What is the role of AADI measurement in the workup of rheumatoid arthritis (RA) of the cervical spine?

Updated: Aug 13, 2019
  • Author: Steven R Garfin, MD; Chief Editor: Jeffrey A Goldstein, MD  more...
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Traditionally, the anterior atlantodental interval (AADI) has been used to monitor patients with rheumatoid arthritis over time. The AADI is the distance from the posterior margin of the anterior ring of C1 to the anterior surface of the odontoid. An AADI of more than 3 mm in an adult or 4 mm in a child is considered abnormal. Various authors have recommended surgery for values of more than 8 mm, 9 mm, or 10 mm (see Surgical Management).

Although the AADI was used commonly to monitor patients with cervical involvement, a number of investigations have shown that the AADI does not reliably discriminate between patients who are neurologically intact from those with neural deficit. [42] This is due in part to the 3-dimensional changes that take place with progressive subluxation. As the deformity progresses, the anterior arch of the atlas displaces in an anteroinferior direction as superior migration of the odontoid (SMO) combines with atlantoaxial instability (AAI). With continued SMO, the AADI decreases, although this vertical translocation is associated with a more unfavorable prognosis.

Boden and associates demonstrated this pitfall by examining the sensitivity, specificity, accuracy, and positive and negative predictive values of the AADI in predicting paralysis based on varying critical intervals (see the table below). [22] For example, raising the cut-off value for the AADI from 8 mm to 10 mm increases the specificity from 58% to 90% but decreases the sensitivity from 59% to 35%.

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