The size of the synovial cavity of the hip can be depicted and measured 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). Normally, the synovial cavities are barely or invisible with US, whereas they can be easily evaluated when a thickening of the synovial membrane or a joint fluid collection occurs. The communicating bursae have a further biomechanical function: they decrease the endoarticular pressure when there is a fluid collection in the joint cavity. 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 circulating. The bursae can be visualized almost solely in pathologic conditions, because they physiologically contain a slight film of synovial fluid. The bursae are virtual spaces localized in specific regions of the joint where high friction between closely opposing structures occurs. The synoviocytes do not actively proliferate under basal conditions, while the speed of cellular division is considerably increased after trauma and acute hemarthrosis. 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. 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. The synovial membrane is made of a cellular intima lying on a fibrovascular subintimal lamina consisting of abundant loose areolar tissue, collagen and elastic fibers. 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. 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. 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 discal 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. The articular capsule is internally covered by the synovial membrane. 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 consists of intertwisted bundles of connective fibrous tissue, whose insertion onto bone occurs as a continuous line. The mucin makes the synovial fluid viscous, elastic and plastic. It also contains electrolytes, glucose, enzymes, immunoglobulins and proteins mainly originating from blood, with the addition of mucin - mostly hyaluronic acid - which is well-represented. The synovial fluid is filtered from the blood plasma and it contains a maximum of 200 cell/cc. 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 cavity consists, depending on where it is found, of the joint cavity, the bursae and the tendon sheaths.įig.3.6 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. The synovial cavity 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.
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