Conference Agenda

Rectal prolapse is a condition in which part of the rectum protrudes through the anus, significantly affecting patients' quality of life. This study aims to characterize the in vivo biomechanical properties of the posterior rectal wall using inverse finite element analysis (FEA). Two patient groups are included: one with rectal prolapse with gel and the other with pelvic prolapse without gel (control group). The goal is to quantify these properties through a minimally invasive approach, utilizing supero-inferior displacement measurements from MRI images acquired in the sagittal plane during the defecatory maneuver.

The study begins by simulating rectal prolapse using finite element analysis, modeling the stages of prolapse with properties from the literature to characterize the rectum, through hyperelastic models, and applying pressure that simulates the defecatory maneuver. These simulations provide insight into how the rectal wall deforms under prolapse conditions.

The supero-inferior displacements obtained from MRI scans are used as input in the inverse finite element analysis to optimize the mechanical parameters of the hyperelastic models. This process allows for the derivation of nonlinear and anisotropic properties of the rectal wall tissues, providing more accurate data on the resilience, elasticity, and deformability of the rectal wall.

By comparing the MRI data with simulation results, this study aims to optimize the in vivo biomechanical properties. The ultimate goal is to improve the understanding of rectal prolapse mechanics and develop personalized treatment strategies, particularly for surgical interventions.

In conclusion, this study integrates numerical simulations with clinical MRI measurements to better understand the biomechanical properties of the rectal wall in rectal prolapse patients. The findings are expected to contribute to more effective therapeutic interventions for prolapse conditions.

The female perineum plays a crucial biomechanical role during childbirth, undergoing significant stretching that frequently results in tissue trauma. Despite improvements in obstetric care, over 90% of vaginal deliveries are still associated with perineal injuries, underscoring the need for better predictive tools. Computational models offer a non-invasive approach to analyze the complex mechanics of childbirth and inform clinical interventions. However, the predictive accuracy of these models depends on the availability of reliable, physiologically accurate tissue properties, which remain poorly characterized for the female perineum. This study aims to address this gap by calibrating a visco-hyperelastic constitutive model to capture the mechanical behavior of the female human perineal body. To this end, the material parameters of the Holzapfel-Gasser-Ogden model were calibrated, as it effectively represents both nonlinear hyperelasticity and anisotropic behavior.

Perineal tissue samples, specifically from the perineal body, were obtained post-mortem from six women, pre-cooled, and then stored frozen. A cryostat was used to section each sample into 0.6 mm slices, which were then cut into a dog-bone shape. To prevent slippage during testing, custom-designed 3D-printed clamps were used. The specimens were subjected to a series of uniaxial mechanical tests, including stress-relaxation and ultimate tensile tests. Experimental data revealed key mechanical characteristics: stress-relaxation tests demonstrated a 40% reduction in stress over a 900-second interval, confirming the viscoelastic nature of the tissue, while tensile tests revealed non-linear stress-stretch behavior, with an average ultimate Cauchy stress of 224.70 ± 131.69 N and a stretch ratio of 1.97. Histological examination confirmed that the fiber orientation aligned with the direction of loading, supporting the choice of a transversely isotropic constitutive model.

To calibrate the material parameters, finite element simulations were performed using a dog-bone shape geometry and boundary conditions matching the experimental tests. A genetic algorithm was used to iteratively minimize the difference between experimental data and simulation output through a least squares evaluation function. To calibrate the visco-hyperelastic material, the viscous properties of the Generalized Maxwell model were initially determined using the mean normalized relaxation curve obtained from the relaxation part of the experiments. Once the viscous parameters were successfully determined, the next step involved calibrating the hyperelastic properties. This was done by comparing the model’s predictions with the absolute values obtained from the Cauchy stress vs stretch experimental data. Three distinct sets of parameters were calibrated, capturing the minimum, mean, and maximum mechanical responses observed across the donor cohort.

The calibration process demonstrated good agreement between numerical predictions and experimental results, validating the robustness of the chosen model and optimization strategy. Incorporating these material properties into finite element simulations would enable accurate simulation of physiological phenomena involving the perineal body. In the context of childbirth, such simulations could play a crucial role in identifying women at greater risk of perineal trauma and guiding personalized obstetric care. Overall, this study bridges experimental biomechanics and computational modeling, contributing to improve maternal health outcomes.

Traditional metallic orthopedic screws, despite their high mechanical strength and biocompatibility, often require a secondary surgical removal procedure. Bioabsorbable materials represent a compelling alternative, as they provide the necessary mechanical support during bone healing and progressively degrade in vivo. Among these, Mg-based alloys represent a promising combination of biocompatibility, cytocompatibility, biodegradability, and mechanical properties suitable for orthopedic applications. However, a major challenge of bioabsorbable medical devices lies in the controllability and predictability of their degradation rate and associated mechanical response. In such a scenario, advanced computational modeling represents an essential tool for optimizing the design and tailoring their performance in specific clinical settings. Screw degradation in biological tissue entails the solution of a moving interface problem evolving in a space-time domain. Traditional numerical methods, such as the Finite Element Method (FEM), struggle to handle these problems due to the necessity of dynamically updating the non-smooth contact surface in a complex living environment. Accordingly, alternative approaches, e.g., the phase field formulation, have been proposed but are still limited to specific applications and require a local identification of the contact boundary conditions. In the present contribution, we consider a nonlocal peridynamic formulation naturally enclosing the global solution of multi-field and multi-scale interfaces, circumventing the need to specify local contact boundary conditions and requiring ad hoc numerical procedures. In particular, we adopt and optimize the PeriFast/Corrosion scheme [1], originally developed under the assumption of constant corrosion currents, and generalize the numerical routine to handle time-varying currents proportional to the volume loss rate. We consider as a case study an M2 orthopedic screw made of Mg-10Gd alloy and submerged in a generic biologically-like electrolyte. Model tuning was performed by comparing and contrasting the computed volumetric loss with experimental results [2]. Extensive numerical analyses demonstrated the capability of the proposed PD model to capture the overall trend of the degradation process effectively. Specifically, the degraded screw geometry reveals that the corrosive attack initiates on all the wetted surfaces but affects the threading more significantly, as it is the area most exposed to the electrolyte. The study also analyses the mechanical response of the degraded screw at different time steps under axial and bending loads, thus identifying the critical conditions for orthopedics sustainability. Future studies will focus on coupling the proposed PD degradation model with approaches that simulate the simultaneous bone growth and remodeling process. This will allow to reproduce, among other aspects, individual biochemical pathways, paving the way for a patient-specific customization of these devices.

[1] L. Wang et al., «PeriFast/Corrosion: A 3D Pseudospectral Peridynamic MATLAB Code for Corrosion,» Journal of Peridynamics and Nonlocal Modeling, 2023.

[2] A. Hermann et al., «Combining peridynamic and finite element simulations to capture the corrosion of degradable bone implants and to predict their residual strength,» International Journal of Mechanical Sciences, 2022.