• Hubbard TA
• Pickar JG
• Lawrence DJ

• Cervical Atlas
• Cervical Vertebrae / diagnostic imaging
• Mastoid
• Radiography / methods

Objective: The purpose of this study was to measure the frequency with which the atlas transverse process is overlapped by the inferior tip of the mastoid process based upon radiographic analysis of the anterior to posterior open mouth (APOM) cervical spine view.

Methods: This is a retrospective study. Anterior to posterior open mouth radiographs (N = 120) were obtained from patient files at a chiropractic clinic. Dimensions were bilaterally measured: the vertical distance from the inferior mastoid to the superior margin of the C1 transverse process (C1TP) and the vertical distance from the inferior mastoid to the inferior margin of the C1TP. The percentage of the C1TP occluded by the mastoid process was calculated by determining the occlusal distance. These percentages were grouped into 4 categories: no occlusion, 1% to 50%, 50% to 99%, and 100%.

Results: The occlusal distance for the left and right ranged from −7.1 to 19.0 mm and −7.5 to 19.5 mm, respectively. The mean occlusal distance was identical on the left and right sides (4.6 [SD, 5.1 mm] and 4.7 mm [SD, 5.0 mm], respectively). The percentage of occlusion for the left and right transverse processes ranged from 0% to 80% and 0% to 100%, respectively. The mean percentage was 6.4% (SD, 16.4) on the left and 6.2% (SD, 16.3) on the right.

Conclusion: This study shows that the occlusal distance for the left and right ranged from −7.1 to 19.0 mm and −7.5 to 19.5 mm, respectively. A total occlusion of the C1TP occurred in 1 side of 120 participants in this sample.

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