What Is a Hernia?
A hernia is a medical condition that happens when an internal organ pushes through a weak spot in the abdominal wall. There are different types of hernias, for example- incisional hernia, umbilical hernia, femoral hernia, inguinal hernia, ventral hernia, etc. It will often appear as a bulge that cannot be pushed back and will disappear while lying down. Even if it is pushed in, physical activities like laughing, coughing, etc., will make the lump reappear. A hernia can become serious when it gets incarcerated or strangulated.
An incarcerated hernia is one that is hard to touch and does not disappear when pushed back, indicating an obstruction; in a strangulated hernia, the muscles in the abdominal wall will clamp down on the neck of the hernia, leading to a compromised blood supply which is life-threatening.
Both asymptomatic and symptomatic hernias are treated only by surgery; during the procedure, the surgeon will push back the contents of the hernia inside, reinforce the abdominal wall with a surgical mesh for better support and suture it.
What Is a Hernia Mesh?
Surgical mesh, also known as hernia mesh, is a flat sheet of prosthetic material in a lattice design that covers or patches a hernia. It is implanted in the weakened spot of the abdominal wall through which hernia occurs. It gives additional support to the abdomen and prevents the recurrence of a hernia, especially a ventral hernia.
A ventral hernia is the same as any hernia, but it has a higher chance of recurrence when compared to other hernias. Examples of ventral hernia are umbilical hernia and incisional hernia; because of their recurrence, the surgical mesh plays a crucial role in the entire treatment.
The ten-year ventral hernia recurrence rate is 63 % for surgery without mesh, and it is 32 % for surgery with a mesh.
Although surgical meshes play a crucial role in the treatment of ventral hernias, they are not without any drawbacks. Infection from the mesh and adhesions are a few mesh-related complications. However, choosing the right mesh with important biomechanical properties will reduce postoperative complications.
What Are Biomechanical Properties of a Surgical Mesh?
Biomechanical properties are the responses of the surgical mesh to a force (which can be internal or external). The different biomechanical properties are-
1) Biocompatibility- It is defined as the capacity of the mesh to be implanted without causing any adverse effects. This is measured in terms of the mesh's ability to induce an immune reaction. The proteins in the body will adhere to the surface of the mesh and subsequently lose their structure. The immune system will assume these altered proteins as foreign substances and trigger an immune reaction. An ideal mesh should be of a non-toxic material that will induce the lowest amount of immune reaction.
2) Mechanical Properties- These can be further divided into:
- Tensile Strength- It is defined as the maximum amount of strength that a material can withstand when subjected to stretching just before it breaks. The ultimate tensile strength of a mesh should be adequate to resist forces exerted on the abdominal wall. The surgical meshes are made with standard thickness, but their tensile strength is measured as N/cm, taking their width into count. Ideally, a surgical mesh should have at least 32 N/cm of tensile strength in its strongest direction and 16 N/cm in its weakest direction. Almost all the commercially available meshes exceed the required tensile strength and can withstand normal physiological forces in the abdominal wall.
- Stiffness- It is defined as the extent of a material's resistance to deformation in response to a force. An ideal mesh should not be overly stiff, nor should it be extremely elastic. It should be able to comply with maximum load and strain and revert back to its original form.
- Elasticity- It is the property of the material to return to its original shape after being deformed. It is an important requirement for a surgical mesh because if it does not return to its original shape after being stretched out, it will lead to a recurrent hernia. An ideal mesh should have an elasticity of 30 % at a tensile strength of 32 N/cm.
3) Compliance- It is defined as the amount of displacement that occurs in response to a unit force. The surgical mesh should have none or the least amount of displacement to physiological forces.
4) Porosity and Weight- The design of the mesh looks like a lattice with pores between the networks. Meshes with small pore size will encapsulate bacteria and prevent bacterial colonization, thereby preventing infection, but they will also trap the immune cells and granulomas (small areas of inflammation) and make the mesh stiff and reduce its flexibility. Nowadays, meshes with large pore sizes are preferred due to their mild foreign-body reaction.
Meshes with smaller pore sizes weigh around 33 g/m², and the ones with larger pore sizes weigh approximately 100 g/m². Some studies have shown that lightweight meshes have less foreign body burden and biomaterial surface area; the ideal weight for a surgical mesh is yet to be determined.
What Are the Different Kinds of Meshes?
The surgical mesh, based on the material used, can be divided into two types-
1) Synthetic Mesh- These are man-made meshes and are further divided based on their degradability; they are-
- Degradable Synthetic Mesh- The degradability of a synthetic mesh is its ability to dissolve or remodel over time. The space formed after the mesh dissolution is replaced by scar tissue or a regenerative matrix. Materials that can be used to make degradable synthetic meshes are- polyglactin (Vicryl), Polyglycolic acid (Dexon), and poly-4-hydroxybutyrate (Phasix).
- Non-degradable Synthetic Mesh- These are the traditionally used meshes; although some of them can cause inflammatory reactions, they are preferred due to their elasticity, rigidness, and their hydrophilic nature. Materials used to make synthetic meshes are polypropylene (Prolene, Marlex, etc.), polyethylene terephthalate polyester (Dacron, Mersilene), and expanded polytetrafluoroethylene (Teflon, Gore-tex).
2) Biological Mesh- Biological meshes are obtained from naturally occurring sources and are believed to promote tissue regeneration. They are harvested from different sources as larger and consistent sheets, which are later modified based on the requirement. The different sources of biological meshes are-
- Allogeneic- The mesh is derived from the human dermis of a decellularized human; examples are- Alloderm, Allomax, and FlexHD.
- Xenogenic- The mesh is obtained from a different species and transplanted in the abdominal wall; the different species and the locations from which they are grafter are-
- Porcine dermis.
- Porcine intestine.
- Bovine dermis.
- Bovine intestine.
Most surgeons prefer to avoid biological meshes due to their intense immune response, higher recurrence rates, and cost. It was previously thought that biological meshes have great compatibility with the abdominal wall compared to synthetic meshes, but no such evidence has been recorded.
Conclusion:
The principle behind a surgical mesh is replacing the like with like; it should be able to replicate the complex abdominal wall anatomy and its dynamic requirement. However, it is difficult to replicate these properties, but efforts should be made to choose the right mesh for the right setting to prevent a recurrence, which happens at an overwhelming rate in ventral hernias.
