Papillary synovial metaplasia–like change in oral mucoceles: a rare and previously undescribed histopathologic variant of a common oral lesion Angela C. Chi, DMD, a Raymond J. Haigney II, DDS, b Daniel B. Spagnoli, DDS, PhD, c Brad W. Neville, DDS, d and Mary S. Richardson, DDS, MD, e Charleston, South Carolina MEDICAL UNIVERSITY OF SOUTH CAROLINA The development of synovial membrane–like structures has been described previously only in association with breast implants, the bone-cement interface of hip prostheses, tendon implants, testicular implants, and traumatized skin. Previous investigators have theorized that this phenomenon—referred to as “synovial metaplasia”— develops in response to gliding trauma. In some cases, these lesions can exhibit a papillary growth pattern. We report 2 unusual cases of oral mucoceles exhibiting papillary synovial metaplasia–like change: the first arising in the lower lip of an 11-year old African-American boy and the second in the lower lip of a 12-year-old European-American girl. We propose that these cases represent a rare and previously undescribed histopathologic variant of the oral mucocele. These lesions should be distinguished from other oral lesions that may exhibit a papillary cystic growth pattern. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2010;109:268-273) Mucoceles (or “mucus extravasation phenomena”) are among the most common benign soft tissue masses of the oral cavity. 1,2 These lesions result from rupture of a salivary gland duct and extravasation of mucus into the connective tissue. Ductal rupture often is caused by trauma; indeed the lower lip is a site prone to trauma, as well as the most common location for oral mucocele development. 1,2 The classic microscopic finding is mu- cin spillage surrounded by an inflamed granulation tissue response. Synovial metaplasia represents the development of synovial membrane-like structures in locations either adjacent to or distant from joint spaces. Synovial meta- plasia has been reported in association with breast implants, the bone-cement interface of hip prostheses, tendon implants, testicular implants, and scarred or otherwise traumatized skin. 3-5 The alternative term “metaplastic synovial cyst” has been used to refer to skin lesions. 6-8 Most documented cases of synovial metaplasia have been related to trauma. In particular, it has been hypothesized that gliding trauma is an inciting stimulus, as exemplified by the dynamics of the breast implant– capsule interface and the passive gliding of silicon-Dacron–reinforced (Hunter) tendon implants. 9 The typical microscopic features of synovial meta- plasia include a central space surrounded by a mem- brane resembling hyperplastic joint synovium. The membrane surface is composed of a thin band of acel- lular eosinophilic matrix likened to a cuticle. Just be- neath this matrix, there is a condensation of palisaded histiocytes or fibrohistiocytic cells (Fig. 1, A). The cells tend to be aligned perpendicular to the surface. Cellular morphology can range from elongated to rounded or epithelioid. Multinucleated giant cells may be present as well. 4,10,11 In some cases, the membranes may be thrown into papillary or villous folds (Fig. 1, B), a growth pattern referred to as papillary synovial meta- plasia. 10 The microscopic features of the membrane can vary in different stages of lesion development, with early lesions exhibiting marked cellularity with a less organized arrangement, mature lesions exhibiting inter- mediate cellularity with more definite palisading and a smooth surface, and long-standing lesions exhibiting hyalinization with decreased cellularity. 4 Presented in part at the 61st annual meeting of the American Acad- emy of Oral and Maxillofacial Pathology, Kansas City, Missouri, May 5-9, 2007. a Associate Professor, Division of Oral Pathology, College of Dental Medicine, Medical University of South Carolina. b Oral and Maxillofacial Surgeon, Private Practice, Cornelius, North Carolina; Clinical Assistant Professor, Department of Surgery, School of Medicine, Emory University. c Oral and Maxillofacial Surgeon, Private Practice, Charlotte, North Carolina; Clinical Assistant Professor of Oral and Maxillofacial Sur- gery, School of Dentistry, Louisiana State University. d Distinguished University Professor, Division of Oral Pathology, College of Dental Medicine, Medical University of South Carolina. e Professor of Pathology and Director, Surgical Pathology, Depart- ment of Pathology and Laboratory Medicine, College of Medicine, Medical University of South Carolina. Received for publication Jul 11, 2009; returned for revision Aug 28, 2009; accepted for publication Sep 8, 2009. 1079-2104/$ - see front matter © 2010 Mosby, Inc. All rights reserved. doi:10.1016/j.tripleo.2009.09.018 268