Dynamic-MRI quantification of abdominal wall motion and deformation during breathing and muscular contraction

Impact of a prophylactic mesh on the biomechanics of abdominal wall closure: an animal study

BACKGROUND

The use of a prophylactic prosthetic mesh (PPM) to reinforce a midline laparotomy suture closure improves the clinical outcomes, in comparison with primary suture technique. However, understanding how a PPM impacts the biomechanics of the repair is crucial for gaining a deeper comprehension and ultimately improving clinical outcome by decreasing incisional hernia (IH) rates post midline laparotomy. Based on a porcine IH model, this study aimed to assess the biomechanical characteristics of the abdominal wall (AW) midline over time post midline laparotomy, considering sthree repair modalities: no repair, primary suture and onlay mesh reinforcement coupled with suture.

METHODS

31 pigs were enrolled in the study and the repair was characterized using CT-scans based on the distance between the right and left Rectus Abdominis Muscle (RAM). The AW of each animal was explanted at 48 h, 4 and 12 weeks postoperatively and a Stereo Digital Image Correlation (s-DIC)-based method was used to assess the response of the repaired AW (e.g., strain, compliance) when subjected to an inflation test mimicking an increase in intra-abdominal pressure (IAP). Intact AW were included in the study and served as controls.

RESULTS

AWs repaired with a primary suture exhibited a higher RAM distance compared to healthy animals, along with an increased compliance of the repair along the transverse direction over time. AWs repaired with primary suture and reinforced with a PPM exhibited a biomechanical response similar to that of healthy animals in terms of repair strain and compliance.

CONCLUSION

The use of a PPM to reinforce suture was found to better restore the biomechanical properties to the midline of the AW post midline incision. Further investigations are needed to correlate the findings of this study with clinical outcomes, especially long-term recurrence rates.

Biomechanics of the abdominal wall before and after ventral hernia repair using dynamic MRI

PURPOSE

This study aims to investigate the use of dynamic MRI to assess abdominal wall biomechanics before and after hernia surgery, considering that such evaluations can enhance our understanding of physiopathology and contribute to reducing recurrence rates.

METHODS

Patients were assessed using dynamic MRI in axial and sagittal planes while performing exercises (breathing, coughing, Valsalva) before and after their abdominal hernia surgery with mesh placement. Rectus and lateral muscles, linea alba, viscera area, defect dimensions and hernia sac were contoured with semiautomatic process to quantify the abdominal wall biomechanical temporal modifications.

RESULTS

This study enrolled 11 patients. During coughing, the axial area of the hernia sac increased by 128.4 ± 199.2%. The sac increased similarly in axial and sagittal planes during Valsalva. Post-surgical evaluations showed a 26% reduction in inter-recti distance and a lengthening of all muscles (p ≤ 0.05). The post-operative rectus abdominis thickness change was negatively correlated with defect width during breathing (p ≤ 0.05). The largest change in linea alba displacement was observed in the surgical site (p = 0.07). Post-operatively, lateral muscles had a larger inward displacement during Valsalva (p ≤ 0.05). Rectus abdominis had a larger outward displacement during breathing (p = 0.09), reduced with the mesh size (p ≤ 0.05). A large inter-individual variability was observed.

CONCLUSION

Using a semi-automatic methodology, an in-depth analysis of the biomechanics of the abdominal wall was conducted, highlighting the importance of a patient-specific assessment. A broader study and consideration of recurrence would subsequently complete this methodological work.

A Muscle-Driven Spine Model for Predictive Simulations in the Design of Spinal Implants and Lumbar Orthoses

Knowledge of realistic loads is crucial in the engineering design process of medical devices and for assessing their interaction with the spinal system. Depending on the type of modeling, current numerical spine models generally either neglect the active musculature or oversimplify the passive structural function of the spine. However, the internal loading conditions of the spine are complex and greatly influenced by muscle forces. It is often unclear whether the assumptions made provide realistic results. To improve the prediction of realistic loading conditions in both conservative and surgical treatments, we modified a previously validated forward dynamic musculoskeletal model of the intact lumbosacral spine with a muscle-driven approach in three scenarios. These exploratory treatment scenarios included an extensible lumbar orthosis and spinal instrumentations. The latter comprised bisegmental internal spinal fixation, as well as monosegmental lumbar fusion using an expandable interbody cage with supplementary posterior fixation. The biomechanical model responses, including internal loads on spinal instrumentation, influences on adjacent segments, and effects on abdominal soft tissue, correlated closely with available in vivo data. The muscle forces contributing to spinal movement and stabilization were also reliably predicted. This new type of modeling enables the biomechanical study of the interactions between active and passive spinal structures and technical systems. It is, therefore, preferable in the design of medical devices and for more realistically assessing treatment outcomes.

A combined experimental and numerical approach to evaluate hernia mesh biomechanical stability in situ

A ventral hernia involves tissue protrusion through the abdominal wall (AW). It is a common surgical issue with high recurrence rates. Primary stability of hernia meshes is essential to guarantee mesh integration, yet existing meshes often fail to match the AW's complex biomechanics. This study proposes a novel method aiming at understanding post-operative mesh-AW interactions. Three fresh frozen human specimens underwent an open Rives-Stoppa implantation of a synthetic hernia mesh coated with metallic micro-beads. Additional beads were placed into the AW muscle tissue. CT scans were conducted at increasing levels of intra-abdominal pressure to reproduce forced breathing. Beads 3D coordinates were exported from the CT-scans and motion and strain of both the hernia mesh and the AW were calculated. At 30 mmHg, the mesh-muscle motion (or sliding) was 2.3 ± 1.3 mm. Muscle exhibited significantly higher strains (12.9 ± 4.7 %) than the hernia mesh (4.7 ± 1.1 %), most likely due to difference in material properties between the mesh and the AW. A repeatability study was carried out to build confidence in the proposed method. This protocol can bring insights of the hernia mesh use-conditions to improve hernia mesh design requirements and develop safer implants to reduce hernia recurrence.

Towards a better understanding of abdominal wall biomechanics: In vivo relationship between dynamic intra-abdominal pressure and magnetic resonance imaging measurements

BACKGROUND

In vivo mechanical behaviour of the abdominal wall has been poorly characterised and important details are missing regarding the occurrence and post-operative recurrence rate of hernias which can be as high as 30 %. This study aimed to assess the correlation between abdominal wall displacement and intra-abdominal pressure, as well as abdominal compliance.

METHODS

Eighteen healthy participants performed audio-guided passive (breathing) and active (coughing, Valsalva maneuver) exercises. Axial dynamic changes of abdominal muscles and visceral area were measured using MRI, and intra-abdominal pressure with ingested pressure sensor.

FINDINGS

Correlations between abdominal wall displacement and intra-abdominal pressure were specific to participant, exercise, and varying between rectus abdominis and lateral muscles. Strong correlations were found between rectus abdominis displacement and intra-abdominal pressure during breathing (r = 0.92 ± 0.06), as well as lateral muscles displacement with intra-abdominal pressure during coughing and Valsalva maneuver (r = -0.98 ± 0.03 and - 0.94 ± 0.05 respectively). The abdominal pseudo-compliance varied greatly among participants during muscular contraction, the coefficient of variation reaching up to 70 %.

INTERPRETATION

The combination of intra-abdominal pressure and dynamic MRI measurements enables the identification of participant-specific behaviour pattern. Intra-abdominal pressure and abdominal wall dynamic undergo consistent and predictable interactions. However, this relationship is subject-specific and may not be extrapolated to other individuals. Therefore, both intra-abdominal pressure and abdominal wall motion must be measured in the same participant in order to accurately characterise the abdominal wall behaviour. These results are of great importance for mesh design, surgical decision-making, and personalised healthcare.

Numerical modeling of the abdominal wall biomechanics and experimental analysis for model validation

The evaluation of the biomechanics of the abdominal wall is particularly important to understand the onset of pathological conditions related to weakening and injury of the abdominal muscles. A better understanding of the biomechanics of the abdominal wall could be a breakthrough in the development of new therapeutic approaches. For this purpose, several studies in the literature propose finite element models of the human abdomen, based on the geometry of the abdominal wall from medical images and on constitutive formulations describing the mechanical behavior of fascial and muscular tissues. The biomechanics of the abdominal wall depends on the passive mechanical properties of fascial and muscle tissue, on the activation of abdominal muscles, and on the variable intra-abdominal pressure. To assess the quantitative contribution of these features to the development and validation of reliable numerical models, experimental data are fundamental. This work presents a review of the state of the art of numerical models developed to investigate abdominal wall biomechanics. Different experimental techniques, which can provide data for model validation, are also presented. These include electromyography, ultrasound imaging, intraabdominal pressure measurements, abdominal surface deformation, and stiffness/compliance measurements.

Functional Tests of the Abdominal Wall Muscles in Normal Subjects and in Patients with Diastasis and Oblique Inguinal Hernias in a Pilot Study

OBJECTIVES

To identify typical patterns of abdominal wall muscle activation in patients with diastasis recti and inguinal hernias compared to controls during the Valsalva maneuver, voluntary coughing, and physical activity.

METHODS

The study included 15 subjects: 5 with diastasis recti, 4 with inguinal hernias, and 6 healthy controls. The functions of rectus abdominis (RA) and external oblique (OE) muscles were measured by surface electromyography (sEMG). Using ultrasound, the thicknesses of the RA, OE, internal oblique (IO), and transversus abdominis (TA) muscles were assessed as well as the echo intensity (EI) of RA and OE.

RESULTS

We found a significant effect of the type of abdominal wall pathology on the maximum sEMG amplitude (p = 0.005). There was a reliable trend in maximum sEMG amplitude, with the highest one in diastasis recti and a significantly lower one in inguinal hernias. Duncan's test showed a significant difference in muscle thickness, both on the right and left sides, between patients with diastasis and controls, but only on the left side between patients with diastasis and those with inguinal hernia (p < 0.05).

CONCLUSIONS

The abdominal wall pathology results in a change in the function and structure of the abdominal muscles, which can be detected using electromyography and ultrasound examination. The presence of diastasis recti is accompanied by an increase in bioelectrical activity and a decrease in thickness.

Soft tissue material properties based on human abdominal in vivo macro-indenter measurements

Simulations of human-technology interaction in the context of product development require comprehensive knowledge of biomechanical in vivo behavior. To obtain this knowledge for the abdomen, we measured the continuous mechanical responses of the abdominal soft tissue of ten healthy participants in different lying positions anteriorly, laterally, and posteriorly under local compression depths of up to 30 mm. An experimental setup consisting of a mechatronic indenter with hemispherical tip and two time-of-flight (ToF) sensors for optical 3D displacement measurement of the surface was developed for this purpose. To account for the impact of muscle tone, experiments were conducted with both controlled activation and relaxation of the trunk muscles. Surface electromyography (sEMG) was used to monitor muscle activation levels. The obtained data sets comprise the continuous force-displacement data of six abdominal measurement regions, each synchronized with the local surface displacements resulting from the macro-indentation, and the bipolar sEMG signals at three key trunk muscles. We used inverse finite element analysis (FEA), to derive sets of nonlinear material parameters that numerically approximate the experimentally determined soft tissue behaviors. The physiological standard values obtained for all participants after data processing served as reference data. The mean stiffness of the abdomen was significantly different when the trunk muscles were activated or relaxed. No significant differences were found between the anterior-lateral measurement regions, with exception of those centered on the linea alba and centered on the muscle belly of the rectus abdominis below the intertubercular plane. The shapes and areas of deformation of the skin depended on the region and muscle activity. Using the hyperelastic Ogden model, we identified unique material parameter sets for all regions. Our findings confirmed that, in addition to the indenter force-displacement data, knowledge about tissue deformation is necessary to reliably determine unique material parameter sets using inverse FEA. The presented results can be used for finite element (FE) models of the abdomen, for example, in the context of orthopedic or biomedical product developments.

Numerical investigation of a finite element abdominal wall model during breathing and muscular contraction

BACKGROUND AND OBJECTIVE

Ventral hernia repair is faced with high recurrence rates. The personalization of the diagnosis, the surgical approach and the choice of the prosthetic implant seem relevant axes to improve the current results. Numerical models have the potential to allow this patient-specific approach, yet currently existing models lack validation. This work extensively investigated a realistic finite element abdominal wall model including the implementation of muscle activation.

METHODS

A parametric 3D finite element model composed of bone, muscle and aponeurotic structures was introduced. Hyperelastic anisotropic materials were implemented. Two loading scenarios were simulated: passive inflation of the abdominal cavity to represent, e.g., breathing, and passive inflation followed by muscular activation to simulate other daily activities such as cough. The impact of the inter-individual variability (e.g., BMI, tissue thickness, material properties, intra-abdominal pressure (IAP) and muscle contractility) on the model outputs was studied through a sensitivity analysis.

RESULTS

The overall model predictions were in good agreement with the experimental data in terms of shape variation, muscles displacements, strains and midline forces. A total of 34 and 41 runs were computed for the passive and active sensitivity analysis respectively. The regression model fits rendered high R-squared in both passive (84.0 ± 6.7 %) and active conditions (82.0 ± 8.3 %). IAP and muscle thickness were the most influential factors for the selected outputs during passive (breathing) activities. Maximum isometric stress, muscle thickness and pre-activation IAP were found to drive the response of the simulations involving muscular contraction. The material properties of the connective tissue were essential contributors to the behaviour of the medial part of the abdominal wall.

CONCLUSIONS

This work extensively investigated a realistic abdominal wall model and evaluated its robustness using experimental data from literature. Such a model could improve patient-specific simulation for ventral hernia surgical planning, prevention, and repair or implant evaluation. Further investigations will be conducted to evaluate the impact of the surgical technique and the mechanical characteristic of prosthetic meshes on the model outputs.

The effect of breathing on the in vivo mechanical characterization of linea alba by ultrasound shearwave elastography

The most common surgical repair of abdominal wall hernia consists in implanting a mesh to reinforce hernia defects during the healing phase. Ultrasound shearwave elastography (SWE) is a promising non-invasive method to estimate soft tissue mechanical properties at bedside through shear wave speed (SWS) measurement. Combined with conventional ultrasonography, it could help the clinician plan surgery. In this work, a novel protocol is proposed to reliably assess the stiffness of the linea alba, and to evaluate the effect of breathing and of inflating the abdomen on SWS. Fifteen healthy adults were included. SWS was measured in the linea alba, in the longitudinal and transverse direction, during several breathing cycle and during active abdominal inflation. SWS during normal breathing was 2.3 [2.0; 2.5] m/s in longitudinal direction and 2.2 [1.9; 2.7] m/s in the transversal. Inflating the abdomen increased SWS both in longitudinal and transversal direction (3.5 [2.8; 5.8] m/s and 5.2 [3.0; 6.0] m/s, respectively). The novel protocol significantly improved the reproducibility relative to the literature (8% in the longitudinal direction and 14% in the transverse one). Breathing had a mild effect on SWS, and accounting for it only marginally improved the reproducibility. This study proved the feasibility of the method, and its potential clinical interest. Further studies on larger cohort should focus on improving our understanding of the relationship between abdominal wall properties and clinical outcomes, but also provide a cartography of the abdominal wall, beyond the linea alba.

Full-field in vivo experimental study of the strains of a breathing human abdominal wall with intra-abdominal pressure variation

The presented study aims to assess the mechanical behaviour of the anterior abdominal wall based on an in vivo experiment on humans. Full-field measurement of abdominal wall displacement during changes of intra-abdominal pressure is performed using a digital image correlation (DIC) system. Continuous measurement in time enables the observation of changes in the strain field during breathing. The understanding of the mechanical behaviour of a living human abdominal wall is important for the proper design of surgical meshes used for ventral hernia repair, which was also a motivation for the research presented below. The research refers to the strain field of a loaded abdominal wall and presents the evolution of principal strains and their directions in the case of 12 subjects, 8 male and 4 female. Peritoneal dialysis procedure allows for the measurement of intra-abdominal pressure after fluid introduction. High variability among patients is observed, also in terms of principal strain direction. Subjects exhibit intra-abdominal pressure of values from 11 to 21 cmH2O. However, the strain values are not strongly correlated with the pressure value, indicating variability of material properties.