Ultrastructural characteristics of the synovial membrane in osteoarthritic temporomandibular joints

Purpose: This study analyzed the ultrastructural characteristics of the synovial membrane in various stages of osteoarthritis (OA) of the temporomandibular joint (TMJ), and developed a classification of this involvement based on these morphologic characteristics. Patients and Methods: Synovial membrane biopsies were performed during unilateral arthroscopy in 40 patients. Thirty-one TMJs constituted the OA group; nine TMJs that were not involved by OA constituted the control group. During light microscopic (LM) examination, Various variables were recorded and related to the duration of clinical signs and symptoms. Ten synovial membranes from osteoarthritic joints showing histologically visible pathologic changes in various stages and one control synovial membrane were selected for electron microscopic examination. Results: The initial, early, and intermediate stages of synovial membrane involvement in TMJ OA were characterized by intima hyperplasia. In the initial and early stages, active and hypertrophic intimal cells are found. In the intermediate stage, an increased number of both intracytoplasmic and extracellular filaments was predominant. Fibrosis of the subintimal tissue was initiated by an increased number of active fibroblasts. The late stage of synovial membrane involvement in TMJ OA was characterized by a relatively normal synovial intima of normal thickness, whereas extensive fibrosis was seen in the subintimal tissues. Conclusions: Synovial membrane involvement in TMJ OA is characterized by an early proliferative phase with probable growth factor-mediated increases in the cellular activity of the synovial intima cells (resulting in hyperplasia and hypertrophy), of fibroblasts (resulting in increased production of collagen fibrils and fibrosis), and of endothelial cells (resulting in blood vessel growth and hypervascularity). The late phase is characterized by extensive fibrosis of the subintimal tissue, whether caused by sustained production of growth factors or by chronic venous insufficiency, with normal or little cellular activity.