• Mary E McCarty, Charles T Mehlman, Junichi Tamai, Twee T Do, Alvin H Crawford, and Guy Klein.
• Division of Pediatric Orthopaedic Surgery, Cincinnati Children's Hospital Medical Center, Cincinnati, OH 45229-3039, USA.
• J Pediatr Orthop. 2009 Oct 1; 29 (7): 755-9.

BackgroundSpondylolisthesis 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.MethodsFour 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.ResultsSlip 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).ConclusionsDefining 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.Level Of EvidenceDiagnostic level III.

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