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The normal mobility of the coccyx (when sitting down) is either in flexion or in extension. Extension is more rare (25% of the cases). The direction of motion is dependant upon morphometric characteritics of the coccyx which can be approached by the measure of the different angles (see the section "Dynamic films"). Straight coccyges (as opposed to curved ones), a low pelvic sagittal rotation (when sitting down) and a low incidence are the prerequisites for extension. These characteristics are shared with posterior luxation. Of course, a normal extension is not pathologic, but the normality is difficult to determine, due to the infrequency in a control group. My opinion is that normal coccygeal extension is comprised between 0° and 20°. The capacity to extend is lesser than for flexion (the same occurs for the lumbar spine). This figure should be taken as indicative, not as an absolute value for abnormality. As for mild hypermobility, the disc space has to be tender to palpation to engage its responsibility. On this page, we present three cases of posterior hypermobility.

Case #1. A typical case. Note how straight is the coccyx and the low pelvic rotation which is around 30°.

Case #2. The coccyx is more than straight here: it is pointing backward, with a reversed curve. It is close to a luxation.

Case #3. Slight posterior hypermobility. There is a friction of the first coccygeal vertebra on the sacrum, which certainly accounts for the pain.

Case #4. A difficult case. There is no motion in the two first vertebrae. The distal vertebra moves in extension and the posterior disc space disappears on the sitting film (X). This is hardly visible. The value of this finding is enhanced by the fact that, at rectal examination, moving up and down that piece of bone reproduced the familiar pain.

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