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Polypropylene and polypropylene/polyglecaprone (Ultrapro®) meshes in the repair of incisional hernia in rats1

PURPOSE: To compare the inflammatory response of three different meshes on abdominal hernia repair in an experimental model of incisional hernia. METHODS: Median fascial incision and skin synthesis was performed on 30 Wistar rats. After 21 days, abdominal hernia developed was corrected as follows: 1) No mesh; 2) Polypropylene mesh; and, 3) Ultrapro® mesh. After 21 days, the mesh and surrounding tissue were submitted to macroscopic (presence of adhesions, mesh retraction), microscopic analysis to identify and quantify the inflammatory and fibrotic response using a score based on a predefined scale of 0-3 degrees, evaluating infiltration of macrophages, giant cells, neutrophils and lymphocytes. RESULTS: No significant difference was seen among groups in adherences, fibrosis, giant cells, macrophages, neutrophils or lymphocytes (p>0.05). Mesh shrinkage was observed in all groups, but also no difference was observed between polypropylene and Ultrapro mesh (7.0±9.9 vs. 7.4±10.1, respectively, p=0.967). Post-operatory complications included fistula, abscess, dehiscence, serohematic collection and reherniation, but with no difference among groups (p=0.363). CONCLUSION: There is no difference between polypropylene (high-density) and Ultrapro® (low-density) meshes at 21 days after surgery in extraperitoneal use in rats, comparing inflammatory response, mesh shortening, adhesions or complications.


Introduction
The most common complication of abdominal surgery is incisional hernia. Prospective studies have shown that its incidence ranges from 9% to 20% [1][2]  Recurrence after hernia repair constitutes an important complication to treatment 2 . The use of meshes in the US increased from 34.2% to 65.5% in 1999 2 . A prospective, controlled, multicenter study in 2004 showed that recurrence in patients in whom mesh was used was approximately 50% lower compared to suture repair 3 . Although several materials available, no consensus exists on the best mesh for hernia repair.
The most commonly used material for abdominal wall repair is polypropylene mesh. It is low-cost and provides adequate tensile strength, although it induces intense inflammatory reactions, which is a strong stimulus for adhesion 5 .
Lightweight meshes (polypropylene/polyglecaprone) have led to an appreciable improvement in biocompatibility, less retraction, easier handling, and less adhesion; however these materials are more expensive 6 . Recently, lightweight meshes recovered with polyglecaprone film were developed for repair of inguinal hernia, although no studies adress its use in abdominal hernias 7 .
The objective of this study was to compare the biomechanical and inflammatory response of three different meshes on abdominal hernia repair in an experimental model of incisional hernia. In contrast to most studies 8  were acclimated and housed under standard conditions. All animals were allowed standard rat chow and water ad libitum throughout the study. Animals that died from surgery complications were excluded. The experimental design is outlined in Figure 1.

Hernia induction surgery
Animals were anesthetized by intraperitoneal injection in the lower right quadrant of the abdomen with 0.5ml/kg of sodium thiopental 0.2% 9 . Ventral abdominal wall hair was shaved and the field was prepped with alcohol. A 4cm midline skin incision was made and subcutaneous tissue dissected from the muscle fascia transversely and vertically, 1.5cm on each side of the incision. A 3cm midline incision was made on the muscular fascias and peritoneum, followed by subsequent skin closure using non-resorbable 4-0 nylon 9,10 . easily undone by manipulation; Degree 2) Stable adhesions, between intestinal loops, not involving the abdominal wall, resistant to manipulation; Degree 3) Stable adhesions, between the abdominal wall and an organ or a structure, resistant to manipulation; Degree 4) Stable adhesions, between the abdominal wall and more than an organ or structure, resistant to manipulation; and Degree 5) Stable adhesions, between intestinal loops and the abdominal wall, with enteric fistulas, resistant to manipulation.

Histological evaluation
A strip was cut perpendicularly to the lateral flap incision from each rat for subsequent biomechanical analysis.
The samples were immediately fixed in formalin, embedded in paraffin, sectioned, and stained. Hematoxylin-eosin staining was used to identify and quantify the inflammatory and fibrotic response using a score based on a predefined scale of 0-3 degrees, evaluating infiltration of macrophages, giant cells, neutrophils and lymphocytes, according to protocol previous described 12 . A single pathologist, blinded to treatments, analyzed slides.

Statistical analysis
Data were analyzed with IBM SPSS statistic software.
One-way ANOVA was used to determine differences in the incidence of recurrent incisional hernias, weight and shrinkage of the mesh among groups. The Kruskal-Wallis test was used to determine differences in histological evaluation, adherences and complications. Values were reported as the mean±standard error.

Surgical treatment of hernia (D21)
At 21 days following the hernia induction procedure, rats were re-anesthetized and prepared as described above. The skin incision was identified and the prefascial plane reentered. The hernia sac was dissected free from the skin, and excised to the hernia edge, with posterior midline approach by continuous suture with absorbable 4-0 polyglactin.
At this point, the animals were distributed into three groups of ten animals each according to treatment, as follows:  Complications included fistula, abscess, granuloma, sero-hematic collection, and wound dehiscence (p=0.46, Table 1).
One of the rats in No mesh and Polypropylene groups developed recurrent incisional hernias, but no significant difference in hernia recurrence was found among groups (p=0.363).
No mesh group was used to compare recurrence rates of hernia and differences in inflammation caused by the mesh and suture repair. There were no differences between this group and either meshes group.
Adhesions were analyzed by a classification proposed previosly 12,13 . Most authors found that the occurrence of adhesions increased with the use of polypropylene 3,14 and Ultrapro ®3 .
However, none of these meshes was designed for intraperitoneal use. We found no significant difference between the meshes groups in its extraperitoneal use.
The time of euthanasia was based on previous studies that observed the presence of cellular infiltrate, which is characteristic of chronic inflammation 12,15 . All groups still had presence of macrophages besides neutrophil infiltration, in contrast to other studies 15,16 .
In this study, no difference was seen among the groups in fibrosis, macrophages, lymphocytes, giant cells or neutrophils.
However, a tendency toward higher numbers of giant cells was seen in both mesh groups, suggesting a greater reaction caused by the mesh, in agreement with the literature 17 .
Polypropylene mesh is most commonly used because it is easily handled and relatively low cost. Several studies have shown that polypropylene causes a pronounced and persistent inflammatory reaction, is well incorporated in the surrounding tissue of the abdominal wall and causes a strong stimulus for adhesion formation 9 . These studies also reported a higher incidence of fibrosis, adhesions, and fistula compared to light-weight meshes 11 .
Although this mesh was designed for extraperitoneal use, most experimental studies assessed the response and complications of polypropylene intraperitoneal placement 18 .

Conclusion
Polypropylene (high density) and UltraPro ® (low-density) meshes in extraperitoneal use, for hernia repair in rats, showed similar inflammatory response, mesh shortening, adhesions and complications.