To our knowledge, this is the first study to describe a correlation between the female pelvic floor biomechanical parameters and the severity of stress urinary incontinence. Our cross-sectional study found significant differences between SUI severity subgroups divided by PGI-S and MESA questionnaire and selected female pelvic floor biomechanical parameters measured by VTI.

SUI is the most common type of UI. The cause of SUI is more intricate than simplistic theories suggesting single anatomical or neurological injuries during childbirth. These injuries expose susceptibility to stress incontinence, which is influenced both genetically (tissue strength, mechanical, and anatomical relationships) and behaviorally (nutrition, smoking, and exercise) [19]. While the Hammock hypothesis currently explains how observed anatomy relates to vaginal, urethral, and bladder function, the Integral Theory prompts us to consider how anatomy may also contribute to bladder overactivity [20,21,22]. Experts agree that stress urinary incontinence (SUI) can generally be attributed to two primary mechanisms: intrinsic sphincter deficiency and urethral hypermobility.

When assessing a woman with SUI, one of the first steps is to ask the patient to fill out validated questionnaires [23]. The MESA questionnaire is a reliable and validated tool developed as part of the MESA project, an observational study funded by the National Institutes on Aging (NIA) in 1983. Its primary purpose is to identify the urgency- or stress-predominant component of MUI and assess the severity of symptoms [8]. The PGI-S questionnaire asks the patient about the severity of a condition using a 5-point scale, which is not specific to incontinence. However, the severity of the disease can be assessed using the scale: in mild cases, the patient gives a score of 1 or 2, and in severe cases, a score of 3 or 4. In the literature, some articles on studies have used both questionnaires and testing procedures that measure some biomechanical properties of the vagina or pelvic floor. Mariott et al. assessed vaginal pressure profiles before and after prolapse surgery using an intravaginal pressure sensor [24], Pires et al. evaluated maximum voluntary contractions with a perineometer among female athletes [25]. However, a solid-state circumferential catheter or even the Oxford scale can assess simpler biomechanical parameters [26]. Nonetheless, neither of these methods can provide deep enough insight into the pelvic floor’s biomechanical components, which is essential when investigating minor variations or a small number of cases. As shown in the methods section, VTI can measure the appropriate and sufficient biomechanical parameters, as previously demonstrated in the case of POP [12].

Our study demonstrates a positive correlation between the MESA SUI Score and the VTI parameter 4, which refers to the tissues/structures in the anterior compartment at 10–15 mm depth, and parameter 27, corresponding to the displacement of the maximum pressure peak in the anterior compartment. These two parameters align well with urethral hypermobility, a prevalent mechanism in the pathophysiology of SUI [7]. Our results suggest that parameters measured during VTI testing may bring us closer to understanding pathophysiology in a personalized way.

As we aimed to investigate the correlation between the female pelvic floor biomechanical parameters and the severity of stress urinary incontinence, we examined this question using the PGI-S and MESA questionnaires. The PGI-S cannot distinguish between types of incontinence, so all types were investigated in the correlations. Accordingly, several biomechanical parameters of the pelvic floor showed a significant difference when severity was subdivided according to the PGI-S, which we interpreted not as an argument against the use of the PGI-S severity-versus-severity subdivision but as a reason for the different pathophysiological background of the different types of incontinence. In contrast, when the MESA questionnaire was used to divide the study population according to the severity of SUI, only the VTI parameter 49 showed a significant difference between the groups. This parameter can explain the mobility of anterior structures at reflex muscle contraction, which we interpret as further evidence of urethra hypermobility as a pathophysiological factor.

We were unsurprised that the anterior vaginal compartment biomechanical parameters correlate with SUI. The close proximity of the urethra to the anterior vaginal wall allows vaginal assessment by VTI, and also that is it is well-known that hypermobility of the urethra contributes to the pathophysiology of SUI [7]. In contrast, we did not expect to find a correlation between the biomechanical parameters of the posterior vaginal wall and SUI severity. One possible explanation is that the overall weakness of the pelvic floor (both the anterior-posterior parameters are abnormal) may indicate a generally weaker pelvic floor function that may translate into worse SUI. The pathophysiology of SUI is not entirely understood, and our results suggest that VTI may aid in evaluating women with SUI. In addition, VTI technology may help to identify pelvic floor biomechanical weaknesses not identifiable by other methodologies or by urodynamic testing. VTI could further improve the ability to provide appropriately personalized care for SUI patients, thus increasing the chances of a cure for this common disease.

One of our study’s strengths was that we were the first to investigate the association between the female pelvic floor biomechanical parameters and the severity of stress urinary incontinence. We used a non-invasive, easy-to-perform method to assess many critical biomechanical properties of the female pelvic floor. Compared to previous methods, such as manometry or Vaginal Pressure Profile [27], the new method used in the study provides a much more detailed insight into the functioning of the pelvic floor due to the use of several sensors. We have used validated questionnaires to assess UI. Our study has several weaknesses, which need to be offset and improved in further studies. The number of participants was relatively low, resulting in inadequate power to find more minor differences in the biomechanical parameters. In addition, the diagnosis of UI was only established by the questionnaires and not by urodynamics testing. Also, the severity of the SUI was not assessed by pad-test, which is frequently used in research studies but less frequently utilized in everyday clinical practice. A large-scale multicenter study would be required to confirm our results.