Introduction
Hormonal changes during pregnancy cause tissue remodeling, resulting in
connective tissue softening to facilitate vaginal
delivery1. Throughout the female lifespan, the levator
ani muscles and pelvic connective tissues act to close the levator
hiatus. During vaginal delivery, the pelvic floor undergoes stretching
to allow passage of the fetal head2. Biomechanical and
finite element modeling have been used to identify and clarify the
amount and location of the maximum muscle and connective tissue stretch
during vaginal birth3,4. The majority of the research
in this area has concentrated on the pelvic floor musculature, pubic
symphysis, and sacroiliac joints1,5,6. However,
previous work from our lab demonstrated the potential need for tissue
softening at the sacrococcygeal joint to accommodate the fetal head.
During simulations of vaginal delivery, mechanical loads introduced as
the fetal head pushed the tip of the coccyx posteriorly, forcing the
muscles and connective tissues engaged with the coccyx to
stretch7. Posterior motion of the coccyx about the
sacrococcygeal joint has also been noted in studies that conducted
magnetic resonance imaging (MRI) during vaginal delivery, but has yet to
be quantified8.
The sacrum and coccyx were chosen as the focus of this study as they
provide an important attachment point for the coccygeus and levator ani.
The coccygeus inserts on the superior two vertebrae of the coccyx (Co1
and Co2) and the most inferior vertebrae of the sacrum (S5). The
coccygeus is distinct from the levator ani, which contribute to the
support of the pelvic organs and help stabilize the coccyx. Among the
levator ani, the iliococcygeus and anococcygeal raphe insert on the
coccyx. In this way, a homeostatic relationship is likely formed between
the coccygeus and the laxity of the sacrococcygeal joint, which would be
driven by the connective tissues surrounding the joint and the tension
generated by the iliococcygeus and anococcygeal raphe. Therefore, with
any increase in pressure (e.g. increasing intrabdominal pressure
generated by a growing fetus), change in muscle function, or change in
the connective tissue stiffness at the sacrococcygeal joint would
ultimately result in a measurable difference in the orientation of the
maternal coccyx. Additionally, during delivery, physical interaction
between the fetal head and the pelvic floor muscles could also cause a
significant alteration in the orientation of the coccyx.
Other studies have shown significant movement of maternal bony pelvic
structures during pregnancy, which can result in asymmetrical sacroiliac
joints or pain localized to the coccyx5,9,10. While
all women sustain stretching of their pelvic floor during birth, only
some will experience injury resulting in pelvic pain, pelvic organ
prolapse, urinary incontinence, and/or fecal incontinence that can
develop immediately or decades after delivery11,12.
Improved metrics quantifying the degree of remodeling during pregnancy
and following delivery may provide insight into predictors of vaginal
birth-related injuries and/or complications and pelvic floor disorders.
The objective of this study was to define changes in the position of the
coccyx relative to the sacrum and the midsagittal shape of the sacrum
and coccyx in women before, during, and after pregnancy. To do this, we
aimed to measure variations in the combined sacrum-coccyx shape induced
by pregnancy and delivery by comparing midsagittal lengths, angles, and
curvature indices between nulliparous, pregnant, and parous women. We
hypothesized that the orientation of the coccyx would be more posterior
in pregnant women—providing more room in the pelvis for the fetus
during delivery by increasing the anterior-posterior diameter of the
obstetric outlet—and then return, but not completely, towards
nulliparous values in parous women.