Ventral midline hernia formation is a complication following surgical management of abdominal pain in horses with incisional drainage and incisional infection being the largest risk factor for hernia formation [1,2]. In the case presented here, the development of a ventral midline hernia was subsequent to infection of the body wall associated with a previous exploratory celiotomy for treatment of colic. Conventional repair of large ventral midline hernia defects in horses involves subperitoneal implantation of non-absorbable knitted polypropylene or polyester mesh and hernia ring apposition. However, this repair technique has high complication rates (20%–60%), with mortality rates up to 50% due to persistent infection of the implant, rupture of the internal abdominal oblique muscle, and persistent colic, which are all potentially due to the rigidity of synthetic, non-resorbable mesh implants [5,7]. Further concerns regarding currently used non-absorbable mesh implants include tissue erosion, persistent inflammation, infection, and pain following mesh hernioplasty as well as difficulties with potential surgical revision.
Alternative surgical techniques for repair of ventral midline hernias include attempted primary closure of the defect without mesh implantation, subperitoneal mesh placement with fascial overlay, and subcutaneous mesh placement with hernia ring apposition [5-7]. In this case, primary closure of the defect without mesh was previously attempted and likely failed due to the large size of the defect, in addition to the large size of the patient. Subperitoneal placement with fascial overlay involves more aggressive dissection of the hernia sac, allowing more potential exposure of the peritoneal cavity to the mesh implant. Consequences of peritoneal contact with the surface of the mesh include intra-abdominal adhesion of the bowel to the mesh implant, which can lead to chronic colic. In addition, mesh infection would have dire consequences leading to potential septic peritonitis, for which mortality rates range from 40% to 60% in equine patients [9,10]. To help minimize these complications, implantation of subcutaneous mesh following primary closure of the defect has been described . This procedure was not elected in this case, as the significant size of the defect and large patient size would have created a large amount of tension on the ventral midline and body wall, likely causing dehiscence of the primary closure as well as potential failure of the mesh. In addition, as the mesh is placed subcutaneously, there is greater concern for mesh infection.
To our knowledge, this is the first published report describing the use of silk mesh for ventral midline hernioplasty in a large animal. The mesh used in this case is a novel silk-derived bioresorbable scaffold designed to support fibrous tissue ingrowth while allowing long-term bioresorption. Prior to the knitting of the mesh scaffold, the silk is processed to remove the outer sericin coat, which is the major cause of allergenic responses . By removing the sericin, the silk has greater biocompatibility and thus enhanced bioresorption in vivo [8,11]. An earlier version of this silk mesh was evaluated for repair of body wall defects in rodents in comparison to the conventionally used polyester meshes . In contrast to the polyester mesh, the silk mesh had higher reduction in the cross-sectional area over the course of the study, whereas the polyester mesh remained essentially unchanged . Although the silk began to bioresorb, the mesh allowed for significantly greater fibrous tissue ingrowth with biomechanical properties comparable to the polyester mesh at the 3-month termination point of the study . In addition, this study found the inflammatory reaction was similar with both silk mesh and polyester mesh . Subjectively speaking, the silk mesh was extremely easy to work with, less rigid than standard polypropylene or polyester mesh, and less abrasive to the touch. We did not encounter any issues with how the silk mesh handled the suture bites and did not experience any pullout or ripping at the edges during application. Although we believe SERI silk mesh has a soft enough texture not to be abrasive to the bowel if placed intra-peritoneally, we cannot comment on its effect with regards to adhesion formation or bowel irritation.
Gross and colleagues recently published their results of the use of the silk mesh used in this case report for soft tissue support in two-stage breast reconstruction using an ovine model . In their study they were able to demonstrate that the load-bearing responsibility which originally was provided by the mesh was gradually transferred to the new ingrown tissue consisting early on of collagen type 3 and later shifting to collagen type 1. The ingrowth of fibrous tissue resulted in a significant increase in the burst strength of the implant when compared to sham samples . Such increase in strength of the implant has obvious benefits, especially when placed in dependent areas such as in this case report.
In this case report, we aimed to evaluate the potential use of silk mesh in a large horse for ventral midline hernioplasty. In this single case, no complications were encountered when implanting the silk mesh retroperitoneally with hernia ring apposition and closure of subcutaneous tissues and skin. The silk mesh allowed adequate sharing of load between the body wall and mesh, while allowing tissue remodeling and fibrous tissue formation around the defect, as documented by ultrasonography. In our study, remnants of the mesh were visible 2 years following hernioplasty with healthy and organized fibrous scar tissue. In their study, Gross and colleagues were able to document histologically the ingrowth of new collagen fibers, with silk fibrils still visible throughout their 12-month study without evidence of hypersensitivity or immune response . Although we were unable to perform histologic and biomechanical analysis of the repair tissue, the outcome had adequate cosmesis of the ventral abdomen and provided enough biomechanical strength to support the ventral midline of a large horse throughout gestation and parturition of two foals.