Mechanical characterisation of porcine rectus sheath under uniaxial and biaxial tension.

Incisional hernia development is a significant complication after laparoscopic abdominal surgery. Intra-abdominal pressure (IAP) is known to initiate the extrusion of intestines through the abdominal wall, but there is limited data on the mechanics of IAP generation and the structural properties of rectus sheath. This paper presents an explanation of the mechanics of IAP development, a study of the uniaxial and biaxial tensile properties of porcine rectus sheath, and a simple computational investigation of the tissue. Analysis using Laplace׳s law showed a circumferential stress in the abdominal wall of approx. 1.1MPa due to an IAP of 11kPa, commonly seen during coughing. Uniaxial and biaxial tensile tests were conducted on samples of porcine rectus sheath to characterise the stress-stretch responses of the tissue. Under uniaxial tension, fibre direction samples failed on average at a stress of 4.5MPa at a stretch of 1.07 while cross-fibre samples failed at a stress of 1.6MPa under a stretch of 1.29. Under equi-biaxial tension, failure occurred at 1.6MPa with the fibre direction stretching to only 1.02 while the cross-fibre direction stretched to 1.13. Uniaxial and biaxial stress-stretch plots are presented allowing detailed modelling of the tissue either in silico or in a surrogate material. An FeBio computational model of the tissue is presented using a combination of an Ogden and an exponential power law model to represent the matrix and fibres respectively. The structural properties of porcine rectus sheath have been characterised and add to the small set of human data in the literature with which it may be possible to develop methods to reduce the incidence of incisional hernia development.

Lyons M ，Winter DC ，Simms CK 《-》

Uniaxial and biaxial mechanical properties of porcine linea alba.

Incisional hernia is a severe complication post-laparoscopic/laparotomy surgery that is commonly associated with the linea alba. However, the few studies on the mechanical properties of the linea alba in the literature appear contradictory, possible due to challenges with the physical dimensions of samples and variations in protocol. This study focuses on the tensile mechanical characterisation of the porcine linea alba, as determined by uniaxial and equi-load biaxial testing using image-based strain measurement methods. Results show that the linea alba demonstrated a non-linear elastic, anisotropic behaviour which is often observed in biological soft tissues. The transverse direction (parallel to fibres) was found to be approximately eight times stiffer than the longitudinal (cross-fibre) direction under both uniaxial and equi-load biaxial loading. The equi-load biaxial tensile tests revealed that contraction could occur in the transverse direction despite increasing load, probably due to the anisotropy of the tissue. Optical surface marker tracking and digital image correlation methods were found to greatly improve the accuracy of stretch measurement, resulting in a 75% change in the apparent stiffness compared to using strain derived from machine cross-head displacement. Additionally, a finite element model of the experiments using a combination of an Ogden and fibre exponential power law model for the linea alba was implemented to quantify the effect of clamping and tissue dimensions (which are suboptimal for tensile testing) on the results. The preliminary model results were used to apply a correction factor to the uniaxial experimental data prior to inverse optimisation to derive best fit material parameters for the fibre reinforced Ogden model. Application of the model to the equi-load biaxial case showed some differences compared to the experimental data, suggesting a more complex anisotropic model may be necessary to capture biaxial behaviour. These results provide an improved assessment of the mechanical properties of the porcine linea alba for wound closure and other studies.

Cooney GM ，Moerman KM ，Takaza M ，Winter DC ，Simms CK ... -  《-》

Uniaxial and biaxial tensile stress-stretch response of human linea alba.

There are few studies on the stress-stretch behaviour of human linea alba, yet understanding the mechanics of this tissue is important for developing better methods of abdominal wound closure. Published data focuses mainly on porcine linea alba and for human tissue there are conflicting results and no bi-axial data available. This variability is likely due to challenges with the physical dimensions of the tissue and differences in experimental methodology. This study focussed on the tensile mechanical characterisation of the human linea alba using uniaxial and equi-load biaxial testing performed using image-based strain measurement methods. Thirteen freshly frozen human cadaveric abdominal walls were obtained and used to prepare 7 samples in both the transverse and longitudinal directions for uniaxial testing, and 13 square samples for bi-axial testing. The results showed significant anisotropy and for the equi-load biaxial tests the deformation was heavily biased in the longitudinal direction. In comparison with similar tests on porcine tissue from a previous study, it was found that the response of porcine linea alba to uniaxial loading is similar to that of human linea alba, with no statistically significant differences observed. Under biaxial loading human and porcine linea showed no statistical significance in the difference between their means in the transverse direction. However, a significant difference was observed in the longitudinal direction, and further study of the respective tissue structures is needed to better understand this result. These results provide the first data on the biaxial tensile properties of human linea alba and can aid in an improved assessment of wound closure mechanics.

Cooney GM ，Lake SP ，Thompson DM ，Castile RM ，Winter DC ，Simms CK ... -  《-》

A comparison of uniaxial and biaxial mechanical properties of the annulus fibrosus: a porcine model.

Gregory DE ，Callaghan JP 《-》

Characterisation of human posterior rectus sheath reveals mechanical and structural anisotropy.

Our work aims to investigate the mechanical properties of the human posterior rectus sheath in terms of its ultimate tensile stress, stiffness, thickness and anisotropy. It also aims to assess the collagen fibre organisation of the posterior rectus sheath using Second-Harmonic Generation microscopy. For mechanical analysis, twenty-five fresh-frozen samples of posterior rectus sheath were taken from six different cadaveric donors. They underwent uniaxial tensile stress testing until rupture either in the transverse (n = 15) or longitudinal (n = 10) plane. The thickness of each sample was also recorded using digital callipers. On a separate occasion, ten posterior rectus sheath samples and three anterior rectus sheath samples underwent microscopy and photography to assess collagen fibre organisation. samples had a mean ultimate tensile stress of 7.7 MPa (SD 4.9) in the transverse plane and 1.2 MPa (SD 0.8) in the longitudinal plane (P < 0.01). The same samples had a mean Youngs modulus of 11.1 MPa (SD 5.0) in the transverse plane and 1.7 MPa (SD 1.3) in the longitudinal plane (P < 0.01). The mean thickness of the posterior rectus sheath was 0.51 mm (SD 0.13). Transversely aligned collagen fibres could be identified within the posterior sheath tissue using Second-Harmonic Generation microscopy. The posterior rectus sheath displays mechanical and structural anisotropy with greater tensile stress and stiffness in the transverse plane compared to the longitudinal plane. The mean thickness of this layer is around 0.51 mm - consistent with other studies. The tissue is constructed of transversely aligned collagen fibres that are visible using Second-Harmonic Generation microscopy.

Whitehead-Clarke T ，Brown C ，Ail G ，Mudera V ，Smith C ，Kureshi A ... -  《-》