Lines of skin tension

The normal tension that exists in skin is a result of elastic fibers in the dermis. These fibers cause the edges of the wound to retract when the skin is incised, converting a linear wound into an elliptical wound. Lines of skin tension (i.e., Langer’s lines) have been referred to as lines of maximal skin tension. However, because skin is anisotropic (i.e., lacking similar properties in all directions) and because these lines of tension can be influenced by movement of an anatomic part, Langer’s lines are more accurately referred to as static or relaxed lines of skin tension.¹⁹ Different from static forces but also important are the dynamic forces on skin, which are created by the contraction of underlying muscles. Wounds that are at right or oblique angles to these static and dynamic forces gape more widely and heal with a wider scar than do wounds created parallel to these lines. Incisions or lacerations made at oblique angles to the static or dynamic lines of tension take on a curvilinear shape. Incisions made perpendicular to these lines tend to gape widely, are subject to greater tension, and require more sutures to approximate their edges, and consequently tend to develop a wider epithelial scar. A surgeon can determine the direction in which to close a wound or to create a fusiform incision to remove a lesion by assessing the direction of maximal extensibility of the skin. Both static and dynamic lines of tension should be determined. Thus, the ideal incision is made parallel to these lines of tension, and wounds should be closed in a direction that averts or minimizes skin tension.²⁰ Lines of tension of humans²¹ and dogs²² have been well described, but those of horses have been investigated only to a small extent.²³

Inherent extensibility

Inherent extensibility refers to the skin’s normal capacity to stretch while an anatomic part is at rest.^24,25 This property allows the surgeon to excise a small fusiform‐shaped piece of skin and perform primary closure of the resultant wound. A skin biopsy punch wound model showed that lines of maximal skin extensibility of the equine carpus, upon flexion and extension, run in the same direction as Langer’s lines, which are parallel to the limb’s long axis.²⁶ The best direction for an incision, therefore, is straight and parallel to the lines of maximum extensibility. The edges of the wound have the least tension when they are approximated parallel to the direction of maximal extensibility.

Pinching the skin to elevate it over the proposed site of excision of a tumor or scar or over a site of proposed reconstruction of a wound provides a rough estimate of the skin’s inherent extensibility.^4,25,27 Maximal skin extensibility should be assessed with the anatomic part in its normal position, at rest, followed by movement of the part (e.g., flexion and extension of the joints of the limb), which can greatly influence extensibility.²⁸

Mechanical creep and stress relaxation

Mechanical creep and stress relaxation represent two different kinds of biomechanical events that occur, within minutes to hours, in viscoelastic tissue placed under a constant load or strain.²⁹ Mechanical creep is the biomechanical property that allows skin, when placed under a constant load, to stretch, over time, beyond its normal limits of extensibility.²⁵ Creep is calculated by applying a constant amount of tension or load on the tissue and measuring the amount of tissue deformation that occurs over time. Creep is, therefore, described as tissue deformation per given length of time at a constant load. Creep occurs because randomly oriented collagen fibers in the dermis straighten longitudinally and become aligned parallel to the stretching force, allowing the skin to stretch.³⁰

Stress placed on skin, when skin is loaded rapidly to a finite strain while its length is held constant, decreases with time.²⁹ This viscoelastic property of skin is referred to as stress relaxation. To measure stress relaxation, a fixed amount of tension that results in a certain degree of deformation or elongation is applied to the skin. The tension is held constant for a period of time, after which the tension within the tissue is measured. The decrease in tissue tension during this time period represents the stress relaxation of the tissue.²⁹ Stress relaxation can be achieved during surgery by placing the skin under constant or cyclic strain. Over time, the amount of strain required to maintain elongation decreases, and the skin becomes less taut. This biomechanical property of skin explains why the tension on a suture line decreases within hours after surgery.^4,25 The amount of tension required to successfully achieve stress relaxation in the clinical situation is unknown, but excess tension on a suture line may compromise blood flow to the skin edges, resulting in ischemia and dehiscence. Sound clinical judgment is required, therefore, to make use of this important property of skin.

The time‐dependent reorientation of collagen fibers that occurs within a viscous matrix was assumed to account for all of the viscoelastic behavior of extensible connective tissue.³⁰ A recent study, however, found no obvious reorientation of collagen fibers, when low loads of tension were applied to skin during the time course of the stress‐relaxation and creep experiments. These results suggest that viscoelastic properties of skin reside not only within the collagen fibers but also at the fiber–extracellular matrix interface.³¹ The skin’s time‐dependent deformation is significantly influenced by the dermal viscoelasticity primarily, up to a critical load beyond which it is controlled by the outermost aspect of the epidermis, particularly the stratum corneum.³²

Biologic creep

Biologic creep is different from mechanical creep in that the skin is expanded rather than stretched and thinned.²⁵ Biologic creep is the slow expansion of tissue, over weeks to months, that accompanies pregnancy or obesity. It also occurs when a subcutaneous mass, such as a tumor, enlarges slowly. The skin is not thinned, as with mechanical creep, but gradually responds by increasing epidermal mitotic activity and in‐growth of blood vessels and cells into the dermis in proportion to the growth of the mass; both of these processes allow more skin to develop and an increase in the blood supply to the expanded skin.^25,33

Developing a surgical plan

Mobilizing adjacent tissue to cover a full‐thickness skin defect requires careful planning. Skin extensibility should be determined while the affected anatomic part is in a normal relaxed position (inherent extensibility), as well as during movement (maximal extensibility) by puckering the skin either over or adjacent to the wound or lesion to be removed (Figure 10.1). In some cases, extensibility can be evaluated sufficiently by elevating a lesion to be excised with the thumb and forefingers. Evaluating skin extensibility in a systematic manner delineates the regions of skin available for relief of tension and mobilization and for forming a skin flap.

The lines of skin tension and maximum skin extensibility on the limbs are parallel to the long axis.^12,26 Wounds that are parallel to the limb’s long axis, therefore, are not subjected to the same amount of tension during flexion or extension as are wounds that are oblique or at a right angle to the limb’s long axis.³ Wounds located on the dorsal or palmar/plantar surface that are perpendicular or oblique to the limb’s axis typically separate widely when the limb is flexed or extended (Figure 10.2), and consequently, postoperative immobilization of such wounds is usually required. When removing a lesion from a limb, the longitudinal axis of excision should be parallel to the long axis of the limb whenever possible. Skin is then mobilized in a transverse plane (lateral to medial) rather than from proximal to distal, which would subject the wound to greater stress.

To determine in what direction to suture a wound or excise a lesion from the proximal portion of a limb, the trunk, or the neck, the affected structure should be manipulated through its full range of motion prior to making a final decision regarding skin extensibility (Figure 10.3a). During surgery, the affected structure should be maintained in a position that creates the greatest tension so that the appropriate surgical approach is selected to close the wound and reduce the risk of postoperative dehiscence (Figure 10.3b). The choice of direction of excision is dictated by available skin and the configuration of the defect. A sterile ruler, sterile marking pen, or sterile methylene blue applied with a sterilized instrument can be used to draw this pattern on the skin (Figure 10.4). After incision, the skin is mobilized, or flaps are created by separating tissues in their natural cleavage planes deep within the subcutaneous tissue.

Presuturing

Presuturing is a technique whereby sutures are placed preoperatively to plicate skin over a proposed site of excision or to relieve tension on the wound’s margin.^34,35 The technique is performed by spanning the wound with heavy sutures, placed in an interrupted or mattress Lembert suture pattern, hours before a lesion is to be excised or a wound is to be debrided and sutured. It is most applicable to lesions or wounds of the body and proximal region of the limbs but sometimes may be useful for closing wounds of the distal portion of the limb.

Using heavy, non‐absorbable suture material, such as size 1 or 2 polypropylene or nylon, suture bites are placed in the skin perpendicular and 2 cm to 6 cm from each side of the lesion or the edges of the wound.^34,35 Tension applied to the sutures elevates and folds skin over the lesion or wound (Figure 10.5). The procedure is performed between 2 and 8 hours before surgery, with the horse sedated, after desensitizing the surgical site by using local or regional anesthesia. Placing these sutures 24 hours or more in advance of surgery may cause tissue surrounding the wound to become edematous, complicating closure.^{25,26,30,33–35,37} Presuturing relies on creep, which permits skin to be stretched beyond its inherent ability to stretch.³⁸ When skin is stretched to a certain length, the force required to maintain that length gradually diminishes via stress relaxation. Presuturing experimentally created wounds on pigs showed that closing a presutured wound required 40% less force than that required to close a non‐presutured wound.²⁵ When compared with simply undermining the surrounding skin, presuturing resulted in some tissue gain and an initial decrease in closing tension.³⁷

Although the original description of this technique suggests that presuturing avoids the potential complications of undermining,²⁵ the combination of presuturing and undermining (discussed later in this chapter) may provide advantages beyond presuturing alone. Because presuturing reduces skin tension, less undermining of adjacent skin is required to close a defect.⁴

Mesh overlay

Just as mesh can be used to bolster repair of an abdominal incisional hernia, so too can it be used to bolster repair of a wound sutured under excessive tension (Figure 10.6) or of a wound located in an area subjected to constant movement, such as a nostril or lip. Using this technique, a mesh [e.g., woven polypropylene (Bard, Davol), or knitted polyester (Mersilene, Ethicon)] is placed over the sutured wound. One edge of the mesh is sutured to the skin using 2‐0 non‐absorbable suture material placed in a simple interrupted pattern (Figure 10.6b). The open side of the mesh is sutured to the skin on the opposite side of the sutured wound using pre‐placed sutures of the same material, placed in a Mayo mattress pattern (Figure 10.6c). The pre‐placed sutures are tightened in unison and tied to transfer tension from the sutured wound to the mesh. To distribute tension over a wider area, two or more staggered rows of simple interrupted sutures anchoring the mesh to skin can be placed axial to each sutured edge of the mesh (Figure 10.6d). To minimize interruption of blood supply to the sutured wound, sutures anchoring the mesh should be placed perpendicular to the sutured wound, unless the mesh is placed in a highly vascular area, such as a lip or nostril.

Adjustable suture technique

Problems associated with delayed closure of a wound – particularly a wound on the distal aspect of a limb of a horse, such as retraction and reduced pliability of the wound margins and ineffective wound contraction – can be obviated to a large degree by placing constant tension on the wound’s margin with an adjustable suture, a technique referred to as augmented wound contraction.³⁸ To perform this technique, the wound and surrounding tissue are desensitized using local infiltration of anesthetic solution or regional anesthesia, after which a heavy suture is placed intradermally to span the wound’s margins using a continuous horizontal mattress pattern. Each suture bite is advanced slightly, so that the suture passes at an angle across the wound, allowing the suture to slide easily through the dermis. Each end of the suture exits the skin, and by applying tension to these suture ends, the skin on one side of the wound is drawn toward that of the opposite side. Each end of the suture is threaded through a button, and after tension has been applied to the margin of the wound by tightening the suture, the ends are anchored to the button using an umbilical clamp or split‐shot.

The margins of the wound are pulled closer together by tightening the suture daily. The most rapid decrease in the size of the wound occurs during the first 3 days.³⁸ Subsequently, the skin edges are apposed, if possible, or the wound is left to heal by second intention. The technique can be used not only to augment contraction but also to prevent a fresh wound from expanding. A commercially available wound closure system (Dynamic Tissue Systems™, Southmedic Inc) uses multiple anchors that are inserted in the skin on opposite sides of the wound. Tension on the wound’s edges is created when elastomer bands are affixed to the anchors.

Tension suturing techniques

Tension suturing techniques are most often used in conjunction with other skin mobilization techniques (e.g., undermining, which is discussed later).

Tension on skin sutures may be relieved by inserting deep dermal sutures with buried knots before inserting superficial skin sutures. These dermal sutures also align the skin edges, easing insertion of the superficial skin sutures. The surgeon should place only as many sutures as are necessary to reduce tension on the wound, especially when the wound is contaminated, because these deep dermal sutures act as foreign bodies and can become a nidus for infection.

Sutures placed in a horizontal or vertical mattress pattern, well away from the skin margins, can also be used to decrease tension at the wound margins. The wound margins are then approximated, without tension, with sutures placed closer to the margin, using a simple interrupted or a vertical mattress pattern. The vertical mattress pattern is less likely to interfere with blood supply than is the horizontal mattress pattern. Sections of rubber tubing (supports) placed under the loops of the tension sutures distribute pressure and may prevent interruption of blood flow to skin underlying these sutures (see Figure 9.19).

The use of rubber tubing to distribute pressure, however, should be reserved for regions that cannot be supported by bandaging (e.g., the neck, trunk, and proximal portion of the limb) because placing a pressure bandage over the sections of rubber tubing may cause pressure necrosis in the skin underlying these supports. The tension sutures are removed in 5–7 days, by which time tension on the approximating sutures should have greatly dissipated.

The far–near–near–far pattern serves as both a tension suture, by providing a pulley‐like effect, and an approximating suture.³⁹ The far component relieves tension on the near component, which approximates the margins of the wound. The loop in the suture greatly increases the strength of the suture and of the tissue in which it is placed. Excessive tightening of the suture tends to invert the skin edges. Tension on a sutured wound may also be relieved by closing it with a walking suture pattern. For a complete discussion of suturing patterns and techniques, see Chapter 9.

External stretching devices for primary or delayed primary closure

Externally applied devices designed to pull, thus stretch, the skin surrounding a wound or excisional site are commercially available (e.g., Sure‐Closure^TM, SyntheMed, Inc.; Wisebands Skin Closure Device^TM, 4 Med Ltd; TopClosure®, IVT Medical Ltd).^40–42 These skin‐stretching devices apply controlled traction to skin surrounding a large tissue deficit thereby allowing the deficit to eventually be sutured.

The use of a skin‐stretching apparatus composed of Velcro (Velcro USA Inc) has been described in dogs. This apparatus uses pads of Velcro, attached to the skin using cyanoacrylate glue and connecting cables of Velcro.⁴² Placement of the adherent pads in relation to the skin edge can be adjusted according to the amount of skin needed to cover the wound; thus, large areas of skin may be mobilized adjacent and more distant to the wound. Tension on the cables is adjusted every 6–8 hours, and in most cases, enough skin is recruited to allow closure within 48–72 hours. The system can also be left in place for 2–4 days after wound closure to offset excessive tension at the suture site. Separation of the pad from the skin during the process of stretching is the most common complication associated with the use of this system. This apparatus may be applicable to horses, although to the authors’ knowledge, its use to stretch the skin surrounding wounds of horses has not been reported.

Another system (Sure‐Closure^TM) attaches the skin to the undersurface of the device using hooks that face the wound and abut against an 8‐cm long intradermal needle inserted on each side and parallel to the wound. A 7.5‐cm long screw‐driven tension rod is used to increase the tension (Figure 10.7).⁴¹ A tension‐gauge on the device is not engaged until tension applied to skin exceeds 1 kg, and when the tension exceeds 2.5 kg, a safety clutch disengages the device, thus neutralizing tension. The advantages to this system are that: (1) the device can be applied, using sterile technique, by local infiltration of anesthetic solution or regional anesthesia; (2) undermining of the skin edges is not required; (3) the stretching force on the skin margin is spread over a wide area (i.e., over 8‐cm long intradermal needles); (4) traction can be applied incrementally with periods of tension relaxation; and (5) the skin can be sutured while the device is in place. The disadvantage is that applying tension to a wound wider than 7.5 cm is difficult.

The Wisebands Skin Closure Device^TM is composed of a stretching device and a 50‐cm long polypropylene band that encircles the skin and underlying tissues (subcutaneous fat, fascia, and muscle), allowing closure of complex wounds.⁴⁰ A gauge permits application of 1 kg/cm² of tension to tissues surrounding the wound; if tension on the tissue exceeds 1 kg/cm², the tension placed by the device relaxes to a lesser tension. Tissue is stretched intraoperatively by tightening the band, pausing until the tissue relaxes, thereby releasing tension on the device. The band is then retightened until the wound edges can be approximated with sutures. The device may be left in place for several days if the sutured wound is at risk of dehiscence because of moderate tension. If the edges of the wound cannot be approximated completely during the operation, the open wound is dressed appropriately, and the wound is stretched daily until the wound can be sutured. The advantage to this system is that it may be applied to assist the closure of complex wounds, no matter their shape or depth, and it can be applied to most regions of the body. The disadvantages are that the horse must be anesthetized for the device to be applied, and, because the gauge allows only 1 kg/cm² of tension to be applied, which is considerably less than the 3 kg/cm² of tension that can be safely applied to stretched skin (at least in humans),⁴¹ a longer period is required to close the defect than when using the Sure‐Closure^TM device. The Wisebands device^TM is supplied for single use only.

The TopClosure® system is comprised of two attachment plates, which are placed on either side of the wound and opposite each other, and a connecting approximation strap, which spans the wound. The attachment plates are secured to the skin, 1.5–2 cm from the wound edge, using the adhesive on the underside of the attachment plate, and can be further secured, if skin tension is great, by using skin staples inserted through holes in the application plate. After the application plates are secure, the approximation strap is inserted through the lock/release mechanism (LRM) on one attachment plate and crossed over the wound to lock in the opposite attachment plate’s LRM. The number of TopClosure® sets needed, the incremental stretch intervals, and the period of application, depend on the elasticity of the skin, the size of the wound, and the location of the defect. The TopClosure® system incorporates a safety mechanism to limit tension to ~3 kg, which prevents over‐stretching of the skin and collapse of LRM on the system. Heavy nylon sutures (size 0 to 2) may be indicated for high‐tension closure.⁴³

Tension sutures, using non‐absorbable 0 to 2 nylon or polypropylene, can be applied to this system to further reduce tension for high‐tension skin closure. Tension sutures are placed by penetrating the skin through one of the designated holes in the attachment plate, passing the suture through the deep subcutaneous tissue on both sides of the wound, and exiting the suture through the skin and the hole in the attachment plate on the opposite side of the wound. Advantages to this system are that: (1) it can be applied using local anesthesia; (2) undermining of the skin edges is not required; (3) traction can be applied incrementally with periods of tension relaxation; (4) the skin can be sutured while the device is in place; and (5) the device can be left in place for days to weeks to reduce the tension on the sutures used to close the wound primarily.

A simple mechanically assisted method of closure that uses hypodermic or spinal needles or Kirschner wires (K‐wires) combined with sutures has been described for use in dogs (Figure 10.8).⁴⁵ Using this technique, the Kirschner wire or long needle is inserted along each side of the wound, approximately 0.5 cm from the wound’s edge. The wire or needle is inserted parallel to wound’s margin in such a manner that the wire or needle is exposed at regular intervals. The wound is covered with an appropriate dressing, a gauze pad, and a cylinder, such as the barrel of a syringe or a syringe case, into which holes have been drilled. Heavy, simple interrupted non‐absorbable sutures are passed around the exposed portions of the needles or wires on each side of the wound and passed through the holes in the cylinder and tied together under tension. As with the TopClosure® system, the incremental stretch intervals and the tension applied to the needles or wire depend on the elasticity of the skin, the size of the wound, and the location of the defect.

These mechanically assisted systems of closure rely on mechanical creep and stress‐relaxation properties of skin. In general, the greatest gain in skin stretching is achieved within 48–72 hours after applying any one of these systems.⁴² When skin and subcutaneous tissue of acute clean wounds are normal, however, the skin can be stretched rapidly (~20–30 minutes).⁴¹ In a clinical study, using the TopClosure® system, acute stress relaxation was achieved during surgery by repeated load cycles of 2–3 minutes of high tension, which inflicted either blanching or shininess at the wound edges, followed by 5 minutes of relaxation. Mechanical creep was achieved by daily pulling the wound edges toward each other in increments of a few millimetres over a period of a few days.⁴³ The time required for stretching skin surrounding chronic wounds is often prolonged as a result of edema and fibrosis along the wound’s margin. Applying tension in cycles, with periods of relaxation between loadings, allows for far greater elongation of tissues, beyond their intrinsic capabilities to extend.^41,46,47 The use of these devices, to the authors’ knowledge, has not been reported in the horse. Regardless, the authors believe these systems may hold some promise for difficult‐to‐close wounds of the body, proximal portion of the limbs, and possibly some wounds of the head.

Intraoperative tissue expansion

Intraoperative tissue expansion has been advocated as a means of decreasing tension on a wide wound while the wound is sutured. The technique involves rapid expansion of skin surrounding a wound by applying tension with towel forceps attached to the skin edges or by advancing the skin edges by inserting sutures in a near‐and‐far pattern temporarily until the edges of the wound have been approximated. Skin on either side of a wound can also be expanded intraoperatively by inserting a monofilament suture in a simple continuous pattern across the wound.⁴⁸ Both ends of the suture are left free. One free end is grasped at the surface of the skin with a hemostat, and tension is placed on the suture by pulling on the other end with a second hemostat. A third hemostat is placed proximal to the second hemostat, just above the skin, and is left in place long enough to produce stretching by stress relaxation. The second and third hemostats are then used in an alternating pulling‐and‐holding fashion until the skin can be approximated with interrupted sutures placed within the simple continuous suture. These techniques of intraoperative tissue expansion cause creep and stress relaxation but to a lesser extent than does preoperative tissue expansion.

Another intraoperative technique of wound expansion involves inserting an inflatable balloon (e.g., a Foley catheter) subcutaneously for several minutes before beginning suturing.⁴⁹ The original theory, postulating that this technique causes mechanical creep, has been challenged because the histologic changes normally attributed to mechanical creep are lacking.⁵⁰ The undermining of the skin required to place an inflatable balloon probably accounts for the decrease in closing tension on sutures.^51,52 An experiment designed to compare the amount of force required to close a wound before and after undermining its edges and after intraoperative expansion using an inflatable balloon, found that undermining resulted in a significant decrease in the force required to close the wounds, whereas intraoperative expansion did not significantly decrease the closing tension on sutures.⁵²

Silicone elastomer (bag) expander

A technique that expands the normal tissue adjacent to the defect by stretching it 2‐ to 3‐fold has been described.^53–55 This is accomplished by inserting a silicone bag (Tissue Expander^TM, Cox UpHoff) into the subcutaneous tissues adjacent to a cosmetic defect that is to be reconstructed (Figure 10.9a). As a rule of thumb, the area of the elastomer expander’s base should be 2.5 times that of the defect to be closed.^56,57 Attached to the expander by a tube is a self‐sealing injection dome, which is implanted conveniently nearby, yet remote to the silicone expander (Figure 10.9b). After the incision created to insert the implant has healed (~15 days), tissue is expanded by injecting sterile saline solution into the injection dome to inflate the expander until the skin becomes taut (Figure 10.9c). The skin over the injection portal is cleansed with alcohol prior to administering the injection. Similar injections are carried out at 4–7‐day intervals until the skin has expanded sufficiently to permit closure of the wound (Figure 10.9d). The volume injected is dependent on the size of the expander and on signs of discomfort displayed by the horse. To account for skin retraction, which occurs after the removal of the expander, the area of expanded tissue should be at least 20% greater than that estimated to be required to cover the defect.⁵⁸ After expansion is sufficient, the expander and injection dome are removed, and the expanded skin is mobilized to cover the defect. Microscopic changes seen in the skin are consistent with biologic creep.⁵⁰

Generally, the skin can be expanded to three times its original surface area, but this must be done slowly to prevent ischemic necrosis. The epidermis responds to tissue expansion by increasing its mitotic activity, resulting in a net increase in epidermal tissue. The dermis thins in response to expansion, and the thinning is compensated for by formation of a thick fibrous capsule surrounding the expander.⁵⁹ The thickness of expanded skin plus the fibrous capsule, which forms in a matter of days, is the same thickness as normal unexpanded skin.⁵⁰ The fact that normal skin with the same hair color and texture directly adjacent to the wound can be used to cover the defect makes this technique somewhat attractive, although more experience is required before its full benefit and limitations for use in horses become known.

Undermining surrounding skin

Undermining the skin can be used alone or in combination with other tension‐relieving techniques (e.g., mesh expansion, skin flaps, etc.) and is a time‐honored approach for reducing tension on a sutured wound. Undermining releases the dermis from its attachments, permitting the edges of the wound to be approximated with less force. It is commonly used in conjunction with suturing techniques that reduce tension on the primary suture line (i.e., the sutures that approximate the skin edges).

The skin surrounding a wound on an extremity is undermined by separating the skin and subcutaneous tissue from their underlying attachments to fascia; occasionally, the cutaneous muscle is also undermined where it exists in the neck and trunk. As with any surgery, the tissue should be handled gently, and every effort made to protect its blood supply. The wound should be debrided and cleaned before its edges are undermined so that undermining does not spread debris and bacteria deeper into tissues.

Skin can be undermined using blunt or sharp dissection. Blunt dissection is accomplished by opening the blades of a scissor after the scissor has been inserted, with its blades closed, into the desired tissue plane. The procedure is repeated along the margin of the wound until the tissue has been adequately undermined. Tissue can also be separated bluntly by inserting the handle of a scalpel and moving the handle in a back‐and‐forth motion. Blunt dissection has the advantage of minimizing the damage to the cutaneous blood supply, but it tears tissue, creating more trauma, and is of little use when attempting to undermine the margin of chronic fibrotic wounds. The authors find this approach most useful when undermining skin surrounding a wound in a relatively loose‐skinned region (e.g., the neck and some parts of the trunk).

Even though sharp dissection using a scalpel (Figure 10.10) or scissor (Figure 10.11) has the disadvantage of potentially transecting blood vessels, disruption of blood supply is unlikely to be consequential when undermining skin surrounding a wound on an extremity of a horse, making this technique the authors’ preferred method of undermining skin of the extremities. When using a scalpel, one deep, cleanly made incision, following the contour of the anatomic part, to separate the attachment of the subcutaneous tissue from the underlying fascia, is preferred to multiple small, shallow incisions, which create more trauma and a greater likelihood of hemorrhage. When using a scissor, rather than making multiple small snips to separate the tissue, the authors prefer to insert the sharp blades in a partially open position, which, with pressure, converts it into a cutting instrument (Figure 10.11). In the authors’ opinion, multiple snips with a scissor creates more tissue trauma and, therefore, should be avoided.

The depth at which the skin is undermined depends on the neurovascular anatomy of the region, so this should be reviewed prior to surgery to avoid injury to neurovascular structures. The dissection should be as deep as possible between the subcutaneous tissue and deep fascia when undermining skin of the distal portion of the limbs.^12,20,27 Skin of the trunk should be elevated beneath the cutaneous muscle when that muscle is adhered to the dermis; if subcutaneous tissue is present, it is dissected in the manner described for dissection of subcutaneous tissue surrounding a wound in the distal portion of the limb. In most cases, these planes of tissue are easily identified; an exception is an old, scarred wound in which readily identifying a tissue plane is difficult until dissection has been continued for some distance. In this situation, beginning with an incision in normal skin adjacent to the wound, or extending the dissection until normal skin is encountered, is appropriate. This enables the surgeon to identify the proper tissue plane.²⁷

The amount of skin that must be undermined can be roughly determined by pulling the skin edges together with towel forceps. As a general rule, for fresh wounds, a distance equal to the width of the defect itself should be undermined on each side of the wound.²⁰ For instance, if the defect is 5 cm from edge to edge, each edge can be undermined 5 cm. The skin edges are then drawn together using towel forceps, and the tension is assessed. If the tension appears too great, undermining can be extended half as much again (i.e., 2.5 cm on each side of the wound).^4,20 Given the potential for disrupting the cutaneous blood supply when undermining skin, careful judgment is needed. If continued undermining is likely to compromise the blood supply to the skin, combining undermining with another skin mobilizing procedure (e.g., relaxing incisions, Z or V–Y plasty) should be considered.

The extent to which the skin of horses can be undermined without damaging the blood supply and causing necrosis has not been established. The skin of companion animals may be undermined quite extensively without causing necrosis as long as the blood supply remains intact, and clinical experience suggests the same is true for horses.^27,39,60 Used in conjunction with a mesh expansion technique on the limbs of dogs, 360‐degree undermining of the skin was not associated with any major complication.⁶¹

If hemorrhage appears to be a problem after undermining skin, the surgeon should consider using a drain and a properly applied pressure bandage to prevent the deleterious effects of a hematoma, such as increased tension/pressure on the suture line and the presence of a medium for bacterial growth. For more information regarding the use and application of drains, see Chapter 9. For information regarding pressure bandaging, see Chapter 7.

Severing subcutaneous tissue attachments to the underlying fascia allows the wound’s edges to be pulled toward one another and makes use of the elastic properties of skin. The closing tension for a defect varies greatly among anatomic regions and is correlated to the amount of loose skin in the area.⁵² The amount of tension relief derived from undermining diminishes as the extent of undermining increases. Frequently checking the amount of tension relief achieved is important to avoid unnecessary undermining or to prompt the decision to use other methods of relieving tension prior to the onset of deleterious effects of undermining (e.g., interruption of the cutaneous blood supply and formation of a hematoma). Much of the elasticity of skin surrounding a chronic, scarred wound has been lost, making extensive undermining necessary to achieve the desired result.