CT Scan Study of Atlantoaxial Rotatory Mobility in Asymptomatic Adult Subjects: A Basis for Better Understanding C1―C2 Rotatory Fixation and Subluxation

Study Design. Normal rotation was evaluated in a group of 40 asymptomatic adults. Objective. To determine the normal rotational limits of C1―C2 in adults and define when a rotatory fixation occurs in the limits of normality or in subluxation. The term subluxation should be used only when C1―C2 is rotated beyond normal limits. Summary of Background Data. Concepts about rotatory fixation were established by accepting that it may occur within the limits of normal range of motion. Although nowadays CT is the current image method used to evaluate any case of torticollis, no study has been performed in adult population on what really normal rotation look like in CT scans. Methods. The study included the measurement of the rotational movement of the neck and a CT scan study of the articular processes of C1―C2 in maximal, left and right, active rotation. A superposition of 6 consecutive slices was carried out, obtaining a linear contour of the axial view of C1―C2. Rotation angle and contact surface loss were measured. Results. The average neck rotation angle was 79° (range: 74° to 81°). The superposition of the images taken in every rotational direction showed a wide contact loss between the correspondent C1―C2 articular surfaces (42.4%―85.7%; average: 70%). The report of these images, carried out by 3 independent radiologists, concluded that there was a rotatory subluxation in all these cases. Conclusion. Our results coincide with our previous published ones conducted in children, and lead us to conclude that a CT scan showing wide― but incomplete― rotational facet displacement is not sufficient to define subluxation. We perceive that there is a risk of overdiagnosis and overtreatment (C1―C2 arthrodesis) when evaluating upper cervical spine rotational problems. The concept of both rotatory fixation and subluxation should be revised, and quantifying the rotational angle and contact surface loss between C1―C2 can be very useful.