Selection and use of currently available Suture Materials and Needles

Introduction

Surgeons rely on suture materials to provide critical support of healing tissues during wound repair. A wide variety of suture material types have been developed, each with their own advantages and limitations. The general performance of suture materials is based on their distinct physical properties, handling characteristics, and biological properties. An ideal suture should have acceptable handling characteristics, knot security, and tensile strength. Besides predictable performance, sutures should remain strong enough to prevent disruption of the wound until healing is complete and, ideally, the suture should undergo complete resorption over time. The suture should be sterile, nonallergenic, noncarcinogenic, stable in a contaminated environment, and it should elicit minimal reaction when buried in tissue. In most cases, there are many suture material choices that would be acceptable for wound repair because many have similar general characteristics but are developed by separate manufacturers. However, there is no ideal suture for every procedure, largely because each wound is different and must be considered individually. An otherwise identical wound created in a similar body region may require different suture considerations due to such factors as degree of bacterial contamination, whether there is a local or systemic factor which would delay healing, and even how active the patient may be after surgery. The most critical factors related to the choice of suture include how long the suture is needed to support the wound, and the mechanical and healing properties of the tissue undergoing repair. The surgeon must understand the nature of the suture material, the biological forces in the healing wound, and the interaction of suture and tissues when selecting suture material. This chapter reviews the characteristics of commonly used and newer suture materials, and needles in small animal surgery. Various wound related factors are discussed, which provide the rationale for choosing appropriate suture materials and needles.

Suture Classification and Definitions

Suture materials are classified as absorbable or nonabsorbable, natural or synthetic, monofilament or multifilament, according to their structure and composition (Table 1-1).

Absorbable suture materials undergo degradation and rapid loss of tensile strength within 60 days, whereas nonabsorbable suture materials retain significant strength past 60 days. This definition can be misleading with respect to silk, cotton, linen, and multifilament nylon sutures because these materials are considered nonabsorbable, yet they lose a portion of their tensile strength within 4 to 6 weeks after implantation. Natural materials (chromic gut, silk) are absorbed by enzymatic degradation and phagocytosis, while the newer synthetic sutures are more predictably absorbed through nonenzymatic hydrolysis. In addition, synthetic sutures generally cause less tissue reaction than natural ones. Monofilament sutures are made of a single strand so they resist harboring of bacteria. Multifilament or braided sutures are woven or twisted from many smaller strands. In general, multifilament suture materials are easier to handle than monofilaments. Multifilament sutures (particularly uncoated ones) often create more friction (chatter) as they are passed through tissues when compared to the smoother monofilaments. Excess friction can cause suture-tissue sawing and cutout, especially when suturing friable tissues with a continuous pattern. Multifilament sutures can be capillary, or act as a wick. This quality is undesirable since fluid and bacteria can travel along the suture and contaminate adjacent areas. The chemical composition and coating influence the capillary nature of a suture. For example, coated caprolactam transports nearly twice as much fluid as uncoated polyester of the same suture size. Waxed silk is not capillary, in contrast to the highly capillary nature of uncoated virgin silk. Capillary suture materials are not recommended when sutures could penetrate or become exposed to contaminated or infected areas.

Suture Selection and Use

When choosing a suture material, certain general principles based on the strength of the tissue being closed, the rate of gain in wound strength after closure, and various biological and mechanical suture characteristics should be considered. After considering these factors, the surgeon may have several choices of appropriate suture material that would be acceptable for use in the wound. Selection can then be made on the basis of familiarity with the material, its ease in handling, and other subjective preferences, such as color, or needle selection.

Strength of Tissue

A suture should be at least as strong as the tissue through which it passes. A tissue’s ability to hold sutures without tearing depends on its collagen content and on the orientation of collagen fibrils. This explains why ligaments, tendons, fascia, and skin are strongest, muscle is relatively weak, and fat is weakest. Muscle has little suture-holding capability across its fibers and even less in the direction of the fibers. Visceral tissue, in general, ranks between fat and muscle in strength. Bladder and colon are the weakest hollow organs of the body, and stomach and small intestine are among the strongest. Tissue strength varies within the same organ and with the age and size of the animal.

The choice of suture size is based on the tensile strength of the tissue as well as of the suture material. Catgut and synthetic suture materials are sized according to either United States Pharmacopeia (USP) or metric gauge (Table 1-2). A larger numeric USP value means a larger-diameter suture. Stated numerically, the more zeros (0s) in the number, the smaller the strand. (e.g., 2 polypropylene is larger than 0, and 2-0 is larger than 4-0). The metric gauge is the actual suture diameter expressed in millimeters multiplied by 10. Stainless steel suture can be sized by USP, metric gauge, or Brown and Sharpe wire gauge. Ranges of suture size recommendations for various tissues and surgical applications are provided in Table 1-3. These guidelines are general and are based on currently available literature and my experience. Larger sizes are used in heavier animals, in critical suture lines such as the abdominal fascia, or in tissues closed under excessive tension. The surgeon should strive to use the smallest suture size possible for wound closure since this will result in less tissue trauma, allow smaller knots to be tied, and encourage the surgeon to handle the sutures and tissue more carefully. Oversized sutures can actually weaken the wound through excessive tissue reaction and tissue strangulation. To maintain maximum suture strength once the suture is removed from the packet, certain suture handling rules are suggested (Table 1-4).

Loss of Suture Strength and Gain of Wound Strength

To use absorbable sutures safely, the loss of suture strength should be proportional to the anticipated gain in wound strength. The relative rates of suture strength loss and simultaneous wound strength gain are important to consider. Fascia, tendons, and ligaments heal slowly (50% strength gain in 40-50 days) and are under constant tensile force. For these tissues, nonabsorbable sutures or the prolonged-degrading, synthetic absorbable sutures are indicated. Maxon® and PDS II® sutures can be used whenever an absorbable suture is needed, but these should be considered especially in wounds that are expected to require suture support for more than 3 weeks (such as abdominal wall fascia). Because visceral wounds heal relatively fast, often achieving most of their strength in 21 days, rapid to intermediate-degrading absorbable sutures (Table 1-1) are good choices. Rapidly-degrading synthetic sutures (Caprosyn®, Monocryl®, Vicryl Rapide®) are indicated in rapidly healing tissues such as the mucosal lining of the mouth or urogenital tract where suture removal is not possible or undesirable. The more intermediate-degrading sutures such as (Vicryl®, Dexon®, and Biosyn®) are often chosen to close wounds that are expected to heal within 3 weeks, such as the subcutaneous tissue and muscle. Monofilament nonabsorbable sutures are suggested for skin closure because they induce little foreign body response and skin sutures should remain strong since they are subject to chewing and wear. These sutures also provide long-term stability in procedures involving fascia, tendons, and vascular prostheses. Systemic and local factors affecting wound healing must also be considered before an appropriate suture is selected. For example, catgut in the presence of infection or gastric secretions, or when placed in a catabolic patient can be degraded within days, rendering the wound closure susceptible to dehiscence. When healing is expected to be delayed, prolonged absorbable sutures or nonabsorbable sutures are better choices.

Healing Considerations

Surgeons must consider how the suture alters the biologic processes in a healing wound environment. Regardless of its composition, suture material is a foreign body to tissues in which it is implanted, and to a greater or lesser degree will elicit a foreign body reaction. The amount of reaction depends on the nature of the suture implanted (e.g., surgical gut versus inert, stainless steel), the amount of surface area and coating of the suture, the type and location of tissue closed (intestinal viscera and skin react strongly to silk, whereas fascia reacts minimally to silk), the length of implantation (polyglycolic acid, or Dexon II®, is moderately reactive early but within months is relatively inert), and the technique of suture placement (excessive suture tightening causes tissue strangulation). Excessive suture-induced tissue reaction increases the likelihood of suture-tissue cutout by softening surrounding tissues, increases the risk of infection, and delays the onset of fibroplasia. Sutures causing excessive tissue reaction are contraindicated in areas in which exuberant scar formation can cause a functional problem (e.g., for vascular repair or ureteral anastomosis) or a cosmetic problem (e.g., in skin). The surgeon should strive to inflict the least amount of trauma necessary for the operation, to reduce contamination, and to use sutures that cause the least tissue reaction to avoid excessive inflammation and delayed wound healing. Relatively speaking, it is not the suture material but the surgeon that causes inflammation within a wound, since most reaction is induced during tissue manipulation and the act of suturing.

All suture materials are capable of increasing wound susceptibility to infection. The suture’s filamentous nature, capillarity, chemical structure, bioinertness, and ability to adhere to bacteria all play a role in suture related infection. In a classic experiment, a single silk suture reduced the total contaminating dose of Staphylococcus required to induce wound infection 10,000 fold. On the other hand, the byproducts of nylon and polyglycolic acid suture degradation in tissues may have beneficial bactericidal effects. A newer synthetic absorbable suture with an antibacterial coating has been developed specifically for use in contaminated wounds (see discussion under newly developed sutures). In general, sutures that induce the least foreign body reaction in tissues, such as monofilament synthetic absorbable and nonabsorbable sutures, produce the lowest incidence of infection in contaminated wounds. If possible, suture should not be implanted in highly contaminated wounds or wounds with a high risk of infection.

Multifilament nonabsorbable suture materials induce chronic sinus formation more often than absorbable or monofilament sutures. Multifilament nonabsorbable sutures harbor bacteria within the suture interstices, creating an effective barrier to phagocytosis. These sutures should never be used in contaminated wounds. Wound infection also increases the rate of loss of strength of suture material. If wound contamination is suspected, synthetic absorbable sutures should be chosen because these sutures are more stable and have predictable absorption rates in contaminated tissue, when compared to chromic catgut. If long-term wound support is required of the suture material, synthetic monofilament nonabsorbables or synthetic (prolonged-degrading) absorbable sutures such as PDS II® or Maxon® are indicated.

The presence of any suture material within the lumen of the biliary or urinary tract can act as a nidus and induce calculus formation or chronic infection. Thus, more rapidly absorbable sutures are recommended in these areas, since they will not persist indefinitely in tissue. Silk and nonabsorbable polyester material, because of their documented calculogenic effects, should never be placed in contact with urine or bile. General guidelines to avoid suture-related complications in surgery are listed in Table 1-5.
 

Mechanical Properties of Suture and Tissue

The mechanical properties or functions of the suture should be similar to those of the tissue being closed. For example, polybutester (Novafil®), is a suture material that is very pliable and elongates and is most suitable for skin closure because it remains flexible and stretches with movement. More inelastic suture materials, such as those composed of polyester or nylon fibers, are more applicable for anchoring prosthetic materials or for joint imbrication. Similarly, inelastic suture material such as stainless steel should not be used in tissues that stretch or are under constant motion because premature suture-tissue cutout or suture breakage could occur.

Newly Developed Sutures

Newer synthetic sutures have been developed to improve suture strength profiles without negatively affecting suture handling or knot security. The newer synthetic monofilament absorbable sutures are more pliable and better handling. Multifilament sutures may convert a contaminated wound into an infected one, so antibacterial coatings have been developed to inhibit bacterial growth in and around multifilament suture.

Polyvinylidine Pronova® (Ethicon)

This unique synthetic nonabsorbable monofilament suture is made of two polyvinylidine polymers, with a special extrusion process. This produces an optimal balance between suture strength and handling characteristics throughout the range of suture sizes. Pronova® suture sizes, 10-0 through 4-0, are composed of an 80/20 polymer blend, that emphasizes tensile strength without compromising handing in smaller sizes. Pronova® suture sizes, 2-0 through #2, are composed of a 50/50 polymer blend that improves handling in these larger sizes, without compromising tensile strength. This suture will remain secure in critical surgical procedures where life-long strength is desired, particularly in delicate applications where fine sutures are used. Tensile and knot strengths of Pronova® suture meet or exceed those of polypropylene suture in all sizes. The suture has excellent resistance to breakage, fraying, and instrument damage, and has reduced package memory. It is an excellent alternate choice when polypropylene suture is indicated. The suture is best for general soft tissue approximation and ligation including cardiovascular, ophthalmic, and neurologic applications. [Ethicon, Product Information; http://jnjgateway. com/home]

Polyglactin 910 and Triclosan Coated Vicryl Plus Antibacterial® (Ethicon)

This synthetic multifilament absorbable suture has an antiseptic coating (Triclosan) that creates a zone of inhibition around the suture site that decreases bacterial colonization of the suture or tissue. The suture performs and handles similarly to Coated Vicryl® suture. Vicryl Plus® is available in suture sizes, 5-0 through 0. It elicits a similar tissue reaction as other synthetic absorbable sutures, and considerably less inflammation than chromic gut sutures, but it should not be used close to the eye (Triclosan may be irritating to the eye). The manufacturer suggests using the suture in procedures that have a higher risk of infection. Few clinical studies have been conducted to substantiate the beneficial effects of this suture.

Glycomer 631 Biosyn® (Syneture)

This absorbable monofilament suture is prepared from a synthetic polyester composed of glycolide, dioxanone, and trimethylene carbonate. The advanced extrusion process gives the suture excellent initial strength and knot security and minimal memory. This suture elicits minimal acute inflammatory reaction in tissues. Like other synthetic absorbable sutures, eventual absorption is predictable by means of hydrolysis. Biosyn® sutures are available in sizes #1 through 6-0. The suture maintains 75% strength at two weeks and approximately 40% at three weeks after implantation. Similar to Dexon® and Vicryl®, this suture should not be used where extended approximation of tissue is required.

Polyglytone 6211 Caprosyn® (Syneture)

This absorbable monofilament suture is prepared from a synthetic polyester composed of glycolide, caprolactone, trimethylene carbonate, and lactide. It has very good handling and knot tying characteristics due to its excellent pliability, and has low tissue reactivity. Caprosyn®, similar to Monocryl®, is useful for general subcutaneous tissue closure, urogenital surgery particularly in the urinary bladder, and where the benefits and rapid absorption may play a role in postoperative success.

Suture Knots

A knot consists of a minimum of 2 throws (sometimes termed simple knots). As a knot is created, the material is deformed, and depending on the properties of the material, this deformation may weaken the suture by as much as 50% of its original strength. Therefore, the knot is the weakest part of a suture. The technical performance of the knot is critical to the security of the wound closure as well as the strength of the stitch. A square knot is least likely to untie or loosen so it is the knot of choice for most suture lines. Depending on how the throws are placed, three different knots can be formed (square knot, granny knot, or a half hitch shown in Figure 1-1). The latter two knots tend to slip and are generally avoided. Square knots are produced by reversing direction on each successive throw while maintaining equal tension on both strands as they are held parallel to the plane of the tissue. Failure to reverse direction of successive throws will result in granny knots. If one strand is pulled under more tension away from the plane of the knot than the other strand, with successive throws, a half hitch (or slip knot) is formed. Sometimes surgeons using monofilament sutures intentionally apply half hitch knots (especially if the wound is under tension) and this allows precise control of intrinsic suture tension. All half hitch knots must be completed with several square knots to prevent loosening. A surgeon’s knot is similar to the square knot except one strand is fed through the loop twice on the first throw. The additional pass of suture in the loop produces increased friction. This knot is especially useful when attempting to knot a stitch when tissues are under tension. Multifilament absorbable sutures such as polyglycolic acid or polyglactin 910 may require surgeon’s knots when used to close abdominal fascia. This knot is avoided when using gut since the increased friction tends to fray the material and excessively weakens it. Caution should be exercised with using surgeon’s knots during vessel ligation, since the bulk of the first throw may not allow complete occlusion of the vessel, and the knot is less reliable than the standard square knot. Surgeon’s knots have increased bulk and are asymmetric, so this knot is used only when necessary.

Additional factors that influence knot security are the material coefficient, the length of the suture ends (ears), as well as the structural configuration of the knot, mentioned previously. Knots that swell (chromic catgut) or knots formed from stiff suture (ones with memory), require longer knot ears in general. Multifilament sutures possess a higher coefficient of friction, and have better knot-holding properties than the monofilaments in general; however, coating the strands to reduce friction or chatter in tissue also reduces knot security. Three single reversed throws are generally sufficient to secure knots in suture materials with high coefficients of friction and minimal tension. When using monofilament sutures (such as nylon or polydioxanone), or coated multifilament sutures, four or more throws should be applied. In a continuous suture line, the final knot (consisting of a loop and single strand) should have a minimum of 5 throws to be secure. General knot tying rules are included in Table 1-6.

Suture Needles

Surgical needles are manufactured in a variety of sizes, shapes, and types. Needles are selected to ensure that the tissues being sutured are altered as little as possible by the needle. The needle chosen should allow tissue passage without excessive force and without disruption of tissue architecture. The hole created by the needle should be just large enough to allow passage of the suture material. The needle should be rigid enough to prevent bending, yet flexible enough to bend before breaking.

Regardless of their intended use, all surgical needles have three basic components: the eye (or suture attachment), the body (or shaft), and the point. There are two types of needle eyes commonly used in practice, the economical closed eye (suture is fed through the eye) and swaged (eyeless). Needles permanently connected to suture (swaged needles) produce significantly less tissue trauma and are easier to handle compared to eyed needles; sutures supplied with needles, expectedly, are more expensive.

The bodies or shafts of needles vary in shape and size. The body should be as close as possible to the diameter of the suture material. The cross-sectional configuration of the body may be round, side-flattened rectangular, triangular, or trapezoidal. Some needle bodies are ribbed to prevent rotation and provide better stability of the needle in the jaws of needle holders. Easily accessible tissues such as the skin may be sutured by hand with straight needles but most surgeons prefer curved needles because they are easier to use with instruments. Curved needles are supplied in 1/4, 3/8, 1/2, and 5/8 circle configurations (Figure 1-2). Choice of length, width, and curvature of the needle is dependent on the size and depth of the area to be sutured. Quarter circle needles have limited use, primarily for eye surgery. Three-eighths circle needles are most commonly used in veterinary surgery and are suitable for most superficial wounds. Half circle needles are preferred for deeper wounds and in body cavities. Five-eighths circle needles are applicable for suturing wounds in confined areas such as the oral, nasal, and pelvic cavities.
 

The needlepoint extends from the extreme tip of the needle to the maximum cross-section of the body. Three general types of needlepoints include: cutting, tapercut, and taper (or round point) (Figure 1-3). Cutting needles provide edges that will cut through dense connective tissue. They are most suitable for skin, tendon, and fascial closure. Like the conventional cutting needle, the reverse cutting needle has a triangular shaped cross-sectional area; however, rather than possessing a sharp edge on the inner curvature that is weaker and tends to cut tissue as the needle is passed, it has a flat inner curvature with an edge along the outer curvature of the needle point and shaft. Spatula point (side cutting) needles are flat on the top and bottom. They are used primarily in special ophthalmic operations. A tapercut needle combines a cutting point with a round shaft. The cutting point readily penetrates tough tissue but the shaft will not cut through or enlarge the needle hole when inserted. This needle is indicated when ease of penetration is important (vascular grafts, intestine) or when a delicate tissue is sutured to a more dense one (such as urethra to skin closure for a urethrostomy). Taper point or round needles have no edges to cut through tissue. The point pierces and spreads tissue without cutting. They are used for suturing easily penetrated soft tissues such as muscle, viscera, or subcutaneous tissue. Blunt pointed taper needles have a rounded point so they are most useful for suturing friable parenchymal organs such as the liver or kidney. General principles of needle use are list in Table 1-7.

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