A Model of Free Tissue Transfer: The Rat Epigastric Free Flap

The aim of this paper is to present the steps required to perform an epigastric free flap in the rat. Free tissue transfer has been increasingly used in legal practice for reconstructing missing tissues since the 1970s. However, even today, surgical trainees are frequently daunted by the complexity of several steps involved in raising, transferring and insetting a free flap.

In fact, to become a proficient micro surgeon extensive experimental practice, in an animal model, is considered mandatory. The rat is the most commonly used model for micro surgical research and training as it is relatively inexpensive, easy to keep and their mandible to frequent manipulation. This photograph shows the left epigastric region of a rat, previously injected with a red latex solution, in the arterial system and with a blue latex solution in the venous system.

It is possible to observe that the epigastric region receives an axial blood supply from the superficial epigastric artery and vein. These vessels originate from and drain into the femoral artery and vein, respectively. This skin represents the region of the abdominal wall, supplied by the superficial epigastric vessels that can be mobilized in the epigastric flap.

This flap can be up to five centimeters in length and three centimeters in width. This hematoxylin eosin stain section of the epigastric region, shows that the epigastric flap is composed of the integument of this region that covers the abdominal wall muscles. Thus, this flap is a composed block of tissues.

It contains a superficial layer of skin formed by the dermis and epidermis, beneath the skin, there is a layer of fat tissue, named panniculus adiposus. Below this layer, there is a layer of striated muscle, know as panniculus carnosus, below the panniculus carnosus, there is deep fascia that covers the larger abdominal muscles. Regarding the preoperative preparation, the rat is anesthetized with a mixture of ketamine and diazepam, the dose used is five milligrams per kilogram of ketamine and zero point 25 milligrams per kilogram of diazepam.

The hair over the ventral surface of the abdomen is removed with depilatory cream. The skin is disinfected with an alcoholic solution. The flap is drawn on the surface of the abdomen, with a surgical skin marker, as are the neck incisions.

Firstly, the flap margins are elevated. The flap is raised from medial to lateral, with an electric cautery. The perforated vessel to the deep surface of the flap are ligated.

Then, the flap's pedicle is carefully dissected. A retractor is placed in the caudal aspect of the wound, to facilitate vascular dissection. The vascular pedicle is carefully dissected by teasing away the loose, rotting tissues.

We can see the femoral artery, the femoral vein, the superficial epigastric artery and vein and finally, the lateral circumflex artery and vein. The origin of these later vessels is variable. However, in most cases, they originate from the superficial epigastric vessels.

The femoral vessels are then, temporarily, clamped. A final trimming of this rotting tissue and a future placement of the anastomoses is performed. Next, the superficial epigastric artery and vein are cut.

Subsequently, the neck incisions are made. First, the external jugular vein is exposed and dissected. Several of its collision branches are ligated, allowing for subsequent vein mobility, in order to perform the anastomoses.

Next, in a deeper plane, the lateral surface of the sternocleidomastoid muscle is exposed. This is followed by the deep surface of this muscle. Thus exposing the carotid artery and the vagus nerve.

The carotid artery is bluntly dissected from the vagus nerve. This picture shows the carotid artery and the vagus nerve. Next, the carotid artery is separated from the vagus nerve and a piece of sterile balloon is placed beneath the carotid artery.

We have noticed that making an incision in the middle portion of the sternocleidomastoid muscle, not only facilitates exposure but also prevents excessive tension on the artery and anastomoses, which can lead to flap loss. The flap is then brought to the neck incision. We perform a lateral incision in the lateral aspect of the carotid artery after placing a 9/0 nylon stitch.

After cutting the vessels and before carrying out the anastomoses, the vessel lumen is washed with a solution containing heparin. A 9/0 or 10/0 nylon suture line is used to perform the anastomoses with an interrupted suture. The venous anastomoses is made in the same way.

Sometimes, it is necessary to dilate the superficial epigastric vein, in order to make its caliber similar to that of the larger external jugular vein. After the anastomoses are made, the clamps are removed, starting from the venous clamps and from the flap vessels. The superficial epigastric artery can be moved to relieve existing vascular spasm.

It is possible to watch the typical aspect of a patent and competent arterial anastomoses as well as a well fueled venous system. This picture shows the final vascular architecture of the flap insetting. The superficial epigastric artery is connected termino-laterally to the carotid artery.

Finally, the superficial epigastric vein is connected termino-terminally to the external jugular vein. The flap is then sutured to the adjacent skin with interrupted 5/0 nylon stitches. The donor zone is closed in a similar way, using the lax abdominal skin to close the abdominal defect.

Typically, five days after surgery, the rat has already removed all stitches as shown in this anesthetized animal. Usually, at this time, it is possible to observe that the entire flap is viable. 14 days after surgery, the hair has already started to grow back on the flap surface, as can be seen here.

Two months postoperatively, only the neck scar shows the flaps original boundaries. The flap can be evaluated whenever needed by simple visual inspection. To facilitate exposure of the eventual aspect of the neck and those of the flap, a food treat can be presented over the head of the rat.

Alternatively, a gentle touch can be applied in the interscapular region. While an assistant, placed in front of the rat, observes and photographs the flap. An 80 percent nearly complete, survival rate was obtained in the last 20 procedures performed by the first author, as can be seen in this graphic.

The rate of flap survival depends, somewhat, on the experience of the surgeon. Generally speaking, if the technical aspects, as described above are taken into consideration, a nearly complete survival rate of around 70 percent of flaps is to be expected. Around 10 percent of flaps present partial necrosis.

And about 20 percent of flaps suffer complete necrosis. Our experience of more than 10 years of using this flap for teaching and research purposes suggests the rat epigastric flap is a respectable model of free tissue transfer. In addition, it is also a good teaching and training model for micro surgery trainees.

Moreover, this model can be used in studies of tissue profusion, tissue repair and surgical infection.