Learning Outcomes

• Master the anatomic and physiologic peculiarities of the omentum.
• Understand the great variety of its field of application.
• Foresee the potential for unpublished uses in the veterinary field.

The omentum is a double peritoneal leaf constituted of 2 parts: the smaller and the greater omentum. The latter spreads down to the bladder where it reflects on itself to reach the dorsal part of the stomach. It forms a cavity called the omental bursa. It is a mesothelium on which lymphoid spots "milky spots" are dispersed.

The omentum was long thought to be vascularized in dogs and cats by branches of the right and left gastroepiploic arteries, as it is in people. Recent study suggests that the main vascular supply in dogs is not derived from the marginal omental vessels. In dogs, the left marginal omental vessels (both in the superficial and the deep leaf) are directly derived from the splenic artery and its continuation in the omental leaves. The main vascular supply of the right marginal artery is derived directly from the right gastroepiploic artery in the superficial leaf, whereas in the deep leaf the right marginal artery arises directly from the gastroduodenal artery. In the same study, anastomosing arteries between the superficial and deep leaves were weak and inconsistent. The venous system runs parallel to the arterial one. The lymphatic vessels from the milky spots drain into subpyloric, splenic and cœliac lymph nodes and then into the thoracic duct or directly from the omental bursa through the visceral surface of the diaphragm.

The omentum is a rich source of angiogenic and neurotrophic factors, acts as a reservoir of peritoneal immune cells, is important for peritoneal lymphatic drainage, and has adhesive properties, contributing to encapsulation of inflammatory processes and hemostasis. Given the extraordinary features of the omentum and facilitated by its size and plasticity, reconstructive surgery can greatly benefit from its use. In people, new clinical applications such as omental transposition to improve bone healing or to enhance survival of transplanted pluripotent stem cell-derived cardiomyocytes, are being explored. Its milky spots are rich in lymphocytes that produced antibodies. Through its immune function, the omentum helps to control infection and wound healing of the peritoneal cavity. Not only macrophages, mastocytes and lymphocytes participate in the production of an angiogenetic factor, but also the capillary network at its surface brings a vascular support to the intraperitoneal organs. Finally, the omentum helps forming adhesions through fibrinolytic inhibition. Adhesion of the omentum over a wound allows early revascularization, and isolation of the contaminants.

In small animal surgery the omentum has been used in the following indications:

• Omental wrapping for covering intestinal anastomosis as well as for any vascular and tissue reinforcement (urinary tract) or for hemostasis
• Omental wrapping to synthetic mesh
• Omental transposition for abdominal or thoracic wall reconstruction
• Drainage of hepatic, prostatic, pancreatic cysts and abscesses
• Drainage of the thoracic cavity
• Support to non-healing wounds, exposed bone or fistulous tracts

Omental Flap and Graft

In people, surgical lengthening of the omentum is necessary for transposition into the pelvic cavity or exteriorization beyond the peritoneal cavity. This is rarely the case in dogs. However, omental pedicle flaps are described in order to reach extra-abdominal applications as chronic axillary and inguinal lesions. The omentum can be exited through a mini-laparotomy and then tunneled under the skin to reach distant regions. It can also be brought through the diaphragm to reach the chest. Most reports on lengthening techniques are based on the omental pedicle extension technique published in dogs by Ross and Pardo.

In the first stage of this lengthening technique, the deep leaf is freed from its pancreatic attachment and flipped caudally. In a subsequent lengthening step, Ross and Pardo suggested an inverse L-shaped incision, beginning from the left just caudal to the splenic portion of the omentum, across approximately 50% of the omental width, and then continuing caudally parallel to the remaining omental vessels. The limited width of the tip of the pedicle and the decrease in available omental tissue is an acknowledged drawback of this lengthening technique. New anatomic knowledge questions the need to mobilize the gastroepiploic arch to allow incorporation in the omental pedicle, reducing the risk of hematoma formation because of ligation of the numerous gastric branches arising from the arch. If the region of interest cannot be reached merely by transposing the intact omentum, the novel pedicle technique based on the splenic arterial blood supply should be considered, with the additional benefit that the width of the pedicle tip can be extended considerably by unfolding the leaves of the pedicle.

Finally, although the omentum could be implemented as a free graft with microvascular transplantation, free omental grafting is not commonly used in veterinary surgery and so far graft survival rates in dogs are poor.

As one can see, the omentum can be used in a variety of conditions the abdominal and thoracic cavities as well as in reconstructive surgery, it deserves its nickname: "the surgeon's best friend."

References

1.  Adams W, Ctercteko G, Bilous M. Effect of an omental wrap on the healing and vascularity of compromised intestinal anastomoses. Dis Colon Rectum. 1992;35:731–738.

2.  Baltzer WI, Cooley S, Warnock JJ, et al. Augmentation of diaphyseal fractures of the radius and ulna in toy breed dogs using a free autogenous omental graft and bone plating. Vet Comp Orthop Traumatol. 2015;28:131–139.

3.  Bigham-Sadegh A, Mirshokraei P, Karimi I, et al. Effects of adipose tissue stem cell concurrent with greater omentum on experimental long-bone healing in dog. Connect Tissue Res. 2012;53:334–342.

4.  Brockman DJ, Pardo AD, Conzemius MG, et al. Omentum-enhanced reconstruction of chronic nonhealing wounds in cats: techniques and clinical use. Vet Surg. 1996;25:99–104.

5.  Doom M, Comillie P, Simoens P, Huyghe S, de Rooster H. The omental pedicle rap in dogs revised and refined: a cadaver study. Vet Surg. 2016;45(6):746–753.

6.  Hosgood G. The omentum - the forgotten organ: physiology and potential surgical applications in dogs and cats. Comp Cont Educ Vet Pract. 1990;12:45–51.

7.  Logmans A, Schoenmakers CH, Haensel CM, et al. High tissue factor concentration in the omentum, a possible cause of its hemostatic properties. Eur J Clin Invest. 1996;26:82–83.

8.  Karl S, Dupré G. Omentalisation of the head in cats: a cadaver study. J Feline Med Surg. 2012;14:295–298.

9.  Kawamura M, Miyagawa S, Fukushima S, et al. Enhanced survival of transplanted human induced pluripotent stem cell-derived cardiomyocytes by the combination of cell sheets with the pedicled omental flap technique in a porcine heart. Circ J. 2013;128:87–94.

10.  Lascelles BDX, Davison L, Dunning M, et al. Use of omental pedicle grafts in the management of non-healing axillary wounds in 10 cats. J Small Anim Pract. 1998;39:475–480.

11.  Ross WE, Pardo AD. Evaluation of an omental pedicle extension technique in the dog. Vet Surg. 1993;22:37–43.

12.  Shrager J, Wain J, Wright C, et al. Omentum is highly effective in the management of complex cardiothoracic surgical problems. J Thorac Cardiovasc Surg. 2003;125:526–532.

13.  Pap-Szekeres J, Cserni G, Furka I, et al. Transplantation and microsurgical anastomosis of free omental grafts: experimental animal model of a new operative technique in dogs. Microsurgery. 2003;23:414–418.

14.  Valat B, Moisonnier P. The omentum: “the surgeon's friend.” Eur J Comp Anim Pract. 2004;14:57–67.