A Medline search was conducted on October 2018 using the following as subject headings, keywords, and text words: (stress urinary incontinence OR pelvic organ prolapse) AND (surgical mesh OR polypropylene) AND (infection OR wound infection OR post-operative complications OR intraoperative complications). No time limits were applied to the search. A total of 168 abstracts were retrieved. All relevant articles were included. In addition, reference lists of selected manuscripts were checked manually for eligible articles.
With regard to infectious complications of transvaginal mesh surgeries, the most recent PROSPECT trial [ 22 ] demonstrated that the rate of infectious complications with the vaginal mesh was less than 1% [ 22 ], although higher rates have been reported of up to 8% [ 9 ]. However, in a series of mesh explantation surgeries after treatment of SUI and POP, mesh exposure without signs of infection was responsible for 44 of 83 cases, with 30 of 84 meshes excised due to infection [ 23 ].
Hence, although the transvaginal route has been the most commonly used route for POP repair, the safety of mesh augmented transvaginal POP repair procedures is now widely questioned with a mesh erosion rate of 8% in 1–3 year follow-up and which can go up to 42% in longer term follow-up [ 19 ]. There appears to be a consensus on lack of safety with transvaginal mesh implantation for POP. In contrast, currently, tension-free vaginal tape procedures for SUI have long-term subjective cure rates of up to 93% [ 20 ] with mesh-related complications occurring in 4% of patients [ 21 ]. Current expert opinion suggests that the benefits of these operations still outweigh the risks with a high level of evidence.
The transvaginal mesh tape insertions for SUI are slightly different than other transvaginal mesh insertions, because in these operations, a smaller surface area of the mesh lies in close proximation to the vaginal skin, but maybe more importantly, the theoretical basis for use of the mesh for SUI is better studied with better defined targets for surgical treatment. For example, placement of a synthetic tape underneath the mid urethra was conceptualized with the introduction of mid-urethral sling surgeries with demonstration of pre- and post-operative urethral pressures.
In transvaginal POP repair procedures, the vaginal support structures, mainly at level II, are attached to stronger ligaments in the pelvic floor (e.g., sacrospinous or uterosacral ligaments) or are augmented with a suture repair in pubocervical or rectovaginal fascia (anterior and posterior colporrhaphy procedures). This fascia is mainly composed of smooth muscle and collagen/elastin, which are the active biomechanical components of the pelvic floor that are probably subjected to not well-defined multidimensional forces. More importantly, during these operations, the mesh traverses a clean contaminated surgical field which increases the chances of contamination. In addition, transvaginal repairs are essentially mesh ‘onlay’ procedures, particularly anterior and posterior colporraphy procedures, which make them prone to colonization by vaginal microbial flora, as they lie very close to the skin [ 18 ].
For transabdominal implantations, namely, abdominal sacrocolpo(histero)pexy operations, the mesh material is used to attach the apex of the vagina or uterus to the sacrum, replacing defective or weak cardinal-uterosacral ligaments constituting level I support structures [ 13 ]. These ligaments are thick and strong collagenous fibres extending both vertically and posteriorly towards the sacrum, meaning that it is not necessarily flexible, but strong in the vertical direction which matches with the mechanical properties of the surgical mesh. In addition, in these operations, the mesh does not traverse a clean contaminated surgical field and it does not lie in close proximation to skin. Thus, the mesh in abdominal implantations is biomechanically more fit for purpose for this application and the chances of contamination during implantation is less compared to vaginal implantations. The success of transabdominal repairs is very good at 97–100% [ 14 ], although mesh erosion still occurring in up to 6% by 2 years [ 15 ] and 10% in 7 year follow-up [ 16 ]. Mesh infection rates are also thought to occur less in abdominal POP repairs compared to vaginal POP repairs, since the first approach avoids contamination of the mesh during insertion [ 17 ]. Furthermore, avoiding a hysterectomy during abdominal sacrocolpopexies is recommended to reduce likelihood of mesh complications by preventing the contact of the mesh with vaginal microbial flora.
Vaginal mesh is used in urogyneacological surgeries mainly to treat SUI and POP. It is used in the female pelvic floor in three main ways: transvaginal treatment for SUI, transabdominal repair of POP, and transvaginal repair of POP.
Occurrence of vaginal mesh-related complications, as we see in the daily clinical practice, are probably multifactorial including the inherent complexity of pelvic floor disorders that are still not incompletely understood [24], the material and biomechanical properties of the mesh being unsuitable for use in pelvic floor, limitations pertinent to the surgical techniques used, and failure of regulatory processes for approval and surveillance of implantable medical devices.
Infectious complications of the vaginal mesh can be thought of as a clinical entity with specific signs and positive culture results, but also it can be a subclinical infection affecting the normal host response to the mesh and its’ tissue integration. Alternatively, we can observe complications associated with an inflammatory reaction to the mesh material with a completely sterile mesh without infection. In this section, we will review the available evidence on the host tissue response to the PPL mesh and how this could relate to clinical outcomes.
Surgical mesh became available as a material after the plastics revolution and started to be used in hernia repair [4]. Plastic materials provided significant advantages over the metal prosthesis, the only available alternative then used in soft-tissue reconstruction because of their better ductility and strength. Plastics, however, came with a new set of material properties that was initially problematic when used with traditional material design strategies and available surgical techniques of implantation. Some of these properties needed to be optimised over the years to obtain the best treatment outcomes [5]. These improvements were made in the context of hernia surgeries over 50 years before mesh was used in pelvic floor repair.
The biocompatibility of the mesh is mainly determined by its textile properties, namely, the porosity and the pore size. Lighter weight meshes with large pores are known to integrate better into host tissues with less foreign body reaction, fibrosis, and the associated pain sensation [25, 26]. Clinical studies comparing heavy and light-weight PPL mesh materials implanted for inguinal hernia repairs demonstrated less pain and less sensation of a foreign material with lighter meshes [27]. However, lighter weight meshes are more flexible which caused effective loss of pores after mechanical loading in vivo and this led to some issues for the definition of pore size and pore stability [28]. Prolapse meshes are thought to be more likely to lose their pores after implantation in vivo compared to hernia meshes Auxetic materials have been developed for use in prolapse repair [29]; however, their efficacy in reducing mesh-related complications is yet to be explored.
It has also been demonstrated repeatedly that the type of the mesh material affects its biocompatibility. Meshes made of polyester or polytetrafluoroethylene (PTFE) are known to be more susceptible to bacterial colonization, and efforts have been focused on the improvement of their antibacterial properties. For example, PTFE has been modified to release two antimicrobial molecules (silver salts and chlorhexidine) used in vaginal implantations in a small series [30]. However, it is widely accepted today that implantations through the vaginal route increase the risk of contamination and that the best material is monofilament macroporous PPL for this application [31].
In case of the PPL mesh, the host response has traditionally been studied for applications in abdominal hernia repair with recent evidence focused on vaginal implantations. The PPL mesh is known to trigger an inflammatory response characterized by polarization of macrophages towards an M1 phenotype, as opposed to M2. An M1 phenotype leads to a pro-inflammatory response, while an M2 results in a constructive remodelling response [32]. In addition, the M1/M2 ratio has been shown to be less favourable with increased molecular weight PP mesh and with smaller pore sizes, suggesting that the mesh burden (the amount of mesh in contact with tissues) is a factor influencing its biocompatibility [33].
Biocompatibility is defined for each specific application of a biomaterial as its ability to perform with an appropriate host response [34]. The biocompatibility of the PPL mesh for applications in the pelvic floor started to being defined after 2007 in the sheep [35]. The sheep have a vagina that is similar in size to the human vagina allowing larger pieces of the mesh to be implanted and they can spontaneously develop POP. A site-specific host response to PPL mesh in sheep models has demonstrated mesh-related complications (exposure and contraction) to occur significantly more in transvaginal mesh implantations as compared to abdominal implantations, where the same materials caused less than 10% contraction and no erosion [36]. Later on, clinical data from women who underwent vaginal mesh excision due to complications revealed an M1 (pro-inflammatory) macrophage response even years after the implantation of mesh, with a higher expression of proteolytic enzymes in explants of women who had mesh exposure compared to women with pain [37].
The events in the tissue–material interface leading to device-related infections are well studied. Initially, the microorganisms attach and adhere to the surface of the material via physicochemical interactions including Van der Waals forces, hydrophobic, and electrical interactions. Microorganisms can also attach on to the proteins adsorbed on the surface of the material. After attachment, microorganisms proliferate and form multi-layered clusters via specific intercellular adhesion polysaccharides [38]. The presence of such accumulated biofilms has been demonstrated on several implanted devices including these surgical meshes [39].
The presence of a mesh-related infection can modify the host tissue response to the implanted material [40]. As soon as a biomaterial is implanted, a ‘race to surface’ begins between the host cells and the microorganisms. A biomaterial-associated infection will affect the integration of the implant into the host. Although it is easy to distinguish between an implant which is clinically infected and a successfully integrated implant, it is not so easy to detect low levels of infection in an implant. Furthermore, this situation is a dynamic process that can change over years.
Bacteria generally form biofilms on the surface of biomedical implants. Biofilms are aggregates of bacteria with a surrounding extracellular matrix (extracellular polymeric substances) that is tightly attached to the biomaterial surface. Bacteria in biofilms are resistant to antimicrobial therapies and they can easily evade the host immune responses giving rise to a state of chronic inflammation [41]. The relationship between microbial biofilms and capsular contraction with breast implants has been extensively studied in pre-clinical and clinical studies, which is reviewed elsewhere [42]. It appears that biofilm formation is an acknowledged factor increasing the occurrence of capsular contraction. Although the mechanisms underlying mesh contraction by the host tissues are not clear, it can be argued that bacterial colonization of the vaginal mesh can affect the host response against the mesh and can contribute to mesh contraction in the absence of obvious signs of infection.
At the time of writing, there have not been enough studies reported to support or refute this hypothesis. Histological analysis of 100-explanted meshes revealed a periprosthetic tissue reaction identical to that of a periprosthetic abscess, regardless of an infectious cause of mesh explantation, and/or a chronic inflammation rich in giant cells and mononuclear cells [43]. However, experimental studies in rats have demonstrated that both absorbable and non-absorbable meshes shrink more when they are infected [44].
In conclusion, for any given synthetic implant, there will be a host response. The ideal situation is that the biomaterial and the host tissues can find a state of mutually acceptable co-existence.
SUI and POP commonly occur together due to challenges which the female pelvic floor must cope with. A combination of genetic and acquired factors that are most probably aggravated by childbirth lead to the occurrence of SUI and POP. Although the exact mechanisms by which an interaction of these factors results in pelvic floor disorders are not completely elucidated, the clinical picture involves initial mechanical damage to the pelvic floor that generally follows birth trauma, previous pelvic surgeries, menopause, and increasing age. Current surgical treatments for SUI and POP are based on restoration of normal anatomy in the female pelvic floor either augmented by mesh or not.
PPL mesh was first designed for use in the treatment of abdominal hernia. For its use in this application, the material properties of the mesh and the surgical technique of implantation developed hand by hand over years to obtain best outcomes for hernia repairs. For example, in incisional hernia repair, onlay mesh repairs were replaced with sublay repairs, where the mesh is placed underneath a thick muscle tissue (retro-rectus) in a well-vascularized wound bed and away from the skin. Onlay mesh repairs required a large area of the mesh to stay in very close proximity to skin increasing the chances of mesh colonization and infection [45].
When adopting the mesh for vaginal mesh implantations, the design requirements for specific application in the pelvic floor were not considered. In vaginal mesh implantations, the mesh stays in very close proximation to the vaginal mucosa, as there are no natural tissue planes in this region such as subcutaneous or muscle tissue layers, unlike in abdominal implantations, where the mesh material is implanted in-between clearly identifiable fat, muscle, and fascia tissue planes.
Furthermore, there are other observations supporting the argument that an inflammatory reaction to the mesh or a subclinical infection caused by the introduction of the mesh may contribute to the occurrence of mesh-related complications. It has been repeatedly demonstrated that vaginal mesh complications are known to increase with increasing amounts of mesh used [36, 46]. In addition, clinical studies showed that avoiding an overlapping suture line during mesh implantation reduces mesh exposure [47, 48]. In addition, mesh exposure mostly occurs in the midline, suggesting a poor wound healing affected by the presence of the mesh [49]. Taken all together, this implies that mesh erosion can be followed from an abnormal wound healing of the incised vaginal mucosa due to a poorly vascularized wound bed combined with the surgical intervention and the presence of large amount of mesh material.
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