Evaluating mechanical integration at the mesh-tissue interface in ventral hernia repair

Lindsey G Kahan, BS, L M Brunt, MD, Spencer P Lake, PhD, Wen Hui Tan, MD, Jared M McAllister, MD, Jennifer Yu, MD, Dominic M Thompson, MA, Jeffrey A Blatnik, MD. Washington University in St. Louis

Introduction: Synthetic meshes for hernia repair exhibit significant variability in mechanical properties. It is unknown how a mesh’s mechanical match to native tissue contributes to in vivo contraction behavior, a reported mechanism of recurrence, and ex vivo mechanics. We used an innovative model of in vivo 3D strain tracking combined with mechanical analysis to assess effects of mesh properties on a repaired abdominal wall. We hypothesized that implantation of meshes with dissimilar mechanical properties compared to native tissue would alter repaired abdominal wall mechanics through stiffness and contraction differences.

Methods: Seven female minipigs (60lbs) underwent ventral hernia creation and subsequent open repair with Mesh A (N=4) or Mesh B (N=3), commercially-available polypropylene meshes. Following implantation with attached radio-opaque beads, fluoroscopic images were taken at insufflation pressures ranging incrementally from 5mmHg to 30mmHg at post-operative days 0, 7, and 28.

At 28 days, the animals were sacrificed and ex vivo biaxial mechanical testing was performed on full-thickness samples across the repaired abdominal wall. Mechanical testing was also conducted on 13 male minipig controls, and on Meshes A and B separately. Anisotropy, the ratio of stiffness in the transverse versus cranial-caudal orientations, was assessed; a value of 1 indicates isotropy (equal properties), while higher values represent directional-dependence. Two-way ANOVA testing was used for statistical analysis.

Results: 3D reconstructions of the repaired abdominal wall showed strain as functions of pressure and time (Fig. 1). As pressure increased, both meshes expanded, with no differences between groups. Over time, meshes contracted 17.64% (Mesh A) and 4.16% (Mesh B) (p=0.11). Total abdominal wall area expanded with animal growth.

Mechanics of the meshes alone showed that Mesh A deviated from anisotropic native tissue more than Mesh B (Fig. 2). Compared to native tissue, Mesh A was stiffer cranial-caudally, whereas Mesh B exhibited no stiffness differences. Testing of explanted repaired abdominal walls showed an increased stiffness of Mesh A-Tissue Complex (A-TC) and B-TC in both directions against native tissue. A-TC and B-TC became more isotropic over time as mesh properties prevailed over native abdominal wall properties (Fig. 2).

Conclusions: This novel technique enables assessment of 3D strains at the mesh-tissue interface in vivo in an animal model. While the abdominal wall expanded, mesh-ingrown areas contracted, potentially indicating stresses at mesh edges. Ex vivo mechanics demonstrate that repaired tissue adopts mesh properties, suggesting that a better-matched mesh could reduce changes to abdominal wall mechanics and lead to fewer failures.