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Biomechanical properties of synthetic and biologic graft materials following long-term implantation in the rabbit abdomen and vagina

Published:March 16, 2009DOI:https://doi.org/10.1016/j.ajog.2008.12.041

      Objective

      We sought to evaluate the effects of anatomic location and ovariectomy on biomechanical properties of synthetic and biologic graft materials after long-term implantation.

      Study Design

      A total of 35 rabbits underwent ovariectomy or sham laparotomy and were implanted with polypropylene (PP) mesh (n = 17) or cross-linked porcine dermis (PS) (n = 18) in the vagina and abdomen. Grafts were harvested 9 months later and underwent mechanical properties testing.

      Results

      After implantation, PS was similar in strength (P = .52) but was twice as stiff as PP (P = .04) and had a maximal elongation only half that of PP (P < .001). Degradation of PS was associated with decreased ultimate tensile strength (P = .03) and elastic modulus (P = .046). Vaginal PP grafts shrunk more (P < .001) and were less stiff than abdominal PP grafts (P = .049) but were not different in strength (P = .19). Ovariectomy had no effect (P > .05).

      Conclusion

      Cross-linked PS undergoes long-term degradation resulting in compromised biomechanical properties and thus is likely inferior to lightweight PP meshes for pelvic organ prolapse and incontinence procedures.

      Key words

      Traditional procedures for the repair of pelvic organ prolapse have resulted in suboptimal surgical outcomes with reoperations comprising 30% of procedures for prolapse or incontinence; consequently, current techniques in pelvic reconstructive surgery are being reexamined in an effort to find a mechanism to help improve long-term cure rates.
      • Olsen A.L.
      • Smith V.J.
      • Bergstrom J.O.
      • Colling J.C.
      • Clark A.L.
      Epidemiology of surgically managed pelvic organ prolapse and urinary incontinence.
      • Birch C.
      The use of prosthetics in pelvic reconstructive surgery.
      The use of synthetic and biologic graft materials for the transvaginal repair of pelvic organ prolapse has gained increasing interest, with ongoing discussion regarding the optimal material and technique that provide good long-term results with minimal complications.
      • Silva W.A.
      • Karram M.M.
      Scientific basis for the use of grafts during vaginal reconstructive procedures.
      Although graft materials are widely accepted for use in incontinence procedures and abdominal or laparoscopic sacrocolpopexy, their use in transvaginal pelvic reconstructive surgery has remained controversial because of increased postoperative morbidity and a paucity of long-term clinical data evaluating their safety and efficacy.
      • Visco A.G.
      • Weidner A.C.
      • Barber M.D.
      • Myers E.R.
      • Cundiff G.W.
      • Bump R.C.
      • et al.
      Vaginal mesh erosion after abdominal sacral colpopexy.
      • Kohli N.
      • Miklos J.R.
      Use of synthetic mesh and donor grafts in gynecologic surgery.
      • Baessler K.
      • Hewson A.D.
      • Tunn R.
      • Schuessler B.
      • Maher C.F.
      Severe mesh complications following intravaginal slingplasty.
      Some prospective, randomized clinical trials are being conducted,
      • Nguyen J.N.
      • Burchette R.J.
      Outcome after anterior vaginal prolapse repair: a randomized controlled trial.
      • Paraiso M.F.
      • Barber M.D.
      • Muir T.W.
      • Walters M.D.
      Rectocele repair: a randomized trial of three surgical techniques including graft augmentation.
      and many more are needed, to determine which materials, if any, provide better long-term support than traditional colporrhaphy, and to determine which patients may benefit most from the use of these materials.
      • South M.
      • Amundsen C.L.
      Pelvic organ prolapse: a review of the current literature.
      For Editors' Commentary, see Table of Contents
      Because biomechanical and biological characteristics of graft materials may change after implantation, it is important for pelvic reconstructive surgeons to be aware of these alterations when choosing a graft material for their patients. Biomechanical properties of grafts may be affected by inflammatory response, host tissue incorporation, autolysis, and structural reorganization.
      • Dora C.D.
      • DiMarco D.S.
      • Zobitz M.E.
      • Elliott D.S.
      Time dependent variations in biomechanical properties of cadaveric fascia, porcine dermis, porcine small intestine submucosa, polypropylene mesh and autologous fascia in the rabbit model: implications for sling surgery.
      • FitzGerald M.P.
      • Mollenhauer J.
      • Brubaker L.
      Donor fascia in urogynecological procedures: a canine model.
      • Walter A.J.
      • Morse A.N.
      • Leslie K.O.
      • Zobitz M.E.
      • Hentz J.G.
      • Cornella J.L.
      Changes in tensile strength of cadaveric human fascia lata after implantation in a rabbit vagina model.
      • Walter A.J.
      • Morse A.N.
      • Leslie K.O.
      • Hentz J.G.
      • Cornella J.L.
      Histologic evaluation of human cadaveric fascia lata in a rabbit vagina model.
      • Trabuco E.C.
      • Zobitz M.E.
      • Klingele C.J.
      • Gebhart J.B.
      Effect of host response (incorporation, encapsulation, mixed incorporation and encapsulation, or resorption) on the tensile strength of graft-reinforced repair in the rat ventral hernia model.
      The increased vascularity and presence of endogenous microflora in the vagina may have further impact on the host tissue response and biomechanical properties of grafts used in vaginal procedures relative to grafts used in abdominal surgery.
      • Hilger W.S.
      • Walter A.
      • Zobitz M.E.
      • Leslie K.O.
      • Magtibay P.
      • Cornella J.
      Histological and biomechanical evaluation of implanted graft materials in a rabbit vaginal and abdominal model.
      Polypropylene (PP) mesh, a permanent material that is neither absorbed nor degraded, is currently the most commonly used synthetic graft material in gynecologic surgery.
      • Birch C.
      The use of prosthetics in pelvic reconstructive surgery.
      We and others have shown that macroporous, lightweight PP meshes induce a mild chronic inflammatory response with good host tissue incorporation within the grafts.
      • Hilger W.S.
      • Walter A.
      • Zobitz M.E.
      • Leslie K.O.
      • Magtibay P.
      • Cornella J.
      Histological and biomechanical evaluation of implanted graft materials in a rabbit vaginal and abdominal model.
      • Huffaker R.K.
      • Muir T.W.
      • Rao A.
      • Baumann S.S.
      • Kuehl T.J.
      • Pierce L.M.
      Histologic response of porcine collagen-coated and uncoated polypropylene grafts in a rabbit vagina model.
      • Claerhout F.
      • Verbist G.
      • Verbeken E.
      • Konstantinovic M.
      • De Ridder D.
      • Deprest J.
      Fate of collagen-based implants used in pelvic floor surgery: a 2-year follow-up study in a rabbit model.
      The introduction of acellular collagen xenografts, such as porcine dermal (PS) or bovine dermal collagen, porcine small intestinal submucosa, and bovine pericardium, was intended to reduce complications such as erosion, infection, and fistula formation found to occur with the use of synthetic grafts. The host response to xenografts depends mainly on chemical cross-linking and porosity.
      • Trabuco E.C.
      • Klingele C.J.
      • Gebhart J.B.
      Xenograft use in reconstructive pelvic surgery: a review of the literature.
      Noncross-linked xenografts are designed as a scaffold for cellular ingrowth with eventual replacement of graft material by host connective tissue; however, concern about the use of resorbable xenografts is the loss of strength at the repair site during graft remodeling.
      • Trabuco E.C.
      • Klingele C.J.
      • Gebhart J.B.
      Xenograft use in reconstructive pelvic surgery: a review of the literature.
      Chemical cross-linking is intended to increase resistance to degradation by host collagenases, but the long-term fate of cross-linked xenografts in vivo is presently unknown.
      Animal studies are necessary to examine alterations in tensile properties of implanted grafts and to determine the long-term in vivo tissue response to evaluate the safety and efficacy of various materials available on the market. In the current study we evaluated the effects of anatomic location (vagina vs abdomen) and ovariectomy on biomechanical properties of synthetic (PP) and biologic (cross-linked PS) graft materials commonly used in pelvic reconstructive surgery after long-term (9 months) implantation in a rabbit model.

      Materials and Methods

      Animal subjects

      A total of 35 adult New Zealand White female breeder rabbits (approximately 4 kg) were used in this study. Animals were obtained from Myrtle's Rabbitry (Thompson Station, TN) and were housed at our institution's animal facility. Rabbits were assigned randomly to group 1 (PP, intact ovaries, n = 8), group 2 (PP, ovariectomy, n = 9), group 3 (PS, intact ovaries, n = 9), or group 4 (PS, ovariectomy, n = 9). Guidelines for the care and use of these animals, approved by our institutional animal care and use committee, were followed.

      Graft materials and study design

      Rabbits were randomly assigned to undergo ovariectomy or sham laparotomy and were surgically implanted with a macroporous, monofilament PP mesh (Gynemesh; Ethicon Inc, Somerville, NJ) or cross-linked, perforated, acellular PS collagen (PelviSoft Acellular Collagen BioMesh; C.R. Bard Inc, Covington, GA) in the abdomen and vagina (same graft type at both sites). PP and PS were cut into uniform strips with a sterile template measuring 0.8 cm wide and either 2.9 cm in length (PP, n = 11 rabbits) or 2.0 cm in length (PP, n = 6 rabbits; PS, n = 18 rabbits). Grafts were harvested 9 months after implantation and underwent biomechanical properties testing and histologic analysis (detailed histology findings are reported separately).
      All rabbits were implanted with 2 grafts at abdominal sites (either PP or PS), with 1 graft designated for tensile properties testing and the other designated for histology. Rabbits implanted with PP in the abdomen also had 2 PP grafts implanted in the vagina. Preliminary studies determined that the rabbit vagina would not support a piece of PS at least 2 cm in length required for biomechanical properties testing (allowing for potential shrinkage), so only 1 PS graft designated for histologic analysis (1.2 × 0.8 cm) was implanted in the vagina. As a result, tensile properties testing of PS was performed on abdominal sites only. Direct postimplantation comparisons between PP and PS were, therefore, made using abdominally harvested grafts only, potentially minimizing tensile properties discrepancies between PS and PP that may actually occur after vaginal implantation.

      Surgery and tissue collection

      Rabbits were anesthetized as described previously.
      • Huffaker R.K.
      • Muir T.W.
      • Rao A.
      • Baumann S.S.
      • Kuehl T.J.
      • Pierce L.M.
      Histologic response of porcine collagen-coated and uncoated polypropylene grafts in a rabbit vagina model.
      To perform the laparotomy, a ventral midline incision from the umbilicus to the pubis was made and the peritoneal cavity was entered. The uterus was exteriorized allowing visualization and access to the ovaries. Ovaries were identified in all animals but were surgically removed in only a subset. The ovarian ligament with ovarian vessels was ligated and transected on each side, and the ovaries were discarded. Abdominal wall fascia and rectus abdominis muscle were closed with silk or Vicryl (Ethicon Inc) sutures. Lateral to the incision, 2 grafts were implanted and secured without tension to the rectus abdominis muscle and fascia with 4 5-0 Prolene (Ethicon Inc) sutures per graft. The abdominal skin was then closed with 3-0 Vicryl (Ethicon Inc) sutures. Grafts were implanted without tension in the posterior vaginal wall between the fibromuscular layer and vaginal epithelium as described.
      • Huffaker R.K.
      • Muir T.W.
      • Rao A.
      • Baumann S.S.
      • Kuehl T.J.
      • Pierce L.M.
      Histologic response of porcine collagen-coated and uncoated polypropylene grafts in a rabbit vagina model.
      Rabbits were evaluated postoperatively by the investigators at 1 week, 2 weeks, 4 weeks, and monthly thereafter to check for mesh erosion or infection.
      At 9 months after implantation, the abdomen and posterior vaginal wall were opened and grafts designated for biomechanical properties testing were removed noting the degree of adherence to the surrounding host tissue. Harvested grafts were wrapped in sterile gauze soaked in cold saline, refrigerated, and tested within 24 hours of sample collection by investigators (M.A.G. and Y.H.) blinded to treatment. The length, width, and thickness of all grafts were measured with electronic calipers before tensile testing. Results were compared with baseline (unimplanted) values.
      Grafts designated for histologic analysis were harvested with surrounding host tissues and serial full thickness sections were processed with hematoxylin-eosin as described.
      • Huffaker R.K.
      • Muir T.W.
      • Rao A.
      • Baumann S.S.
      • Kuehl T.J.
      • Pierce L.M.
      Histologic response of porcine collagen-coated and uncoated polypropylene grafts in a rabbit vagina model.

      Biomechanical properties testing

      Static mechanical properties of grafts were determined with a TA Instruments RSAIII mechanical analyzer (New Castle, DE) in a tension configuration. Samples were equilibrated for 10 minutes at 37.5°C before analysis and maintained during mechanical testing. Grafts were gripped by clamps at each end (using waterproof sandpaper to prevent slippage) such that the gauge length (distance between clamp faces) was normalized to 10 mm for all samples tested. Tensile strength, or force required for rupture, was measured by application of a gradually increasing tensile load at an elongation rate or crosshead speed of 5 mm/min in a uniaxial direction with the force parallel to the long axis of the grafts. Force (or load) vs displacement (elongation) and stress vs strain curves were subsequently generated. Force-displacement characteristics are dependent on the cross-sectional area of the specimen, but geometric factors are eliminated by normalization of load and elongation to stress and strain, respectively. Stress is load or force per unit cross-sectional area, and strain is the change in original length. From these analyses, measures of graft strength obtained included the following: maximum load to material failure measured in newtons; graft stiffness (relative resistance of the graft to deformation or force applied/unit change in length, N/mm); ultimate tensile strength (load at failure divided by the cross-sectional area of the graft [N/mm2 or megapascal {MPa}]); tensile or elastic modulus (slope of the linear region of the stress vs strain curve, MPa; a measure of graft stiffness); and ultimate strain (percentage elongation of the graft before failure).

      Scanning electron microscopy

      Small sections of grafts were cut using a razor blade. Specimens were freeze-dried in a lyophilizer (Labconco CentriVap Gel Dryer System; Labconco Corp, Kansas City, MO) for 12 hours at -40°C and processed for electron microscopy. Cross sections were viewed with a JEOL JSM 6400 scanning electron microscope (Carl Zeiss Inc, Thornwood, NY).

      Statistical analysis

      Values are expressed as means with SE. The paired t test (or the corresponding nonparametric Wilcoxon signed rank test when indicated) was used to compare abdominal vs vaginal grafts in each animal. The Student t test (or the corresponding nonparametric Mann-Whitney rank sum test) was used to compare PP vs PS graft materials, baseline tensile properties vs those after implantation, and tensile properties of grafts from animals with and without ovaries. A P value of < .05 was considered significant.

      Results

      Scanning electron microscopy images of graft materials used in this study are shown in Figure 1. At 9 months, PP grafts had strong adherence to surrounding host tissue, requiring sharp dissection for removal, and had good host tissue incorporation between fibers. PS grafts were less adherent than PP grafts, requiring blunt and occasionally sharp dissection for removal, and were encapsulated by a layer of connective tissue that was also incorporated into the fenestrations of the PS grafts.
      Figure thumbnail gr1
      FIGURE 1Implanted graft materials
      Scanning electron micrographs of polypropylene (PP) and porcine dermis (PS) grafts before and after implantation. A, Unimplanted PP. Note knitted design of monofilament PP fibers. B, PP after implantation in vagina. Note tissue incorporation between PP fibers. C, Unimplanted PS. Note acellular collagen fibers in interweaving pattern. D, PS after implantation in abdomen. Note encapsulation (top layer) of relatively intact graft. Bar = 200 μm. (Original magnifications: A, ×75; B, C, and D, ×100.)
      Pierce. Biomechanical properties of synthetic and biologic graft materials following long-term implantation in the rabbit abdomen and vagina. Am J Obstet Gynecol 2009.
      All PP and PS grafts designated for biomechanical properties testing were identified at necropsy. However, the host response to PS grafts was more variable than the response to PP, with some PS grafts appearing to be undergoing digestion at 9 months, whereas PS grafts from other animals appeared intact (Figure 2). Degradation of PS grafts occurred in the absence of infection at a gross level. Histology revealed a more intense foreign body reaction in degrading PS grafts, with numerous foreign body giant cells and mononuclear cells invading the PS as if digesting it (Figure 2, D). Interestingly, the vaginal PS graft from the same rabbit shown in Figure 2, B and D, was missing and showed no evidence of inflammation (not shown), likely having undergone complete digestion by 9 months after implantation. Degradation of abdominal PS grafts was observed histologically in 7/18 (39%) rabbits. Six (86%) of these 7 animals with degrading abdominal PS grafts also showed vaginal graft degradation. In turn, 6/13 (46%) rabbits with degrading vaginal PS grafts also showed abdominal PS graft degradation, suggesting that degradation of PS occurs as a result of the overall host inflammatory reaction to this biologic material, and that the vaginal environment may expedite this degradation process. Degrading grafts were obtained from 3 different lots from the manufacturer (lot 06B13-2, n = 2; lot 06B25-1, n = 3; lot 05B32-9, n = 2). Grafts macroscopically or microscopically undergoing digestion were considerably weaker and less resistant to tensile forces than intact grafts (Figure 2 and Table 1).
      Figure thumbnail gr2
      FIGURE 2Variable host response to porcine dermis
      Implications for variable long-term host response to porcine dermis (PS): degradation results in compromised tensile properties 9 months after implantation. Tensile properties testing was performed on 1 abdominal PS graft per animal and histologic analysis was performed on the other. A, Intact abdominal PS grafts at 9 months and C, corresponding hematoxylin-eosin–stained section. B, Abdominal PS grafts from different rabbit experiencing degradation at 9 months and D, corresponding hematoxylin-eosin–stained section. Histology revealed more intense foreign body reaction in grafts from panel B than from panel A, with numerous foreign body giant cells (FBGC) and mononuclear cells invading graft as if digesting it (arrows). Grafts B, undergoing digestion were considerably weaker and less stiff than A, intact grafts. Load at failure = A, 31.4 N and B, 5.6 N; stiffness = A, 11.4 N/mm and B, 0.9 N/mm; ultimate tensile strength = A, 2.2 MPa and B, 0.7 MPa; elastic modulus = A, 12.4 MPa and B, 1.3 MPa; elongation at failure = A, 28.3% and B, 68.8%. Original implants were 2.0 × 0.8 cm. (Original magnifications: C and D, ×200.)
      Pierce. Biomechanical properties of synthetic and biologic graft materials following long-term implantation in the rabbit abdomen and vagina. Am J Obstet Gynecol 2009.
      TABLE 1Comparison of tensile properties between intact and degrading porcine dermis grafts
      Graft characteristicNo of samplesLoad at failure, NStiffness, N/mmUltimate tensile strength, MPa (or N/mm2)Elastic modulus, MPaRelative elongation at failure, %Thickness, mm
      PS baseline (unimplanted)516.27 (0.54)6.78 (0.38)2.31 (0.07)9.18 (0.82)54.75 (4.41)0.85 (0)
      PS intact1117.33 (3.56)6.87 (1.85)1.84 (0.52)7.27 (1.89)35.11 (4.74)1.53 (0.07)
      PS degrading77.74 (1.78)2.88 (0.85)0.57 (0.12)2.35 (0.67)37.02 (5.84)1.72 (0.16)
      PS intact vs baselineP value.82
      Mann-Whitney rank sum test.
      .50
      Mann-Whitney rank sum test.
      .14
      Mann-Whitney rank sum test.
      .52
      Student t test;
      .02
      Student t test;
      < .001
      Student t test;
      PS degrading vs baselineP value.048
      Mann-Whitney rank sum test.
      .005
      Student t test;
      < .001
      Student t test;
      .003
      Mann-Whitney rank sum test.
      .049
      Student t test;
      .003
      Mann-Whitney rank sum test.
      PS intact vs PS degradingP value.06
      Student t test;
      .12
      Mann-Whitney rank sum test.
      .03
      Mann-Whitney rank sum test.
      .046
      Mann-Whitney rank sum test.
      .80
      Student t test;
      .23
      Student t test;
      MPa, megapascal; PS, porcine dermis.
      Values are represented as mean (SE).
      Values for ultimate tensile strength and elastic modulus were calculated using cross-sectional area of grafts harvested 9 months after implantation.
      Degrading PS grafts demonstrated strong foreign body reaction with infiltration of numerous foreign body giant cells and mononuclear cells appearing to digest porcine collagen.
      Pierce. Biomechanical properties of synthetic and biologic graft materials following long-term implantation in the rabbit abdomen and vagina. Am J Obstet Gynecol 2009.
      a Student t test;
      b Mann-Whitney rank sum test.
      The overall biomechanical properties of PP and PS grafts are reported in TABLE 2, TABLE 3. Mechanical testing was performed on 34 PP samples (17 vaginal and 17 abdominal grafts from 9 rabbits without and 8 with ovaries) and 18 PS samples (18 abdominal grafts from 9 without and 9 with ovaries). Three of 17 (18%) of 17 PP grafts eroded in the vagina (from 2 animals without ovaries and from 1 animal with intact ovaries), although the degree of erosion was minimal (< 2 mm exposure each). No erosion of PP or PS grafts occurred in the abdomen.
      TABLE 2Tensile properties of unimplanted (baseline) and implanted polypropylene and porcine dermis grafts
      Graft characteristicNo. of samplesLoad at failure, NStiffness, N/mmUltimate tensile strength, MPa (or N/mm2)Elastic modulus, MPaRelative elongation at failure, %Thickness, mm
      PP baseline58.30 (1.15)2.97 (0.06)2.59 (0.36)9.04 (0.10)40.97 (4.38)0.42 (0)
      PS baseline516.27 (0.54)6.78 (0.38)2.31 (0.07)9.18 (0.82)54.75 (4.41)0.85 (0)
      PP implanted in vagina1712.17 (1.83)1.61 (0.21)1.60 (0.30)3.76 (1.16)87.48 (7.39)1.24 (0.12)
      PP implanted in abdomen1715.68 (1.96)2.42 (0.33)1.93 (0.37)6.18 (2.41)72.68 (6.53)1.26 (0.14)
      PS implanted in abdomen1813.60 (2.50)5.32 (1.25)1.35 (0.35)5.36 (1.30)35.85 (3.58)1.60 (0.08)
      PP vs PS baselineP value< .001
      Student t test;
      .008
      Mann-Whitney rank sum test;
      .46
      Student t test;
      .87
      Student t test;
      .06
      Student t test;
      < .001
      Student t test;
      PP implanted in vagina vs PP baselineP value.28
      Student t test;
      .003
      Student t test;
      .11
      Student t test;
      .004
      Mann-Whitney rank sum test;
      .003
      Mann-Whitney rank sum test;
      .002
      Student t test;
      PP implanted in abdomen vs PP baselineP value.06
      Student t test;
      .03
      Mann-Whitney rank sum test;
      .37
      Student t test;
      .01
      Mann-Whitney rank sum test;
      .02
      Mann-Whitney rank sum test;
      .005
      Student t test;
      PS implanted in abdomen vs PS baselineP value.28
      Mann-Whitney rank sum test;
      .15
      Mann-Whitney rank sum test;
      .02
      Mann-Whitney rank sum test;
      .08
      Mann-Whitney rank sum test;
      .02
      Student t test;
      < .001
      Student t test;
      PP implanted in vagina vs abdomenP value.19
      Paired t test;
      .049
      Paired t test;
      .40
      Wilcoxon signed rank test.
      .16
      Wilcoxon signed rank test.
      .15
      Wilcoxon signed rank test.
      .84
      Paired t test;
      PP vs PS implanted in abdomenP value.52
      Student t test;
      .04
      Student t test;
      .25
      Student t test;
      .76
      Student t test;
      < .001
      Student t test;
      .04
      Student t test;
      MPa, megapascal; PP, polypropylene; PS, porcine dermis.
      Values are represented as mean (SE).
      Values for ultimate tensile strength and elastic modulus were calculated using cross-sectional area of grafts harvested 9 months after implantation.
      Pierce. Biomechanical properties of synthetic and biologic graft materials following long-term implantation in the rabbit abdomen and vagina. Am J Obstet Gynecol 2009.
      a Student t test;
      b Mann-Whitney rank sum test;
      c Paired t test;
      d Wilcoxon signed rank test.
      TABLE 3Tensile properties of implanted polypropylene and porcine dermis grafts harvested from rabbits with and without ovaries
      Graft characteristicNo. of samplesLoad at failure, NStiffness, N/mmUltimate tensile strength, MPa (or N/mm2)Elastic modulus, MPaRelative elongation at failure, %Thickness, mm
      PP implanted in vagina, intact ovaries814.77 (3.09)1.74 (0.39)1.78 (0.39)4.64 (2.36)96.98 (9.58)1.24 (0.13)
      PP implanted in vagina, no ovaries99.85 (1.95)1.51 (0.22)1.45 (0.46)2.99 (0.78)79.04 (10.78)1.23 (0.19)
      PP implanted in vagina, ovaries vs no ovariesP value.19
      Student t test;
      .60
      Student t test;
      .60
      Student t test;
      .96
      Mann-Whitney rank sum test.
      .24
      Student t test;
      .60
      Mann-Whitney rank sum test.
      PP implanted in abdomen, intact ovaries816.00 (3.86)2.53 (0.66)1.89 (0.60)3.75 (1.12)71.58 (11.62)1.26 (0.19)
      PP implanted in abdomen, no ovaries915.39 (1.71)2.32 (0.26)1.97 (0.49)8.34 (4.44)73.66 (7.47)1.27 (0.21)
      PP implanted in abdomen, ovaries vs no ovariesP value.96
      Mann-Whitney rank sum test.
      .67
      Mann-Whitney rank sum test.
      .77
      Mann-Whitney rank sum test.
      .36
      Mann-Whitney rank sum test.
      .88
      Student t test;
      .99
      Mann-Whitney rank sum test.
      PS implanted in abdomen, intact ovaries917.48 (4.27)7.81 (2.12)2.02 (0.61)8.38 (2.13)30.50 (5.21)1.46 (0.08)
      PS implanted in abdomen, no ovaries99.71 (2.15)2.82 (0.74)0.67 (0.14)2.33 (0.59)41.20 (4.49)1.75 (0.11)
      PS implanted in abdomen, ovaries vs no ovariesP value.12
      Student t test;
      .11
      Student t test;
      .02
      Mann-Whitney rank sum test.
      .05
      Mann-Whitney rank sum test.
      .06
      Mann-Whitney rank sum test.
      .06
      Student t test;
      MPa, megapascal; PP, polypropylene; PS, porcine dermis.
      Values are represented as mean (SE).
      Values for ultimate tensile strength and elastic modulus were calculated using the cross-sectional area of grafts harvested 9 months after implantation.
      Pierce. Biomechanical properties of synthetic and biologic graft materials following long-term implantation in the rabbit abdomen and vagina. Am J Obstet Gynecol 2009.
      a Student t test;
      b Mann-Whitney rank sum test.
      At baseline, PP grafts were weaker (force required for rupture for unimplanted PP was half that of PS; P < .001) and less resistant to tensile forces (P = .008) than PS (Table 2). Nine months after implantation, PP grafts increased in elasticity indicated by increased maximal elongation before rupture (vagina, P = .003; abdomen, P = .02; vs baseline) and decreased stiffness (vagina, P = .003; abdomen, P = .03; vs baseline). PP grafts tended to increase in strength (load at failure) after implantation although statistical significance was not achieved. In contrast, PS grafts decreased in maximal elongation from baseline (P = .02). Degrading PS grafts decreased significantly in strength (P = .048) and stiffness (P = .005) from baseline, whereas intact grafts showed no difference (P > .05; Table 1). At 9 months, PS was twice as stiff as PP (P = .04) and had a maximal elongation only half that of PP (P < .001), but PS and PP were similar in strength (P = .52; Table 2).
      Vaginal PP grafts showed significant (P < .001) shrinkage from baseline (20 ± 3% mean decrease in surface area), whereas the surface area of abdominal PP grafts did not change (P = .16). Abdominal PP grafts were stiffer than vaginal PP grafts (P = .049) but were not significantly different in strength (P = .19) or maximal elongation (P = .15; Table 2). Ovariectomy did not have a statistically significant effect on the mechanical properties of PP grafts in the vagina or abdomen (P > .05; Table 3). Ultimate tensile strength (P = .02) and elastic modulus (P = .05) were lower in PS grafts from ovariectomized animals compared with those from animals with intact ovaries, although increased thickness of grafts from ovariectomized rabbits was noted.

      Comment

      Results from this study determined that changes in biomechanical properties of PP mesh and cross-linked, perforated PS occur after long-term implantation in a rabbit model. Considerable variability was observed in the tensile properties of implanted grafts, especially PS, that may be related to the degree of degradation and host tissue incorporation. PP consistently demonstrated good host tissue infiltration between mesh fibers and showed stronger adherence to host tissues than PS. After long-term implantation, PP decreased in stiffness from baseline, increased in maximal elongation, and tended to increase in strength. Encapsulated PS grafts that appeared intact with minimal inflammation did not change in strength or stiffness from baseline, but decreased in maximal elongation. In contrast, degrading PS grafts decreased in strength, stiffness, and maximal elongation from baseline. Although PS was stronger and stiffer than PP before implantation, at 9 months after implantation PS overall was similar in strength to PP, but was twice as stiff as PP and had a maximal elongation only half that of PP.
      Findings from the current study and others
      • Claerhout F.
      • Verbist G.
      • Verbeken E.
      • Konstantinovic M.
      • De Ridder D.
      • Deprest J.
      Fate of collagen-based implants used in pelvic floor surgery: a 2-year follow-up study in a rabbit model.
      • Gandhi S.
      • Kubba L.M.
      • Abramov Y.
      • et al.
      Histopathologic changes of porcine dermis xenografts for transvaginal suburethral slings.
      revealed that cross-linked PS elicits a variable long-term host response that may lead to unpredictable clinical outcomes in patients. In this study, macroscopic and/or microscopic degradation of abdominal PS grafts occurred in 39% of rabbits 9 months after implantation, which was associated with a more intense foreign body reaction and resulted in compromised tensile properties compared with intact PS grafts. This raises concern clinically because PS is marketed as a “permanent” biologic material that undergoes chemical cross-linking to stabilize the collagen and protect it indeterminately from host degradation. In some individuals, however, cross-linked perforated PS may behave more like a resorbable material that loses strength at the repair site during graft remodeling.
      • Trabuco E.C.
      • Klingele C.J.
      • Gebhart J.B.
      Xenograft use in reconstructive pelvic surgery: a review of the literature.
      Likewise, histopathologic analyses of cross-linked nonperforated PS (Pelvicol; C.R. Bard Inc) used for transvaginal suburethral slings in women found that some specimens underwent limited collagen remodeling and had minimal inflammation, whereas others induced a strong foreign body response and contained numerous histiocytes and foreign body giant cells that appeared to be engulfing the porcine collagen grafts.
      • Gandhi S.
      • Kubba L.M.
      • Abramov Y.
      • et al.
      Histopathologic changes of porcine dermis xenografts for transvaginal suburethral slings.
      In addition, in cases of recurrent stress incontinence, no graft remnants were observed at 58 and 67 weeks after implantation, and they appeared to be completely replaced by dense connective tissue without evidence of inflammation.
      • Gandhi S.
      • Kubba L.M.
      • Abramov Y.
      • et al.
      Histopathologic changes of porcine dermis xenografts for transvaginal suburethral slings.
      Similarly, a 2-year study examining Pelvicol (C.R. Bard Inc) grafts implanted in a rabbit abdominal model revealed that the grafts were encapsulated and remained intact for up to 180 days, but after 1 year half the grafts underwent late-onset degradation by a foreign body reaction.
      • Claerhout F.
      • Verbist G.
      • Verbeken E.
      • Konstantinovic M.
      • De Ridder D.
      • Deprest J.
      Fate of collagen-based implants used in pelvic floor surgery: a 2-year follow-up study in a rabbit model.
      The inconsistency in response may provide an explanation for the unreliable long-term surgical outcomes attained with the use of PS grafts in a variety of procedures for pelvic organ prolapse and incontinence.
      • Quiroz L.H.
      • Gutman R.E.
      • Shippey S.
      • et al.
      Abdominal sacrocolpopexy: anatomic outcomes and complications with Pelvicol, autologous and synthetic graft materials.
      • Altman D.
      • Zetterstrom J.
      • Mellgren A.
      • Gustafsson C.
      • Anzen B.
      • Lopez A.
      A three-year prospective assessment of rectocele repair using porcine xenograft.
      • Giri S.K.
      • Hickey J.P.
      • Sil D.
      • et al.
      The long-term results of pubovaginal sling surgery using acellular cross-linked porcine dermis in the treatment of urodynamic stress incontinence.
      Quiroz et al
      • Quiroz L.H.
      • Gutman R.E.
      • Shippey S.
      • et al.
      Abdominal sacrocolpopexy: anatomic outcomes and complications with Pelvicol, autologous and synthetic graft materials.
      found that abdominal sacrocolpopexy was more likely to fail with the use of nonperforated cross-linked PS than with synthetic or autologous grafts, and the median time to anatomic failure was 9 months. In addition, the PS group had a significantly higher rate of graft-related complications including erosion.
      • Quiroz L.H.
      • Gutman R.E.
      • Shippey S.
      • et al.
      Abdominal sacrocolpopexy: anatomic outcomes and complications with Pelvicol, autologous and synthetic graft materials.
      A prospective study of rectocele repair also using PS found an unacceptable anatomic recurrence rate of 41% and bowel-emptying difficulties in the majority of patients at the 3-year follow-up.
      • Altman D.
      • Zetterstrom J.
      • Mellgren A.
      • Gustafsson C.
      • Anzen B.
      • Lopez A.
      A three-year prospective assessment of rectocele repair using porcine xenograft.
      Acellular cross-linked PS was also found to result in a significantly inferior long-term cure rate in comparison with rectus fascia in the treatment of urodynamic stress incontinence (54% cured or improved at 3 years with PS vs 80% treated with rectus fascia).
      • Giri S.K.
      • Hickey J.P.
      • Sil D.
      • et al.
      The long-term results of pubovaginal sling surgery using acellular cross-linked porcine dermis in the treatment of urodynamic stress incontinence.
      The host tissue response to PP is more uniform than that observed with PS, and deposited collagen becomes increasingly more organized and fibrous over time that coincides with increasing tensiometric strength.
      • Claerhout F.
      • Verbist G.
      • Verbeken E.
      • Konstantinovic M.
      • De Ridder D.
      • Deprest J.
      Fate of collagen-based implants used in pelvic floor surgery: a 2-year follow-up study in a rabbit model.
      Comparison of PP grafts implanted in the vagina and abdomen of rabbits in the current study demonstrated that vaginal PP grafts shrink more and are less stiff than abdominal grafts at 9 months, but are not significantly different in strength. The reduction in surface area observed in vaginal PP implants is likely caused by scarring resulting in implant contraction, because PP is a permanent material that is neither absorbed nor degraded. Although the new host tissue incorporated into shrinking PP grafts may “pull” the PP fibers closer together thereby reducing the distance between individual mesh fibers and thus the overall size of the PP implants, the incorporated host tissue has greater elastic properties than the PP itself, contributing to decreased stiffness of the PP graft after implantation. Because biomechanical properties depend mainly on collagen and elastin content,
      • Goh J.T.W.
      Biomechanical and biochemical assessments for pelvic organ prolapse.
      work is in progress to examine differences in collagen subtypes and elastin deposition in vaginal and abdominal grafts.
      In this study, ovariectomy did not have a statistically significant effect on the mechanical properties of PP grafts in the vagina or abdomen. Although ovarian hormones are known to be important in maintaining vaginal tissue structure, contractility, and collagen metabolism,
      • Kim N.N.
      • Min K.
      • Pessina M.A.
      • Munarriz R.
      • Goldstein I.
      • Traish A.M.
      Effects of ovariectomy and steroid hormones on vaginal smooth muscle contractility.
      the biomechanical properties of grafts from ovariectomized rabbits were not compromised and thus ovariectomy did not appear to negatively affect host tissue incorporation and healing in vaginal grafts. Interestingly, ultimate tensile strength and elastic modulus were lower in abdominal PS grafts from ovariectomized animals compared with those from animals with intact ovaries, although increased thickness of grafts from ovariectomized rabbits was noted.
      Ideally, the strength and elasticity of graft materials implanted in the vagina should be similar to that of healthy vaginal tissue to provide proper support, enable expansion, and minimize erosion. Ultimate tensile strength and load at failure are not likely to be clinically relevant measurements, because it is unlikely that grafts would be subjected to enough stress in vivo that they would rupture.
      • Goh J.T.W.
      Biomechanical and biochemical assessments for pelvic organ prolapse.
      • Moalli P.A.
      • Papas N.
      • Menefee S.
      • Albo M.
      • Meyn L.
      • Abramowitch S.D.
      Tensile properties of five commonly used mid-urethral slings relative to the TVT.
      • Lei L.
      • Song Y.
      • Chen R.
      Biomechanical properties of prolapsed vaginal tissue in pre- and postmenopausal women.
      • Cosson M.
      • Lambaudie E.
      • Boukerrou M.
      • Lobry P.
      • Crepin G.
      • Ego A.
      A biomechanical study of the strength of vaginal tissues: results on 16 postmenopausal patients presenting with genital prolapse.
      It is thought that physiologic loads that tissues bear are less than a third of ultimate tensile strength.
      • Goh J.T.W.
      Biomechanical and biochemical assessments for pelvic organ prolapse.
      Strength of implanted grafts in this study exceeds previously published values for that of vaginal tissue.
      • Lei L.
      • Song Y.
      • Chen R.
      Biomechanical properties of prolapsed vaginal tissue in pre- and postmenopausal women.
      Measurements of graft stiffness such as elastic modulus and relative elongation likely are more meaningful clinically. A previous study demonstrated that vaginal tissues from postmenopausal and premenopausal women without prolapse elongated 137% and 168% greater than their initial length, respectively.
      • Lei L.
      • Song Y.
      • Chen R.
      Biomechanical properties of prolapsed vaginal tissue in pre- and postmenopausal women.
      In contrast, at 9 months after implantation, maximal elongation of PS was only 36% and that of PP was 73-87% greater than their initial length.
      Limitations of this study exist. Biomechanical properties of implanted grafts were subjected to a constant rate of elongation until they ruptured or failed, which does not occur under physiologic circumstances. Because biological tissues have elastic, viscous, and plastic properties, future experiments should involve repetitive stress testing within the physiologic range of vaginal wall strength or time-dependent or rate-dependent experiments not to failure.
      • Goh J.T.W.
      Biomechanical and biochemical assessments for pelvic organ prolapse.
      • Lei L.
      • Song Y.
      • Chen R.
      Biomechanical properties of prolapsed vaginal tissue in pre- and postmenopausal women.
      • Cosson M.
      • Lambaudie E.
      • Boukerrou M.
      • Lobry P.
      • Crepin G.
      • Ego A.
      A biomechanical study of the strength of vaginal tissues: results on 16 postmenopausal patients presenting with genital prolapse.
      In addition, data from this study were obtained from a rabbit model using a relatively small sample size. Rabbits possess a high collagenolytic activity and, therefore, may degrade PS grafts more rapidly than do human beings.
      • Claerhout F.
      • Verbist G.
      • Verbeken E.
      • Konstantinovic M.
      • De Ridder D.
      • Deprest J.
      Fate of collagen-based implants used in pelvic floor surgery: a 2-year follow-up study in a rabbit model.
      However, emerging clinical evidence indicates that cross-linked PS also degrades in women by a similar foreign body reaction.
      • Gandhi S.
      • Kubba L.M.
      • Abramov Y.
      • et al.
      Histopathologic changes of porcine dermis xenografts for transvaginal suburethral slings.
      In summary, accumulating scientific and clinical evidence suggests that cross-linked PS grafts are likely inferior to lightweight PP meshes for pelvic organ prolapse and incontinence procedures. PP induces a more uniform response and has better elasticity after implantation compared with PS. Perforated cross-linked PS may undergo long-term degradation by a foreign body response as previously described for nonperforated cross-linked PS,
      • Claerhout F.
      • Verbist G.
      • Verbeken E.
      • Konstantinovic M.
      • De Ridder D.
      • Deprest J.
      Fate of collagen-based implants used in pelvic floor surgery: a 2-year follow-up study in a rabbit model.
      • Gandhi S.
      • Kubba L.M.
      • Abramov Y.
      • et al.
      Histopathologic changes of porcine dermis xenografts for transvaginal suburethral slings.
      which may lead to compromised biomechanical properties of grafts and unacceptable surgical failure rates in women. Clinical studies are needed to investigate the long-term fate of cross-linked xenografts in women undergoing urogynecologic surgery.

      Acknowledgments

      We acknowledge the unconditional donation of Gynemesh Nonabsorbable Prolene Soft Mesh by Ethicon Inc, Somerville, NJ, and PelviSoft Acellular Collagen BioMesh by C.R. Bard Inc, Covington, GA. We also thank the Scott & White Hospital animal care staff for their assistance in management of anesthesia and husbandry of the rabbits used in this study.

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