Uniaxial and Biaxial Mechanical Properties of Porcine Linea Alba

Abstract

Trocar site hernia (or incisional hernia) is a severe complication post laparoscopic 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 8 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.

Analysis of Accuracy of Biaxial Tests Based on their Computational Simulations

“…While hooks are applied most frequently and considered as a standard gripping method (see among others ), clamps were rejected as unacceptable by some authors . However, this rejection was substantiated by numerical or experimental analyses done only for broad clamps used one per specimen edge, while two or more narrow clamps per edge were – according to our knowledge – presented by our group only .…”

Optimized wound closure using a biomechanical abdominal model

“…Mean transverse stretch (λ) was calculated from the dot array using image analysis at each recorded pressure. Stretch was used as a quantitative measure as, from a biomechanical perspective, stretch is a prerequisite for dehiscence/herniation.…”

“…Stretch was also measured in three specimens without an incision to provide a control stretch measure at each pressure level. Suture placements (bite separation and bite widths) were decided based on distances described previously in uniaxial testing. Bite separations of 5, 10 and 15 mm in combination with bite widths of 4, 7, 10, 13 and 16 mm were tested (within a wide range either side of current clinical practice), with a sample size of three throughout.…”

“…A video camera (Samsung Electronics, Gyeonggi-Do, South Korea) was used to record a close-up of the dot array during pressurization. The pressure was increased by 1 kPa every 3 s and the experiment was ended when 20 kPa was reached, as this is approximately the maximum physiological intra-abdominal pressure 31 -35 . Mean transverse stretch (λ) was calculated from the dot array using image analysis 25,26 at each recorded pressure. Stretch was used as a quantitative measure as, from a biomechanical perspective, stretch is a prerequisite for dehiscence/herniation.…”

“…However, these studies all assumed uniaxial suture loading. The abdomen under intra‐abdominal pressure is similar to a thin‐walled pressure vessel and the in situ loading environment therefore must be multiaxial. To correctly assess the effect of suture‐based wound closure on the apposition of a linea alba laparotomy defect, it is necessary to replicate the in situ loading environment as a multiaxial environment.…”