Type II: Artificial microporous, with pores smaller than 10 μm in at least one of its three dimensions. The best example is the mesh Gore-Tex^®, made with PTFE. See Fig. 7.2.

Type III: Prosthetic components, macroporous or microporous multifilament (mixed mesh). Examples are braided polyethylene mesh (Mersilene™, Ethicon Endosurgery, Blue Ash, OH, USA), PP braided (Surgipro™ (Covidien, Dublin, Ireland)), or PTFE perforated (MicroMesh^®, L. Gore & Associates, Newark, DE, USA). See Fig. 7.3.

Type IV: Prostheses with pores smaller than 1 μm, such as silastic, and pericardium or dura substitutes. These meshes are not suitable for hernia repair, although biomaterials in combination with type I can make a surface suitable for intraperitoneal implantation.

Constant evolution has made the Amid classification obsolete. Reticular PP prostheses tend to have a smaller amount of the biomaterial in order to enhance integration into the host tissue and patient comfort, without compromising on the strength of the parietal repair. These prostheses also are associated with absorbable material, forming hybrid prostheses, leaving an even smaller amount of foreign material in the host organism [7].

PTFE also has been subjected to changes in its structure, after initially evolving toward a “multiperforated” patch without developing better biomechanical strength. In subsequent generations of this material, innovations have appeared: incorporation of a roughened surface which promotes integration via its outer face (DualMesh ^®, L. Gore & Associates, Newark, DE, USA), argentic impregnation to prevent bacterial adhesion in the early stages after implantation.

Fully absorbable biomaterials have been incorporated into the context of prosthetic laminar: tissue extracted from its natural environment and serving as support for cell colonization. These are called biological prostheses; they achieve both repair and regeneration of host tissue, ultimately forming a “neopared” for the repair of the hernia defect. These natural prostheses require stringent controls given their animal origin, as they are not risk-free for possible transmission of disease as has happened in lyophilized dura prosthesis. The most commonly used are derived from the submucosa of porcine small intestines and collagen of human or porcine origin.

Finally, the composite type prosthesis or “composite” has opened a wide range of possibilities in the repair of large hernia defects in which the biomaterial must be left in contact with the visceral peritoneum. These prostheses have two components: one is the main or first biomaterial component, to which another is added in order to meet the specific function that can be performed first, so that the first material assumes the role of tissue integration, while the second is in contact with the peritoneal surface and modulates the behavior of this interface. The first component is usually a reticular prosthesis PP or polyester type; the latter may be absorbable or nonabsorbable, constituting a real barrier between the core material and the peritoneum and, in turn, promoting the generation of a mesothelial layer which prevents adhesions [7]. See Fig. 7.4.

Thus, the new classification is as follows:

As previously noted, research in the field of the host response to the implantation of the prosthesis has led to the modification of the PP reticular prosthesis in the sense of creating meshes with fewer biomaterials to expand the size base of pores and lower spatial rearrangement of the filaments, which will facilitate tissue growth between collagen fibers. This has led to the concept of “pore PP prosthesis, broad, and low density,” also called “lightweight,” with a similar behavior in terms of the tissue integration of classic PP meshes; this causes less foreign body reaction and maintains receptor tissue elasticity after implantation [7].

In an extensive and recent review published by Coda et al. [8], a total of 166 products registered as prosthetic biomaterial for abdominal wall repair were identified, of which 80 were made of PP, suggesting the following classification of biomaterials:

This complex classification has been complemented with additional function of molecular weight of the prosthesis, a characteristic, as previously has been mentioned, that is gaining increasing interest. In the aforementioned review, 70 of the 80 registered PP prostheses had no information about molecular weight expressed in g/m², and 40 of these also reported on thickness. This has given rise to proposing a classification that focuses primarily on PP reticular prostheses, which is justified because it is the most universal biomaterial used in the repair of abdominal wall hernias [8]:

These new classifications of biomaterials are justified by the need to systematize the characteristics of the flood of products that the industry offers. The goal should be to agree on a “common language” that is being used by both surgeons and by the manufacturers of meshes, to collect the important features of the prostheses for application (molecular weight, thickness, tensile strength, wing rupture, elasticity, pore size, etc.) and to establish rules regarding the information that companies are required to provide about their products.