniscus is a separate structure. The meniscus is wedge-shaped when viewed in this cross-section. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.13 Confocal images of meniscal collagen structure. (a) Green staining is type I collagen. (b) Red staining is type II collagen. (c) A composite image showing overlap of types I and II collagen (yellow). The arrow indicates a ‘tie fibre’ where the fibres are wrapped together. (Reproduced with permission from )
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5.14 Several cell types are found in the synovium. The type B synoviocytes primarily produce the synovial fluid, but several cell types are involved with immune functions and blood vessels. Nerve endings are present within the synovium and could be activated by painful stimulation. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.15 Structure of a tendon, showing the internal organization of collagen. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.

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