Since open abdomen is a therapeutic option in the treatment of traumatized abdomen or severe peritonitis there is a need for minimizing sequelae . The well-established method of closure with a synthetic resorbable mesh prevents evisceration and chronic foreign body infection, although complication such as massive adhesions and small bowel fistulae are well known [18, 19]. The need for major surgery to reconstruct the abdominal wall and for programmed ventral hernia raises the possibility of further morbidity, increased costs and mortality . With introduction of intra-abdominal negative pressure dressings, delayed primary closure became an option in a high percentage of young trauma patients . Delayed primary closure might be the best option in such patients, but in a majority of older patients with severe peritonitis due to septic focuses this is not feasible . Closure with a tissue transplant could be helpful if adequate mechanical stability could be retained in an open abdomen environment.
Hollinsky and co-workers  measured the tensile strength of healthy human abdominal wall using specimens excised from fresh cadaver tissue. They were able to show that the linea alba fails in longitudinal and transversal direction at 39 N. This was calculated to be equivalent to a tensile strength of 10 N/mm2 and this may be regarded as the maximum strength required in a healthy human under extreme loads. The mean tensile strength of the transplants in the control group was 35 N/mm2 at day 0. Even 25 N/mm2, which was the mean strength after 21 days of incubation should be sufficient for Epiflex® to be able to withstand the maximum anticipated force, more so since the in-vitro test disregards adhesion and integration phenomena.
The breaking strength of hAD samples incubated in blood was not significantly different to that of transplants incubated in Ringer’s solution. Epiflex® should therefore be strong enough to reconstruct ventral hernias in an uninfected situation.
Epiflex® incubated in urine had similar properties. It should therefore be suitable for ventral reconstruction in the presence of an urinoma or urinary tract leakage.
Incubation in a bacteria solution resulted in no significant loss of strength at day 0 and 7 when compared to incubation in Ringer’s solution, but there was a significant difference at day 14. At day 21 there was no longer a significant difference. Typical intestinal flora may be capable of reducing the mechanical strength of Epiflex® within the time frame under investigation to a greater extent than blood or urine, although the residual mechanical strength of 16 N/mm2 still exceeds the calculated requirement.
The study has limitations with regard to the composition of the bacteria solution. Although the solution represents a common mixture of bacteria found in peritonitic patients, different bacteria mixtures could well have significantly different effects on hAD, since different bacteria strains and species excrete different concentrations of active agents such as collagenase.
The samples were incubated in high concentrations of bacteria that would only arise in an uncontrolled septic focus in the open abdomen. In such cases, an attempt at a primary closure would not normally be indicated. The combined effect of bacterial secretions and the inflammatory host response on the mechanical strength of candidate materials for abdominal wall closure cannot be simulated in-vitro.
Upper GI tract secretion had a powerful effect on the mechanical strength of Epiflex®. Loss of mechanical strength was continuous and when compared to the effects of the other incubation fluids, significantly increased at day 7, 14 and 21. The presence of an upper GI leakage, the closure of an open abdomen with Epiflex® might be compromised. At this stage of an open abdomen therapy, a definite closure is rarely indicated. It is unclear whether a pure pancreatic secretion from a pancreas fistula would have the same impact on the mechanical strength of Epiflex®.
The decrease in mechanical strength in the different liquids might be caused by various factors. Upper GI secretion is a mixture of gastric fluid, bile and pancreatic fluid and contains a heterogeneous mixture of digestive enzymes including proteases, lipases and amylases [24, 25]. The upper GI secretion was frozen shortly after collection at −80°C to retard reduction of enzyme activity. Since the extracellular matrix consists of various proteins, glycoproteins and polysaccharides, enzymatic hydrolysis would seem to be a likely contributor to loss of tensile strength [26, 27]. Bacteria also secrete hydrolytic enzymes such as collagenases, whereby the extent and the composition depends on species and strain. Furthermore, microbial organisms can modify the pH of the environment [28–30]. This may influence the degradation of bioresorbable materials [31–33]. It is known that superoxide ions from leukocytes and macrophages accelerate the degradation of absorbable materials . The mechanism leading to loss of mechanical strength in Ringer's solution after 21 days is unclear. Temperature may affect biodegradation [35, 36], however the incubation temperature in our study (37°C) seems unlikely to have exacerbated hydrolysis. Numerous studies demonstrated a significant loss in strength of biodegradable materials in aqueous solutions, presumably by cumulative low-level irreversible hydrolysis [33, 36, 37].
Limitations of the studyy
In the clinical situation wound healing processes such adhesions to the transplant and integration, remodeling, vascularization, inflammation and scaring will have an effect on the mechanical strength of the transplant and the forming abdominal wall. This limits our findings especially after 21 days of incubation. Some of these effects are likely to be positive, although it cannot be ruled out that the remodeling process in itself results periods during healing during which mechanical strength is reduced, if resorption processes advance more rapidly than synthetic processes. The uniaxial tensile stresses applied to the transplants in our study do not ideally represent the stresses that occur in vivo. The latter are dynamic and multidirectional and can best be analyzed in a clinical setting.