Purpose of the study: The mean rotation center (MRC) characterizes the movement of two solids in relation to each other. This parameter has been proposed for the cervical spine to describe the motion of vertebral segments. Two lateral views (flexion and extension) are required to draw the necessary lines and establish the centers of rotation. The process is rigorous but time-consuming. We validated a computerized analysis system for automatic determination of the cervical MRC and study the localizations observed in healthy subjects.

Material and methods: Validation of the computerized system. Accurate angle measurements: nine cervical spines were harvested from anatomic specimens. A K-wire was inserted sagittally into each vertebra. Lateral images were obtain in flexion and extension. The measurements of mobility made by the software were compared with manual measurements. Reproducibility tests (intra- and interobserver): six pairs of flexion and extension views in healthy subjects. Two different observers made fifteen successive measurements of each MRC for each spinal segment. Frequently encountered positions of the MRC in healthy subjects: stress films were obtained in 51 healthy subjects aged 18–40 years. For each spinal segment, the MCR was determined with the computerized system.

Results: Accuracy of the angle measurements: the precision was 1.4° for a 95% interval of confidence. Reproducibility: variability of the position in X and Y for the MRC (expressed in percent of the size of the vertebral body) was: 19.6 and 24.5 for C2–C3; 112 and 15.3 for C3–C4; 7.7 and 9.4 for C4–C5; 9.1 and 9.4 for C5–C6; 13.1 and 11.8 for C6–C7. Positions frequently encountered in healthy subjects: the most frequent position of the MRC varied from one segment to another. There was a frequent position for each segment. These frequent positions were situated in the posterosuperior quadrant of the subjacent vertebra for C2–C3, C3–C4, C4–C5, and C5–C6. For C6–C7, the frequent positions for MRC were at the level of the intervertebral space, behind the center of the disc.

Discussion: The software tested here appeared to provide good measurements for cervical spine from C3 to C7. At these levels, the measures were accurate and reproducible, as were the coordinates for the MCR of each segment. The frequent positions of the MRC found in this study are the same as reported by other authors. This method is easy to apply in routine practice.

Purpose: Rotation dislocation of C1-C2 subsequent to trauma is not often described in adults. The clinical, radiological and computed tomographic diagnostic criteria are not well known and can lead to false positive diagnosis. The Fielding classification was described for children. We report five cases of traumatic rotation dislocation of C1-C2 in adults and propose computed tomographic criteria for diagnosis. The Fielding classification is discussed.

Material and methods: In two cases, the diagnosis was suggested by the clinical presentation and the plain x-rays. In three cases, it was revealed by the systematic CT performed in multiple trauma patients. In three cases, MRI enabled visualisation of ligament tears (transverse ligament, alaire ligament). Finally, the C1-C2 relations in neutral position and in rotation were studied on the CT scans in the study patients and in ten healthy subjects to establish diagnostic criteria. The patients were treated with cervical traction until reduction was achieved (checked with CT) then with an “Indian”collar for 45 days. One patient did not wear the collar and experienced a recurrent dislocation.

Results and discussion: In patients who can be examined, the diagnosis is suggested by suboccipital pain, slight rotation inclination of the head to the contralateral side, impossibility of turning the head to the opposite side beyond the mid line. The open-mouth x-ray can be a source of false positive diagnosis but can be suggestive. The CT scan must be performed under precise conditions: patient positioned without rotation or inclination of the head (false positive); superposition of the two slices passing through the C1 and C2 faces (unilateral loss of congruency); sagittal reconstruction. In case of doubt, homo and contralateral rotation slices can provide more sensitive images. The five dislocations were uin-lateral (Fielding type II) with posterior displacement in two cases, a finding not described in this classification. In addition, type I could be a variant of the normal (as seen in control scans). Treatment in the early phase is conservative with reduction by simple cervical traction (verification on CT), followed by complementary immobilisation until ligament healing.

Conclusion: The diagnosis of traumatic rotation dislocation of C1-C2 in adults is based on CT evidence. Certain injuries should be added to complete the Fielding classification. When recognised early, this rotation dislocations can be treated conservatively.