title: Atypical location of an osteoid osteoma with atypical anterior knee pain.
abstract: BACKGROUND
  An osteoid osteoma is a painful tumor that most commonly affects the extra-articular parts of the long bones. An intra-articular location of an osteoid osteoma is rare. Various differential diagnoses may arise in connection with such an unusual location because it causes atypical clinical signs.
  
  
  METHODS
  A 24-year-old male developed pain in the central region of the right knee. Magnetic resonance imaging (MRI) showed no clear pathology in the knee joint. A technetium bone scan and computed tomography (CT) were then ordered and confirmed the presence of an osteoid osteoma in the knee joint. The patient was treated through an anteromedial approach to the knee, and the lesion was removed by excisional biopsy under fluoroscopy.
  
  
  CONCLUSIONS
  The diagnosis of intra-articular osteoid osteoma is challenging because the clinical presentation can be misleading. MRI is often requested as the first imaging method when dealing with knee symptoms, and radiologists are often unaware of the clinical presentation. Edema seen on MRI can be misleading with respect to the location of the nidus. CT is considered to be the best imaging method because it usually allows for clear visualization of the nidus. Different treatments have been proposed, ranging from open excision to arthroscopic resection.
  
  
  CONCLUSIONS
  Osteoid osteoma should be considered in young adult patients with chronic knee pain and no history of trauma.
authors.affiliation: Department of Orthopaedics, Taksim Training and Research Hospital, Istanbul, Turkey. Electronic address: drharunmutlu@yahoo.com.
authors.forename: Mutlu
authors.initials: M
authors.lastname: Harun
authors.affiliation: Department of Orthopaedics, Taksim Training and Research Hospital, Istanbul, Turkey.
authors.forename: Yaldız
authors.initials: Y
authors.lastname: Hayrettin
authors.affiliation: Department of Orthopaedics, Kanuni Sultan Suleyman Training and Research Hospital, Istanbul, Turkey.
authors.forename: Mutlu
authors.initials: M
authors.lastname: Serhat
authors.affiliation: Department of Orthopaedics, Baltalimani Training and Research Hospital, Istanbul, Turkey.
authors.forename: Cetinkaya
authors.initials: C
authors.lastname: Engin
authors.affiliation: Department of Orthopaedics, İnebolu Goverment Hospital, Kastamonu, Turkey.
authors.forename: Cepni
authors.initials: C
authors.lastname: Kamil
authors.affiliation: Department of Orthopaedics, Tatvan Goverment Hospital, Bitlis, Turkey.
authors.forename: Arslan
authors.initials: A
authors.lastname: Armagan
authors.affiliation: Department of Orthopaedics, Taksim Training and Research Hospital, Istanbul, Turkey.
authors.forename: Parmaksızoglu Atilla
authors.initials: PA
authors.lastname: Sancar
keywords: Intra-articular; Knee pain; Nidus; Osteoid osteoma