Background: Synovial chondromatosis (SC) of temporomandibular joint (TMJ) occupies 3% SC cases. In other joints like hip and knee which were composed hyaline cartilage (HC), loose bodies (LBs) were reported to be a HC feature. However, condyle surface and disc in TMJ are fibrous cartilage (FC). Therefore, we proposed a different pathogenesis of TMJSC.Methods: LBs and synovium were collected from seven TMJSC patients, and histological and immunohistological examinations were performed.Results: Three ways of HC formation were discovered: regular-shaped cartilaginous nodules (CNs) in sublining layer (SL) of vascularized synovium, regional chondrification of SL, and finger-like tissue with a tail attaching to synovium. Detached LBs could fuse and were only positively stained by aggrecan. Without synovium attachment to LBs, fused LBs remained a hyaline extracellular matrix (ECM). However, after synovium attachment, transformation from HC to FC occurred. Two types of FC were observed. First type FC was featured by vertical-distributed type I collagen fibers imbedding few chondrocytes, suggesting mature phase with superior mechanical features. Second type FC was featured by medium-density chondrocytes with type I collagen and aggrecan-positive ECM, suggesting primary phase. The transformation process started in appearance of 2nd type FC deriving from synovium covering LB, and gradually replaced HC from periphery to center.
Conclusions: Three ways of HC formation were closely related. Different with SC in other joints, hyaline ECM in LBs of TMJSC could be replaced by FC deriving from synovium, during which 2nd type FC first replaced HC and then transformed to 1st type FC. K E Y W O R D S angiogenesis, fibrous cartilage, hyaline cartilage, loose bodies, synovial chondromatosis 1 | INTRODUCTION Synovial chondromatosis (SC) is an uncommon, benign and proliferative disorder of subsynovial cartilaginous metaplasia. 1 It is characterized by formation of hyaline cartilaginous nodules (CNs) in sublining layer (SL) of synovium, subsequently separation from synovium and formation of loose bodies (LBs) in joint compartment. 2 Though large joints such as knee, elbow, and hip are usually affected by SC, 3 SC in temporomandibular joint (TMJ) only occupies 3% cases. 4 More than 200 cases of TMJSC had been presented in literatures. 5 The existence of LBs in TMJ causes joint pain, limited mouth opening and degenerative changes for disc, condyle and glenoid fossa 6 even skull base, 7 which impedes normal function of TMJ.
