Study design, participants and sample size: This was an observational study. Study covered the period between June 2019 and December 2020 at the Institute for the Health of Mother and Child in Prague, Czech Republic. This tertiary perinatology centre performs over 5000 deliveries per year. In this study, we included non-pregnant women without history of previous pregnancy and without morphological a functional signs of pelvic floor dysfunction. Exclusion criteria were: a) history of previous pregnancy and delivery, b) previous vaginal or perineal surgery. We use a set of 67 Caucasian nulliparous women. Clinical information is listed in Table 1. The age was available for all 67 subjects, the height and weight for 48 subjects.
Table 1
Clinical data of subjects.
| Min | Max | Mean | Median |
Age | 22 | 38 | 27.7 | 27 |
Height | 153 | 184 | 168.7 | 168 |
Weight | 44 | 120 | 65.2 | 65 |
Procedures, data analysis and outcome measures: High-resolution MRI allows direct non-invasive evaluation of the muscular and ligamentous pelvic floor support structures in multiple planes without the use of ionizing radiation. Stronger 3 Tesla signal allows greater spatial resolution, increased contrast detection and produces a higher soft-tissue resolution image in comparison to the 1.5 Tesla scanner. The MRI imaging protocol was a high-resolution axial 3Tesla MRI scan (Phillips Achieva TX series), taken in the supine position. The imaging parameters were as follows: repetition time 5331 milliseconds, 375 phase encodes, 24-cm field of view, and 2-mm slice thickness, no gap in axial, coronal and sagittal projections. MRI sequences were performed at rest, the women were asked not to contract the pelvic floor during imaging and had not received bowel or bladder preparation. The participants had to micturate 30 minutes before the examination. All scans were evaluated independently by two researchers (MJ and LHH).
For each subject, the MRI data were loaded into open source Slicer software (slicer.org) [16] that allows exporting landmark coordinates in the three main axes. The landmarks were exported into a single file by means of SlicerMorph module [23]. All sets of landmarks were marked according to our methodology by the same person in order to avoid errors due to different approach by various people. From obtained set of landmarks, the dimensions were calculated as distance between corresponding landmarks. The concerned internal and external landmarks are defined in Table 2 and the internal and external dimensions are defined in Table 3.
Table 2
Bony pelvis landmarks addressed in the study.
Abbreviation | Landmarks |
PSA | Pubis symphysis anterior (upper edge of the pubic symphysis) |
PSP | Pubis symphysis posterior (back of the pubic symphysis) |
PSI | Pubis symphysis inferior (inferior border of the pubic symphysis) |
L5 | Spinous process of the fifth lumbar vertebra |
PR | Promontory |
S3 | Superior surface on the body of the third sacral vertebrae |
SJ | Sacro-coccygeal joint |
SB | Sacro-coccygeal joint (external) |
ILL, ILR | Left and right most widest points on illiopectineal line |
BCL, BCR | Distantia bicristalis (left and the right external margins of the iliac crest |
BSL, BSR | Distantia bispinalis (left and the right external tips of the iliac spine) |
ISL, ISR | Left and the right ischial spines |
ITL, ITR | Left and right ischial tuberosity |
TIL, TIR | Left and right tuber ischiadicum |
PSISL, PSISR | Left and right dimples |
Table 3
Internal and external bony pelvis dimensions addressed in the study.
Plane | Position | Abbreviation | Dimension | Landmarks | ICC |
Sagittal | External | COE | Conjugata obstetrica externa | PSA, L5 | 0.96 |
DSB | Distance between PSA and SB | PSA, SB | 0.95 |
Internal | APD | Anterior-posterior diameter (conjugata obstetrica vera) | PSP, PR | 0.93 |
CW | Pelvic canal width | PSP, S3 | 0.87 |
SCIPP | Sacro-coccygeal inferior pubic point line | PSI, SJ | 0.93 |
Transverse | External | DBC | Distantia bicristalis | BCL, BCR | 0.91 |
DBS | Distantia bispinalis | BSL, BSR | 0.87 |
DIS | Ischial spine diameter | ISL, ISR | 0.92 |
DIT | Ischial tuberosity diameter | ITL, ITR | 0.95 |
DTI | Distance between left and right tuber ischiadicum | TIL, TIR | 0.84 |
PSIS | Distance between left and right dimples | PSISL, PSISR | 0.97 |
TD | Transverse diameter | ILL, ILR | 0.98 |
Angle | | IPA | Perineal shoulder angle | PSI, ITL, ITR | 0.92 |
In the sagittal plane, anterior-posterior diameter (APD) defines the birth canal in the plane of pelvis inlet, whilst the plane of pelvis outlet is defined by sacro-coccygeal inferior pubic point line (SCIPP). Additionally, pelvic canal width (CW) defines the pelvis width in the middle section of the birth canal in the sagittal plane. We consider conjugata obstetrica externa (COE) as the major external dimension in the sagittal plane. Additionally, we propose DSB (distance between PSA and SJ), where PSA refers to the upper edge of the pubic symphysis and SJ refers to the sacro-coccygeal joint, as an additional external anterior-posterior dimension in the sagittal plane.
In the transverse plane, transverse diameter (TD) is the internal dimension defining the birth canal. We consider two external dimensions, particularly distantia bicristalis (DBC) and distantia bispinalis (DBS) as the major external dimensions in the transverse plane. Additionally, we address ischial tuberosity diameter (DIT), distance between left and right tuber ischiadicum and distance between left and right dimples (PSIS) as additional external dimensions in the transverse plane. We also calculated the correlation between TD and the average of DTI and DBC. Finally, we consider IPA as the internal pubic angle, defined as an angle between ischial tuberosities and inferior pubic point.
The pelvic type is defined by APD (anterior-posterior diameter) and TD (transverse diameter) [8][17], therefore our hypothesis aims to find a correlation between APD and a chosen external dimension in the sagittal plane, and between TD and a chosen external dimension in the transverse plane. As the birth canal geometry changes along the sagittal plane, we address the upper part (inlet), the middle part and the lower part (outlet) of the birth canal. The pelvic type (also referred as inlet shape) is recognized by means of a relationship value (ReV) [8], which is computed by subtracting APD from TD. The inlet shape is classified as dolichopellic (anthropoid) if APD is longer than TD, which results in negative ReV. It is classified as mesatipellic (gynecoid) if APD is equal to TD or TD is no more than 10 mm longer than APD, therefore, ReV ranges between 0 and 10. It is classified as brachypellic (android) if TD is more than 10 mm and less than 30 mm longer than APD, which results in ReV ranging between 10.01 and 29.99. Lastly, inlet shape is considered platypellic (platypelloid) if TD exceeds APD by more than 30 mm, which results in ReV exceeding 30.
Statistical analysis: All statistical analyses were performed using MATLAB software (version R2020a). We assess the correlation of the chosen internal dimensions with the external dimensions by means of the Pearson correlation coefficient ρ. We expect that the correlation is independent for dimensions on the sagittal and the transverse planes. For each subject, we further determine the particular pelvic type and compare it to the estimated pelvic type based on the achieved correlation. Interobserver (LHH and MJ) agreement were obtained for all parameters in a test-retest series in blinded fashion. The Intraclass correlation coefficient (ICC) was used for this purpose. The value of ICC for particular pelvic dimensions is listed in Table 3.