TRUE SYNOVIAL METAPLASIA OF BREAST IMPLANT CAPSULES - A LIGHT AND ELECTRON-MICROSCOPIC STUDY

The formation of true synovial-lined membranes at tissue sites not int imately related to an articulation or a tendon sheath has been describ ed in a variety of pathologic and postsurgical conditions, but until r ecently has not been well recognized to occur in association with tiss ue surrounding silicone breast implants. Of 15 cases with resected per iprosthetic breast capsules, 7 (47%) demonstrated true synovial metapl asia with capsule-implant interfaces lined by typical synovial cells. Histochemical and immunohistochemical staining reactions were essentia lly identical to those observed in synovial control cases and featured positive reactions to Alcian blue-periodic acid-Schiff, reticulin, an d vimentin. Focal positive immunoreactivity was observed with alpha(1) -antitrypsin, alpha(1)-antichromotrypsin, lysozyme, and CD68. No immun oreactivity was observed with cytokeratin AE1/AE3, S-100 protein, carc inoembryonic antigen, or basement membrane antigens. Transmission elec tron microscopy of the lining cells confirmed their true synovial natu re with the type A (macrophage-like) cells, type B (fibroblast-like) c ells, and intermediate forms or type AB cells identified. We conclude that the cellular lining surrounding silicone breast implants is a tru e synovial membrane, that synovial metaplasia may occur in nearly one half of all resected periprosthetic capsules, and that awareness of th is entity will enable the surgical pathologist to render an accurate h istopathologic diagnosis.