Intercoccygeal angle measurement may be used to quantify the degree of anterior angulation of the coccyx. It was originally described on lateral sacrococcygeal radiographs but can also be measured on cross-sectional imaging in the sagittal plane 1.
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Measurement
On a midline sagittal image, a straight line is drawn bisecting the first coccygeal segment parallel to its long axis. The intercoccygeal angle is formed at the intersection between this line and a similar straight line drawn bisecting the last coccygeal segment 1.
Interpretation
Multiple studies have reported average values of the intercoccygeal angle in males and females but these vary substantially across different ethnic populations and also with increasing age 2-4. Intercoccygeal angle measurement has yet to be extrapolated to quantifying coccygeal angular mobility on dynamic imaging 5.
The intercoccygeal angle measurement has value as a descriptive tool when it is reported alongside the expanded Postacchini and Massobrio classification of coccygeal angulation.
Practical points
the sacrococcygeal joint or the sacral and coccygeal cornua may be fused in up to 50% of patients, which may complicate the identification of the first coccygeal segment 6,7
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