title: Surface and boney landmarks for sacral neuromodulation: a cadaveric study.
abstract: OBJECTIVE
  Standard external landmarks have been suggested as a guide for in-office percutaneous nerve evaluation (PNE), but validity of these landmarks has not been assessed. Our objective was to determine whether the standard 9 cm from the tip of the coccyx indicates the position of the S3 sacral foramen and whether other boney landmarks and measurements improved positioning.
  
  
  METHODS
  Measurements and distances between external boney landmarks were obtained in 22 embalmed cadavers. Spinal needles were placed 9 cm superior to the coccyx and 2 cm lateral to midline bilaterally. After dissection, internal measurements relating to sacral length, position of S3, and location of the needle in relation to S3 were recorded. Correlations among measured variables were assessed using descriptive statistics.
  
  
  RESULTS
  Mean distance from the tip of coccyx to S3 was 9.26 cm (±0.84), from S3 to midline 2.30 cm (±0.2); from needle to S3 1.25 cm, and needle placement was as likely to be placed above or below S3; and S2-S3 and S3-S4 interforamenal distance 1.48 cm (±0.30) and 1.48 cm (±0.24), respectively. Mean distance from S3 to sacroiliac joint (SIJ) was shorter than S2 to SIJ. All associations between external measurements and length from tip of coccyx to S3 were not significant.
  
  
  CONCLUSIONS
  A distance 9 cm from the tip of the coccyx is a reasonable starting landmark for in-office blind PNE. However, given the variability in coccyx length, caution should be taken; also, sensory-motor response is necessary to confirm proper placement.
authors.affiliation: Division of Female Pelvic Medicine and Reconstructive Surgery, Department of Obstetrics, Gynecology, and Women's Health, University of Louisville School of Medicine, Louisville, KY, USA.
authors.forename: Nicolette E
authors.initials: NE
authors.lastname: Deveneau
authors.forename: Miriam
authors.initials: M
authors.lastname: Greenstein
authors.forename: Abhijit
authors.initials: A
authors.lastname: Mahalingashetty
authors.forename: Nicole R
authors.initials: NR
authors.lastname: Herring
authors.forename: Lioudmila
authors.initials: L
authors.lastname: Lipetskaia
authors.forename: Ali
authors.initials: A
authors.lastname: Azadi
authors.forename: Donald R
authors.initials: DR
authors.lastname: Ostergard
authors.forename: Sean L
authors.initials: SL
authors.lastname: Francis
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D059925:Anatomic Landmarks / Q000033:anatomy & histology*; D002102:Cadaver; D003050:Coccyx / Q000033:anatomy & histology*; D004599:Electric Stimulation Therapy; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012445:Sacrococcygeal Region / Q000033:anatomy & histology*; D012446:Sacroiliac Joint / Q000033:anatomy & histology; D012447:Sacrum / Q000033:anatomy & histology*; D013126:Spinal Nerve Roots / Q000033:anatomy & histology