Microvascular Surgical Instrumentation and Application

Introduction

Microvascular surgery in veterinary medicine is indicated for free tissue transfer such as skin and muscular flaps and in kidney transplantation. Microvascular instrumentation development began in the 1930s and progressed further in 1952 with the creation of the Microsurgical Instrumentation Research Association.¹ Thanks to the work of Acland, Buncke, Tamai, and others, many instruments have been designed specifically for varying microsurgical needs. This wide variety of microvascular instruments includes both basic and sophisticated instrumentation that is necessary for correct tissue handling during surgery.^1,2

Microsurgical instruments have fine tips like ophthalmic instruments, but they differ in that they are a more standard length, whereas ophthalmic instruments are generally shorter than conventional surgical instrumentation. Plastic and reconstructive surgery usually involves a superficial operative field and the average length of the instruments is 14 to 16 cm.² The majority of instruments are spring loaded to reduce cramping of the hand muscles during long procedures that can lead to shaking and tremors. The handles are generally rounded to facilitate maneuvering the instruments in the fingers and allowing them to be rolled in the fingers, as necessary for suture placement and tissue manipulation. Many microsurgical instruments are grooved near the head to make them conform to the notch created between the surgeons’ thumb and index finger. This groove allows the instrument to rest in the notch without being actively held, to minimize muscle fatigue from grasping the instrument, which can result in tremors. Additionally, many instruments are counterbalanced with a weight at the head of the instrument to minimize finger fatigue caused by prolonged gripping of the instrument (Figure 8-1).

Instruments for microvascular surgery are generally made of stainless steel with the tips of the instruments containing chromium to increase their strength. Some surgeons advocate the use of titanium instruments, which are lighter, stronger, and more importantly have antimagnetic properties that prevent the fine microneedles used in suturing from sticking to the instrument. Microsurgery is performed with the surgeon in a sitting position to minimize fatigue and muscle tremors. The surgeons’ antebrachium rests on the table, with the heels of the hands resting comfortably on the table as well. The instruments are held as one holds a pen or pencil, with most operative maneuvers carried out by the fingers while the wrists remain motionless on the table.
 

This chapter describes instrumentation and suture materials that are most commonly used when performing veterinary microsurgery. In addition, descriptions of free skin transfer, free muscle tissue transfer and their indications are presented.

Jeweler’s Forceps

Jeweler’s forceps consist of two flat, narrow legs connected at the head that narrow to form the jaws of the instruments.^1-3 The contact surface at the tips is referred to as the bit and the distance between the jaws is approximately 8 mm. Jeweler’s forceps are numbered according to the width of the bit and legs and their overall shape. Five basic jeweler’s forceps are used in microvascular surgery: Nos. 2,3,4,5, and 7 (Figure 8-2 A,B,C). The No. 2 forceps have the largest contact surface and are advocated for use as needle holders. The No. 3 forceps are used for testing vessel patency. The Nos. 4 and 5 forceps are useful for delicate tissue handling; the No. 4 forceps have a slightly larger bit. The No. 7 forceps have the unique feature of having curved tips, which are useful to access obstructed areas or to prepare small vessels for anastomosis (Figure 8-2C).

Special care must be taken to avoid bending the tips of jeweler’s forceps. The tips should be examined under a microscope before the beginning of a surgical procedure to assess the alignment of the tips because bent tips may catch on adventitia, tear vessel walls, and inhibit proper handling of the microneedle. The tips of some jeweler’s forceps are pointed or rough, leading to tissue or vessel damage and inadvertent cutting of suture material. For these reasons, it is recommended to gently file the tips of jeweler’s forceps with an emery board or Arkansas stone before their first use.
 

Jeweler’s forceps are inexpensive and have a wide range of styles and usefulness during microvascular surgery; however, they do not have round handles, are not counterbalanced, and are of short length. In contrast, microvascular forceps are available in a variety of styles and designs but are considerably more expensive than jeweler’s forceps. Microvascular DeBakey forceps, microring tipped forceps, and a variety of curved or straight microforceps are available. These forceps are appropriate in length, have round handles, and are counterbalanced.

Needle Holders

Number 2 jeweler’s forceps are used as needle holders for their simplicity, ease of knot tying, lack of concern about entrapment of the suture material in the lock mechanism, and low cost. The major disadvantage of jeweler’s forceps is that the needle is not held securely and may slip at an inopportune moment. Additionally these forceps do not have rounded handles, lack a grooved head, and are not counterbalanced. Rounded shanks are particularly important in needle holders because passage of the microneedle through the vessel wall requires that the instrument be rolled in the fingers.

The three basic parts of the needle holder are the jaws, the lock, and the shank. The jaws are usually flat and not grooved. Generally, curved needle holders are used because they have less of a tendency to obstruct the surgeon’s view of the operating field. Ratchetless needle holders are used exclusively in microsurgery because of the delicate nature of the microneedles. Additionally, the locking and unlocking of the ratchet causes motion in the tips that can damage the vessel.

Scissors

Microvascular scissors are among the more expensive instruments in the microvascular surgical pack. They should have rounded shanks, be spring loaded, and have fine, delicate tips. They are used for delicate dissection, for cutting suture, and for trimming adventitia during vessel preparation.

Scissors are composed of blades, lock, and shanks. The blade tips are pointed or slightly rounded, and the blades are only sharp along their inner surface. The blades may be straight, curved, or angled at 45°. The shanks are spring loaded so the blades are open at rest, and when the shanks are compressed, they come together with a cutting action. These instruments are used for blunt dissection by closing the blades, inserting them into the fascial plane, and allowing the spring action to open the blades within the tissue plane. Scissors must be thoroughly cleaned, well protected when not in use, and their sharpness constantly maintained.

Vessel Dilators

Vessel dilators are modified jeweler’s forceps with a narrower, smoother, nontapering tip (Figure 8-3). The tips of this instrument are inserted into the vessel lumen and are opened slightly to dilate the vessel gently as part of vessel preparation. Dilators may also be used as a counterpressor when suturing vessels. They should be inspected under high magnification to ensure alignment of the tips. The tips must be smooth and unbent to prevent injury to the vascular intima when they are inserted into the vessel lumen.

Microvascular Clamps

Microvascular clamps are used to occlude the vessel and prevent intraoperative hemorrhage. These clamps must be atraumatic yet have adequate closing pressure to prevent hemorrhage from the vessel. The blades should be flat to disperse the pressure evenly across the vessel, and they should have a rough surface to hold the vessel securely. Clamps should be easy to apply with finger pressure or applicator forceps (Figure 8-4). Most clamps are small enough to fit in the operative field but large enough to be easily manipulated. Clamps are available in various sizes with varying closing pressure to accommodate variation in vessel size. The closing pressure of the clamps should be less than 30 gm/mm to avoid endothelial damage. The surfaces of the clamps are usually dull, to minimize light reflection.
 

The approximating clamp facilitates retraction and reapproximation of vessels for suturing. The purpose of the approximator clamps is to decrease the amount of tension between two vessels being anastomosed, thereby allowing for atraumatic vascular anastomosis. An approximating clamp is composed of two microvascular clamps joined by a connecting bar. The clamps may be movable along the connecting bar to allow for the distance between vessels to be adjusted (Figure 8-5) or fixed in position to the connecting bar, a position requiring that the clamps be placed at the appropriate distance along the vessels because the interclamp distance cannot be adjusted. The entire clamp should fit in the operating field, yet be large enough to be easily maneuvered and turned over for suturing both sides of the vessels. The Acland framed nonmovable approximator clamps have two cleats on the frame that facilitate vessel anastomosis, especially when a surgical assistant is not available (Figure 8-6). Because they are expensive microvascular instruments, extreme care should be taken when cleaning and storing microvascular clamps and approximator clamps to prevent damaging them.
 

Coagulators

Hemostasis is essential for creating a clear field for microvascular surgery. Because of the magnification required to perform surgery, even small amounts of blood can obscure the operating field making surgery virtually impossible. Unipolar coagulators damage surrounding tissue because the current passes from the cautery tip, through surrounding tissues, into the patient, and out to the ground plate. This dissipation of current and associated heat generation can damage the parent vessel of interest. Bipolar cautery has the advantage that both current and heat are only produced in the small space between the tips of the coagulating forceps. This restricts the amount of tissue damage, yet it provides for accurate hemostasis. A thin layer of sterile petrolatum applied to the tips of bipolar cautery forceps helps to prevent charred tissue from adhering to the tips of the forceps. If bipolar coagulation is not available, jeweler’s forceps can serve as cautery forceps. Although this application is monopolar, it is more precise and minimizes the amount of lateral heat and damage to adjacent tissues compared with the standard cautery pencil.

The amount of cautery used in microsurgery should be kept to a minimum, to avoid damage to vessels or other important structures that may be in the vicinity of the operating field. For vessels larger than 1.5 mm in diameter, hemostatic clips are effective in achieving hemostasis without damaging adjacent structures. Clips are used judiciously because too many hemostatic clips can interfere with the surgical procedure.

Suction

Vacuum suction is an optional tool in microvascular surgery. If mechanical suction is used, care must be taken to avoid contact with vessels or nerves. Endothelial damage from suction can lead to complete thrombosis of the vessel and surgical failure. Standard suction tips are generally too large for microsurgical application. A 20-gauge catheter may be connected to appropriately sized Silastic tubing and connected to the suction unit to create a fine tipped suction device. A small fenestration created in the Silastic tubing allows the surgeon some control over the strength of the vacuum. The surgeons’ finger is placed over the hole to occlude the fenestration partially or completely, thereby adjusting the amount of suction at the catheter tip. This control over the strength of suction aids in minimizing vascular injury. Sterile applicators can also be used for fluid absorption, but care must be taken to avoid damaging vessels or nerves.

Irrigators

Irrigation of the wound is essential in microvascular surgery to decrease the amount of desiccation caused by the intense light source of the operating microscope. Irrigation is also used to remove clots and to float the vessel edges apart. Standard irrigation syringes are too bulky and flood the microsurgical field. A simple irrigator can be made for microsurgery using a 10-ml syringe attached to a 20-gauge needle or catheter using either saline or heparinized saline. Irrigation is applied in a gentle stream. The catheter tip is not inserted into the vessel, to avoid damaging the vascular endothelium.

The Bishop-Harmon anterior chamber irrigator is used extensively in ophthalmic surgery and is applicable to microvascular surgery. Many cannulas are available and the advantage of this system is that it is easier to operate and to control the flow of the fluid with the small bulb than with a syringe.

Background Material

When performing microvascular surgery, a background is used to set the vessels out from surrounding structures. Background material is placed behind the structures of interest to improve their visualization through the operating microscope. Various colors are advocated to maximize visualization of the structures of interest. Use of dark colors, such as green or blue, enhances visualization of the artery and the vein, as well as the suture material. Background materials are commercially available, but a rectangular section of a balloon can be sterilized and used as an inexpensive background.

Counterpressor

Counterpressors are used to avoid suturing the opposite wall of a vessel during a vessel anastomosis. When the surgeon passes the needle through the vessel wall, counterpressure must be applied, or the wall is pushed away. The counterpressor provides resistance for passing the microneedle. The instrument must be sturdy, small enough to fit in a vessel, and easily maneuverable. The counterpressor has either a circular or a double-pronged tip, so the microneedle can be passed through the circle or between the tips. A counterpressor can be constructed by twisting 34-gauge wire onto itself, creating a tiny loop at the end. The free end is connected to a disposable tuberculin syringe or a metal bar to serve as a handle.

Maintenance of Instruments

Microvascular instruments are delicate and easily damaged. Extreme care is exercised when cleaning and storing instruments. After use, instruments are soaked in warm water containing a commercially available enzymatic cleaner, rinsed in distilled water, and air dried. Ultrasonic instrument cleaners offer the best way of cleaning microinstruments. Care should be taken when instruments are dried with a cloth, because the delicate tips of the microinstruments bend easily. After the instruments are thoroughly cleaned and dried, tipped instruments should be covered with rubber tubing to protect them from bending. Because of the amount of electrical instrumentation in the operating room, microinstruments become magnetized, causing the microneedle to become attracted to clamps and other instruments during surgery. This problem is prevented by subjecting the instruments to a demagnetizer coil before packing and autoclaving takes place.

Storage boxes should contain specially shaped, trough like receptacles made of foam to prevent damage to instruments. Instruments must not be stored where they are in direct contact with metal or other instruments. Gas is the preferred method of sterilizing micoinstruments because steam damages the instruments over time.^1-3

Microvascular Suture

The creation of microsuture enabled surgeons to anastomose vessels with a diameter of 1.0 to 2.0 mm. The microvascular needle consists of a point, blade, and body, and swage. The needle may be straight or curved, and the curve may be one-half, three-eights, one-fourth circle, or progressive. A 3-4 mm length needle is used most commonly. The diameter of the needle is important because it is directly related to the amount of trauma the needle inflicts on the vessel. Most microneedles contain a tapered point, which is the least traumatic to tissue. Currently, flat needles are used almost exclusively because the flat needle is more secure in the needle holder than a round needle, which can roll between the microneedle holder jaws. The needle may be made from carbon steel, stainless steel, or other metal alloys, with carbon steel being the strongest and least malleable.

Nylon is the most commonly used suture material in microvascular surgery.^1,2 It is smooth; allowing it to glide easily through tissue, and it has a high tensile strength while causing minimal tissue reaction. The major disadvantage of nylon is that additional throws may be needed to ensure knot security. The most commonly used suture in microvascular surgery is 10-0 nylon on a tapered needle.⁴

Vessel Preparation and Anastomosis

In veterinary medicine the long-term patency of microsurgically anastomosed vessels in the dog is approximately 93 to 95%.^5-7 Performing a microsurgical anastomosis of an artery or a vein has three steps: 1) vessel preparation; 2) vessel anastomosis; and 3) evaluation of vessel patency

Vessel Preparation

Vessel preparation is one of the most critical steps in performing a microvascular anastomosis. Vessel preparation includes proper alignment of the vessel in the approximator clamp, vessel irrigation, trimming the adventitia from the end of the vessel, and vessel dilation. The ends of the vessels must be properly oriented in the approximator clamp to ensure that the vessels are not twisted following completion of the anastomosis (Figure 8-7). Blood should be flushed from the vessel lumen flushed, using heparinized saline delivered with a #22 angiocath. This procedure prevents blood located at the ends of the vessels from developing into a thrombus (Figure 8-8). To prepare the vessels, 2-3 mm of adventitia is removed from the end of each vessel. Adventitia is removed with jeweler’s forceps or microsurgical forceps and microscissors under 10-16x magnification. Once the adventitia is draped over the vessel end, a small hole is made in the adventitia with microsurgical scissors, and one blade of the scissors is placed into this hole (Figure 8-9). The scissor blade is moved adjacent to the attachment of the adventia on the vessel, and the adventitia is excised around the circumference of the vessel wall. This prevents adventitia from being caught in the lumen during anastomoses. The framed approximator clamp is then applied, bringing the two ends of the vessels close enough so that there is little tension during the anastomosis. It is important to remember that the clamps will be flipped after the near side of the anastomosis is complete. Vessel spasm can be reversed with topical lidocaine or gentle dilation. Vessel dilation results in temporary paralysis of the smooth muscle in the vessel, thereby preventing vasospasm at the anastomotic site (Figure 8-10). Dilating the vessel also helps to increase the overall diameter of the lumen and helps delineate the near and far wall of the vessel.
 

Preparation of veins is technically much more difficult than arteries due to the relative thinness of the venous wall. Special care must be taken when removing the adventitia of a vein because inadvertent damage to the tunica media may result, thus weakening the vessel. Irrigating the end of a vein with a 22-gauge catheter will help in identifying the lumen of the vessel and the adventitia. Thin-walled veins may be prepared by submerging the vessel in a pool of saline to improve visualization of the vessel lumen.³

End-to-End Vessel Anastomosis

The end-to-end vascular anastomosis is usually performed by using a full thickness simple interrupted pattern with 10-0 nylon on a 100 µm flat-bodied needle. The ends of the vessels are aligned in the approximator clamp to create a 1 to 2 mm gap between the vessel ends. A background may be placed behind the approximating clamps to improve visualization of the vessels and suture material. The needle is grasped using a two-handed technique by grasping the suture with one hand and the needle with the other. The needle is held just beyond its midpoint, 1-2 mm back from the end of the needle holder. Three guide sutures are placed 120° apart, two on the near vessel wall, and one in the far vessel wall (Figure 8-11). The suture tags are left long to help manipulate the vessels with minimal trauma during the anastomosis procedure. It is imperative that the guide sutures be accurately placed, as sutures not exactly 120° will result in twisting at the anastomotic site. With the guides in place, equal numbers of interrupted sutures are placed between each guide suture. Counterpressure may be applied adjacent to the intended exit site of the needle to aid in passage of the needle through the vessel wall (Figure 8-12). Enough sutures are placed so that there is no anastomotic leak. Usually a total of 9 sutures are necessary for the average size artery. Needle placement must be accurate and symmetric. The needle entry point should be twice the thickness of the vessel wall away from the edge and symmetrical entry should be taken on the opposite edge. Uneven placement leads to vessels overlapping and thrombus formation. Needle placement should utilize a two-handed action under 20x to 30x magnification. The needle lumen is cannulated with microforceps or in larger vessels the adventitia is grasped to provide counter pressure as the needle is advanced through the vessel wall. After the needle penetrates the wall, the needle is pulled along its arch. A two-pass technique is used, unless the vessel edges are approximated. Tying the suture correctly also impacts on the likelihood of vascular patency. Knots need to lie flat and the proper amount of tension must be applied each time. Excessive tension damages the vascular intima while inadequate tension hampers proper vessel approximation. Surgeon’s knots are thrown first, followed by a simple square knot. After the near vessel wall is sutured, the clamp is flipped, and the process is repeated on the far vessel wall. Veins are anastomosed in a similar fashion but extreme care must be taken since the walls of veins are delicate and easily torn by suture material. A continuous suture pattern provides the same accuracy and versatility as a simple interrupted suture pattern^8-10 and is significantly faster and associated with less anastomotic leakage.⁹ However, a continuous pattern significantly narrows the vessel lumen. As a result, a continuous suture pattern should be avoided in arteries with a vessel diameter greater than 0.7 mm in diameter and in veins with a diameter greater than 1.0 mm in diameter.⁹ The main application of the continuous suture pattern is for end to side anastomoses performed on large arteries and veins.

Release and removal of the microvascular clamps should be completed in the same order each time. Once both anastomoses are completed the anastomoses are irrigated with a 1% lidocaine solution and the clamp release is started. The arterial clamp is released first, then the venous clamp. This eliminates misinterpreting venous backflow for adequate arterial inflow. Some leakage may occur at the anastomosis, and usually stops with direct pressure. Continued bleeding will occasionally occur and requires additional suture placement.
 

End-to-Side Anastomosis

Careful placement of sutures can accommodate disparities in vessel luminal diameters of up to 2:1.¹¹ The technique involves placing interrupted sutures farther apart on the larger vessel. When vessel diameter differences of 2:1 or 3:1 occur, beveling or spatulation of the vessel with the smaller luminal diameter is advisable.^3,12 When beveling an edge, the oblique cut should not be more than 30° in order to avoid turbulence. Spatulation is performed by creating a longitudinal incision in the cut end of the smaller vessel. When vessel discrepancy exceeds 3:1, end to side anastomosis is required.

End-to-side anastomosis is performed when there is a large vessel diameter mismatch. In order to minimize turbulent flow at the anastomosis site, the angle between the donor and recipient vessel should be as small as possible. Angles less than 60° are preferred. The angle of anastomosis can be decreased by spatulating or beveling the vessel with the smaller diameter. To prepare the recipient vessel, an approximator is placed on the isolated side of the recipient vessel and the adventitia is removed from the proposed arteriotomy site. A properly sized arteriotomy forcep is placed on the recipient vessel and a Dennis blade is used to gently cut the portion of artery held by the arteriotomy forcep or a stay suture technique can be used to create the arteriotomy (Figure 8-13). A single clamp is placed on the donor vessel and the adventitia is removed. Intraluminal blood from both the donor and recipient vessel is flushed with heparinized saline to prevent thrombus formation. Unlike the end to end technique, there are only two guide sutures, each placed 180° apart. The intervening sutures are placed as usual using a continuous or interrupted suture pattern.

Evaluation of Patency

Patency of a vascular anastomosis can be tested in a variety of ways. Venous patency is easily assessed when the vessel is translucent. Direct observation of expansive arterial pulsation is a reliable indicator of patency, whereas longitudinal pulsation usually signifies partial or complete obstruction. In free tissue transfer, examination of the arterial bed of the transplanted tissue flap for pulsation and evaluation of the cut surface of the flap for capillary bleeding can document arterial patency.

The chance of vessel thrombosis is greatest at the site of anastomosis 15 to 20 minutes following completion of the anastomosis. It is therefore advisable to observe the anastomosis and test vessel patency during this period of time. If partial obstruction occurs, gently squeezing the vessel with forceps, or massaging the vessel may break up the thrombus. A complete thrombosis necessitates resection of the damaged area, and repeating the anastomosis. Vascular thrombosis is most commonly due to technical error in suture placement, or the use of a vessel with a damaged intima. Venous rather than arterial thrombosis is the most common cause of flap failure. The thinner venous wall makes the anastomosis more fragile, more compressible, and more likely to twist and kink. After the first 20 minutes, postoperative days 1-3 are also critical for anastomotic patency. In most cases, a flap that is viable at day 5, will likely survive.

Anastomotic Devices

Anastomotic coupling devices may be used in place of hand suturing for microvascular anastomosis. Anastomotic devices reduce anastomotic time by 50%-75%, and have patency rates similar to hand suture techniques.13,14 The device performs well on thin-walled vessels of similar size, but can cause vessel intimal damage on thick-walled arteries. Because of the increased risk of technical errors associated with performing a suture microvascular venous anastomosis a coupling device is routinely used and recommended when performing the venous anastomosis. Some familiarity with the device is necessary for success, but the technique can be quickly learned. The coupling device consists of a pair of polyethylene rings with six small pins on one side of each ring. The anastomosis is performed by pulling the end of the vessel through the ring and impaling the wall of the vessel over the six pins. The other end of the vessel is also impaled on the pins of the second ring, and the two rings are precisely joined together with an anastomotic instrument. This device provides a secure anastomosis with intima-to-intima contact which in turn improves patency and reduces the chance of thrombosis (Figure 8-14). A second major advantage to using a coupling device is shortened overall procedure time which decreases the overall ischemia time of tissue when compared to hand suturing.^13-15 Anastomotic couplers come in sizes of 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, and 3.0 mm diameter.

Free Skin Flaps

Microvascular free skin flaps can be used to reconstruct wounds in almost any location on the body. Some of the described axial pattern skin flaps can be used for this purpose.¹⁶ The requirements for an axial pattern tissue flap to be used as a free flap include a 1 mm pedicle vessel diameter and a 2 to 4 cm vascular pedicle length. Generally, the longer the vascular pedicle, the easier it is to perform the vascular anastomosis. As a result, the omocervical, thoracodorsal, deep circumflex iliac, caudal superficial epigastric, and the medial saphenous fasciocutaneous flaps could be used in free tissue transfer. The skin flaps that are most commonly used for this purpose are the medial saphenous fasciocutaneous and omocervical cutaneous free flaps.^5-7,17-21

Blood Supply Patterns

The skin has two sources of blood supply. In dogs and cats the predominant blood supply is from direct cutaneous arteries. These arteries typically perfuse a very large section of skin. Over the torso they exit the body wall and lie in the well-developed panniculus carnosus muscle known as the cutaneous trunci muscle. In other areas of the body where the panniculus is absent (extremities), the cutaneous arteries run in the subcutaneous fascial layer.²²

The direct cutaneous artery divides into a network of branches, similar to a tree trunk and its numerous branches. All of the tissues that are supplied by this single artery are called the primary angiosome. Other angiosomes called secondary angiosomes are connected to the primary angiosome by choke vessels (Figure 8-15). The skin within the primary angiosome will consistently survive; likewise a large portion of the secondary angiosome usually will survive. Extending the flap into the tertiary angiosome leads to inconsistent survival.²² Since vessels cannot be visualized on the surface of the skin either mapping of the vasculature with Doppler or the use of deep anatomic landmarks are used to define the angiosome of a skin flap. One precautionary note is that the skin of dogs and cats is loose over the torso and may shift during positioning of the patient on the operating table; this will shift the angiosome relative to deep anatomical landmarks. In order to correct for this problem, the skin should be grasped, pulled upward, and then allowed to relax back in position. This should reposition the skin relative to deep anatomical landmarks.
 

Another useful technique to identify the angiosome when elevating a skin flap is to use a transillumination technique. As the skin is dissected off the underlying muscle, it is elevated and illuminated with an operating lamp on the epithelial side of the flap and inspected from the subcutaneous side. The vessels can be easily visualized through the skin. Use of this technique will aid in preventing damage to the perforatoring vessels of the skin.

Omocervical Free Skin Flap^7,18-21

The flap tends to have a substantial amount of subcutaneous fat associated with the skin. As a result, it is best suited for wounds located at the level of or proximal to the stifle; it can also be used for wounds in the forelimb that are located at the level of or proximal to the antebrachial region. If the dog is very lean, the flap likewise will be very thin and can be used in any location on the body. The flap has a thick coat of hair which makes it suitable for reconstruction of a highly visible region.

Flap Designs

• Simple skin flap
• Myocutaneous - skin flap and the cervical portion of the trapezius muscle
• Osteomyocutaneous - skin flap, the cervical portion of the trapezius muscle, and the spine of the scapula

Blood Supply

The blood supply of the omocervical free skin flap arises from the cutaneous branch of the superficial cervical artery and vein. These vessels penetrate the fascia between the omotransversarius and the cervical portion of the trapezius muscles. The superficial cervical artery and vein have 7 named branches, most of which supply the adjacent muscles. The prescapular lymph nodes are intimately associated with the vessels as they traverse medial to the aforementioned muscles. This is in the region of the cranial shoulder depression, which is easily palpated cranial to the scapula. In a large breed dog, the vascular pedicle of the flap is about 5 cm long and the diameters of the artery and vein are about 1.5 mm and 4 to 5 mm, respectively. The vein is very thin walled which can make it more challenging to work with during microvascular anastomosis to a recipient vein. One should be aware that the vascular pedicle does not always course under the omotransversarius, but may travel superficial to it (Figure 8-16). This variant was reported in 1 dog and described in another 2 dogs.¹⁷

Anatomic Boundaries

The cutaneous anatomical boundaries of the angiosome of the superficial cervical artery include the wing of the atlas cranially, dorsal midline, spine of the scapula caudally, and the acromion of the scapula ventrally. The axis of the cutaneous vessels is oriented in a caudoventral to craniodorsal direction, therefore the outline of the flap should be oriented in this direction (Figure 8-17).
 

Procedure

The cervical region in some dogs can be laden with fat. This makes the dissection of this flap very difficult. In order to prevent damage to the vascular supply during the dissection of the flap, the underlying fat should be elevated with the skin. After the skin has been incised around the entire circumference of the proposed flap, the caudal border of the flap is dissected until the intermuscular septum between the cervical portion of the trapezius and the omocervical muscle is identified. The dissection then continues along the dorsal border in a ventral direction.

The fascia between the cranial border of the cervical part of the trapezius and omotransversarius is incised to the level of the acromion, which exposes the superficial cervical artery and vein. The cutaneous branches are visualized and the remaining portion of the skin flap is dissected free from the muscles. The muscular branches of the superficial cervical vessels are ligated and divided. The fat surrounding the vessels is carefully removed (skeletonized) in order to decrease pedicle bulk of the pedicle.

Careful removal of adventitia at the proposed site of vein transection when it is distended with blood can make this process more easily performed, than after the vessel has been transected and deflated. After the vessels have been isolated as far down the pedicle as possible, they are occluded with microvascular clamps, ligated distal to the clamps, and transected.

The wound is closed in layers in order to minimize dead space. It is advisable to place a closed suction drain in the wound for 3 to 5 days, as seroma formation is a common complication in this highly mobile region. The recipient site is protected with a soft padded bandage. The bandage is changed daily as abundant serosanquinous discharge is expected.

The flap is transferred to the recipient wound. Care is taken to ensure that the vascular pedicle is not twisted. The skin flap is then tacked in place with a few sutures in order to ensure proper orientation of the hair (if possible) and vascular pedicle. Microvascular anastomosis of the artery and vein of the flap to recipient vessels is performed.

Medial Saphenous Fasciocutaneous Free Flap^5,6

Uses

This flap is relatively thin therefore it is useful for reconstruction of wounds located on the distal extremities and face. The flap is somewhat sparsely haired in some dogs and the client must be informed about the potential for less hair at the recipient site.

Flap Designs

• Simple skin flap
• Myocutaneous - skin flap and the distal half of the caudal head of the sartorius
• Osteomyocutaneous - skin flap and distal half of the caudal head of sartorius and medial tibial cortex
• Osteocutaneous - skin flap and medial tibial cortex

Blood Supply

The blood supply to this flap comes from the saphenous artery and medial saphenous vein (Figure 8-18). Proximally, these vessels lie under the caudal aspect of the caudal head of the sartorius, then enter the superficial fascia at the level of the distal femur.
 

There are two cutaneous perforators that perfuse the medial saphenous fasciocutaneous flap: a cranial branch and a caudal branch (Figure 8-19). Cadavaric studies have confirmed that the skin on the entire medial aspect of the femorotibial region from the level of the inguinal ligament to the distal tibia is perfused by segmental fascial perforators of the saphenous artery. Two muscular branches are found proximal to the cutaneous branches: one to the distal gracilis muscle and the other to the distal sartorius muscle. The distal 1/2 of the caudal head of the sartorius is consistently perfused by the saphenous artery. The gracilis muscle is not well perfused by the saphenous vessels, as its dominant blood supply is based on the proximal caudal femoral artery and vein.
 

Anatomic Boundaries

The medial saphenous fasciocutaneous free flap generally is based on the proximal two cutaneous branches. If a smaller flap is needed, it can be based on either the cranial or caudal cutaneous branch. The most proximal cutaneous branch supplies the caudal half of the flap and the second cutaneous branch supplies the cranial half of the flap. There may be some variation of the location where the first two cutaneous branches originate off the medial saphenous vessels, thus care must be taken when elevating the flap. The flap generally is centered over the thigh region with the proximal most aspect of the flap being at the junction of the thigh and abdomen. The flap should not be centered over the stifle as this may increase the risk for incisional dehiscence.

Procedure

The proximal, cranial and caudal borders of the flap are incised and the flap is elevated. A transillumination technique is used to identify the cutaneous perforators of the flap. The distal border of the flap should be incised last, as the cutaneous vessels may extend off the parent vessels in a more distal location than expected. Next, the saphenous artery and medial saphenous vein distal to the cutaneous perforators are isolated, ligated and divided. The saphenous vessels are dissected from their fascial attachments between the gracilis and the sartorius muscles. The gracilis muscular branch of the saphenous vessels is ligated and divided. Two muscular branches of the saphenous vessels entering the caudal head of the sartorius muscle are ligated and divided. The pedicle is then completely isolated to the level of the femoral vessels.

The vessels are ligated at the level of the femoral vessels, occluded with microvascular clamps just distal to this region, and sharply divided. The medial saphenous nerve, which is sacrificed at the time of the vessel dissection, is injected with bupivacaine (Figure 8-20). The donor site is closed in two layers: subcutaneous fascia and the skin. A drain is usually not placed in the donor site.
 

A number of precautions should be taken in order to decrease the risk of donor site wound dehiscence:

• The maximum width of the flap should not be greater then 6 cm in a large breed dog; if the flap needs to be wider, harvest a much longer flap as the length of the flap will translate into greater flap width.
• Attempt to keep the location of the flap as proximal as possible.
• Flex and extend the stifle to determine the isometric points of tension and temporarily appose the skin edges with towel clamps at the time of wound closure.
• Close the fascia that is attached to the underlying skin edges with a simple interrupted pattern and close the skin with an interrupted intradermal pattern.
• Protect donor site with a modified Robert-Jones bandage for 10 days after surgery.

Table 8-1 summarizes important differences between the medial saphenous fasciocutaneous and omocervical free flaps.

Free Muscle Flaps

Muscle flaps have a number of characteristics which make them ideal for reconstructive surgery. A muscle flap will revascularize a wound bed rapidly and improve the delivery of antibiotics, antibodies, and components of cell mediated immunity to the area. Oxygen tension in the wound bed is increased which inhibits anaerobic infection and promotes healing. A muscle flap can prevent, and potentially help to eliminate osteomyelitis in open fractures. In humans, open tibial fracture osteomyelitis has almost been eliminated as a postoperative complication, with the use of free muscle transfer. Muscle flaps provide a healthy, well vascularized surface for immediate free skin grafting. Muscle flaps conform well to any shape wound bed and they will atrophy to 40% of their original thickness within two months after surgery.

The advantage of using a muscle flap over a free skin flap is that the angiosome is contained within the specific muscle; therefore there is no “guess work” as to the location of the blood supply. If the skin is shifted or skewed off important deep landmarks, the flap may not be within the primary angiosome.

Selection of the appropriate muscle for wound reconstruction is important in the preoperative planning. A muscle that is expendable with little functional or cosmetic detriment to donor site function should be used. The muscle should fit the size and shape of the wound. The rectus abdominis muscle is ideal for distal extremity wounds. Due to its length, the blood supply of the rectus can be anastomosed to recipient vessels that are well outside the zone of the wound. The cervical part of the trapezius is also acceptable for distal limb extremity. Its vascular leash is relatively long, however, if trauma to the extremity is extensive and recipient vessel integrity is questionable, it should not be considered as a first choice. The latissiumus dorsi myocutaneous flap is useful for very large wounds that require bulk, but is infrequently used in veterinary medicine in free tissue transfer.

All muscle flaps need a cutaneous covering. There are two options: free skin grafting or creation of a composite flap (myocutaneous). One of the primary disadvantages of using a myocutaneous flap to reconstruct a wound on the distal extremity is that the resultant flap tends to be rather bulky, thus it is cosmetically less acceptable. If the patient gains a significant amount of weight, the flap usually will become bulkier due to deposition of adipose tissue. The second disadvantage of a myocutaneous flap based on the perforator system in dogs is that survival of the skin portion is inconsistent. The survival of the skin pedicle when developed on the axial pattern blood supply is consistent. Skin grafting over the muscle flap is more successful in veterinary patients.

Trapezius Free Muscle Flap^23,24

Uses

The cervical portion of the trapezius can be used for reconstruction of distal extremity and facial wounds. It is a fairly sizeable muscle flap and can therefore be used to reconstruct moderately large wounds. The muscle may be harvested with the omocervical skin flap to form a myocutaneous flap. This composite myocutaneous flap, however, tends to be very bulky when used for reconstruction of distal extremity wounds.

Blood Supply

The cervical portion of the trapezius muscle is a relatively thin and broad muscle with the superficial cervical artery and vein serving as the dominant pedicle. This muscle is useful for reconstruction of distal extremity and facial wounds.

The cervical part of the trapezius muscle has a type II blood supply. The dominant pedicle consists of the superficial cervical artery and vein which enters the cranial aspect of the muscle. The blood supply within the muscle can be visualized on the under side of the muscle. The vascular pedicle is about 5 cm long and the artery and vein diameters are approximately 1.5 mm and 4 to 5 mm, respectively. Numerous side branches arising from the superficial cervical artery and vein enter the omotransversarius, deltoid, supraspinatus and brachiocephalicus muscles and need to be ligated and divided during the dissection. The pedicle has a relatively thick cuff of fat that can be safely removed in order to skeletonize the vascular pedicles.

Procedure

A skin incision is made 5 cm cranial and parallel to the full length of the spine of the scapula. Next the fascial attachment between the cervical trapezius muscle and the omotransversarius muscle is incised with a pair of scissors (Figure 8-21).
 

The omotransversarius is retracted ventrally to expose the superficial cervical artery and vein (Figure 8-22). Branches of the superficial cervical artery and vein extending into the omotransversarius, acromial deltoid, supraspinatus and the brachiocephalicus muscles are ligated and divided to free the pedicle. If a skin paddle is not included in the flap design, the direct cutaneous artery and vein are ligated and divided. At this point the fat and prescapular lymph nodes can be removed from the pedicle using very gentle dissection and ligation of any side branches. The attachment of the trapezius muscle to the dorsal spinous processes and the spine of the scapula are incised. The trapezius muscle is flipped over which will expose the blood supply from the superficial cervical artery and vein (Figure 8-23).

A prominent dorsal venous extension from the superficial vein, beyond the branch that enters the trapezius, is ligated and divided. At this point the entire trapezius should be completely free other than being attached to its vascular pedicle.
 

Rectus Abdominis Free Muscle Flap^25,26

Uses

The rectus abdominis muscle is very useful for distal extremity, facial, and intraoral reconstruction. Because the flap is long, it can be revascularized to recipient vessels that are distant to the primary wound bed.

Blood Supply

The rectus abdominis muscle is thin and flat and extends from the first rib to the brim of the pelvis. The abdominal portion of the rectus abdominis can be used as a free flap. The muscle has multiple tendinous intersections located along its length. The rectus muscle has a type 3 blood supply. The blood supply to the rectus abdominis is from three sources: the cranial epigastric, caudal epigastric, and segmental lateral perforator arteries and veins. The caudal epigastric vessels join the caudal superficial epigastric vessels from the mammary chain to form the pudendoepigastric vessels. In some dogs the pudendoepigastric vessels are absent, leaving the caudal superficial epigastric and the caudal epigastric vessels to originate directly from the deep femoral artery and the external iliac vein.

The caudal two-thirds of the abdominal part of the muscle is perfused by the caudal epigastric artery and vein. The primary angiosome based on the caudal vascular pedicle extends approximately to the third tendinous intersection. Based on the fact that the caudal pedicle is perfusing a much larger portion of the muscle, a free rectus muscle flap should be based on this set of vessels.

The caudal epigastric artery and vein enter the caudolateral aspect of the rectus abdominis muscle near the inguinal ring. The pedicle is about 2 to 3 cm long, and the artery and vein diameters are 1 mm and 2.5 mm, respectively; by harvesting the pudendal artery and vein, the diameters of the vessels are greatly increased.

Surgical Procedure

A ventral midline skin incision is made from the xiphoid process to the cranial border of the pubis. In male dogs a parapreputial incision is made. The initial skin incision is deepened to the level of the linea alba. Subcutaneous tissues are then dissected off the superficial rectus sheath.

The superficial rectus sheath is incised starting at the external inguinal ring and extended cranially over mid portion of the muscle. The muscle is dissected out of its sheath with a combination of blunt and sharp dissection. Dorsally, the deep rectus sheath, which is less adherent to the muscle is bluntly dissected. Perforators entering the lateral aspect of the muscle are ligated and divided. The flap is transected at the cranial border and is reflected caudally.

The caudal epigastric artery and vein are ligated just proximal to the caudal superficial epigastric vessels and divided. The superficial rectus sheath is closed with 0 PDS in a simple continuous suture pattern. Subcutaneous tissues and skin are closed routinely.

Table 8-2 summarizes important characteristics of the trapezius and the rectus abdominis muscle flaps.
 

  
References

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