title: Three dimensional MRI study: Safety of short versus long needle peribulbar anesthesia.
abstract: BACKGROUND
  The standard technique of Peribulbar block is to use 25 g 25 mm needle at the junction between the lateral one third and medial two third of the lower orbital rim in the infero-temporal quadrant of the orbit. Theoretically, insertion of longer needles increases the potential of injury to important structure; however, safety of the shorter needle had never been demonstrated. This study describes the anatomy of the orbital structures with magnetic resonance imaging (MRI) using the three-dimensional constructive interference in steady state (3D CISS) sequence to present a morphological basis for needle entry at 12.5 and 25 mm lengths. Statistical comparisons were performed at the 12.5 versus 25 mm depths. Statistical significance was indicated by P < 0.05.
  
  
  METHODS
  Fifty patients free of orbital pathology with normal axial length were selected for MRI with the 3D CISS sequence. Original axial and multiplanar image reconstruction (MPR) images were selected for image interpretation. Orbital structures were identified at 12.5 and 25 mm depths from the orbital rim to compare significant differences in anatomy between the two imaging planes at the expected needle depth and to assess the size of the globe and the orbit.
  
  
  RESULTS
  The cross sectional area of the extraocular muscles were statistically significantly smaller at the 12.5 mm plane (P = 0.001). The area of inferotemporal fat was statistically significantly larger at the 12.5 mm plane (P = 0.033). There was no statistical difference in the inferonasal and superonasal fat areas at different depths (P = 0.34, P = 0.35 respectively). The size of the orbit and globe was significantly larger at 12.5 mm (P = 0.001). There was no difference between depths in the presence or absence of neurovascular bundles and supporting structures including the intramuscular septae.
  
  
  CONCLUSIONS
  There is a larger structure-free space at a depth of 12.5 mm than at 25 mm. Therefore, the inference is that a needle inserted in the infero-temporal zone to a depth of 12.5 mm is less likely to injure the eyeball or extra-ocular muscles than one advanced to 25 mm.
authors.affiliation: Department of Radiology, King Khaled Eye Specialist Hospital, Riyadh, Saudi Arabia ; Mansoura Faculty of Medicine, Diagnostic Radiology Department, Egypt.
authors.forename: Sahar M
authors.initials: SM
authors.lastname: ElKhamary
authors.affiliation: Department of Anesthesia, St. Michael's Hospital, 30 Bond Street, Toronto, Ontario M5B 1W8, Canada.
authors.forename: Waleed
authors.initials: W
authors.lastname: Riad
keywords: MRI 3D-CISS; Peribulbar anesthesia; Short needle