| Objective1. To establish the animal model of polypropylene mesh implantation and to observe the biomechanical behavior of the mesh-muscular tissue unit and the degree of mesh shrinkage.2. To observe the integration of the mesh according to the biocompatibility evaluation index,3. To observe the ultra-structure of mesh and connective tissue by the scanning electronic microscope.MethodEight New Zealand female rabbits graded as clean were selected, which were randomized into 4 groups. The abdomen was divided into 12 quadrants,10 of them were implanted with polypropylene mesh while the rest two were not and assigned as the control. Under intraperitoneal anesthesia, polypropylene mesh patches (prolift mesh), measuring 10mm×40mm were implanted in the abdominal skin of the rabbits after proper disinfection. There were totally 10 patches implanted in one rabbit. Mesh and the surrounding muscular tissue were obtained at 30 days (group B),90 days (group C),180 days (group D) and muscular tissue from control group (group A) respectively. There were 15 pieces of mesh-muscular tissue from group B, group C and group D measuring 10 mm×40 mm and 16 pieces of muscle tissue from group A. Simultaneously patches shrinkage were measured. The mesh area was calculated according to the measured length and width. The percentage of mesh shrinkage was calculated based on the preoperative-patch area (eg:400 mm2). Area shrinks %=(postoperative mesh area/preoperative mesh area) x 100%. They were biomechanically assessed by MTS-813.10 and we measured the tissue biomechanical properties, including stress relaxation, creep and stress-strain relationship. All mesh-muscular tissue samples in group A were properly prepared for pathological evaluation by light microscope. Five samples from group B, group C and group D were divided into 2 sections, one for pathological evaluation under light microscope, grading the scar plate formation, inflammation and tissue in-growth from 1 to 4 (1=normal,4=severe) while in the other section the tissue surrounding the mesh was observed under electronic microscope. All data was shown as the mean±standard deviation (x±s) and they were processed with spss16.0 software for statistical analysis. Single factor analysis of variance was applied for comparison between groups with significance level P=0.05.Results1. Stress-relaxation reflects the reduction of internal stress at a certain stress level. In the study period, the longer the implantation is, the less reduction of the remaining stress is. The internal stress diminished rapidly in response to stress in the specimens in all group. The stress relaxation normalized curve was identical in group B and group D. The instant stress reduction rate was slower than Group A and C. Residual stress were 0.21±0.05MPa,0.22±0.05MPa, 0.15±0.03MPa and 0.27±0.02MPa in group A, group B, group C and group D respectively with significant difference between groups (P<0.05). Residual stress in group C showed the least, while group D showed the greatest which with significant difference compared to the other groups(P<0.05). However, there were no significant difference between group A and group B.2. Creep reflects the size stability of mesh-muscle tissue at stress. In the study period, the size of mesh-muscle tissue tends to be more stable along with time at a certain stress. When it reached a stable level about 0.6MPa, samples in all the groups creep slowly (t>50s). The normalized creep curves in group B, group C and group D were identical, of which the speed and relative strain variation are smaller than those in Group A.Among those, group D is the least possible to creep.3. Elastic modulus reflects the flexibility of mesh-muscle tissue. In the study period, mesh-muscle tissue tends to be less flexible along with time. The elastic modulus increased with time under relatively low stress level. The elastic modulus were 0.54±0.20,0.76±0.16,1.28±0.35 and 1.79±0.59 in group A, group B, group C and group D respectively at 0.2MPa and 0.61±0.16,0.96±0.20,1.59±0.25 and 2.10±0.59 at 0.3MPa. There was significant difference in the elastic modulus between groups at 0.2MPa and 0.3MPa, respectively (P＜0.05). 4. The mesh shrank to a greater extent with time, while group D shrank greatest, with significant difference while comparing to group B and group C (P＜0.05). There was no statistically significant difference between group B and group C (P>0.05).5. There were no signs of infection observed on the surgical sited in all the rabbits. Grossly, there were no erosion, exposure or hematoma formation. Pathological study showed stable scar plate formation 90 days post-operatively. There was significant difference compared group B to group C and group D (P<0.05). Inflammatory reaction became moderate with time. There was significant difference compared group B to group C and group D (P<0.05).There were no obvious esophylic and atrophic changes in the muscular tissue in all the groups.6.Ultra-structural study showed that connective tissue was well integrated into the mesh and fiber capsule was becoming thinner with time. No fracture occurred in the mesh fiber.Conclusion1. Stable size and tension strength were observed in the mesh-muscular tissue unit at 0.6MPa. The elasticity reduced with time at low level of stress (σ=0.2 MPa and 0.3 MPa). The in-growth of connective tissue assists in maintaining the stability of size and tension strength while decreasing its elasticity, leading to tissue stiffness, thus preventing tissue from recovering.2. Inflammatory response and tissue rejection was mild which suggested well biocompatibility of polypropylene mesh.3. Mesh shrank to a greater extent with time. It is more important to pay attention to the complications related to the shrinkage in the long run.
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