Objective
We aimed to characterize the effect of vaginal or abdominal mesh insertion and of different collagen augmentation of polypropylene mesh in a sheep model. Outcome measures were passive and active biomechanical properties and semiquantitative morphometry.
Study Design
Forty-two Texel sheep were used: 6 were nonimplanted controls (n = 6), the rest were implanted with polypropylene mesh (n = 12; Avaulta Solo; Bard Medical, Covington, GA) or collagen-coated meshes: Avaulta Plus (n = 12; Bard Medical) and Ugytex (n = 12; Sofradim International, Trevoux, France). Through a single incision, the rectovaginal septum was dissected and a 35 × 35–mm mesh was sutured to the underlying tissues. Abdominally, a 50 × 50–mm mesh was laid over a primarily sutured full thickness 40-mm longitudinal incisional defect. Animals were explanted after 60 or 180 days (n = 6 per group). Outcome measures were passive biomechanics by biaxial tensiometry, active contractility of vaginal explants, and histologic evidence.
Results
Vaginal explants were 2.4 times stiffer compared with native vaginal tissue ( P < .001), but without differences in comfort zone stiffness or slope of the load-elongation in the physiologic range between the products that were tested. Collagen coating was associated with a 16-fold reduction in contractile force at 180 days, compared with native vaginal tissue, both for Avaulta Plus ( P = .032) and Ugytex ( P = .015). Abdominal explants were 1.3-times stiffer compared with native abdominal wall tissue ( P < .001) and were 1.9-times stiffer compared with vaginal explants.
Conclusion
Vaginal mesh implantation yields less stiff explants compared with abdominal explants. Vaginal mesh implantation also alters the passive and active biomechanical properties compared with native vaginal tissues. Collagen matrices did not reduce the number of graft-related complications.
Pelvic organ prolapse is a widespread condition; up to one-half of parous women display some degree of genital prolapse. The lifetime risk for prolapse surgery is 19%. As women get older and yet are more active, the need for durable and functional repair will increase. Synthetic prolapse meshes initially were modifications of abdominal hernia meshes; later trocars and cannula systems were developed to facilitate standardized transvaginal mesh placement through a single incision. The severity, difficulty to treat, and overall occurrence rate of graft-related complications (GRCs) have become a serious concern for patients, clinicians, and health authorities. As a result, the International Urogynecological Association and the Food and Drug Administration have issued health warnings and have called for a more controlled introduction of new implants before their use in the clinic, which includes preclinical research.
These guidelines suggest preclinical studies. Animal models are used to study the effects of mesh implantation, initially typically in rodents that undergo abdominal wall reconstruction. Having many limitations, larger animal models that include rabbits, sheep, and pigs have been proposed. Larger animal models allow simultaneous implantation of larger and multiple meshes and the possibility of long term follow-up evaluations. This, in turn, allows for better characterization of the passive biomechanical properties. A sheep is a large, near clinical size animal model that can be used to assess vaginal implantation, which may be more relevant for the field of urogynecology. de Tayrac et al described a vaginal exposure rate as high as 25% for certain polypropylene meshes but could not demonstrate a reduction of GRC by collagen coating. We used the sheep model to compare the effect of abdominal and vaginal implantation and concluded that both implant location and mesh size influence the occurrence of GRCs and the biomechanical properties of the explant. However, in that study, we examined only a single synthetic mesh type. The objective of the present study was to assess the effects of the collagen coating of polypropylene implants on the occurrence of GRCs and biomechanical properties of vaginal explants. We also compared outcomes to explants from abdominal-wall reconstruction.
Materials and Methods
We used 3 commercially available macroporous meshes: (1) Avaulta Solo (weight 58 g/m 2 ; Bard Medical, Covington, GA), (2) Avaulta Plus (Bard Medical), the same textile as Avaulta Solo but with a sheet of hydrophilic cross-linked porcine acellular collagen matrix (ACM; 0.5-mm thick and 1.8-mm pores; weight up to 100 g/m 2 ), and (3) Ugytex (38 g/m 2 ; Sofradim International, Trevoux, France) that has its polypropylene filaments coated with atelocollagen, polyethylene glycol, and glycerol. All meshes that were used in this experiment were graded clinically and came sterile in their original packing.
All animals were treated according to protocols approved by the KU Leuven Animals Ethics Committee. Thirty-six parous Texel sheep underwent surgical implantation of mesh; 6 primiparous sheep were used as nonoperated controls. Implanted animals were divided randomly into 3 groups: Avaulta Solo (n = 12), Avaulta Plus (n = 12), and Ugytex (n = 12). For abdominal and vaginal mesh implantation, we followed a previously described protocol. Briefly, with general anesthesia, antibiotic prophylaxis, aquadissesction of the rectovaginal space, and a single incision, sufficient space was dissected to allow insertion of a 35 × 35–mm flat mesh (1225 mm 2 ). It was fixed to the underlying perirectal tissue with multiple interrupted 4/0 polypropylene (Prolene; Ethicon, Zaventem, Belgium). The vaginal incision was closed with continuous 2/0 polyglactin 910 (Vicryl; Ethicon). For abdominal implantation, a 40 × 40–mm full-thickness fascial incision was made, primarily repaired with continuous polydioxanone (PDS II; Ethicon) and overlaid with a 50 × 50–mm mesh that was fixed with 4/0 interrupted Prolene. Subcutis and skin were closed with continuous 2/0 Vicryl. Sixty or 180 days later, the vaginal and abdominal implants and surrounding tissues were removed en bloc (n = 6 per group) and referred to as explants. The presence of any of the following GRC was noted at the time of explantation: herniation, encapsulation, adhesions, exposure, or clinically obvious infection. Contraction of the mesh was determined by comparing the flat surface area of the mesh at explantation with the use of a digital micrometer (accuracy 0.01 mm; Mitutoyo Corporation, Kawasaki, Japan), to its preimplantation dimensions (expressed as a percentage).
Histologic information
The 1.0 × 0.5–cm explants were fixed in 10% formalin, embedded in paraffin, cut into 6-μm sections, and stained with hematoxylin and eosin and Movat for morphometry, as previously described. Hematoxylin and eosin–stained slides were scored by 2 individuals who were blinded to the surgery for foreign body giant cells (FBGC), polymorphonuclear cells (PMN) and vessels. Collagen was assessed on Movat stains for its organization, composition, and amount. All scores (unitless) ranged from 0 to 3 and a higher score referred to a higher number of cells and vessels from hematoxlyin and eosin stains. A higher score on a Movat stain meant more organization, quantity, and mature collagen. Infection was classified as either low-grade (≥15 polymorphonuclear cells without clinical evidence of infection) or high-grade (presence of [micro] abscesses, inflammatory infiltrate, bleeding, and necrosis).
Polymorphonuclear cells
Biaxial tensiometry was performed with a previous established protocol. Briefly, we used a 500-N Zwicki tensiometer (Zwick GmbH & Co KG, Ulm, Germany) with a 200-N load cell and a spheric plunger (diameter, 11.5 mm). Control and vaginal or abdominal explants that had been specified for biomechanical testing were square specimens of 25-×-25 mm, wrapped in a saline solution–soaked gauze, and stored at –20°C. On the day of testing (within 1 week after explantation), each sample was thawed to room temperature and clamped.
The sample was preloaded (0.1N) and then loaded at a rate of 20 mm/min until 200 N or failure occurred. The plunger test resulted in a bilinear load-elongation relationship. These 2 regions have been described previously as the “comfort” and “stress” zone. We limited our analysis to the “comfort zone.” We calculated the slope of the load-elongation curve or the comfort zone stiffness (Newton/millimeters) and the elongation when the comfort zone ends, which was defined as the comfort zone length (millimeters).
The active properties of the vagina after implantation were assessed by a contractility test. Briefly, an approximate 10 × 8–mm vaginal explant that was oriented along the circumferential axis was isolated at explantation and placed into a heated (37°C) physiologic Krebs solution. Samples were subjected to a small (0.1 mN) preload and allowed to equilibrate for 1 hour in the heated bath. After 1 hour, each tissue sample was subjected to potassium (80 mmol/L KCl). The contractile force was normalized to the measured tissue volume (milliNewton/cubic millimeter).
Statistics
With a Kolmogorov-Smirnov test, mesh contraction was distributed normally therefore represented as mean ± standard deviation; all other outcomes were nonparametric. A 1-way analysis of variance was used to compare mesh contraction with a Sidak post hoc test. Nonparametric data are represented as median (interquartile range) and a Kruskal-Wallis test with a Mann-Whitney post hoc. All statistics were done with SPSS software (version 16.0; IBM, Armonk, NY) and with a significance level set to a probability value of < .05.
Results
We first compared outcomes of vaginal vs abdominal explants for each mesh type and then compared the biomechanical properties of vaginal explants to native vaginal tissues.
Avaulta solo
Vaginal exposure occurred in 50% of the animals after 60 days and in 16.7% of the animals after 180 days (total, 33%; Figure 1 ). There were no abdominal exposures ( Table 1 ). The average contraction rate of vaginal explants was 30% ± 15.9%, which was higher than abdominal explants (11% ± 10%; P = .007).
