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Eur Spine J. 2009 Apr;18(4):577-82. doi: 10.1007/s00586-008-0877-5. Epub 2009 Jan 23.

Interobserver reliability and intraobserver reproducibility of powers ratio for assessment of atlanto-occipital junction: comparison of plain radiography and computed tomography.

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Department of Orthopedics, Spine Service, Massachusetts General Hospital, Boston, MA 02114, USA.


Powers ratio, as assessed on plain radiographs or computed tomography (CT) images, appears to have clinical and prognostic value. To date, the validation of this assessment tool has been limited to a small number of observers at a single site. No study has examined the intraobserver reproducibility and interobserver reliability of the Powers ratio measurement on plain radiographs or CT images among a large cohort of spine surgeons. This type of validation is critical to allow for the broader use of the Powers ratio methodology in research studies and clinical applications. Plain radiographs and spiral CT images of the cervical spine of 32 patients were assessed, and the Powers ratio was determined by five spine surgeons. Each surgeon performed three readings, 7 months apart. In the first round of measurements, the observers used only the Powers' method of instruction. The second and third measurement sets were obtained after an interactive teaching session on the methodology. The order of the images was altered for the second and third set of measurements. The coefficient of variation (Cv) was calculated to determine the intraobserver repeatability and interobserver reliability for each imaging technique. A Bland-Altman plot was then used to assess the agreement between the two imaging techniques. For interobserver reliability, the mean Cv of the Powers ratio was 9.09 and 4.31% for plain radiographs and CT, respectively. The Cv mean value for intraobserver reproducibility averaged 4.95% (range 1.39-9.08) when CT scans were used and 14.17% (range 7.54-34.30) when plain radiographs were used. For intraobserver reproducibility, the lowest and highest Cv mean value of five raters was 1.39 and 9.08% using CT scans and 7.54 and 34.3% using plain radiographs. The Bland-Altman plot, demonstrated that the two methods were in close agreement on the -0.8 and 0.89% interval for limits of agreement (bias +/- 1.96sigma). The intraobserver reproducibility and interobserver reliability of Powers ratio measurement was acceptable (<5%) with CT scans but not with plain radiographs. However, despite the statistically inferior reliability and repeatability, the Bland-Altman plot analysis showed that given the -0.8 and 0.89% limits of agreement, the two methods may be used interchangeably in clinical practice.

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