Ventral Hernia Repair with Synthetic,Composite,and Biologic Mesh: Characteristics,Indications,and Infection Profile

Abstract

Background:

A variety of mesh materials are available for ventral hernia repair (VHR), each with a unique set of characteristics. Surgeons are offered an ever-expanding selection of products, making a review of the available materials timely.

Methods:

Current surgical literature is reviewed to describe the different types of synthetic mesh, the indications for the use of each type, their relative risks of infection, and other benefits and shortcomings. We also review clinical studies demonstrating outcomes, efficacy, and use of the meshes in different surgical settings, including laparoscopic hernia repair and special situations such as infection or large abdominal wall defects.

Results:

Three main types of prosthetic mesh are available. Synthetic mesh, such as polypropylene (PP) or polyester, is characterized by high tensile strength and vigorous tissue ingrowth, but is unsuitable for intra-abdominal placement because of its tendency to induce bowel adhesions. Composite, or barrier-coated, mesh is a dual-sided prosthetic having a synthetic parietal side to promote a strong repair and a visceral surface that repels tissue ingrowth and decreases adhesion formation. Biologic mesh is a collagen-based human, porcine, or bovine scaffold that may be implanted in the extra- or intra-peritoneal position. Biologic mesh is used frequently in the setting of infected or contaminated surgical incisions.

Conclusions:

Synthetic PP mesh is an appropriate, durable material for extra-peritoneal placement in uncomplicated, clean VHR. Expanded polytetrafluoroethylene and composite meshes are suitable for intraperitoneal placement during laparoscopic VHR. Biologic meshes may be appropriate for contaminated fields or other special situations, but there is no consensus on when or how to use them.

More than 100,000 ventral hernia repairs (VHRs) are performed annually in the United States [1]. Such repairs may be performed with or without mesh, and via an open or laparoscopic technique. Simple repairs and repairs involving separation of the components of the abdominal wall to achieve fascial re-approximation without mesh are appropriate in patients with small defects, and in those without factors predisposing to hernia recurrence, such as obesity, smoking, corticosteroid use, or advanced age [2]. However, in patients with large abdominal wall defects, or in primary repairs that would result in tension on the closure, use of a prosthetic mesh is indicated.

Over time, VHRs have evolved from predominantly primary suture repairs to tension-free repairs with mesh. Open suture repairs are associated with recurrence rates as high as 63% at 10 years' follow-up [3]. The advent of mesh repair has led to a significant decrease in this unacceptably high recurrence rate; one recent Cochrane meta-analysis of three studies demonstrated a pooled recurrence rate of 16% [4].

Despite a substantial decrease in the recurrence rate after tension-free repair, the use of mesh is not without its own complications. Infection is the most important and difficult to treat, typically necessitating explantation of the mesh and leaving the patient with a colonized abdominal wall defect [5 –7]. Laparoscopic mesh repair is associated with infection rates as high as 3.6%, whereas rates as high as 10% have been cited with open mesh repairs [8 –11]. Seroma formation, adhesions, chronic pain, erosion into bowel, fistula formation, and bowel obstruction are other known complications of VHR with mesh.

In the light of the known benefits and complications of mesh repair, along with the variety of prosthetic mesh materials now available, this review of the current surgical literature describes the different types of synthetic mesh used for VHR, the indications for the use of each type of mesh, their infectivity profiles, and their relative benefits and shortcomings.

Mesh Characteristics

The types of meshes used for VHR have evolved rapidly over the past century. Early use of metals such as silver, tantalum, and stainless steel has given way to lighter weight and more flexible synthetic meshes of polyester and polypropylene (PP). This in turn evolved into the increasing use of more biocompatible grafts, such as the composite meshes of coated polytetrafluoroethylene (PTFE) and coated PP, and biologic meshes, including autografts, allografts, and xenografts from human, bovine, and porcine sources.

The ideal characteristics of implantable mesh for VHR were first described in the early 1950s by Cumberland and Scales and refined in 1985 by Hamer-Hodges and Scott. The ideal mesh has eight key characteristics: It is noncarcinogenic, chemically inert, resistant to mechanical strain, capable of being sterilized, unresponsive to body and tissue fluids, able to limit foreign-body reaction, adaptable and modifiable to the size of the individual defect, and unlikely to cause an allergic reaction [12]. Later, with the advent of biologic mesh, three additional characteristics were described: Resistance to microbial infection, ability to provide a barrier to visceral adhesion formation, and the capacity to respond in a fashion similar to native tissue [13]. In short, an optimal mesh would be highly biocompatible, non-adhesiogenic, and resistant to infection.

Today's prosthetics may be classified broadly into three categories: Synthetic, composite, and biologic. Each type of mesh features many of the aforementioned characteristics, and the choice of prosthetic is determined by its likely performance in a particular clinical situation. Mesh composition, biodegradability, source, handling characteristics, and post-harvest processing all influence performance in vivo.

Synthetic Meshes

Several permanent synthetic meshes are used for VHR, of which monofilament PP is the most popular [14]. Introduced in the 1960s, PP is a carbon-based mono-, dual-, or multi-filament prosthetic that is pliable and resistant to biologic degradation [15]. Moreover, it is low cost, easy to handle, and incorporated well into tissue [16].

Polypropylene is available in both lightweight and heavyweight varieties. Heavyweight PP (such as monofilamentous Marlex® (C.R. Bard, Inc., Murray Hill, NJ) and bifilamentous Prolene® (Ethicon, Inc., Somerville, NJ) incites a more vigorous foreign-body and chronic inflammatory response, occasionally culminating in contracture, scar formation, loss of compliance, and chronic pain. Lightweight mesh (such as Vypro®; Ethicon) is thinner and has a larger pore size and a smaller amount of material than heavyweight mesh. It causes less host tissue reaction and results in greater patient comfort and pliability.

Lightweight mesh also displays elasticities and tensile strengths closer to the physiologic range. Normal strains on the abdominal wall, generated by activities such as coughing, lifting, and jumping, result in forces as high as 27 N/cm. Heavyweight PP meshes are engineered to withstand pressures greater than 100 N/cm, but are associated with paresthesias, a sensation of stiffness, and a perception of the mesh in the abdominal wall [16 –18]. Lightweight mesh, on the other hand, is less “over-engineered” and typically can withstand pressures as much as double that seen in physiologic settings without patient discomfort [18]. Despite these findings, heavyweight PP is the mesh implanted most frequently around the world; this is attributed to its ease of use and resultant feeling of a “strong” repair [18].

The high tensile strength and macroporous structure of many PP meshes allow fibrous scar tissue to surround their fibers, incorporating the mesh into the abdominal wall to form a well-integrated, durable repair. Unfortunately, these characteristics also induce scarring of intra-abdominal structures, resulting in dense abdominal adhesions and making PP unsuitable for cases in which mesh may be exposed to bowel. Adhesions, bowel obstruction, and fistula formation may result from intraperitoneal placement of PP mesh, and the foreign-body reaction at the implant site may cause mesh shrinkage and reduction of abdominal wall compliance. Additionally, PP mesh is vulnerable to bacterial inoculation. In several retrospective studies investigating VHR, infection was among the most common reasons for mesh explantation, suggesting that heavy-weight, multifilamentous PP should be used with caution, particularly in the setting of intra-abdominal contamination [19 –21].

Polyester mesh (technically poly[ethylene terephthalate] mesh) is a second type of synthetic mesh, engineered to have greater pliability and reduced adhesiogenic properties [22]. Similar to PP, polyester is a non-absorbable carbon-based polymer, but it is less susceptible to oxidative stress and less likely to contract after implantation [23]. Common polyester meshes are Dacron® (DePoy Intl., Leeds, United Kingdom, and others), introduced in the 1950s, and Mersilene® (Ethicon), a macroporous mesh consisting of interlocking Dacron fibers, introduced in the 1960s [24].

It is unclear whether polyester meshes offer an advance over traditional PP mesh. One frequently cited study from 1998 found that Mersilene was associated with higher rates of bowel obstruction, fistulization, and recurrence than mesh repairs utilizing PP [25]. However, complete omental coverage of bowel resulting in bowel protection from the mesh was clearly accomplished in only 24% of patients in this study, perhaps encouraging complications [25]. A later study, published in 2007, demonstrated equivalent 10-year outcomes for Mersilene and Prolene in a Rives-Stoppa hernia repair, in which the prosthesis is placed between the rectus abdominis muscle and the posterior sheath [26]. These authors concluded that because there is no direct contact of the mesh with the abdominal viscera, there is a reduced risk of complications with either mesh.

The risk of infection with polyester mesh may be less than that with PP mesh. A recent in vivo study of six frequently applied surgical meshes contaminated with Staphylococcus aureus demonstrated that polyester mesh showed a relatively low infection rate compared with PP [27]. A second group of investigators reviewed unprotected polyester mesh repairs retrospectively and concluded that there was no greater risk of post-operative infections or fistula formation [28]. These findings have been replicated by other groups, suggesting that polyester mesh may be used for elective repair of incisional hernias with the expectation of a minimal risk of infection [29,30].

Expanded polytetrafluoroethylene (ePTFE) is a third type of synthetic, non-absorbable mesh. This inert, fluorocarbon-based polymer has a macroporous ventral side characterized by ridges and depressions, promoting tissue ingrowth and high tensile strength. The microporous visceral side is smooth and non-erosive, preventing adhesions and promoting resistance to infection, and can be placed safely in direct contact with the intestines. In addition, ePTFE is soft and flexible and generates minimal patient discomfort [31 –34]. Teflon® (multiple manufacturers under license from E.I. DuPont de Nemours and Company, Wilmington, DE) and GORE-TEX® (W.L. Gore & Associates, Wall Township, NJ) are two commonly employed ePTFE meshes.

First applied to abdominal wall defects in newborn infants, the use of ePTFE expanded quickly to adult patients with incisional hernias [35,36]. With the advent of laparoscopic hernia repair, ePTFE showed favorable results when placed in the intra-peritoneal position. Several studies demonstrated its safety and efficacy, with minimal rates of adhesion or enterocutaneous fistula formation [10,37 –40]. However, higher rates of hernia recurrence were observed at the interface of ePTFE and fascia; this has been attributed to the decreased inflammatory response elicited by ePTFE in comparison with PP and polyester [41]. An additional shortcoming of ePTFE is that the microporous visceral layer reduces the accessibility of antimicrobial agents and host immune cells to bacteria. Indeed, as vascular ingrowth does not occur in any synthetic mesh, these repairs are susceptible to colonization and infection, as white blood cells and antibiotics are unable to reach the mesh [5]. This undesirable feature often leads to explantation of the synthetic mesh [6,7,9,36,42 –44]. In vivo studies support this finding as well. The morphology of the ePTFE mesh positively impacts the formation of a bacterial biofilm [45]. This, in combination with the niches in the mesh, generated higher rates of infection than are observed with polyester mesh [27]. Seroma formation is common with the use of ePTFE; this has been hypothesized by some to be associated with the documented higher infection rates [16].

Composite Meshes

Several recent studies documented the difficulty of re-operation in patients with intra-abdominal PP mesh. The risk of small-bowel resections, surgical site infections, and complicated perioperative courses was significantly higher in patients who had undergone intra-abdominal repairs with PP mesh than in those patients with pre-peritoneal mesh placement [46]. Another study found that the risk of unplanned enterotomy and small-bowel resection with its resultant increased lengths of stay, post-operative complications, return to the operating room, and risk of enterocutaneous fistula was higher in patients who had undergone mesh VHR [47].

Designed in response to these complications, and stimulated by the increasing popularity of laparoscopic VHR, with its requisite intra-abdominal mesh placement, composite mesh, also known as “second-generation” or “barrier” mesh, helped eliminate some of the complications observed with synthetic mesh. On its ventral side, a permanent synthetic mesh anchors placement on the abdominal wall; on its visceral (dorsal) side is a barrier layer that decreases adhesion formation between the viscera and the underlying mesh. Several in vivo animal studies investigating a variety of composite meshes support these findings [48 –51].

Composite mesh may be classified into two categories: (1) Coated mesh with a temporary barrier coating, or (2) dual-sided mesh with a permanent barrier layer. Examples of coated mesh are Proceed^™ (Ethicon), a lightweight PP supplemented with oxidized cellulose that degenerates over 28 d; Sepramesh^™ (C.R. Bard), a PP coated with Seprafilm® (Genzyme, Cambridge, MA), a hyaluronate carboxymethylcellulose layer that degenerates over 7 d; and Physiomesh^™ (Ethicon), a PP encapsulated with polydioxanone that degrades over 240 d. Dual-sided meshes are Composix® (C.R. Bard), a lightweight or heavyweight PP combined with a visceral layer of ePTFE, and DualMesh® (W.L. Gore & Associates), a double-sided ePTFE with a parietal textured surface encouraging tissue incorporation and a visceral smooth surface that minimizes tissue attachment.

Coated meshes, with their temporary barrier coating, exhibit more adhesion formation over time [47]. Proceed^™ mesh, in particular, demonstrated a prolonged active inflammatory response and fibroblast influx, thereby promoting adhesion formation; several studies have shown that it is more likely to generate adhesions than are other types of composite mesh [48,52,53].

Another notable finding is that both dual-sided and coated meshes cause adhesions preferentially at the site of fixation to the abdominal wall or at the cut edges of the mesh [51]. Indeed, it can be difficult to avoid exposure of the PP mesh edges to intra-peritoneal structures with pre-laminated composites when an inlay repair technique is used. Composite mesh implants usually are cut by the surgeon to fit the defect size and shape, which can expose the abrasive cut edges of the mesh to the intra-peritoneal contents. Prosthetic mesh edge exposure is a major source of adhesions, particularly when the mesh edge is adjacent to or within the peritoneal cavity [54].

Several retrospective studies have investigated the incidence of infection after composite mesh repair. In one study, 3.3% of patients undergoing VHR with a composite mesh required mesh explantation because of infection [55]. A second study found that infected composite mesh can be salvaged with conservative office-based methods, including dressing changes, local wound debridement, partial mesh excision, wound vacuum, and antibiotic therapy. None of the patients with a salvaged infected composite mesh developed hernia recurrence in a three-year follow-up [56]. These two studies stand in contrast to a retrospective review that reported a 10% infection rate among 206 patients who underwent elective composite mesh hernia repair. The majority of infections were caused by Staphylococcus, and all but two necessitated mesh removal [57]. In light of the contrasting results of these studies, additional research is required on the risk of infection with composite mesh.

Biologic Meshes

The development of biologic mesh was born of the success of autologous tensor fascia lata grafts for necrotizing abdominal wall infections, enteric fistulae, or exposed prosthetic material after VHR [58]. In the setting of a colonized or infected abdominal wall, reconstruction with additional synthetic material is contraindicated because of the high infection rate and subsequent need for abdominal wall debridement [59]. Synthetic materials do not promote vascular ingrowth, leaving them susceptible to chronic infections [60]. Biologic grafts, in contrast, theoretically support angiogenesis, allowing delivery of antibiotics and white blood cells.

Biologic meshes are composed of an acellular, soft-tissue extracellular matrix, consisting predominantly of a collagen backbone that provides integral strength and structure. Other components are elastin, which lends the graft compliance, as well as glycoproteins and growth factors that are important in mediating a tempered host immune response. Vascular channels are preserved in unmodified matrices and are essential to revascularization.

The origin of biologic mesh may be human autografts or allografts or bovine or porcine xenografts. Donated human dermis is known commercially as AlloDerm® (Lifecell Corporation, Branchburg, NJ) or DuraDerm^™ (C.R. Bard). Porcine xenograft may be derived from either dermis (XenMatrix^™ [C.R. Bard]; Zenoderm [Ethicon]; CollaMend^™ [C.R. Bard]; Strattice^™ [Lifecell]) or small-intestinal mucosa (Surgisis® BioDesign^™ [Cook Medical Inc., Bloomington, IN]), whereas bovine xenograft is pericardial in origin (Veritas® [Synovis Surgical Innovations, St. Paul, MN]; Tutopatch [Tutogen Medical, Inc., Alachua, FL]).

Biologic mesh may be classified further into two types. Grafts may be chemically cross-linked—this prevents collagenase access to the collagen; or they may be non-cross-linked, composed of natural, unprocessed collagen [61]. Cross-linking prevents degradation of the collagen-based backbone; un-cross-linked mesh may be incorporated and resorbed within 3 mos [61]. There are other differentiating characteristics of biologic mesh: Depending on processing technique, each biologic prosthetic may be stored in a different fashion (wet, dry, at room temperature, refrigerated) or require rehydration prior to implantation.

After harvesting, biologic grafts are processed to remove cellular debris and reduce the foreign-body reaction. All major elements responsible for skin alloantigenicity, including keratinocytes, Langerhans cells, and vascular endothelial cells (which express major histocompatibility complex type II antigens) are removed, leaving intact the collagen fiber network and glycosaminoglycans [62]. Removing cells and cellular debris eliminates the most potent cause of rejection while maintaining the extracellular matrix to guide autologous cells to repopulate the graft. Theoretically, this fosters regeneration and remodeling rather than inflammation and a foreign-body response, and a protective barrier is formed between the mesh and intra-abdominal structures [58]. Potential benefits of this tissue layer include reducing the incidence of visceral adhesions and fistula formation by preventing direct, ongoing exposure of intraperitoneal structures to mesh fibers, and facilitating reoperation if necessary by providing a safe plane of dissection [63].

An ideal biologic mesh does not trigger an immune response but does stimulate new collagen growth, resist or survive infection, and maintain durability over time. It would be used to reinforce high-risk standard laparotomy wounds or large-surface-area wound re-constructions (i.e., component separation). Additionally, it would reduce the recurrence rate and risk of adhesion formation and bowel injury observed with traditional synthetic mesh. In short, it would promote tissue ingrowth for strength but decrease the chronic inflammatory response inherent in synthetic mesh repairs. However, many extant studies have been conducted only in the setting of difficult clinical situations, and of the 13 commercial products, only four have been subjected to published, peer-reviewed studies evaluating their outcomes in abdominal wall reconstruction [18,62]. Perhaps most importantly, there have been no randomized clinical trials evaluating VHR with biologic mesh [64,65].

Compounding the matter further are contradictory results from published series. For instance, when investigating VHR with Surgisis—a non-cross-linked porcine xenograft—one recent study demonstrated a recurrence rate of 3.7% over five years of follow-up [66]. This is in contrast to separate studies reporting recurrence rates as high as 33% [67,68]. These conflicting reports have compelled several authors to call for long-term investigations before these materials are used widely as a primary mesh [20,64].

Bioprostheses are not impervious to infection [69]. The effect of bacterial infection on biologic mesh is still under investigation. One group reported that an in vivo model of biologic mesh infection demonstrated a significant reduction in tensile strength and elasticity, and those investigators cautioned against placement of such meshes in a contaminated field [70]. Other authors suggest that infection rates are dissimilar among the various biologic mesh scaffolds, with cross-linked porcine biologics showing higher infection and explantation rates [71,72]. Local wound management was ineffective because the mesh became encapsulated rather than incorporated, necessitating graft removal [72]. Although most xenografts are used by surgeons in the setting of contamination, none of these “devices” has received a U.S. Food and Drug Administration (FDA) indication for use in this situation. One particularly interesting study reviewed the FDA database of adverse events associated with biologic mesh. One hundred fifty adverse events were identified, with 80% described as infection and 90% necessitating reoperation [73]. The authors concluded that the intrinsic properties of mesh and their relation to infection remain poorly understood, necessitating further evaluation of these products, a view echoed by other commentators [64,65,73].

Special Situations

Two special situations deserve attention: Use of prosthetic mesh in contaminated abdominal wounds, and in complex abdominal wall reconstructions. Much discussion focuses on the use of mesh in infected abdominal wounds. As noted, the use of synthetic mesh in a contaminated abdominal wall repair is associated with high rates of explantation and hernia recurrence. Closure with absorbable mesh is a temporary measure with a high risk of enterocutaneous fistula formation. Several authors have reported that the use of biologic mesh in this setting, particularly human acellular dermal matrix (HADM), results in a lower recurrence rate and frequency of explantation [74 –76]. In one retrospective study of 75 patients undergoing VHR in the setting of a contaminated or clean-contaminated incision, five patients required mesh explantation, four because of enterocutaneous fistula formation and one because of mesh infection; two patients suffered recurrence [74]. A second retrospective analysis demonstrated similar results: Among 67 patients with contaminated abdominal wounds who underwent repair with HADM, two required mesh removal because of infection and failure of graft incorporation, and 12 developed a recurrence [75]. The investigators in both studies noted that whereas overall infection rates were equivalent to those observed with synthetic mesh repair, the management of these infections usually consisted only of local wound debridement.

A second important topic is the use of prosthetic mesh in complex abdominal wall reconstructions, such as in patients who have sustained loss of domain, undergone unsuccessful skin grafting, are recovering from open-abdomen management, have a colostomy or ileostomy, or have substantial co-morbidities such as obesity or a connective tissue disorder. Although a complete discussion of this topic is outside the scope of this article, it is worth noting that three publications suggest that biologic mesh may be beneficial in these situations [77 –79]. One group found a significant reduction in the recurrence rate when HADM was added as an overlay in primary closure of patients with complex, medium-sized hernias [77]. A second study evaluating the use of HADM in high-risk abdominal wounds supported these findings, showing successful repair in 26 of 29 patients. The three patients who experienced a hernia recurrence had body mass indices greater than 50 and underwent combined ventral–peristomal hernia repairs [78].

Conclusions

There is a wide variety of prosthetic meshes available for use in VHR. Synthetic mesh, such as PP or polyester, is appropriate for clean cases when it is placed in an extra-peritoneal position; otherwise, it generates an unacceptably high risk of adhesion formation and bowel obstruction. Composite mesh and coated ePTFE are useful in laparoscopic hernia repair, particularly when care is taken to avoid exposing the macroporous edges to the abdominal viscera. Biologic mesh, whether from a human, porcine, or bovine source, is a relatively new addition to the surgeon's armamentarium of prosthetic hernia repair materials, and holds promise in complex abdominal wound closure and infected abdominal wounds. Indeed, there is an ever-expanding selection of products, offering surgeons unprecedented choice of repair material. Continued innovation in this field may yield long-term data analyzing the effect of implantation, particularly of novel biologic materials.

Footnotes

Author Disclosure Statement

Dr. Itani has received institutional research support from Life Cell.

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