Evidence for the Presence of Synovial Sheaths Surrounding the Extensor Tendons at the Metacarpophalangeal Joints

Background. MRI-detected inflammation around the extensor tendons of metacarpophalangeal (MCP-)joints is prevalent in RA and poses a markedly increased risk of RA-development when present in arthralgia patients. Such inflammation is called ‘peritendinitis’ since anatomy literature reports no presence of a tenosynovial sheath at these tendons. However, the presence or absence of tenosynovium at these extensor tendons has never been studied. Methods. Therefore, an anatomical and histological study of extensor tendons at the MCP-joints of two embalmed human hands was performed. Routine histology with Haematoxylin-Eosin staining and immunohistochemical staining was performed. Results. Immunohistochemistry showed presence of markers for synovial macrophages and fibroblast-like synoviocytes bordering a natural dorsal space next to the extensor tendon, suggesting the presence of a synovial lining. Conclusion. This implies that contrast-enhancement on MRI around extensor tendons at MCP-joints observed in early RA and pre-RA likely represents tenosynovitis and that inflammation of this synovial tissue is an early feature of RA.