title: Monthly swelling of the knee - Case report and review of the literature.
abstract: BACKGROUND
  We report a 17-year old with monthly swelling of her knee. The complaints are associated with the menstrual cycle. After physical examination, radiographs, MRI and an arthroscopy with biopsies, a diagnosis of intra-articular endometriosis and menstrual arthritis was made.
  
  
  CONCLUSIONS
  Both an intra-articular manifestation of endometriosis and menstrual arthritis are very rare diagnoses. Extraperitoneal lesions of endometriosis are rare and the mechanism for spreading outside the retroperitoneal space is unknown. A similar situation exists for menstrual arthritis, it is thought that cytokines, which are produced as a reaction to retrograde menstruation, are the trigger for menstrual arthritis. A review of literature is given.
  
  
  CONCLUSIONS
  A monthly recurrent, painful swelling of a joint, can and should give rise to thorough investigation. Differential diagnostic one should think of intra-articular manifestation of endometriosis and menstrual arthritis. We give a proposal for treatment.
authors.affiliation: Department of Orthopaedic Surgery, Zuyderland Ziekenhuis, Henri Dunantstraat 5, 6401 CX Heerlen, The Netherlands. Electronic address: jetsejelsma@gmail.com.
authors.forename: Jetse
authors.initials: J
authors.lastname: Jelsma
authors.affiliation: Specialty Registrar Trauma and Orthopaedics, Ninewells Hospital, Dundee, Scotland DD1 9SY, United Kingdom.
authors.forename: Alistair
authors.initials: A
authors.lastname: Mayne
authors.affiliation: Department of Orthopaedic Surgery, Zuyderland Ziekenhuis, Henri Dunantstraat 5, 6401 CX Heerlen, The Netherlands.
authors.forename: Bjorn
authors.initials: B
authors.lastname: Steffanie
mesh_terms: D000293:Adolescent; D018771:Arthralgia / Q000209:etiology / Q000503:physiopathology*; D001168:Arthritis / Q000175:diagnosis* / Q000503:physiopathology; D004487:Edema / Q000209:etiology / Q000503:physiopathology*; D004715:Endometriosis / Q000175:diagnosis* / Q000503:physiopathology; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000503:physiopathology*; D008597:Menstrual Cycle; D012008:Recurrence
keywords: Endometriosis; Menstrual arthritis; Monthly knee pain