Composition

Sutures are made from naturally occurring substances, synthetic polymers, or metallic fibers. Natural materials, such as cotton and silk, tend to elicit a substantial inflammatory reaction and have mostly been replaced by synthetic materials.

Morphology

Sutures can be broadly classified as monofilament or multifilament. Sutures constructed of one filament (i.e. single‐stranded) are referred to as monofilament, whereas those constructed of multiple filaments (i.e. multi‐stranded) that are either braided or twisted together are referred to as multifilament. Multifilament sutures have greater strength and pliability than monofilament sutures of the same material and thus are easier to handle and manipulate for tying knots and are less susceptible to catastrophic damage from crushing or nicking. Moreover, less tension is required to ensure good apposition of tissues when using multifilament sutures, because they have a higher coefficient of friction than monofilament sutures; this characteristic also improves knot security. Multifilament sutures, however, have greater tissue drag and thereby injure tissue further when inserted and removed. Multifilament sutures have interstices between fibers, making them more capillary than monofilament sutures, which in turn, makes them more likely to absorb and retain fluid and bacteria.^1,2,4,5 Recent studies reported that biofilms show a preference for growing in the braid grooves formed between filaments; thus, multifilament sutures might be a predisposing factor for the formation of biofilms.^6,7

Suture materials, especially multifilament sutures, can be coated with a variety of compounds, broadly characterized as water soluble or insoluble, including antibiotics, to increase pliability, reduce capillarity, decrease tissue drag, improve tying characteristics, facilitate knot formation, and lessen the likelihood of surgical site infection (SSI). Nevertheless, coatings, especially water‐insoluble ones, tend to reduce knot security compared with similar uncoated suture.

Antibiotic coatings are now available on a number of sutures. The most common antibiotic coating contains triclosan (Coated Vicryl Plus Antibacterial; Monocryl Plus Antibacterial; Polydioxanone Plus Antibacterial – Ethicon, Johnson & Johnson Company, Inc.), a bacteriostatic drug that has been shown to inhibit the in vitro growth of both wild‐type and methicillin‐resistant Staphylococcus aureus and Staphylococcus epidermidis ⁸ and to reduce the incidence of SSI after clean, clean–contaminated, and contaminated surgery⁹ without affecting tissue reaction, healing response, or the absorption profile of coated sutures.¹⁰ The use of triclosan in many consumer products including soaps, toothpastes, shower gels, deodorants, etc., over the years has, however, led to the emergence of strains of bacteria that are triclosan resistant. A new synthetic antimicrobial suture coating, poly[(aminoethyl methacrylate)‐co‐(butyl methacrylate)] (PAMBM), inspired by natural antimicrobial peptides (AMPs), has been shown to provide bactericidal activity against both wild‐type and methicillin‐resistant S. aureus compared with the bacteriostatic property of triclosan in vitro. Because of their simple physical mechanism of action, AMPs display broad‐spectrum activity against Gram‐positive and Gram‐negative bacteria, and it appears difficult for bacteria to develop resistance against them.¹¹ Such products have not yet been tested in vivo and are, therefore, not available commercially.

A variant in the morphology of suture material is the presence of surface barbs, created by placing linear nicks along the suture material. Self‐anchoring, barbed monofilament sutures have been developed (V‐Loc^TM 90, 180 or PBT Wound Closure Device, Covidien, Medtronic; Quill^TM PDO Knotless Tissue‐Closure Device, Angiotech, Surgical Specialties Corporation, Inc.) in an attempt to eliminate knots, thus providing increased tensile strength and a stronger repair, and shortening surgical times. Knots are known to be the weakest portion of a suture line. Indeed, implanting, tying, and especially knotting of suture decrease its strength by up to 40%.⁴ Knot‐tying techniques are described later in this chapter, and the influence of various suture materials on security of the knot and suture line is summarized in Table 9.3. Contrary to expectations, studies report that knotless absorbable barbed suture devices yielded wound strength and tissue reaction scores that were comparable to traditional non‐barbed sutures secured with knots.^12,13 According to Rosen,¹⁴ barbed sutures have, however, revolutionized the approach of plastic surgeons not only by shortening surgical times but also by enhancing their ability to quilt and powerfully lift tissue and to eliminate surgical drains. Although little is yet known about the use of barbed sutures in the management of wounds of horses, the security and utility provided by this suture look promising.

Degradation behavior

Absorbable sutures degrade and lose tensile strength, usually within 60–90 days after implantation, either by hydrolysis, enzymatic digestion, or phagocytosis.⁴ Their rate of absorption varies depending on their composition, coating, size, biologic environment, and species into which they were implanted. The environment surrounding the suture material is a major determinant of the rate at which the material is absorbed. Thus, although general times for absorption are offered as guidelines for clinical use, considerable variations in the rate of absorption and residual strength are noted. As an example, exposure to bodily fluids, such as urine, especially infected urine,¹⁵ or gastric secretions, substantially accelerates the rates of degradation and absorption.

Non‐absorbable sutures retain tensile strength for longer than 90 days, and, consequently, they are mainly used when extended support of the wound is required. Non‐absorbable sutures are not degraded substantially after implantation and are usually encapsulated or walled off by fibrous tissue.

It is widely accepted that synthetic absorbable and non‐absorbable suture materials are superior to the natural suture materials because the latter elicit more reaction, including inflammation; thus, the use of synthetic suture materials should be favored. The most important characteristics of commonly used absorbable and non‐absorbable, natural and synthetic, monofilament and multifilament suture materials are summarized in Table 9.1 and Figure 9.1.

Suture size

Suture size is standardized to U.S. Pharmacopeia (USP) standards. The USP size (expressed with zeroes, with 12‐0 being the smallest and 7 the largest) indicates a specific diameter necessary to produce a predetermined tensile strength, which varies among the different categories of suture material. The tensile strength of a suture material is the maximal stress that the suture can withstand before breaking. Tensile strength varies among suture materials and is correlated to suture diameter. The smaller the suture size, the less tensile strength the suture has. Sutures should be as strong as the tissue through which they are placed.² The tensile strength of the selected suture material, however, need not exceed that of the tissue it secures, and the rate at which the suture material loses tensile strength and that at which the wound gains strength should concord as much as possible to avoid dehiscence.² Consequently, the smallest‐diameter suture that adequately apposes and holds the wound’s edges during healing should always be preferred.^1,19

Using an unsuitably large suture introduces more foreign material into the wound than is necessary, interferes with the blood supply, and may cause excessive inflammation, all of which alter the tissue’s architecture and ability to resist infection.¹ Excessive tension on sutures leads to tissue necrosis, which favors wound dehiscence by interfering with local blood supply and increasing edema of the wound and surrounding tissues.^20–23 Most suture materials are recognized by the body as foreign and, therefore, can potentiate the risk of wound infection when used in excess (i.e., too much suture, too large a diameter, or too many knots).^1,24

If suturing alone cannot reduce the tissue tension to an optimal level, other tension‐relieving techniques to mobilize the wound’s edges should be used before closing the wound.^1,19,25 The reader is referred to Chapter 10 for more information about mobilizing skin to reduce tension upon closure.

Interrupted versus continuous suture patterns

The choice of an interrupted over a continuous suture pattern is somewhat contentious because both types of patterns have advantages and disadvantages. The major advantages of using an interrupted suture pattern are the ability of interrupted sutures to precisely control tension at each point along the wound, the possibility of making adjustments to improve alignment of the edges of an irregularly shaped laceration, and the minimal interference of the suture pattern with the skin’s blood supply.^1,19 Disadvantages of using an interrupted suture pattern include increased surgical time (to tie multiple knots and cut the suture ends), increased volume of foreign material within the wound when the sutures are buried, increased risk of wound dehiscence,³⁷ and poor economic use of suture material.¹⁶

In contrast, a continuous suture pattern is quickly placed, thus reducing surgical time; distributes tension evenly along the entire length of the wound; uses less suture material, thus reducing cost; and minimizes the number of knots, thus reducing the amount of foreign material within the wound (Figure 9.13).¹⁶ A continuous pattern also forms a better seal against fluid and air.³⁸ On the downside, the tension on a continuous suture line cannot be varied to the same degree as can tension on an interrupted suture pattern, and failure of the knot or suture material can have a disastrous effect on the closure. Suture used in a continuous pattern, therefore, must be handled carefully. Instrument‐induced trauma to the suture should be avoided (e.g., grasping the suture with needle holders or thumb forceps), throws of the knots should be applied correctly, and the knots should be secure. Because of their design, continuous suture patterns tend to compromise the microvascular supply to the edges of the wound.³⁹ If the compromise is marked, it may lead to the formation of edema that, in turn, can prolong the inflammatory phase of wound healing and delay the wound’s gain in tensile strength. The Ford interlocking suture pattern represents a compromise between interrupted and continuous suture patterns (Figure 9.16).

Inverting versus everting suture patterns

Disrupted tissue may be apposed using a suture pattern that inverts or everts tissues, depending on the wound’s location. Inversion is usually desirable only to close hollow viscera to prevent leakage, but excessive inversion reduces luminal diameter. Slight eversion to counter the tendency of edges of a cutaneous wound to invert during healing is desirable because slight eversion leads to the most cosmetic outcome after sutures are removed.¹

Tension sutures

Wounds suffering from a substantial loss of tissue are difficult to close without creating tension on the suture line. Some suture patterns offer mechanical advantages over others, by requiring less force to close the wound.⁴⁰ Although clinical experience suggests that a moderate amount of tension is acceptable in sutured wounds of horses,¹⁹ when the forces exerted by individual sutures increase to the point where sutures cut through tissue and restrict blood flow, tension sutures should be used. Tension sutures draw the wound edges together while minimizing the risk of vascular damage, which leads to necrosis and dehiscence.

Tension sutures, which are usually everting sutures (Figures 9.10a, 9.12a), placed well away from the wound edges to avoid strangulation of tissue, can be used either alone (Figure 9.20) or in combination with an appositional suture pattern (Figures 9.17, 9.18, 9.19). Tension sutures are usually preplaced. Skin edges are brought into apposition with the aid of towel clamps, and the preplaced tension sutures are tied. The edges of the laceration or incision are apposed using an appositional suture pattern (Figures 9.17b,c, 9.18, 9.19).¹

Walking sutures (Figure 9.24) are buried tension sutures that: (1) move skin progressively toward the center of the wound or the opposite margin of the wound; (2) distribute tension; and (3) obliterate subcutaneous dead space, thereby preventing the formation of a serum pocket.³