title: A reference line on temporomandibular joint MRI.
abstract: OBJECTIVE
  The aim of this study was to establish a reference line and the 12 o'clock position on sagittal MRI images of the temporomandibular joint (TMJ) for close observation of early changes in disk position.
  
  
  METHODS
  The study included 106 joints of 53 consecutive male and female patients (mean age 13.3 years) with available MRI and limited cone-beam computed tomography (LCBCT) images, from a pool of postorthodontic patients who had finished phase I or phase II orthodontic treatment between March 2006 and March 2008 in a private orthodontic office. High-resolution (0.1 pixel) LCBCT images taken in natural head position in the same time period and adjusted to the same magnification were superimposed on corresponding MRI images. The true horizontal line (THL) determined by natural head position on the LCBCT image was transferred to the MRI image. A clinically easy-to-use reference line (RL) approximating THL was drawn by connecting two readily locatable anatomical landmarks, the most inferior point of the articular eminence and the center of the auditory canal, and correspondence between THL and RL was assessed.
  
  
  RESULTS
  The mean angle formed by THL and RL on the MRI images of the TMJs studied was 2.2° (SD 2.8°) clockwise around the center of the auditory canal, and the angulations of THL and RL on MRI images of the TMJs studied demonstrated a high positive correlation (r = 0.84).
  
  
  CONCLUSIONS
  THL can be established directly on MRI images using the THL-RL angle obtained by this study in patients without advanced disk displacement resulting in bony changes of the joint. The deepest point on the glenoid fossa that meets the THL can be used as the 12 o'clock position for evaluation of incipient disk position change.
authors.affiliation: Hillside View Orthodontic Office, Tokyo, Japan.
authors.forename: Kazumi
authors.initials: K
authors.lastname: Ikeda
mesh_terms: D000293:Adolescent; D059925:Anatomic Landmarks / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D002508:Cephalometry / Q000379:methods; D002648:Child; D054893:Cone-Beam Computed Tomography / Q000379:methods; D004424:Ear Canal / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000592:standards*; D008297:Male; D012015:Reference Standards; D012189:Retrospective Studies; D013701:Temporal Bone / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D013704:Temporomandibular Joint / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D019224:Temporomandibular Joint Disc / Q000033:anatomy & histology* / Q000000981:diagnostic imaging
keywords: 12 o'clock position; MRI; TMJ; disk displacement; reference line