Methods
The funding for this research came from the Swanson School of
Engineering Undergraduate Research Grant and NSF GRFP Grant #1747452.
Funding sources had no involvement in the study design, data collection,
analysis and interpretation of data, writing of the report, or decision
to publish.
This retrospective study was approved by the Institutional Review Board
at the University of Pittsburgh and Northshore University HealthSystem.
Images from 63 female patients between the ages of 20 and 49 that had a
pelvic MRI scan with or without contrast at Magee-Womens Hospital or
Northshore University HealthSystem between 2005 and 2018 were included
in this study. Exclusion criteria were history of pelvic surgery (not
including cesarean delivery (CD)), pelvic masses, and incomplete scans
(did not include the necessary bony anatomy) or incomplete birth
history.
Subjects were categorized into groups based on parity and gravidity,
which resulted in 23 nulliparous, 14 pregnant and vaginally nulliparous,
and 26 parous women: For the purposes of this study the women that were
placed in the nulliparous and parous groups were not currently pregnant
and were at least one year postpartum To delineate the effects of
pregnancy on the sacrum-coccyx shape, we then examined the pregnant and
parous groups by their respective number of deliveries (CDs for the
pregnant group, and combined CDs and vaginal deliveries for the parous
group) and normalized those values with respect to the nulliparous
average. These will be referred to as the pregnant and parous subgroups.
The reasoning behind the relatively small sample of pregnant women is
likely due to one of the exclusion criteria. Because we wanted to
isolate the effects of pregnancy, we eliminated pregnant subjects that
were vaginally parous. This is why all previous births of women in the
pregnant group were from CD.
Using HOROS v3.3.5 (Nimble Co LLC, Annapolis, MD USA) the midsagittal
plane of the sacrum and coccyx was identified. The sacrum was defined as
the first 5 vertebrae inferior to the sacral promontory. Any remaining
vertebrae were defined as the coccyx. This resulted in 3, 4, or 5
coccygeal vertebrae. Twelve length, angle, and curvature measurements
were made using definitions from previous literature to define the
sacrum, coccyx, and combined sacrum-coccyx shape13.
These measures included a count (the number of coccygeal vertebrae),
length (measured as both a straight and curved length), angle, and
curvature index. A curvature index was defined as a ratio between a
straight length (the shortest distance between the top and bottom of a
structure) and curved length (the average of the anterior and posterior
borders of a structure) multiplied by 100. A curvature index of 100
indicates that a structure is perfectly straight. The sacrococcygeal
straight length, curved length (anterior, posterior, and average), and
curvature index are shown in Figure 1a. Angles were defined as the
included angle between two straight lines, thus making an angle closer
to 180 degrees straighter. The sacrococcygeal angle is shown in Figure
1b. The sacral angle was excluded from this study as many scans did not
include necessary sacral landmarks13.
All of these measures were defined and measured in previous literature
that intended to quantify the change in shape of the sacrum and coccyx
that aimed to investigate and define normal adult sacrococcygeal
morphometry and were as follows: Coccygeal curved and straight lengths
were measured from the middle of the upper border of Co1 to the
coccygeal tip; sacral curved and straight lengths were measured from the
middle of the upper border of S1 to the middle of the of the inferior
border of S5; sacrococcygeal curved and straight lengths were measured
from S1 to the tip of the coccyx; sacrococcygeal angle was the included
angle between the middle of the superior portion of S1, the middle of
the superior portion of the Co1, and the tip of the coccyx (Figure 1b);
and the coccygeal angle was the included angle between the line drawn
through the middle of the superior and inferior edges of Co1 and the
line drawn through the middle of the superior and inferior edges of the
most inferior coccygeal vertebrae.
Statistical analyses were conducted in IBM SPSS Statistics v25 (IBM
Corp., Armonk, NY USA) and consisted of a One-Way Independent MANCOVA
followed by univariate ANOVAs with multiple comparisons and
Benjamini-Hochberg (BH) corrections post-hoc14. The
covariate was the age of the patient. In a BH correction, an allowable
false discovery rate (the rate at which a null hypothesis is rejected
incorrectly) is chosen (10% for this study). A critical value is
calculated using the rank of the p-value from the MANCOVA analysis, the
allowable false discovery rate, and the number of measurements. A
p-value smaller than the critical value is considered significant. Those
variables were then considered in a univariate analysis. Using the rank
of the p-values instead of the numerical value of them means that a BH
correction is a less conservative alternative to a Bonferroni
correction. Measures with significant differences between groups were
followed-up with additional multiple comparisons. Homogeneity of
variances were tested, and independent samples were assumed.