Basic Physiology

For radiation synovectomy, ^-emitting radiocolloids are used for intra-articular application. Immediately after administration, the colloids are taken up by phagocytosis by type-A synoviocytes which partly build the surface layer in the synovial membrane as well as by phagocytosing immunocompetent cells such as macrophages. Therefore, the size of the radiocolloidal particles is an important factor determining the biokinetics of the radiopharmaceutical. The colloid has to be small enough to be taken up by the synovial cells and to achieve a homogenous distribution within the whole joint. But it also has to have a particle diameter, which is large enough to avoid leakage out of the joint cavity by venous or lymphatic drainage, which would result in an increased irradiation of the whole body and particularly of the locoregional lymph nodes, the liver, and the spleen. The most appropriate particle size was described to be between 2 and 5 mm (8). Using such radiocolloids, animal studies in rabbits demonstrated a homogenous distribution throughout the synovial tissue by autoradiography (9,10) while no uptake in the articular cartilage was observed. Most of the energy of the ยก3 particles is deposited within a range of millimeters up to 1 cm into synovial tissue depending on the chosen radionuclide. The biological effects within the irradiated tissue are caused by both direct damage and indirect interaction. Secondary oxygen radicals generated by tissue irradiation are known to destroy the cellular membranes by lipid peroxidation and result in DNA strand damage and breaks. Oxygen radicals are also potent inductors of apoptosis. All these interactions result in fibrinoid necrosis, sclerosis, and fibrosis of the synovial stroma, the inflammatory cells, and the capillaries within the synovial membrane. A reduction in volume of the synovial tissue together with a significant reduction of the inflammatory pannus tissue is seen within a few weeks (11). The occlusion of capillaries of the synovial membrane decreases the secretory activity markedly (12,13). Thus, the clinical effects of radiosynoviorthesis for the patient are reduction of pain, swelling, and effusion.

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