Change in levator ani muscle stiffness and active force during pregnancy and post-partum.

INTRODUCTION AND HYPOTHESIS: It is assumed changes occur to the biomechanics and viscoelastic response of the levator ani muscle during pregnancy; however, there is limited evidence of this. This study used instrumentation and clinical measures to determine the stiffness and active force capacity of levator ani muscle during pregnancy and post-partum, investigated any associations with delivery outcomes, and explored the biomechanical properties associated with symptoms of pelvic floor dysfunction. METHODS: This was a prospective observational study, with nulliparous women with a singleton low-risk pregnancy. Data were collected at two stages during pregnancy and post-partum. Measurements included the Australian Pelvic Floor Questionnaire, palpation of active force, and elastometry measurements. Post-partum, 3D/4D ultrasound measurements were included. Repeated measures ANOVAs, pairwise comparisons, Pearson correlation coefficients, and Student's t-tests were used as appropriate. RESULTS: Fifty-nine women took part in the study. Active force was significantly different over the pregnancy and post-partum, measured with instrumentation (p = 0.002) and palpation (p = 0.006 right, p = 0.029 left). There was no significant change in muscle stiffness during pregnancy. Post-partum muscle stiffness was significantly different between women who gave birth vaginally vs. caesarean section (p = 0.002). Post-partum there were differences in levator hiatal area, symptoms of bladder dysfunction, prolapse symptoms, and sexual dysfunction symptoms. CONCLUSIONS: Active force of the levator ani muscle was significantly reduced during pregnancy and in the post-partum period, while muscle stiffness reduced only in those who had vaginal deliveries.

Is it time to rethink using digital palpation for assessment of muscle stiffness?

AIM: Physiotherapists typically use digital palpation to determine residual tension in a muscle, referred to as muscle stiffness or tone. These assessments are subjective, and little is known about their accuracy or repeatability. Despite this, it is standard practice to base clinical treatment on these findings. The aim of this study was to assess physiotherapists' ability to assign a seven-point palpation scale to quantitative stiffness values generated by a novel device. METHODS: Prospective observational study involving 125 musculoskeletal and pelvic floor physiotherapists. A novel device was developed that replicates the haptic feedback that clinicians assess as muscle stiffness. Measurements of displacement, force, and stiffness were recorded. RESULTS: There was wide overlap between each scale category assigned to the stiffness values, from low stiffness at -3 (119 [106, 132] N/m) to moderate stiffness at 0 (462 [435,489] N/m); to high stiffness at +3 (897 [881,913] N/m). Consistency in applying the scale was poor, and the probability of a similar value of stiffness being assigned to the same scale category by different participants was low. CONCLUSIONS: While palpation is used globally by physiotherapists as a readily available and low-cost method of assessing muscle stiffness, these results indicate that it should be used with caution in diagnosing and defining patient care. Clinical assessment of muscle stiffness requires a validated and reliable palpation scale if this metric is to be used to diagnose pathology and develop treatment protocols. Training in this scale should then be recommended to improve reliability in patient assessment.

Characterizing levator-ani muscle stiffness pre- and post-childbirth in European and Polynesian women in New Zealand: a pilot study.

INTRODUCTION: The influence of levator-ani muscles on second-stage labor is poorly understood. The ability of these muscles to stretch without damage may affect birth outcomes, but little is known about material properties, effects of pregnancy and/or ethnicity on levator-ani stiffness. There are strong associations between muscle damage and subsequent pelvic floor disorders. This study aimed to quantify levator-ani muscle stiffness during the third trimester of pregnancy and postpartum in European and Polynesian women. Associations between stiffness, obstetric variables, and the risk of intrapartum levator-ani injury (avulsion) were investigated. MATERIAL AND METHODS: This was a prospective observational pilot study. A total of 167 (106 European and 61 Polynesian) nulliparous women were recruited antenatally; 129 returned postnatally. Participants were assessed between 36 and 38 weeks' gestation and three to five months postpartum. Assessments included pelvic floor ultrasound, elastometry testing, and validated questionnaires on pelvic floor function. Logistic regression, Student t-, Chi-square and Mann-Whitney tests were used as appropriate. RESULTS: There are significant differences between antenatal and postnatal muscle stiffness measurements (p < 0.01). Stiffness was significantly higher in the European cohort (p = 0.03). There were more avulsion injuries in European (20%) than in Polynesian (9%) women. There were no significant differences in antenatal stiffness between women with and without avulsion, but change in stiffness (antenatal to postnatal) was significantly less in the avulsion group. There were no associations between stiffness, and other obstetric variables, epidural anesthesia seemed protective (p = 0.03). CONCLUSIONS: Quantification of levator-ani muscle stiffness is feasible. Muscle stiffness is significantly different before and after birth.

Mathematical modeling of the female reproductive system: from oocyte to delivery.

From ovulation to delivery, and through the menstrual cycle, the female reproductive system undergoes many dynamic changes to provide an optimal environment for the embryo to implant, and to develop successfully. It is difficult ethically and practically to observe the system over the timescales involved in growth and development (often hours to days). Even in carefully monitored conditions clinicians and biologists can only see snapshots of the development process. Mathematical models are emerging as a key means to supplement our knowledge of the reproductive process, and to tease apart complexity in the reproductive system. These models have been used successfully to test existing hypotheses regarding the mechanisms of female infertility and pathological fetal development, and also to provide new experimentally testable hypotheses regarding the process of development. This new knowledge has allowed for improvements in assisted reproductive technologies and is moving toward translation to clinical practice via multiscale assessments of the dynamics of ovulation, development in pregnancy, and the timing and mechanics of delivery. WIREs Syst Biol Med 2017, 9:e1353. doi: 10.1002/wsbm.1353 For further resources related to this article, please visit the WIREs website.

Modeling the second stage of labor.

Vaginal delivery is the primary cause of levator ani muscle injury, which is in turn the leading factor contributing to pelvic floor disorders including pelvic organ prolapse and urinary stress incontinence. Existing biomechanical models of childbirth have provided some understanding of pelvic floor function during delivery and have helped in the investigation of preventative strategies. The modeling frameworks for childbirth simulation are described with emphasis on (1) the recent advances in medical imaging quality and computational power; (2) improvements in the anatomical representation of the pelvic floor and fetal head; (3) more realistic boundary conditions for delivery; and (4) mechanical properties determined from experiments. Researchers have used these models to analyze childbirth mechanics and identify anatomical and mechanical features of the maternal pelvic floor, shape of the fetal head, and delivery techniques that potentially contribute to a difficult labor and higher risk of levator ani muscle injuries. The challenges to be addressed for these frameworks to be clinically useful are also discussed, including: (1) the improvements required to more accurately simulate the second stage of labor; (2) automatic segmentation of medical images and creation of customized computer models; (3) acquisition of individual specific pelvic floor mechanical properties; and (4) construction of statistical models for rapidly predicting the indices of childbirth mechanics. Within the next decade, it is likely that biomechanical models of childbirth will be sufficiently well informed and functional for personalized birth planning, and as educational tools for clinicians. WIREs Syst Biol Med 2016, 8:506-516. doi: 10.1002/wsbm.1351 For further resources related to this article, please visit the WIREs website.

Effects of fetal head shape variation on the second stage of labour.

Fetal head geometry plays an important role in the mechanics of childbirth during the second stage labour. Large heads have been shown to be associated with difficult and prolonged childbirth. However, the relationship between the fetal head geometry and childbirth mechanics has not been quantitatively analysed. To address this, our study used finite element (FE) modelling techniques and biomechanical simulations to analyse the contribution of fetal head shape and size on the mechanics of childbirth. X-ray computed tomography (CT) images from 26 newborn infants (less than 9 days old) without skull abnormalities were used to construct individual-specific FE models of the fetal skull. Simulations of childbirth were conducted using each model of the skull and a customised pelvic floor model based on magnetic resonance imaging (MRI) of a healthy nulliparous woman. The force required for delivery, the maximum principal stresses, and the maximum principal stretch ratios at the left and right pelvic floor muscle-pubic bone interfaces were quantified. Partial least squares regression (PLSR) models for predicting these mechanical indices were constructed using: (i) either the FE geometries of the fetal heads or the biometrical parameters (biparietal diameters and fetal head circumferences) as inputs; and (ii) either a linear or a quadratic function for the inner relation. The predictabilities of the mechanical indices using the PLSR models were quantified using a leave-one-out analysis. Quantitative associations were found between the geometric parameters of the fetal head and the indices of childbirth mechanics. When using the full FE geometries as inputs, the PLSR model using a linear inner relation gave better predictability than the model using a quadratic inner relation. This could be attributed to the quadratic inner relation correlating response to the noise in point-to-point correspondence. When using the biometrical parameters of the skull as inputs, the PLSR model using a quadratic inner relation gave the best overall predictability. Such a model could be implemented in a clinical setting as a predictive model for childbirth planning and as an educational tool for clinical training.

An automated hand-held elastometer for quantifying the passive stiffness of the levator ani muscle in women.

AIM: Design and develop an automated, hand-held instrument (elastometer) to assess in vivo passive stiffness of the pelvic floor muscle. MATERIALS AND METHODS: The elastometer system consisted of a hand piece, real-time controller, and laptop computer. A cable connected the hand-piece to the controller, which communicated with a laptop computer via an ethernet connection. Force sensitivity calibration and displacement accuracy were determined experimentally using a spring load and an Instron mechanical tester. A test re-test series quantified the in vivo repeatability (within a procedure) and reproducibility (between procedures after a 5 min delay) of passive stiffness in volunteers (n = 20). Stiffness was determined from the gradient of the force-displacement curve for each cycle. RESULTS: The force-aperture spring measurements from the elastometer showed consistent (r(2) = 1.0000) agreement with those measured by the Instron. The difference between spring stiffness as measured by the elastometer and the Instron (388.1 N/m cf. 388.5 N/m, respectively) was negligible. The intra-class correlation coefficient for repeatability within procedures was 0.986 95% CI (0.964-0.994) n = 20, and reproducibility between procedures ICC 0.934 (95% CI 0.779-0.981) n = 12. Bland-Altman analysis determined a bias of 0.3 and 18.5 N/m, for repeatability and reproducibility respectively. Neither bias is likely to be clinically significance. CONCLUSION: The elastometer demonstrated very good repeatability and accuracy in the measurement of force/displacement during in vitro testing. There was a high degree of repeatability and reproducibility in stiffness measurements in a test re-test series. Our results demonstrate the elastometer is accurate and reliable and thereby suitable for larger clinical trials.

Clinical evaluation of a high-fidelity wireless intravaginal pressure sensor.

INTRODUCTION AND HYPOTHESIS: A wireless intravaginal pressure sensor (IVPS) has been developed to quantify abdominal pressure (P(abd)) changes during exercise and activities of daily living to guide post-operative advice given to women. In this pilot study, we aimed to compare IVPS performance, comfort, retention, and acceptability to a standard fluid-filled intrarectal pressure catheter currently used to measure P(abd) during routine urodynamics. METHODS: A Life-Tech 3-mm urodynamic intrarectal catheter and IVPS were inserted concurrently in volunteers attending a urodynamics clinic. The IVPS was positioned above the levator plate and the intrarectal catheter positioned in routine fashion well above the anal sphincter. Routine urodynamics was undertaken, with women asked to perform star jumps if coughing or Valsalva did not invoke leakage. Subjects rated device comfort using a visual analogue scale (0-10). Repeated measures Bland-Altman analysis determined level of agreement (LOA) between the two devices for peak pressures for each activity. RESULTS: Twenty-seven women were recruited, 67% of the participants preferred the IVPS, 18% the intrarectal catheter, while 15% had no preference. Mean comfort score was 0.9 ± 1.7 and 2.1 ± 2.6 (p = 0.049) for the IVPS and intrarectal catheter respectively. Bland-Altman analysis demonstrated minimal bias for cough and Valsalva, however LOA were wide. Differences were more prominent during star jumps where rapid dynamic pressure changes occurred. CONCLUSIONS: The IVPS had a higher comfort score and was well retained. The LOA between the two systems was moderate, but the high sampling rate and lower susceptibility to motion artefacts of the IVPS may provide more accurate information that will be important clinically.

Characterizing the ex vivo mechanical properties of synthetic polypropylene surgical mesh.

The use of synthetic polypropylene mesh for hernia surgical repair and the correction of female pelvic organ prolapse have been controversial due to increasing post-operative complications, including mesh erosion, chronic pain, infection and support failure. These morbidities may be related to a mismatch of mechanical properties between soft tissues and the mesh. The aim of this study was to gain a better understanding of the biomechanical behavior of Prolene polypropylene mesh (Ethicon, Sommerville, NJ, USA), which is widely used for a variety of surgical repair procedures. The stiffness and permanent deformation of Prolene mesh were compared in different directions by performing uniaxial tensile failure tests, cyclic and creep tests at simulated physiological loads in the coursewise (0°), walewise (90°) and the diagonal (45°) directions. Failure tests suggest that the mechanical properties of the mesh is anisotropic; with response at 0° being the most compliant while 90° was the stiffest. Irreversible deformation and viscoelastic behavior were observed in both cyclic and creep tests. The anisotropic property may be relevant to the placement of mesh in surgery to maximize long term mesh performance. The considerable permanent deformation may be associated with an increased risk of post-operative support failure.

Comparison between transperineal ultrasound and digital detection of levator ani trauma. Can we improve the odds?

AIMS: To investigate the predictive ability of four digital assessment parameters to detect levator ani (LA) muscle defects (avulsion injury) and compare these to transperineal tomographic ultrasound images. METHODS: This was an observational study imbedded in a larger quasi-experimental cohort study for women with urinary incontinence. Seventy-two women, ≥ 60 years who had attended or were going to attend physiotherapy for treatment of urinary incontinence, were included in the study. Inclusion criteria from the parent study were symptoms of stress, urge or both types of urinary incontinence. The predictive ability of the following digital parameters: direct palpation of a discontinuity of the LA muscle from insertion on the pubic ramus; palpation of the distance between the muscle insertion sites; palpation of LA strength; palpation of LA tone, were analyzed against findings from tomographic transperineal ultrasound images. Correlation between methods was measured using Cohen's kappa for each of the individual parameters. RESULTS: Seventeen women (24%) presented with a complete or partial avulsion of the puborectalis muscle as diagnosed with tomographic ultrasound imaging. Nine women (13%) had complete avulsions, one of which was bilateral. The predictive ability of the digital assessment parameters varied from poor (k = 0.187, 95% CI [0.02-0.36]) to moderate (k = 0.569, 95% CI [0.31-0.83]). The new parameter of 'width between insertion sites' performed best. CONCLUSIONS: Adding the parameter of "width between insertion sites" appears to enhance our ability to detect avulsion of the levator ani (LA) muscle by digital examination however it does not distinguish between unilateral or bilateral avulsion.

Anisotropic effects of the levator ani muscle during childbirth.

Pelvic floor dysfunction and pelvic organ prolapse have been associated with damage to the levator ani (LA) muscle, but the exact mechanisms linking them remain unknown. It has been postulated that factors such as vaginal birth and ageing may contribute to long-term, irreversible LA muscle damage. To investigate the biomechanical significance of the LA muscle during childbirth, researchers and clinicians have used finite element models to simulate the second stage of labour. One of the challenges is to represent the anisotropic mechanical response of the LA muscle. In this study, we investigated the effects of anisotropy by varying the relative stiffness between the fibre and the matrix components, whilst maintaining the same overall stress-strain response in the fibre direction. A foetal skull was passed through two pelvic floor models, which incorporated the LA muscle with different anisotropy ratios. Results showed a substantial decrease in the magnitude of the force required for delivery as the fibre anisotropy was increased. The anisotropy ratio markedly affected the mechanical response of the LA muscle during a simulated vaginal delivery. It is apparent that we need to obtain experimental data on muscle mechanics in order to better approximate the LA muscle mechanical properties for quantitative analysis. These models may advance our understanding of the injury mechanisms of pelvic floor during childbirth.

Effects of nonlinear muscle elasticity on pelvic floor mechanics during vaginal childbirth.

The role of the pelvic floor soft tissues during the second stage of labor, particularly the levator ani muscle, has attracted much interest recently. It has been postulated that the passage of the fetal head through the pelvis may cause excessive stretching of the levator ani muscle, which may lead to pelvic floor dysfunction and pelvic organ prolapse later in life. In order to study the complex biomechanical interactions between the levator ani muscle and the fetal head during the second stage of labor, finite element models have been developed for quantitative analysis of this process. In this study we have simulated vaginal delivery using individual-specific anatomical computer models of the pelvic floor interacting with a fetal head model with minimal restrictions placed upon its motion. Two constitutive relations were considered for the levator ani muscle (of exponential and neo-Hookean forms). For comparison purposes, the exponential relation was chosen to exhibit much greater stiffening at higher strains beyond the range of the experimental data. We demonstrated that increased nonlinearity in the elastic response of the tissues leads to considerably higher (56%) estimated force required for delivery, accompanied by a more homogeneous spatial distribution of maximum principal stretch ratio across the muscle. These results indicate that the form of constitutive relation beyond the presently available experimental data markedly affects the estimated function of the levator ani muscle during vaginal delivery, due to the large strains that occur. Further experimental data at higher strains are necessary in order to more reliably characterize the constitutive behavior required for modeling vaginal childbirth.

Modeling childbirth: elucidating the mechanisms of labor.

The process of childbirth and the mechanisms of labor have been studied for over a century, beginning with simple measurements of fetal skull and maternal pelvis dimensions. More recently, X-rays, ultrasound, and magnetic resonance imaging have been used to try and quantify the biomechanics of labor. With the development of computational technologies, biomechanical models have emerged as a quantitative analysis tool for modeling childbirth. These methods are well known for their capabilities to analyze function at the organ scale. This review provides an overview of the state-of-the-art finite element models of the mechanics of vaginal delivery, with detailed descriptions of the data sources, modeling frameworks, and results. We also discuss the limitations and improvements required in order for the models to be more accurate and clinically useful. Some of the major challenges include: modeling the complex geometry of the maternal pelvic floor muscles and fetal head motion during the second stage of labor; the lack of experimental data on the pelvic floor structures; and development of methods for clinical validation. To date, models have had limited success in helping clinicians understand possible factors leading to birth-induced pelvic floor muscle injuries and dysfunction. However, much more can be achieved with further development of these quantitative modeling frameworks, such as tools for birth planning and medical education.

Modelling childbirth: comparing athlete and non-athlete pelvic floor mechanics.

There is preliminary evidence that athletes involved in high-intensity sports for sustained periods have a higher probability of experiencing a prolonged second stage of labour compared to non-athletes. The mechanisms responsible for these differences are not clear, although it is postulated that muscle hypertrophy and increased muscle tone in athletes may contribute to difficulties in vaginal delivery. In order to test these hypotheses, we have constructed individual-specific finite element models of the female pelvic floor (one athlete and one non-athlete) and the fetal head to simulate vaginal delivery and enable quantitative analysis of the differences. The motion of the fetal head descending through the pelvic floor was modelled using finite deformation elasticity with contact mechanics. The force required to push the head was compared between the models and a 45% increase in peak force was observed in the athlete model compared to the non-athlete. In both cases, the overall maximum stretch was induced at the muscle insertions to the pubis. This is the beginning of a quantitative modelling framework that is intended to help clinicians assess the risk of natural versus caesarean birth by taking into account the possible mechanical response of pelvic floor muscles based on their size and activation patterns prior to labour.

Alterations in levator ani morphology in elite nulliparous athletes: a pilot study.

BACKGROUND: A difficult vaginal birth is known to be one of the causes of damage to the pelvic floor muscles. Prolonged second stage of labour is thought to be one of the implicating factors in this damage. If the muscles of the pelvic floor were hypertrophied in response to repetitive high impact training, then the 'suspected phenomenon' of a more difficult childbirth for very athletic women might be related to these muscles reducing pelvic floor compliance. AIM: To document the muscle morphology of the pelvic floor using magnetic resonance imaging (MRI) in a group of nulliparous female athletes and to compare this to the findings in a similar group of age-matched nulliparous nonathletic women. METHODS: Static Magnetic Resonance (MR) imaging of the pelvic floor was performed with particular reference to the muscle morphology of levator ani and puborectalis. The cross-sectional area of the urogenital hiatus and the diameter of the bony pelvic outlet were also assessed. RESULTS: Significant differences in the cross-sectional area and width of the pelvic floor muscles, measured in the line of the anal canal, were found between the athletic group and the controls. There were no differences found between the two groups in the cross-sectional area of the urogenital hiatus, and the adequacy of the pelvic outlet was established with reference to normative data. CONCLUSIONS: The differences shown in the pelvic floor muscles in the athletes and nonathletes provide a focus for further research into this area. This change might influence the function of the pelvic floor muscles in this subgroup of women.