Spondylolisthesis: intraobserver and interobserver reliability with regard to the measurement of slip percentage.

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.

LEVEL OF EVIDENCE

Diagnostic level III.