Technique development and measurement of cross-sectional area of the pubovisceral muscle on MRI scans of living women

A unified pelvic floor conceptual model for studying morphological changes with prolapse, age, and parity

Several 2-dimensional and 3-dimensional measurements have been used to assess changes in pelvic floor structures and shape. These include assessment of urogenital and levator hiatus dimensions, levator injury grade, levator bowl volume, and levator plate shape. We argue that each assessment reflects underlying changes in an individual aspect of the overall changes in muscle and fascial structures. Vaginal delivery, aging, and interindividual variations in anatomy combine to affect pelvic floor structures and their connections in different ways. To date, there is no unifying conceptual model that permits the evaluation of how these many measures relate to one another or that reflects overall pelvic floor structure and function. Therefore, this study aimed to describe a unified pelvic floor conceptual model to better understand how the aforementioned changes to the pelvic floor structures and their biomechanical interactions affect pelvic organ support with vaginal birth, prolapse, and age. In this model, the pelvic floor is composed of 5 key anatomic structures: the (1) pubovisceral, (2) puborectal, and (3) iliococcygeal muscles with their superficial and inferior fascia; (4) the perineal membrane or body; and (5) the anal sphincter complex. Schematically, these structures are considered to originate from pelvic sidewall structures and meet medially at important connection points that include the anal sphincter complex, perineal body, and anococcygeal raphe. The pubovisceral muscle contributes primarily to urogenital hiatus closure, whereas the puborectal muscle is mainly related to levator hiatus closure, although each muscle contributes to the other. Dorsally and laterally, the iliococcygeal muscle forms a shelflike structure in women with normal support that spans the remaining area between these medial muscles and attachments to the pelvic sidewall. Other features include the levator plate, bowl volume, and anorectal angle. The pelvic floor conceptual model integrates existing observations and points out evident knowledge gaps in how parturition, injury, disease, and aging can contribute to changes associated with pelvic floor function caused by the detachment of one or more important connection points or pubovisceral muscle failure.

Comparison of Cross-Sectional Area of Pubovisceral Muscle in Nulliparous and Primiparous Women

INTRODUCTION AND HYPOTHESIS

The main risk factor for pelvic floor disorders is vaginal delivery, which may cause levator ani muscle (LAM) injury and denervation. LAM includes pubovisceral muscle (PVM, pubococcygeus), puborectalis muscle (PRM), and iliococcygeus muscle. We hypothesize that primiparous women with low pelvic floor muscle contraction have a reduced PVM cross-sectional area (CSA) compared to nulliparous women.

METHODS (SAMPLE SIZE AND STATISTICAL APPROACHES)

This single-centre prospective observational study compared healthy nulliparous (n = 40) to primiparous (n = 40) women after vaginal delivery without LAM avulsion and Oxford score ≤ 3. Demographics, questionnaires (ICIQ-UI-SF, OAB-Q-SF, PISQ-12), POP-Q, Oxford score, ultrasound measurements (minimal anteroposterior and lateral diameters, hiatal area, PRM thickness, levator-urethra gap) and magnetic resonance imaging (MRI)-PVM CSA were evaluated. Normality was tested, and an appropriate test was used to compare the groups. Power calculation suggested 40 participants per group.

RESULTS

The primiparous group was older, had a higher BMI, and their hiatal area on ultrasound at contraction was larger compared to the nulliparous group. The CSA of the left-sided PVM (1.15 ± 0.50 cm2) was larger compared to the right side (1.03 ± 0.50 cm2), p = 0.02 in nulliparous women. The PVM CSA of primiparous women with low Oxford score was reduced compared to nulliparous (0.87 ± 0.30 versus 1.09 ± 0.50 cm2, p = 0.006). The intra-rater reliability for PVM CSA had an ICC of 0.90 and inter-rater ICC of 0.77.

CONCLUSIONS

Primiparous women after vaginal delivery with low pelvic floor contraction force had reduced PVM CSA on MRI images compared to nulliparous women.

Three-Dimensional Volume Rendering of Pelvic Floor Anatomy with Focus on Fibroids in Relation to the Lower Urogenital Tract Based on Cross-Sectional MRI Images

We aimed to assess the feasibility of developing three-dimensional (3D) models of pelvic organs using cross-sectional MRI images of patients with uterine fibroids and urinary symptoms and of obtaining anatomical information unavailable in 2D imaging modalities. We also aimed to compare two image processing applications. We performed a feasibility study analysing MRI scans from three women, aged 30 to 58 years old, with fibroids and urinary symptoms. Cross-sectional images were used to render 3D models of pelvic anatomy, including bladder, uterus and fibroids, using 3D Slicer and OsiriX. Dimensions, volumes and anatomical relationships of the pelvic organs were evaluated. Comparisons between anatomical landmarks and measurements obtained from the two image processing applications were undertaken. Rendered 3D pelvic models yielded detailed anatomical information and data on spatial relationships that were unobtainable from cross-sectional images. Models were rendered in sufficient resolution to aid understanding of spatial relationships between urinary bladder, uterus and fibroid(s). Measurements of fibroid volumes ranged from 5,336 to 418,012 mm3 and distances between the fibroid and urinary bladder ranged from 0.10 to 83.60 mm. Statistical analysis of measurements showed no significant differences in measurements between the two image processing applications. To date, limited data exist on the use of 3D volume reconstructions of routine MRI scans, to investigate pelvic pathologies such as fibroids in women with urinary symptoms. This study suggests that post-MRI image processing can provide additional information over standard MRI. Further studies are required to assess the role of these data in clinical practice, surgical planning and training. Three-dimensional reconstruction of routine two-dimensional magnetic resonance imaging provides additional anatomical information and may improve our understanding of anatomical relationships, their role in clinical presentations and possibly guide clinical and surgical management.

The variation in shape and thickness of the pelvic floor musculature in males and females: a geometric-morphometric analysis

INTRODUCTION AND HYPOTHESIS

In women, the risk of pelvic floor prolapse is known to be associated with age and parity. Different studies suggested that it is also related to pelvic dimensions, e.g. biomechanical modelling showed that a larger pelvic canal results in higher values of displacement, stress and strain in the pelvic floor muscles, which can increase the risk of pelvic floor disorders. To better understand the multiple factors contributing to pelvic floor disorders, we assessed how age, body weight, body height, parity (in women), pelvic canal size and overall muscle development affected pelvic floor geometry.

METHODS

A comprehensive geometric morphometric analysis of variation in pelvic floor muscle shape was conducted based on a dense set of 3D landmarks measured on CT scans in a cohort of 49 deceased men and 52 deceased women. The multivariate association between biological variables (parity, dimensions of the true pelvis, age, body weight, height) and pelvic floor muscle morphology was explored by reduced rank regression in both sexes.

RESULTS

In women, advanced age, high body weight relative to body height and a large pelvic canal were associated with a deeper pelvic floor. Surprisingly, parity did not have any strong association with overall pelvic floor shape. In men, high body weight was associated with a deep pelvic floor. Age had little effect on male pelvic floor shape, except for the thickness of the ischiocavernosus muscle, which reduced with age.

CONCLUSION

These results suggest that age, relative body weight and the size of the pelvic canal contribute to the risk of female pelvic floor disorders via their effect on pelvic floor shape, independently of birth-related factors such as injury and avulsion of pelvic floor muscles.

Aging effects on pelvic floor support: a pilot study comparing young versus older nulliparous women

INTRODUCTION AND HYPOTHESIS

We sought to determine age-related changes to the pelvic floor in the absence of childbirth effects.

METHODS

A case-control study was conducted from June 2017 to August 2018 comparing two groups of nulliparous women: <40 years old and ≥ 70 years old. Clinical evaluation included POP-Q, instrumented speculum testing, and handgrip strength. Dynamic 3D-stress MRI was performed on all women to obtain genital and levator hiatus (LH) lengths, LH area, and levator bowl volume. LH shape was quantified using a novel measure called the "V-U index." Pubovisceral muscle (PVM) cross-sectional area (CSA) was also measured. Bivariate comparisons between the two groups were made for all variables. Effect sizes were calculated for MRI measurements.

RESULTS

Twelve young and 9 older nulliparous women were included. Levator bowl volume at rest was 83% larger in older women (108.0 ± 34.5 cm³ vs 59.2 ± 19.3 cm³, p = 0.001, d = 1.82). MRI genital hiatus at rest was larger among the older group (2.7 ± 0.6 cm vs 3.5 ± 0.6 cm, p = 0.007, d = 1.34). V-U index, a measure of LH shape where 0 = "V" and 1 = "U," differed between groups indicating a more "U"-like shape among older women (0.71 ± 0.23 vs 0.35 ± 0.18, p = 0.001, d = 1.72). Handgrip strength was lower in the older vs young group (23.2 ± 5.2 N vs 33.4 ± 5.2 N, p < 0.0001); however, the Kegel augmentation force and PVM CSA were similar (3.2 ± 1.1 N vs 3.3 ± 2.2 N, p = 0.89, and 0.8 ± 0.3 cm² vs 0.7 ± 0.2 cm², p = 0.23 respectively).

CONCLUSIONS

Levator bowl volume at rest was over 80% larger among older women, reflecting a generalized posterior distension with age.