Reproducibility and diagnostic value of a new method using ratios to diagnose anterior atlanto-axial subluxation on plain radiographs.

Measures on conventional radiography are used to detect, especially in rheumatoid arthritis, upper cervical spine instabilities (CSIs) with the anterior and posterior atlanto-dental intervals (AADI and PADI) measurements. Our objective was to evaluate the diagnostic performance and reliability of AADIs and PADIs extrapolated based on ratios in assessing anterior atlanto-axial subluxation (aAAS) when plain radiographs do not allow the measures. Radiographies of 119 patients were randomly selected. Two blinded observers performed two measurements of the odontoid sagittal diameter (O), axis body base sagittal diameter (C2), AADI, PADI, Clark station and Ranawat index, and the AADI/O, AADI/C2, PADI/O and PADI/C2 ratios were calculated. The diagnostic value of AADI and PADI extrapolated from the AADI/O, AADI/C2, PADI/O and PADI/C2 ratios was evaluated using ROC curves, with AADI>2.9mm used as the gold standard. Among the 119 patients, 12 patients had aAAS (AADI>2.9mm), 6 of them had severe aAAS (AADI>8.9mm and/or a PADI<14mm), and 6 patients had vertical AAS (Clarks station=2 or 3 and/or Ranawat index<13mm). The AADI extrapolated from the AADI/O and AADI/C2 ratios has excellent intra- and inter-observer reproducibility. The diagnostic value of the extrapolated AADI was high for aAAS (sensitivity 92%; specificity of 100%) and severe aAAS (sensitivity75%; specificity 100%). The diagnostic value of the extrapolated PADI was good but lower than the diagnostic value of the extrapolated AADI. Extrapolated AADI can be used instead AADI to detect aAAS and severe aAAS.

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