Elongation of textile pelvic floor implants under load is related to complete loss of effective porosity, thereby favoring incorporation in scar plates € hl,4 Uwe Klinge1 Jens Otto,1 E. Kaldenhoff,2 R. Kirschner-Hermanns,3 Thomas Mu 1
Department for General, Visceral and Transplant Surgery at the University Hospital of the RWTH Aachen, Germany Department of Urology at the University Hospital of the RWTH Aachen, Germany 3 Department of Neuro Urology at the University Hospital of the Rheinische Friedrich-Wilhelms University of Bonn, Germany 4 Laboratory for Electrical Measurement Technique at the FH Aachen University of Applied Sciences, Germany 2
Received 14 December 2012; accepted 17 April 2013 Published online 11 June 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.a.34767 Abstract: Use of textile structures for reinforcement of pelvic floor structures has to consider mechanical forces to the implant, which are quite different to the tension free conditions of the abdominal wall. Thus, biomechanical analysis of textile devices has to include the impact of strain on stretchability and effective porosity. ProliftV and Prolift 1 MV, developed for tension free conditions, were tested by measuring stretchability and effective porosity applying mechanical strain. For comparison, we used Dynamesh-PR4V, which was designed for pelvic floor repair to withstand mechanical strain. ProliftV at rest showed moderate porosity with little stretchability but complete loss of effective porosity at strain of 4.9 N/cm. Prolift 1 MV revealed an increased porosity at rest, but at strain showed high stretchability, with subsequent loss of effective porosity at strain of 2.5 N/cm. Dynamesh R
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PR4V preserved its high porosity even under strain, but as consequence of limited stretchability. Though in tension free conditions ProliftV and Prolift 1 MV can be considered as large pore class I meshes, application of mechanical strain rapidly lead to collapse of pores. The loss of porosity at mechanical stress can be prevented by constructions with high structural stability. Assessment of porosity under strain was found helpful to define requirements for pelvic floor devices. Clinical studies have to prove whether devices with high porosity as well as high structural stability can improve the C 2013 Wiley Periodicals, Inc. J Biomed Mater patients’ outcome. V R
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Res Part A: 102A: 1079–1084, 2014.
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Key Words: effective porosity, pelvic floor repair, mesh implant, biomechanics, PP, PVDF
€ hl T, Klinge U. 2014. Elongation of textile pelvic floor How to cite this article: Otto J, Kaldenhoff E, Kirschner-Hermanns R, Mu implants under load is related to complete loss of effective porosity, thereby favoring incorporation in scar plates. J Biomed Mater Res Part A 2014:102A:1079–1084.
INTRODUCTION
After introduction of synthetic textiles for the reinforcement of pelvic floor structures as slings or flat meshes, mainly in the 1990s, several enthusiastic reports rapidly could confirm the feasibility of these techniques. Unfortunately most of these early studies based on short-follow-up results with complication rates that probably could not reflect long-term outcome as well. However, an increasing number of reported serious complications like acute and chronic infection, tissue contraction due to mesh shrinkage, erosion or exposure of the mesh into adjacent structures, and dyspareunia limiting sexual activity1 initiated a controversial debate, ending up in a warning of the FDA2,3 and not least the withdraw from sale of two commonly used devices, the ProliftV - and Prolift 1 MV system by Johnson&JohnsonV this summer. Although it should be undisputable that some serious events may be caused by inappropriate technique of the R
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surgeon or by severely impairment of the patients’ biology, an impact of the device cannot be excluded in all cases. Already in 2007 Gomelsky and Dmochowski in this journal presented a review on biocompatibility assessment of synthetic materials4 pointing out that many textile structures had been developed for the tension free condition of abdominal wall hernias and not for pelvic floor repair, which may not be considered tension free. Correspondingly, the textile structure of the ProliftV system, the Gynemesh PSV (GMPS) is identical to the Prolene Soft MeshV for hernia repair, and the textile structure of the Prolift 1 MV (GMM) system is copied from the UltraproV hernia mesh.5 Both meshes can be classified as large pore class I mesh with an effective porosity that is sufficient to prevent bridging of scar tissue throughout the entire pore.6 However, as this may change under tension, in the present study we tested the biomechanical properties of the devices in response to mechanical strain, with special regard to elongation and R
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Correspondence to: J. Otto; e-mail:
[email protected]
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[Fig. 1(a)]. The blue-marking stripes indicate the warp direction, the main course of the warp filaments in the textile structure. After implantation, the warp direction runs perpendicular to the vaginal axis. Although the fixation arms are not in the course of the warp direction nor perpendicular to these and the entire structure is punched out of a large peace of mesh, the course of the filaments differs remarkable in the fixation arms, mainly in arm three. All three arms were measured in rest as well as under load. The center part was only measured in rest, due to the defined probe dimensions under load conditions. The measurements under uniaxial load were made with a Gynemesh PSV (10 cm 3 15 cm), according to the identical structure as described in the FDA 510(k) documents. The shape and geometry of the Gynemesh MV mesh used in the Prolift 1 MV system [Ethicon, Hamburg, Germany; Fig. 1(b)] are quite similar to the Gynemesh PS mesh from the ProliftV system. However, the used mesh is identical to the UltraproV hernia mesh. Most of the pores are far larger than 1 mm [Fig. 1(b)], which makes the mesh more stretchable in both the directions. It was constructed of polypropylene monofilaments and absorbable monocryl monofilaments, so that the amount of material and thus the stability of the structure decreases and the stretchability increases parallel to the degradation of the monocryl. Although the weight of the mesh decreases to only 58% after resorption of the monocryl, the effective porosity did not increase remarkable. In the current study, we used the device as it is used for implantation, and thus cannot exclude any further change, once the monocryl has disappeared. The Dynamesh-PR4V [FEG-Textiltechnik Aachen, Germany; Fig. 1(c)] includes a center part and two arms on both sides. In contrast to the previous meshes, this structure with its arms was not cut out of a larger piece of mesh but was knitted in its present form, showing smooth selvedges at the fixation arms. Like the other systems, the warp direction of the structure is situated after implantation perpendicular to the vaginal axis. In contrast to the other structures, the fixation arms are in the course of the warp direction, thus showing the highest resistance to elongation in the direction of loading during implantation. The concept has been focused on large pores [Fig. 1(c)] as well as limited elongation and deformation of the pores even under load. The mesh is made of monofilament polyvinyl fluoride, claiming a higherbiocompatibility and biostability than polypropylene.7 According to the description of the manufacturer DynaMesh-PR4V mesh is the same mesh as DynaMesh-PRV; therefore, for strain measurements of the small center part, a DynaMesh-PRV mesh (10 cm 3 15 cm) was taken. Because of the identical structure and form of the both arms, only one arm was tested. R
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FIGURE 1. a, Prolift device as compound of a central flat mesh area with three arms on each side for mesh placement and fixation. The V blue markings indicate the course of the warp fibers. b, Prolift 1 M device as compound of a central flat mesh area with three arms on each side for mesh placement and fixation. The blue markings indiV cate the course of the warp fibers. c, Dynamesh-PR4 is made of monofilament polyvinyl fluoride with a center and two arms on both sides. Warp fibers are running in line with the main direction of the arms. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] R
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effective porosity. For comparison we used the DynameshPR4V PVDF-mesh (DMPR), as it was specifically designed for the use in the pelvic floor area with large pores and pronounced resistance to mechanical stress. R
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MATERIALS AND METHODS
The three different devices used in this study represent three different concepts of textile construction and thus stretchability and structural stability: The Gynemesh PS/Prolene Soft Mesh of the ProliftV device [Ethicon, Hamburg, Germany; Fig. 1(a)], made out of polypropylene monofilaments, consists of a flat mesh area in the center, and three different fixation arms on both sides. Every pore was additionally crossed by two monofilaments, which enhances the isotropy of the mesh properties, reduces the pore size and makes it less stretchable R
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Measurement setup The measurement setup consisted of a probe-fixing device, a position control unit, a probe illumination, a camera system, a control and evaluation unit, the mechanical load by weights, and an evaluation software, which determines the
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textile and effective porosity of the probe. According to Conze et al.,8 for polypropylene filaments, a minimum distance of 1000 lm between filaments was set for the calculation of the effective porosity and a minimum distance of 600 mm was used in case of using polyvinyl fluoride. The measurement setup was periodically tested with a perforated plate, and the results were compared with mechanical measurements to ensure reproducible and reliable results. On this basis, measurement uncertainties of the absolute values were estimated: probe length 60.5 mm, probe width 60.2 mm, and porosity 63%. In addition, other influencing factors were filament transparency, probe position, and probe flatness. The value for the effective porosity was dependant of the probe section and was sensible to small changes in mesh production and probe handling. Therefore, six measurements of each mesh center were carried out to evaluate the reproducibility of the porosity results. Mechanical load to the center part was applied by weights of 500 g, 1000 g, and 2000 g. Considering the 4 cm width of the samples this corresponded to 1.23 N/cm, 2.45 N/cm, and 4.90 N/cm. Initial sample length between the clamps was 8 cm for all measurements. Loading of the fixation arms was done by weights of 250 g, 500 g, and 1000 g. Considering the different width of the arms, ranging from 12 mm (DMPR) to 19 mm (GMPS and GMM), this corresponded to varying values of N/cm and in general a higher loading of the smaller DMPR fixation arms. The loading was applied to the probes of the center parts either in warp direction or perpendicular to the warp direction. In the arms due to the various courses of the fibers the force was applied only in longitudinal direction of the arms, corresponding to the loading direction during implantation. RESULTS R
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ProliftV system: Gynemesh PSV/Prolene Soft MeshV The central mesh part of the Gynemesh PSV/Prolene SoftV showed the least effective porosity of 24.3% at rest and a moderate elongation of 14.5% in the warp direction and 22% perpendicular to the warp direction under the maximum load of 4.9N/cm. Correspondingly, the narrowing under load was more pronounced in the perpendicular to warp direction, with a discreet rolling in at the probe edges (Fig. 2). Effective porosity under the maximum load decreased to 18% in warp direction and completely vanished perpendicular to these direction. In comparison with the center the arms showed different behavior under load, due to the various fiber orientations. Any effective porosity disappeared under the loading with 1000 g in arm 1 and arm 2, and in arm 3 already at 500 g. The elongation for arm 1–3 at the maximum load reached 22%, 25%, or 27%, respectively. Arm 1 and arm 2 shows similar narrowing under load with a remarkable rolling in while the loading of arm 3 yields a total collapse of all pores and a rope like appearance. Overall, in the flat mesh area the GMPS showed only slight anisotropy in terms of elongation and effective R
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porosity and a remarkable deformation of the arms under load (Fig. 3). R
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Prolift 1 MV system: Gynemesh MV/UltraproV Without any strain the effective porosity was 57.5%, and the majority of pores had a diameter of larger than 1000 mm. The effective porosity almost disappeared already at slight loading of 2.5 N/cm regardless whether applied in warp or perpendicular to warp direction. The elongation was markedly more in the perpendicular to warp direction and reached 74.3% at 4.9 N/cm. In contrast in the warp direction, the mesh only showed an elongation of 19.1% at a load of 4.9 N/cm (Fig. 2). Effective porosity in all three arms disappeared already under a mechanical loading with 250 g accompanied by an elongation of 10.9%, 10.4%, and 9.5% for arms 1–3. Under the load of 1000 g the elongation in the arms 1–3 rises to 20%, 17.9%, and 20.6% and shows a high strangulation and single fiber tips pointing outwards the tape (Fig. 3). Overall, the textile showed marked anisotropy with highest elongation if stressed perpendicular to the warp direction, predominantly seen in the analysis of the flat mesh area. The fixation arms show low form stability and a total collapse of the effective pores even under low load. However, as the different pattern of the blue markings indicate considerable differences in structure, it cannot be excluded that these values differ furthermore in the various sections of the arms. R
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Dynamesh PR4V/Dynamesh PRV Without any strain the effective porosity of the center of Dynamesh PR4V was 56.5 3 1.2%. The effective porosity did not decline under strain for up to 4.9 N/cm, and maintained a high level of more than 50%. Furthermore, the mesh showed only little elongation at a load of 4.9 N/cm with stretching of 4% in warp direction and 9% in perpendicular direction (Fig. 2). No observable narrowing of the mesh under load reflected this high form stability. The fixation arms, showed a similar behavior under strain when compared to the warp direction of the center. With an elongation of only 4% at a load of 4.9 N/cm effective porosity of the arm remained widely constant with 55%. Due to the small width of only 12 mm the loading with the weight of 1000 g yields the highest force of 8.2 N/cm. Even under these load condition the arms stay flat and shows the highest structural stability and no signs of rolling in (Fig. 3). Overall the textile showed a marked structural stability with only little elongation and no loss of porosity under strain. Similar results for the two directions of the fibers reflect the considerable isotropy of the textile characteristics (Table I). R
DISCUSSION
The present characterization of the three devices with measuring effective porosity under strain unravels the different concepts of their textile construction. Porosity has been identified to be decisive for proper tissue integration.9,10 In small pores, the foreign body reaction bridges
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FIGURE 2. Illustration of the anisotropic elasticity of the central part of Prolift , Prolift 1 M , and Dynamesh PR4 at rest (0 N/cm) and under strain (4.9 N/cm). a, Elongation (%) when strain was applied in warp direction. b, Elongation (%) when strain was applied perpendicular to warp direction. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
from one filament to the other and leads to pores completely filled with scar, whereas in structures with sufficiently large pores local physiologically regenerated tissue, for example, fat can be seen. Large pores with a diameter of more than 1000 mm for polypropylene or 600 mm for
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polyvinyl fluoride have been found to prevent the undesirable bridging of scar tissue throughout the pore.8 The corresponding pore sizes of a device can be characterized by the concept of effective porosity developed by M€ uhl et al.,11 which for the first time considers the geometry of the pores
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FIGURE 3. Illustration of the anisotropic elasticity of textile arms of Prolift , Prolift 1 M , and Dynamesh PR4 at rest (0 N/cm) and under strain (4.9 N/cm) in warp direction.
and is able to identify the “good” pore area as area with sufficiently large pores. The Gynemesh PSV/Prolene Soft MeshV used in the ProliftV system was a device with moderate stretchability and effective porosity, however, showed rapidly reduction of effective porosity under strain. The Gynemesh MV/UltraproV mesh in the Prolift -MV system has increased porosity at rest, and showed a higher stretchability, but only if the load was applied perpendicular to the warp direction as any elongation derived from deformation of pores, consecutively a collapse of pores even at low mechanical load was found. These devices once have been developed for reinforcement of tissues under tension free condition of the abdominal wall. Correspondingly, without mechanical stress they can be classified as large pore devices (class I)6 with little risk to get embedded in a scar plate. However, this changes significantly when mechanical stress is applied. Already at low loads of less than 5 N/cm the pores collapsed. As the effective porosity disappeared, the meshes now have to be grouped as small pore meshes (class II) with inherent risk for bridging of scar tissue and higher risk for shrinkage or folding of the textile structure. In contrast to these two devices, which meanwhile are withdrawn from the market, the Dynamesh PR4V construction preserved its high R
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porosity and structural stability even under strain, though with the consequence of limited stretchability. These conceptual differences were already apparent in the flat mesh center of the device but even more prominent in the sling like structure of the arms. The variable and dynamic impact of tissue ingrowth or a deteriorated stability under dynamic load12 make it even more difficult to assess precisely the biomechanical forces with its subsequent deformation of the device correctly. However, there are some assumptions that may help to define the requirements for a textile pelvic floor device. Whereas a limited stability of the textile structure, not necessarily exceeding 16–20 N/cm13,14 may be widely acceptable, the demand for elasticity is still under discussion. In general, stretchability is assumed to be in a range of 20– 100%, depending on the recipient tissue.13,15 However, any device with a high elasticity may lead to less resistance to withstand a prolapse. In particular, if intended to replace the ligaments, a high stretch ability seems to be dispensable, and thus may force the subsequent loss of effective porosity, as we could confirm in this study. Nevertheless, perfect mimicking of the biomechanical properties of tissues by textile implants is difficult, and has to consider that almost all knitted textiles show some
TABLE I. Analysis of the Central Part of the Meshes in Regard to Textile Porosity, Effective Porosity, and Elongation (%) in Rest and at Strain (500 g, 1000 g, and 2000 g), Applied Either in Warp or in Cross Direction Prolift Center Strain (g)
Warp Direction
Cross Direction
Textile porosity/effective porosity/elongation (%) 0 63.3/26.4/0 63.2/24.3/0 500 61.6/23.3/3.6 62.7/25.7/6.4 1000 60.7/20.7/7.9 58.2/18.0/12.7 2000 57.3/18.0/14.5 49.9/0/22.0
Prolift 1 M Center
Dynamesh PR4 center
Warp Direction
Cross Direction
Warp Direction
Cross Direction
64.3/57.2/0 49.5/8.9/11.8 41.7/0/15.2 33.8/0/19.1
63.5/57.7/0 46.8/0.2/47.2 23.5/0/61.6 9.0/0/74.3
59.7/52.7/0 59.7/51.8/1.1 59.9/52.8/2.9 59.9/52.5/3.5
61.0/52.4/0 61.3/52.0/2.2 61.0/51.8/2.2 62.7/54.7/8.9
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anisotropy. Most knitted textiles made by either warp knitting (GMP) or chrochet technique (GMM and DMP) exhibit low elongation in warp direction and an up to 10 times higher strechability perpendicular to this direction, depending on the textile construction and the material used. If strain is applied in the direction perpendicular to the warp course, then as seen with the GMM, an enormous elongation is possible, which mainly results from a slit-like deformation of the large pores, build by the single knits.11 Any attempt to stabilize the pores by shoring cross-filaments as was realized for GMP reduces the pore diameter with the effective porosity, and enhances the risk of scarry bridging. Thus, if a tension condition for the proper function of a textile device has to be expected, the structure should provide sufficient structural stability that it prevents collapse of pores. This can be done by the size of the filaments and the type of knitting as realized with the PVDF mesh. In particular, in case of highly anisotropic meshes, the orientation of the mesh in vivo has to consider that stretchability may be seen only in one direction.16 The anisotropy of the textile structures usually is not reflected by uniaxial testing, but may be estimated by performing the tests in two perpendicular directions.17 The present study demonstrates that even uniaxial strain in the two directions together with the analysis of the highly relevant effective porosity is able to give a precise textile characterization.18 A further consequence of elongation of a textile due to deformation of the pores is a narrowing of the width combined with an enrolment of the borders. Even initially wide and flexible arms then become rigid and slim, and may be followed by an increased potential for erosion and cutting into the adjacent tissue. Therefore, devices that may be submitted to tension should provide a sufficient stability to avoid enrolment or folding if charged by tensile stress. Maybe, it already is the strain applied during placement that is responsible for the doubling of the textile, which can frequently be seen when investigating explanted meshes histologically.19,20 The relevance of structural stability of course has to be defined by further studies, but there is a undisputable need for the adoption of the textile characteristic of pelvic floor implants to the special requirements of application and technique, not necessarily following the tension free conditions of hernia surgery. However, the present study offers a first characterization of textile pelvic floor implants under unixial mechanical stress, which may allow a better relation to clinical results as standard testing of effective porosity without any tension can do,17 and in future might help to improve devices for this specific surgical procedure.
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