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Page 4 of 8 Nessel et al. Mini-invasive Surg 2023;7:21 https://dx.doi.org/10.20517/2574-1225.2023.21

at a given load, but they may experience more repetitions [20-22] .

What are important boundary conditions?
A load-limit curve involves the number, magnitude, and duration of pressure elevations [18,25] . The resistance
of reconstruction depends on the energy distribution between mesh, tissue, and fixation elements . The
[5]
elasticity of both tissue and mesh is critical for the regional energy distribution and the formation of areas of
high tissue distorsion [5,18] . These areas, referred to as “hot spots”, can initiate failure mechanisms such as
creeping motion when the elastic-plastic deformation does not reach a shakedown state [5,26] . In the context
of shakedown, it implies that plastic deformation is finally reached, after which loads are taken up and
energy is dissipated in a purely elastic manner without further deformation. If further deformation occurs,
the effect is called “ratchetting shakedown”, leading to the failure of the reconstruction. This concept holds
true for both primary closure and hernia repair . To prevent slippage, it is crucial to enhance the energy
[27]
[28]
distribution in these hotspots . Additionally, careful attention should be given to notches and cracks
during reconstruction, as these structural weaknesses can initiate failure [29,30] .

Which factors determine the durability of an interface between tissue, mesh, and fixation material?
The gripping coefficients vary significantly, with tissues exhibiting an 18fold variation, hernia meshes
showing a 14fold variation, and different sutures, tacks, and adhesives displaying a 14fold variation.
Suturing a defect increases the retention force by up to 3fold. About half of the patients experience overall
tissue distensions of up to 350% or more. Regional variation can be high. Recalculating data from
multicentric randomized studies on primary sutures, incisional hernia rates drop with better GRIP
values . Due to the magnitude of the influences, a durable combination can be chosen from the variables
[2-8]
mentioned above. From a physical perspective, this combination should permit shakedown within a given
timeframe and volume at given elasticity, strain, and pressure levels . For practical purposes, a toolkit of
[26]
meshes and fixation elements should be at the disposition of the surgeon. In critical cases, computed
tomography at rest and during the Valsalva maneuver can be used to assess individual tissue elasticity [2,17] .

Can notch effects and stress concentration be assessed to strengthen weak spots?
Incisional hernia repair deals with defect cut-outs in multi-layered polymeric composite structures under
multiaxial tensile loads [9,31,32] . Using the cyclic load bench test described above, round, rhomboid, and
elliptical defects of the same sizes were closed with standardized sutures and tested for durability. After 425
DIS strains, no reconstruction held tight after round defect closure, whereas 30 and 100% were durable after
suturing the other defect shapes. Recent work points to a change in stress distribution during loading
conditions . Concepts derived in material sciences from fracture mechanics, such as fracture toughness,
[33]
elastic fracture mechanism, and stress concentration or intensity, can be applied to porcine muscle tissue
and tendons. Tear resistance, buttonhole formation, suture slackening or fixation retention force, and other
effects commonly observed in surgical practice are not fully explained by current theory and warrant further
investigation [34,35] . Most likely, a better word than “fracture toughness” should be used in the surgical field to
characterize the ability of a material to maintain strength despite the presence of a macroscopic crack. Since
all work is based on tissue or mesh, “cyclic tear resistance” might be used. At least, tissue and mesh
resistance or “toughness” should be distinguished. Using the existing cyclic load bench test, standardized
suturing was successfully used to close round defects up to diameters of 7.5 cm durably . Larger defects
[27]
develop tears in the tissue lateral to the suture line, most probably due to local stress concentrations . Mesh
[36]
[37]
augmentation is successful as long as the stress concentration is considered . This is an area where the
scientific evaluation has just started. A cyclic load bench test augmented by tension assessment might be the
next step to further improve incisional hernia repair.