CHAPTER 97 Mesh Hernioplasty
Mesh hernioplasty is indicated when the body wall has been disrupted in a manner that is not compatible with closure techniques aimed at obtaining primary tissue apposition. Most commonly in equine surgery, this method is used to repair chronic noninfected hernias associated with ventral midline celiotomy incisions. Meshes can also be used to strengthen tissues and aid in the closure of anatomic spaces. For example, a mesh can be used to reinforce closure of the inguinal canal in the laparoscopic repair of inguinal or scrotal hernias, and they have been used to aid in nephrosplenic space ablation in horses with recurrent left dorsal colonic displacement.
ABDOMINAL INCISIONAL HERNIA
An abdominal incisional hernia is an acquired hernia related to a previous incision and is defined as an abnormal protrusion of an organ, or part of an organ, through the abdominal wall. An incisional hernia usually starts as an asymptomatic partial disruption of the linea alba and the deeper layers of a laparotomy wound during the immediate or early postoperative period.
Because of peri-incisional edema, this event may go unnoticed if the skin wound remains intact, and only after several weeks will the protrusion of abdominal viscera become noticeable, depending on the size of the defect. Most incisional hernias are noticed within the first 3 months after surgery but can be diagnosed as early as 2 weeks postoperatively.
In horses the incidence of incisional hernia following abdominal surgery ranges from 1% to 16%. Several factors have been associated with the development of an incisional hernia, the most prevalent including incisional suppuration, use of chromic gut suture, postoperative pyrexia, and leukopenia. In one study, the odds of incisional herniation were 62.5 times greater in horses that had postoperative incisional drainage compared with those that did not have postoperative incisional drainage.
The occurrence of an incisional hernia may be detrimental to the horse for a number of reasons. Intestinal segments may become entrapped within the hernia sac, a situation that may lead to strangulation obstruction and colic. Alternatively, the weakened body wall and dependent nature of the hernia may be subject to traumatic injury in active horses. Finally, the obvious cosmetic blemish associated with this complication may be of concern to horse owners.
Repair of incisional hernias is commonly done after the acute phases of wound healing have passed. This approach allows the abdominal fascia to heal appropriately and regain a holding strength that approaches that of normal tissue. The strength of celiotomy incisions depends on collagen deposition across the wound. For the first 4 weeks after surgical closure, collagen is initially absent from the wound and then is poorly organized and immature, rendering ventral midline incisions weak in tension. Beginning at 8 weeks after surgery, however, the linea alba has strengthened to be comparable to normal tissue. Unfortunately, studies aimed at assessing tissue strength and holding power of suture material in compromised abdominal incisions are lacking. For this reason most clinicians elect to repair an incisional hernia at least 3 months after complete healing of any incisional complications such as an infection or area of cutaneous trauma. Particularly when a mesh implant is used, it is important to operate on healthy tissue because prosthetic implants may support infection.
MESH IMPLANT BIOLOGY
The use of a synthetic mesh allows the surgeon to close a soft tissue defect in a tensionless manner. Understanding the biological properties of the various types of meshes helps the clinician select the material appropriately, adopt the best placement technique, and predict the interactions that will occur between the mesh and host tissue. Although no single material is perfect, the basic function of all meshes is to ensure mechanical stability and reinforce the abdominal fascia. For this purpose, the mesh should be strong in tension, biologically inert, and resistant to infection, and it should favor neovascularization and ingrowth of connective scar tissue without causing intra-abdominal adhesions.