Degradation, infection and heat effects on polypropylene mesh for pelvic implantation: what was known and when it was known.

Many properties of polypropylene mesh that are causative in producing the complications that our patients are experiencing were published in the literature prior to the marketing of most currently used mesh configurations and mesh kits. These factors were not sufficiently taken into account prior to the sale of these products for use in patients. This report indicates when this information was available to both mesh kit manufacturers and the Food and Drug Administration.

There has been a lack of dissemination of information regarding many of the characteristics of polypropylene mesh especially the many factors which are implicated in the complications that our patients experience postoperatively. The first polypropylene mesh kit cleared by the US Food and Drug Administration (FDA) for implantation was that used in the transvaginal tape (TVT®) procedure for the treatment of stress incontinence. This clearance was granted in 1998. Previously in 1996, a woven polyester mesh kit was cleared and further meshes and mesh kits meshes were granted clearance in the ensuing years. All FDA information regarding clearance for marketing dates is available at http://www.fda.gov/MedicalDevices/default.htm. I will concentrate here on those factors known to influence the behavior of mesh in vivo until 2003, when many more new mesh kits were cleared by the FDA. Heat effects and degradation will be summarized.

Relevant information has accumulated since the 1950s and was available in the medical literature for many years before FDA clearance of various meshes and mesh kits as outlined below (PP: polypropylene; SEM: scanning electron microscopy; FBGC: foreign body giant cells):

Any implanted device must not be physically modified by tissue fluids, be chemically inert, not incite an inflammatory or foreign body cell response, be non-carcinogenic, not produce allergic reactions, stand up to mechanical stress, be fabricated in form required at low cost and be capable of sterilization [1].

PP monofilament suture had high tensile strength, good flexibility and resistance to fatigue along with good knot retention along with being inert with excellent chemical resistance [2].

One hundred bacteria were enough to cause infection of a multfilament suture and monofilament suture withstood infection [3].

Monofilament suture is better than multifilament suture in wound infections [4].

Granulation formation related to friction between tissue and implant [5].

Immobile bacteria propagate inside multifilament suture and this plays a role in the spread of infection [6].

Bacteria are protected in interstices of material [7].

Bacterial adherence to multifilament suture 5-8 times greater than monofilament suture as documented with SEM [8].

Pore size is important for tissue incorporation [9].

Bacteria are protected in interstices from phagocytosis since leukocytes cannot readily enter the small pores of multifilament suture which supports infection and may result in sustained and prolonged infection [10, 11].

Multifilament sutures harbor bacteria at 70 days after implantation as shown with SEM [12].

Heat exposed PP releases biologically active degradation products affecting normal metabolic events [13].

Degradation of PP suture known as seen with SEM [14].

Immediately upon insertion of a mesh there is a race to the mesh surface between bacteria and host defense cells [15].

Bacteria adhere more to hydrophobic surfaces and produce a biofilm which further protects them from phagocytosis and antibiotics [16].

Multifilament mesh with a histiocytic reaction and unstable fixation which promotes infection [17].

Bacteria migrate along synthetic polymeric fibers [18].

ProteGen® Sling Mesh Kit FDA Clearance Letter Dated November 15, 1996

Multifilament Surgipro® mesh has more FBGCs than monofilament PP mesh [19].

High and low responders indentified by tumor necrosis factor measurements [20].

TVT® FDA Clearance Letter Dated January 28, 1998

Bacteria adhere to biomaterials using a biofilm [21].

PP mesh shrinks 30-50% after 4 weeks [22].

A multifilament mesh must be removed with infection [23].

Surface roughness promotes wicking of bacteria [24].

Ten bacterial colony forming units are enough to infect 15% of multifilament meshes [25].

Prolene Soft Mesh® FDA Clearance Letter Dated May 23, 2000

Bacterial colonization found in 33% of explanted meshes [26].

IVS® FDA Clearance Letter Dated April 4, 2001

SPARC® FDA Clearance Letter Dated October 26, 2001

Greater pore size leads to more deposition of mature collagen with increased tensile strength and vascularity. Pores <12 microns prevent vascularization [27].

The abdominal wall stiffens after mesh insertion [28].

All Other Meshes/Kits Have FDA Clearance Letters Dated after 2001

The extent of bacterial adherence depends on the mesh surface area. Multifilament meshes have a 205% increase in surface area compared to monofilament meshes. This may explain infection months to years after implantation [29].

Heat sterilization causes degradation [30]. Figures 1 and 2.

Fig. 1

The control polypropylene mesh. Note the smooth surface with minimal striations as seen under SEM at 1500x. Reprinted from The American Journal of Surgery, 195(3), Kemal Serbetci et al, Effects of resterilization on mechanical properties of polypropylene meshes, pages 375–9, Copyright 2007, with permission of Elsevier and the author

Fig. 2

Degradation of polypropylene mesh after three autoclavings. Note the more pronounced irregularities with small protrusions on the surface of the polypropylene fiber as seen in SEM at 1500x. Reprinted from The American Journal of Surgery, 195(3), Kemal Serbetci et al, Effects of resterilization on mechanical properties of polypropylene meshes, pages 375–9, Copyright 2007, with permission of Elsevier and the author

Degradation occurs in all currently used meshes [31]. Figures 3 and 4.

Fig. 3

Degradation of a non-knitted, non-woven mesh removed from a patient seen in SEM at 850x. Note the nearly completely broken fiber in the center and other degraded fibers with deep cracks in the background. Grateful acknowledgement is given to patient S. A. Y. who gave permission to reproduce this SEM

Fig. 4

Degradation of a single polypropylene fiber as seen in SEM at 1000x. Note the deep cracks in the surface of the fiber. Grateful acknowledgement is given to Henri Clavé from the Department of Gynecologic Surgery, St. George Clinic, Nice, France for permission to reproduce this SEM

An abundance of information was available for both the FDA and mesh manufacturers prior to the FDA clearance of most meshes. Many publications detailed degradation mechanisms including heat exposure during manufacture and bacterial colonization of the polypropylene used in pelvic repair meshes.