title: Spondylolisthesis: intraobserver and interobserver reliability with regard to the measurement of slip percentage.
abstract: BACKGROUND
  Spondylolisthesis is often diagnosed and treated on the basis of measurements obtained from radiographs. Many physicians will perform surgery regardless of the patient's symptoms above a specific slip percentage. However, current methods used to assess slip percentage are vague and lack appropriate standardization, leaving physicians to devise personal evaluation techniques. This study presents a defined method to calculate slip percentage that takes advantage of modern technology, is fast and simple to perform, and shows excellent intraobserver/interobserver reliability.
  
  
  METHODS
  Four pediatric orthopaedic attendings each reviewed 30 radiographic cases of spondylolisthesis (grades 1 to 4) at the L5 to S1 level. The radiographs were measured twice through computer using PACS information management software with an interval of 2 days to 2 weeks between sessions. Using the PACS line tool, observers superimposed 6 lines onto each radiograph from which measurements were derived. The numerator in slip percentage (anterior displacement) was determined by 2 methods: the distance between a line outlining the posterior border of the sacrum and A: a line outlining the posterior border of L5 or B: a line parallel starting at the inferior, posterior corner of L5. The denominator in slip percent was determined by 2 methods. C: length of the inferior border L5 or D: length of the superior border of L5. This resulted in 4 different equations of slip percentage: A/C, A/D, B/C, and B/D. Analysis was performed using intraclass correlation coefficient.
  
  
  RESULTS
  Slip percentage=A/D resulted in the highest intraclass correlation coefficient for both intraobserver and interobserver reliability (0.87 and 0.85, respectively). Slip percentage=B/C showed the poorest intraobserver reliability (0.69). Slip percentage=B/C and B/D had equally poor interobserver reliability (0.59).
  
  
  CONCLUSIONS
  Defining the numerator in slip percentage as the distance between a line outlining the posterior border of the sacrum and a line outlining the posterior border of L5 (A) results in the highest intraobserver/interobserver reliability. Defining the denominator in slip percentage as the length of the superior border of L5 (D) results in the highest intra/inter observer reliability.
  
  
  METHODS
  Diagnostic level III.
authors.affiliation: Division of Pediatric Orthopaedic Surgery, Cincinnati Children's Hospital Medical Center, Cincinnati, OH 45229-3039, USA.
authors.forename: Mary E
authors.initials: ME
authors.lastname: McCarty
authors.forename: Charles T
authors.initials: CT
authors.lastname: Mehlman
authors.forename: Junichi
authors.initials: J
authors.lastname: Tamai
authors.forename: Twee T
authors.initials: TT
authors.lastname: Do
authors.forename: Alvin H
authors.initials: AH
authors.lastname: Crawford
authors.forename: Guy
authors.initials: G
authors.lastname: Klein
mesh_terms: D002648:Child; D006801:Humans; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging*; D015588:Observer Variation; D011856:Radiographic Image Enhancement / Q000379:methods*; D015203:Reproducibility of Results; D012447:Sacrum / Q000000981:diagnostic imaging*; D012720:Severity of Illness Index; D013168:Spondylolisthesis / Q000000981:diagnostic imaging*