The Redlund-Johnell Criterion is the distance between the inferior margin of C2 vertebral body and a line drawn from the posterior tip of the hard palate (white arrow) to the caudal cortical margin of the occiput (McGregor Line).
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Redlund-Johnell I, Pettersson H. Radiographic measurements of the cranio-vertebral region. Designed for evaluation of abnormalities in rheumatoid arthritis. Acta Radiol Diagn (Stockh). 1984; 25:23–28