Main Findings
The midsagittal sacrum-coccyx measures differed across nulliparous,
pregnant, and parous groups with the sacrum-coccyx assuming a straighter
shape during pregnancy as the coccyx rotates posteriorly about the
sacrococcygeal joint. Although not significantly different from the
nulliparous group, parous values tended to straddle the nulliparous and
pregnant groups, suggesting that the sacrum-coccyx shape of some parous
women does not completely return to the nulliparous range even years
postpartum.
Interestingly the most significant statistic was the coccygeal curvature
index, while the least significant was the coccygeal angle. At first
glance, they appear to describe the same shape changes, but the
curvature index is much more sensitive to subtle changes. Figure 3
demonstrates how the curved length (simplified as 2 lines) can remain
the same while the coccygeal angle and straight length increase. The
increase in the simplified straight length, and by extension the
curvature index, (4.3%) is larger than the increase in angle (1.7%).
This suggests that when evaluating coccyx shape the curvature index may
highlight differences that the coccygeal angle would not capture.