Arthrocentesis: Quick cytologic diagnosis of orthopedic conditions (Proceedings)
Synovial fluid analysis is an essential test for diagnosing joint disease in dogs and cats. It can distinguish inflammatory from non-inflammatory arthropathies, and may help determine the underlying cause of either. Arthrocentesis is a relatively quick and simple procedure to perform. It requires some basic knowledge of joint anatomy, minimal equipment, and can be performed under sedation. It is low risk for the patient, and has the potential for high yield results.
The workup for joint disease begins with a good history, physical examination, and radiographic imaging. Synovial fluid analysis may be indicated for any dog with joint pain or effusion. Most commonly, we use it to definitively diagnose inflammatory arthritis, either infectious or non-infectious. These animals may present with generalized stiffness ± fever, with pain and effusion in multiple joints, or with a lateralized lameness.Animals with immune-mediated joint disease typically have multiple joint involvement. Most often, the distal limb joints- carpus and tarsus- are more affected. Rickettsial arthritis can be polyarticular as well. It is important to document range of motion in all joints, as some affected joints may only show a decreased range of motion or pain with extreme flexion or extension. If polyarthritis is suspected, multiple joints should be aspirated. Some authors recommend sampling of a minimum of 6 joints, to help ensure positive findings in at least 2 joints.
Septic arthritis is more commonly monoarticular, and often affects the more proximal joints- stifle, elbow, shoulder, hip. These cases may have severe joint effusion, and sometimes exhibit accompanying cellulitis.
Joints are comprised of two opposing surfaces of articular hyaline cartilage. The joint is supported by a joint capsule (synovial membrane or synovium) and bathed in synovial fluid. Cartilage is composed of a proteoglycan matrix, collagen, and water. Routine cartilage metabolism and maintenance produce degradative enzymes such as MMP-3 and proteoglycan fragments, which can be found in the synovial fluid. Synovial fluid is a highly viscous ultrafiltrate of blood plasma. The synoviocytes of the joint capsule play a critical role in maintaining the composition of this fluid. Type A synoviocytes are mobile resident macrophages of the joint. They are responsible for phagocytosis and antigen-presentation. Type B synoviocytes are fixed in the villi of the synovium, and are primarily secretory. They produce hyaluronic acid, collagen, and fibronectin. Hyaluronic acid provides the viscous property of joint fluid. As a group, synoviocytes compose the synovial membrane, providing selective permeability to molecules in plasma <12kD in size. Larger molecules, such as the plasma protein fibrinogen, are not present in joint fluid. Thus, because of filtration, secretion, and cartilage metabolism, normal joint fluid differs from plasma by the absence of plasma proteins, and the presence of hyaluronate, mononuclear cells, proteoglycans, and inflammatory mediators. In addition to lubricating the joint, synovial fluid enhances joint stability and provides nutrients to articular cartilage.
The pathogenesis of joint effusion is similar regardless of the specific inciting event. Chondrocytes and synoviocytes release cytokines, which cause vasodilation of the synovial capillaries. This vasodilation and the accompanying increased vascular permeability permit fluid, protein, and inflammatory cells to enter the joint. These white blood cells contribute to the inflammatory cascade, causing release of degradative enzymes from additional cells. The type and number of leukocytes determine the clinical features of the arthritis.