For over five decades, hernia mesh manufacturers have claimed that the polypropylene (PP) utilized to make their hernia mesh products is inert. An inert object will not change or degrade over time. Hernia mesh implants are life-long medical devices; therefore, it is of the utmost importance that the material utilized to make the hernia mesh is inert.
Polypropylene is known to degrade through a process called oxidation. It was initially believed that polypropylene would not oxidize enough after implantation to undergo any degradation. Before implantation, it has been well accepted that polypropylene can be oxidized by exposing the polypropylene to gamma irradiation. Therefore, most polypropylene hernia meshes undergo sterilization with ethylene oxide instead of gamma irradiation to prevent the oxidation and degradation process. However, the Proceed hernia mesh continues utilizes gamma irradiation to sterilize the polypropylene, which causes the Proceed mesh to degrade even before implantation.
There is now a growing body of scientific research and literature demonstrating that the polypropylene utilized in hernia mesh implants does oxidize and degrade after implantation. The life-long inflammatory process that polypropylene incites, results in oxidization and degradation of the polypropylene. The inflammatory, oxidation, and degradation process is even more pronounced in coated hernia meshes placed closer to the bowel.
Scientific Articles on Polypropylene Degradation After Implantation
The authors conducted the study in order to gain a better understanding of the mechanism of polypropylene mesh deterioration. In doing so, they “developed in vitro methodology, which mimics the oxidative environment at the tissue-implant interface after surgery.” Below are some of the studies most significant findings.
“It was shown that structural changes in polypropylene meshes exposed to oxidative stress may involve formation of cross-links between the polymer chains, chain scissions, and hydrogen bonds between the carboxyl groups, which are formed in the material during the oxidation. These effects result in mesh stiffening, ultimately leading to chronic post-operative pain.”
“A growing body of evidence points to oxidative stress as a key factor in the development of the complications associated with post-hernioplasty chronic pain. When exposed to the myriad of oxidants and enzymes after the implantation, hernia repair meshes may undergo various structural, chemical and mechanical changes. This can cause alterations in the mesh’s linear dimensions. It was shown that mesh shrinkage or expansion is associated with the development of chronic pain and increased chance of the recurrent hernia. Moreover, mesh stiffening can lead to mechanical irritation of the surrounding tissues, thus provoking further excessive immune response.”
30 polypropylene hernia mesh explants were obtained from the Columbia, MO University Hospital Department of Surgery’s hernia mesh explant repository. The 30 polypropylene hernia mesh explants were removed because of pain, discomfort, and/or hernia recurrence that may cause the mesh to be a potential source of further complications. The implant duration ranged from 9 to 181 months. The authors found that “these results indicate in vivo degradation of PP hernia mesh… More importantly, these measurements indicate the degree of material transformation in vivo, which is important for understanding the biocompatibilty and durability of an implant. For instance, a material exhibiting high levels of surface oxidation is likely to exhibit embrittlement and surface cracking resulting in a decrease in material compliance and an increase in the surface area exposed to oxidative degradation, respectively. Enlarged surface areas due to surface cracking may intensify the inflammatory response. [Modulated differential scanning calorimetry] and [thermogravimetric analysis] measurements indicate the changes of the bulk material due to chain scission and/or crosslinking. These changes in bulk material properties can also lead to alteration of mesh compliance as well as variation in mechanical strength and degradation resistance. Reduction in the compliance of a material could induce pain due to mesh rigidity during patient movement.”
The authors conclude that “the lack of correlation between the other patient factors and characterization techniques could suggest that PP mesh is extremely susceptible to oxidation regardless of the patient population but it also suggests the need for continued analysis of explanted hernia mesh.”
164 explanted hernia meshes were examined using conventional microscopy and transmission electron microscopy. Below are the most significant photos and findings of the study.
“It is important to know the effect of degradation of an implanted material on the body and on the long-term performance of the device. When physical and chemical characteristics of a material undergo changes in the body its applications should include planning for safe and complete removal with minimal tissue damage. This exit-strategy is especially important in younger patients, proximity to organs and large vessels, and anatomical sites which are difficult to reach.”
“Another clinically important aspect of degradation is the potential for bacterial colonization of the fissures within the degraded material. It is known that irregularities of polymer surface promote bacterial adherence.”
“polypropylene degradation was observed across a large range of devices, produced by different manufacturers, explanted from different anatomical locations and due to different clinical complications.”
“For descriptive purposes, the uniform circumferential nature, fissuring and partial peeling of the layer resembled tree bark.”
“Myeloperoxidase is an oxidative enzyme expressed by the inflammatory cells together with an array of other oxidative substances… [Myeloperoxidase] was detected deposited on the surface of the “bark,” but was not observed mixed within it. This finding further indicated incompatibility of the “bark” material with water-soluble proteins. It also indicated an oxidative environment immediately around the fibers.”
“polypropylene degradation is likely mediated by the foreign body reaction, which is ongoing until the device is removed. Our observations of adherent macrophages on the polypropylene surface are consistent with the previous studies reporting chronic inflammation in explanted polypropylene mesh several years after implantation. We observed strong staining for oxidative enzyme myeloperoxidase produced by the macrophages in the tissue surrounding the mesh fibers. This indicated that, the surface of the polypropylene was exposed to reactive oxygen species (ROS) while oxidation of polypropylene as a result of the foreign body reaction has been suggested as the mechanism of degradation by earlier reports.”
“There was a good correlation between the thickness of the degradation layer and the duration of in vivo exposure indicating that the thickness of degraded material grows while the mesh is in the body.”
“Degradation related stiffening of the mesh is expected to increase over time.”
“A described effect of degradation and wear of medical devices is the release of material particles. The debris form prosthetic joints is well known to cause tissue necrosis, inflammation and fibrosis around the joints. For polypropylene meshes, we observed occasional particles of degraded polypropylene in the surrounding tissue and macrophages.”
“The fact that the “bark” was melted during excision surgery indicated that the degradation layer was formed in the body before the excision surgery. The finding also revealed that the fiber core and the outer “bark” are composed of materials with similar chemical compositions that are miscible with each other when heated.”
“The [blue] granules were seen in the non-degraded core of the fibers as well as to a variable degree in the outer degradation layer. In the latter, they were detected within the “bark” remaining on the core as well as in the segments on the “bark” separated form the core… which suggest that they also undergo degradation and lose color. The finding was a direct confirmation that the “bark” originated from the same material as the core of the fibers.”
The authors concluded by stating, “We have shown that a focused examination of explanted specimens can reveal features which have been overlooked for decades. Specifically, polypropylene degradation can be detected by readily available conventional light microscopy. A number of features indicated that polypropylene degrades while in the body. Both physical and chemical aspects of polypropylene degradation need to be studies more extensively for their roles in the development of these complications.”
The study concisely summarizes the degradation process of polypropylene, “Immediately upon insertion an acute inflammatory reaction begins. Neutrophils are the first cells to arrive. They begin to produce oxidants, including hydrogen peroxide and hypochlorous acid, which continue the heat induced oxidative process begun during manufacture. In the environment of the human body polymers undergo varying degrees of degradation. Oxidation of the PP chains produces free radicals, which cause various events, including depolymerization (breakdown of the molecular chain), cross-linking, oxidative degradation, additive leaching (which may include toxic substances), hydrolysis and stress cracking. This process produces more free radicals, which perpetuate the chain reaction. Enzyme catalysts allow these reactions to occur at body temperature. Ultimately due to the breakdown of the structural integrity of the PP polymer chains the mesh shows signs of surface alterations initially and then deep cracking of the fibers. The end result is lessened structural integrity with changes in molecular weight and crystallinity. The mechanical integrity of the implanted mesh is also decreased.”
The authors of the study conclude that “based on the available evidence it is clear that [polypropylene] alters in vivo after implantation. It undergoes various processes that lead to degradation, including oxidation, cross-linking, depolymerization and embrittlement. These processes result in various degrees of degradation and the loss of mechanical and physical properties, [polypropylene] is not inert.”
The authors of the study note that “despite all the improvements to hernia repair in terms of the surgical approach, many complications still exist of which a large subset can be attributed to a lack of material inertness in vivo.” The authors conclude that “[polypropylene] and ePTFE underwent oxidation and crosslinking, respectively. Collectively, the results of these experiments provide evidence of in vivo degradation of the [polypropylene], PET, and ePTFE composites… Identification of in vivo degradation processes might stimulate the design of more inert hernia mesh materials and reduce the need for revision surgeries stemming from material incompatibility.”
The authors note many complications associated with polypropylene hernia meshes and note that the “prolonged inflammatory response is thought to cause fibrosis and a rigid scar plate to form around the mesh material, particularly in the case of polypropylene meshes, leading to chronic pain and reduced mobility. As a result of this chronic inflammatory response, the mesh material is exposed to a continuous bath of oxidants. Aliphatic hydrocarbons such as polypropylene are known to be highly susceptible to oxidative attack… Continuous exposure of polypropylene to these oxidants may lead to chain scission, production of free radicals, and overall degradation of the material, both physically and chemically. This degradation is evidence by fissures, micro-cracks, with a build-up of hydroxyl and carbonyl grouops on the surface of the material, changes in thermal properties such as decreased glass transition and melting temperatures, weight loss, and changes in mechanical properties such as embrittlement and reduced compliance.”
13 explanted polypropylene hernia meshes were examined. Nearly all explanted polypropylene meshes had a decrease in the compliance, ranging from 4 to nearly 30 times less compliance. The authors concluded that “the explanted polypropylene meshes did undergo degradation while in vivo, most likely due to oxidation.”