title: Detection and prevalence of variant sciatic nerve anatomy in relation to the piriformis muscle on MRI.
abstract: OBJECTIVE
  To determine whether known variant anatomical relationships between the sciatic nerve and piriformis muscle can be identified on routine MRI studies of the hip and to establish their imaging prevalence.
  
  
  METHODS
  Hip MRI studies acquired over a period of 4 years at two medical centers underwent retrospective interpretation. Anatomical relationship between the sciatic nerve and the piriformis muscle was categorized according to the Beaton and Anson classification system. The presence of a split sciatic nerve at the level of the ischial tuberosity was also recorded.
  
  
  RESULTS
  A total of 755 consecutive scans were reviewed. Conventional anatomy (type I), in which an undivided sciatic nerve passes below the piriformis muscle, was identified in 87% of cases. The remaining 13% of cases demonstrated a type II pattern in which one division of the sciatic nerve passes through the piriformis whereas the second passes below. Only two other instances of variant anatomy were identified (both type III). Most variant cases were associated with a split sciatic nerve at the level of the ischial tuberosity (73 out of 111, 65.8%). By contrast, only 6% of cases demonstrated a split sciatic nerve at this level in the context of otherwise conventional anatomy.
  
  
  CONCLUSIONS
  Anatomical variations of the sciatic nerve course in relation to the piriformis muscle are frequently identified on routine MRI of the hips, occurring in 12-20% of scans reviewed. Almost all variants identified were type II. The ability to recognize variant sciatic nerve courses on MRI may prove useful in optimal treatment planning.
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California, San Francisco, 505 Parnassus Avenue, San Francisco, CA, 94143, USA. vanja.varenika@ucsf.edu.
authors.forename: Vanja
authors.initials: V
authors.lastname: Varenika
authors.affiliation: Department of Radiology, Stanford University School of Medicine, 300 Pasteur Drive, Stanford, CA, 94305, USA.
authors.forename: Amelie M
authors.initials: AM
authors.lastname: Lutz
authors.affiliation: Department of Radiology, Stanford University School of Medicine, 300 Pasteur Drive, Stanford, CA, 94305, USA.
authors.forename: Christopher F
authors.initials: CF
authors.lastname: Beaulieu
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California, San Francisco, 505 Parnassus Avenue, San Francisco, CA, 94143, USA.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Bucknor
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006615:Hip / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D015995:Prevalence; D012189:Retrospective Studies; D012584:Sciatic Nerve / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D055815:Young Adult
keywords: MRI; Piriformis; Sciatic nerve; Variant anatomy