This paper describes the steps required to raise a fasciocutaneous epigastric free flap and transfer it to the neck in the rat.
Free tissue transfer has been increasingly used in clinical practice since the 1970s, allowing reconstruction of complex and otherwise untreatable defects resulting from tumor extirpation, trauma, infections, malformations or burns. Free flaps are particularly useful for reconstructing highly complex anatomical regions, like those of the head and neck, the hand, the foot and the perineum. Moreover, basic and translational research in the area of free tissue transfer is of great clinical potential. Notwithstanding, surgical trainees and researchers are frequently deterred from using microsurgical models of tissue transfer, due to lack of information regarding the technical aspects involved in the operative procedures. The aim of this paper is to present the steps required to transfer a fasciocutaneous epigastric free flap to the neck in the rat.
This flap is based on the superficial epigastric artery and vein, which originates from and drain into the femoral artery and vein, respectively. On average the caliber of the superficial epigastric vein is 0.6 to 0.8 mm, contrasting with the 0.3 to 0.5 mm of the superficial epigastric artery. Histologically, the flap is a composite block of tissues, containing skin (epidermis and dermis), a layer of fat tissue (panniculus adiposus), a layer of striated muscle (panniculus carnosus), and a layer of loose areolar tissue.
Succinctly, the epigastric flap is raised on its pedicle vessels that are then anastomosed to the external jugular vein and to the carotid artery on the ventral surface of the rat’s neck. According to our experience, this model guarantees the complete survival of approximately 70 to 80% of epigastric flaps transferred to the neck region. The flap can be evaluated whenever needed by visual inspection. Hence, the authors believe this is a good experimental model for microsurgical research and training.
Transferencia de tejido libre ha sido cada vez más utilizado en la práctica clínica para la reconstrucción de tejidos desaparecidos desde la década de 1970 1-5. Esto ha permitido la reconstrucción de defectos complejos y de otra manera no tratables resultantes de extirpación de tumores, traumatismos, infecciones, malformaciones o quemaduras 1-7. Los colgajos libres de este tipo son particularmente útiles para la reconstrucción de regiones anatómicas de alta complejidad, como los de la cabeza y el cuello, la mano, el pie, y el perineo 1,4.
Sin embargo, aún hoy en día los cirujanos en formación se amilana con frecuencia por la complejidad de varios pasos involucrados en la crianza, la transferencia y insetting un colgajo libre con el uso de técnicas e instrumentos de microcirugía 8,9. Además, es ampliamente aceptado que se convierta en un microcirujano competentes, extensa práctica experimental en un modelo animal es obligatorio 4,8-13.
Por otra parte la investigación, básica y traslacionalen el ámbito de la transferencia de tejido libre es de gran 8,14-16 potencial clínico. No obstante, los investigadores están con frecuencia decidan a utilizar modelos de microcirugía de transferencia de tejido debido a la falta de información con respecto a los aspectos técnicos involucrados en los procedimientos operativos 4,8-14. La rata es un buen modelo animal para la investigación y la formación de microcirugía, ya que es relativamente barato, fácil de mantener, y susceptibles de manipulación frecuente 8,11,13,14,17,18.
Aunque varios colgajos óseos libre, musculares y de la piel se han descrito en la rata 18-24, la aleta fasciocutáneo epigástrica libre es el más ampliamente utilizado con fines didácticos 9,12,13,18,25. Este colgajo libre fue descrito por primera vez en 1967 por Strauch y Murray y ha ganado cada vez más popularidad desde entonces, debido a varios factores, a saber, la anatomía vascular constante, la facilidad relativa de la disección, los vasos de nutrientes importantes, y la redundancia de la piel en la zona donante, WHich permite el cierre primario del defecto resultante de la elevación del colgajo 4,9-11,13,17,18,25-28.
Solapa de Anatomía e Histología
El colgajo epigástrico es suministrada por la arteria epigástrica superficial y la vena (Figura 1). Estos vasos se originan en y drenan en la arteria femoral y la vena, respectivamente. En promedio, el calibre de la vena epigástrica superficial es de 0,6 a 0,8 mm, lo que contrasta con los 0,3 a 0,5 mm de la arteria epigástrica superficial (Figura 2) 17,18. La arteria epigástrica superficial emite dos ramas principales: un lateral y una rama medial que a su vez se dividen en múltiples ocasiones, originando redes capilares que suministran la mayor parte del tegumento de la región epigástrica. Estos capilares drenan en los afluentes de las venas epigástricas superficiales que tienen un curso paralelo al árbol arterial (Figura 2) 13,17,18. El diagrama de la Figura 3 representa la región de la pared abdominal ventrolateral suministrada por los vasos epigástricos superficiales que se pueden movilizar en la solapa epigástrica. Este colgajo puede ser de hasta 5 cm de largo y 3 cm de ancho 13,17,18.
Histológicamente, la solapa se compone del tegumento que cubre los músculos de la pared abdominal ventrolateral (Figura 4) 13,17,18. Contiene una capa superficial de la piel, formada por la dermis y la epidermis. Debajo de la piel hay una capa de tejido graso llamado panículo adiposo. Por debajo de esta capa hay otra capa de músculo estriado conocido como panículo carnoso 18,28,29. Por debajo del panículo carnoso hay tejido laxo que es superficial a la fascia profunda que cubre los músculos abdominales más grandes. Por lo tanto, la aleta es un bloque compuesto de los tejidos, que contiene todas estas capas, a excepción de la fascia muscular profundo (Figura 5) 13,17,18,27-31.
The most important aspect to obtain consistent flap survival is paying attention to detail in various steps of the microsurgical technique. For example, to obtain good visualization of the vessels, of the surgical instruments and of the fine suture lines, it is very helpful to place underneath the vessels, a sterilized colored plastic background. As many researchers, we prefer to use sterilized fragments of yellow or green balloons (Figures 7 and 11). This background provides the additional advantage of minimizing adherence of suture lines to the adjacent structures, which sometimes leads to the need of pulling the suture line with too much tension, which may in turn lead to vascular tearing. Finally, the use of a background has the additional advantage of decreasing the probability of inadvertently dragging potential thrombogenic tissue debris to the anastomosis site.
Considering that the flap’s vessels are very fine and fragile, it is important not to pinch the entire width of the vessels, in order to avoid intimal lesion that, in turn, will lead to intravascular thrombosis and flap failure. To prevent inadvertent injury to both the flap’s vessels and to the recipient site’s vessels, it is safer to liberally ligate and divide neighboring tributaries, which will allow an easier manipulation of these vessels.
Before starting the anastomoses, it is vital to place the vessels in their definitive position, striving to prevent vascular kinking or torsion of the flap’s pedicle. Given the small caliber and delicate consistency of the vessels, these are often difficult to exclude unequivocally. One helpful trick is to secure the flap in its final position with 3 stitches placed away from the site of the anastomoses. Next, if in doubt, temporarily open the vascular clamps placed at the flap’s pedicle, and fill the vessels’ lumen with heparinized normal saline in a concentration of 10 IU/mL until they become engorged. This leads vessels to assume the configuration they will present after being perfused by blood, as when the clamps are removed after anastomoses completion.
Moreover, it is of paramount importance to detect any air bubbles, even if small, inside the vessels during the entire procedure and particularly before tying the final stitches. If these bubbles are distant from the vascular section, the vessels can be milked gently with microsurgical forceps. If they are located close to the anastomotic sites, simple irrigation leads the less dense bubbles to be easily expelled from the vascular lumen. Failure to acknowledge the presence of air bubbles can cause irreversible flap ischemia and necrosis, no doubt due to the fine caliber of the flap vessels.
Additionally, it cannot be overemphasized the need for meticulous care while passing and tying the stitches, in order to: include the three layers of the vessels (intima, media and adventitia); obtain good vessel eversion to ensure adequate intimal contact, which is vital to anastomosis sealing and endothelial regrowth; avoid loose vascular contact, which will result in anastomotic incompetence, i.e., bleeding; and avoid grabbing too much vascular tissue, which will lead to anastomosis stenosis and proclivity to thrombosis, which in turn will result in venous congestion or poor flap perfusion, if the vein or artery are involved, respectively.
Finally, it is essential to ensure perfect hemostasis, during the entire procedure, especially when raising the flap in its deep surface. Otherwise hematoma formation and rat death are likely to ensue.
Modifications and troubleshooting of the technique
The authors observed that making a transverse incision in the middle portion of the SCM using an electric cautery, not only allows a better exposure of the carotid artery, but also minimizes the risk of undue tension over the future arterial anastomosis.
Another important technical tip is to start the anastomosis from the vessels’ back wall, in order to minimize the risk of unwillingly catching this wall when placing the stitches in the more easily exposed front wall. If the back wall is sutured to the anterior aspect of the anastomosis, lack of vascular patency will almost invariably result either immediately due to mechanical reasons or after only a few hours as a result of thrombosis8.
If the anastomoses of the epigastric vessels of the rat are considered too technically challenging due to the small caliber of these vessels, the femoral vessels can be ligated distal to the origin of the epigastric vessels and used as the vascular pedicle of the epigastric flap. In this way, larger vessels will be used (the femoral artery has a caliber of 1.0 to 1.2 mm; and the femoral vein has a caliber of 1.2 to 1.5 mm). Moreover, by dissecting and ligating the other tributaries of the femoral vessels, a vascular pedicle length of over 2 cm can be obtained, which will facilitate flap insetting18,34,35.
Reproducibility
Our experience of more than ten years of using this flap for teaching and research purposes strongly suggests that the rat epigastric flap is a reproducible model of free tissue transfer11,13,17,18,26. It can be easily incorporated in microsurgical courses, as it is a good teaching and training model for microsurgery trainees11,13,17,18,26. In our experience, although technically challenging in the beginning for the novice in microsurgery, after some training, the free epigastric flap can be successfully transferred to the neck of the rat with minimal to no subsequent necrosis in 70 to 80% of cases. These results concur with those generally reported in the literature13,18,36.
Significance with respect to existing methods
Numerous free flaps have been described in the rat10,16,18,37-39. The most commonly used for teaching and research purposes have been the transverse rectus abdominis myocutaneous flap, the latissimus dorsi and serratus anterior muscle flaps, the hind limb replantation model, and the epigastric (groin) flap18,35. These flaps have been favored, due to their consistent anatomy and sizeable vascular pedicle. The epigastric flap is arguably the one associated with lesser donor site morbidity, as it dissected above the muscle fascia18. Moreover, the epigastric flap, described in 1967, was the first flap to be described in rats34,35. This occurred only 4 years after the first description of an experimental flap in an animal by Goldwyn. Interestingly, this flap was a groin flap in the dog34.
Limitations of the technique
The two main limitations of this model are the need for microsurgical skills in order to carry out the surgery, and the presence of significant necrosis in 20 to 25% of cases, according to different authors13,18,36. Another potential limitation of the model herein presented is the auto cannibalism of the flap. However, as the authors above, this is an infrequent finding that almost only occurs in cases of total flap necrosis.
Future applications of the technique
The rat epigastric free flap can be used in experimental studies of tissue perfusion, tissue repair and surgical wound infection40,41. Its nutrient vessels are particularly suitable for intravascular injection of solutions containing substances of interest, namely drugs, viral vectors or liposomes, that will mostly produce a local or regional effect30,31. In addition, beneath the flap, pathogens, foreign bodies, radioactive seeds or chemicals can also be placed, mimicking several disease processes and potential treatments30,31.
The authors have nothing to disclose.
Uno de los autores (Diogo Casal) recibió una subvención del programa de Advanced Medical Education, que está patrocinado por la Fundación Calouste Gulbenkian, la Fundación Champalimaud, Ministério da Saúde e Fundação para a Ciência e Tecnologia, Portugal.
Los autores desean agradecer la ayuda técnica del Sr. Alberto Severino en la filmación y edición del vídeo. Los autores también agradecen a D. Octavio Chaveiro, Sr. Marco Costa y el Sr. Carlos Lopes, por su ayuda en la preparación de los especímenes de animales presentados en este documento.
Por último, los autores desean agradecer a la Sra Gracinda Menezes por su ayuda en todos los aspectos logísticos relacionados con la adquisición y el mantenimiento de los animales.
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