A Brief Overview of Needles, Sutures, and Knots
Mark Turrentine
Surgical interventions during obstetrics may be required in the antepartum (e.g., cervical/abdominal cerclage and open or minimally invasive maternal and/or fetal surgery) or peripartum periods (e.g., perineal and/or pelvic laceration repair at the time of vaginal delivery, cesarean birth, or even hysterectomy). An obstetric provider’s armamentarium should include an understanding of the use of the tools (such as needles and sutures) and techniques (knot tying) necessary to perform these surgical interventions. The objective of this chapter is to provide a brief overview of surgical needles and sutures commonly used in operative obstetrics, the general concepts of suture knot securing, and a description of the typical needles and sutures used in the most commonly performed obstetrical procedures.
THE ANATOMY OF THE NEEDLE
A surgical needle consists of three parts: The attachment end (also known as the eye or swage), the body, and the point (Figure 1.1.1) (1,2,3).
The attachment end is where the suture is fastened to the needle and may be swaged or closed eye. Most contemporary needles are the former type. The closed-eye needle must be threaded but allows the use of different suture types and are used in specialized obstetric procedures such as cerclage. Swaged sutures join the needle and suture together as a continuous unit. The suture attachment may be cut or easily released when using controlled-release needles.
The body of the needle connects the attachment end to the point and determines the shape of the needle. The needle body may be straight or curved, but most curves are 1/4, 3/8, 1/2, or 5/8 of a circle.
The needlepoint extends from the tip of the needle to the maximum cross section of the needle body, with the more common types being cutting or taper needles.
Cutting needles have three cutting edges and are either conventional (the cutting edge is on the inside of the concave curvature) or reverse (the cutting edge is on the outer convex of the curvature). Cutting needles are commonly used for skin sutures or through tough tissue.
Taper needles (also known as round needles) pierce and spread tissue without cutting it. These are used in easily penetrated tissues (such as the uterus or fascia).
Figure 1.1.1. The composition of a needle. (Data from Rose J, Tuma F. Sutures and needles. In: StatPearls. StatPearls Publishing; 2020; Byrne M, Aly A. The surgical needle. Aesthet Surg J. 2019; 39(S2):S73-S77; and Moy RL, Waldman B, Hein DW. A review of sutures and suturing techniques. J Dermatol Surg Oncol. 1992; 18:785-795.)
Taper cutting needles have a sharp, short cutting point that blends into a round body. These needles puncture more tough tissues and subsequently disperse the tissues as it pushes through. It is used where penetration is required but tearing of tissues is to be avoided, commonly used for suturing dense, fibrous connective tissue such as the fascia.
Spatula tip needles or side cutting needles have a unique design, flat on both the top and bottom, which eliminates the undesirable tissue cut out of other needles. Spatula cutting needles lend themselves well to surgery that requires the needle to split through thin layers of tissue and travel within the plane between them.
Blunt point needles (a variation of the taper point) have a tapered body with a rounded, blunt point that will not cut through tissue, which are designed for use in friable tissue, but also advocated to reduce the risk of penetrating the surgeon’s or assistants’ glove or skin.
Straight needles allow direct manipulation with precision with easily accessible tissues in a less confined surgical field such as with abdominal wall closure.
SUTURES
Sutures are widely used in many surgical contexts but most frequently in wound treatment. Typical applications aim at closing an injury or an incision by approximating its open margins to allow for the formation of new structures. However, a number of different sutures are encountered in surgery, often chosen based on the surgeon’s experience and intended application.
Sutures are further characterized by various properties they acquire in response to their composition (Table 1.1.1).
Suture sizes are commonly denoted using the United States Pharmacopeia (USP) system (Table 1.1.2).
This numbering system epicenters around the “0” suture. Suture sizes increase from USP size 0 (“zero”) to size 1, commonly called “number-1,” to USP size 2 (“number-2”),
and upward. In contrast, suture sizes decrease in size, again from 0, to USP size 2-O, commonly called “two O,” 3-O (“three O”), and downward (4). Smaller diameter sutures are associated with less tensile strength, and a balance between the size of suture and maintenance of tissue approximation must be determined.
Table 1.1.1 Properties of Suture Materials
Property
Description
Elasticity
The suture’s ability to return to its original length after stretching
Memory
The suture’s ability to return to its original shape after deformation
Coefficient of friction
The suture’s ability to slide through tissue
Tensile strength
The force required to break the suture
Knot security
The force necessary for a knot to slip
Inflammatory potential
The likelihood the suture will evoke an inflammatory response
Data from Rose J, Tuma F. Sutures and Needles. StatPearls Publishing; 2020; Firestone DE, Lauder AJ. Chemistry and mechanics of commonly used sutures and needles. J Hand Surg Am. 2010;35(3):486-488; Moy RL, Waldman B, Hein DW. A review of sutures and suturing techniques. J Dermatol Surg Oncol. 1992;18(9):785-795; and Yag-Howard C. Sutures, needles, and tissue adhesives: a review for dermatologic surgery. Dermatol Surg. 2014;40(S9):S3-S15.
Table 1.1.2 Varying Sizes of Synthetic Suture Used in Obstetric Procedures According to Their United States Pharmacopeia (USP) System Designation, Terminology, and Diameter (in Millimeters)
USP Size
Terminology
Diameter (mm)
2
Number-2
0.500-0.599
1
Number-1
0.400-0.499
0
Zero (0)
0.350-0.399
2-O
Two O
0.300-0.339
3-O
Three O
0.200-0.249
4-O
Four O
0.150-0.199
Data from Byrne M, Aly A. The surgical suture. Aesthet Surg J. 2019;39(S2): S67-S72.
Sutures are classified according to their absorption properties and may be either absorbable or nonabsorbable (1,4,5,6).
Absorbable sutures are used to approximate wound edges temporarily until they have healed sufficiently to withstand normal tissue stress. Absorbable sutures can be grouped as natural and synthetic sutures. Absorbable sutures are digested either by body enzymes (natural sutures) or hydrolyzed in body tissue (synthetic sutures) (1). Absorbable sutures lose most of their tensile strength over variable periods of time from a few weeks to several months.
Monofilament sutures are a single strand of material, whereas multifilament sutures consist of several strands twisted or braided together. Monofilament sutures have less resistance as they pass through tissue, have less inflammatory response, and tie down easily.
Multifilament sutures have greater tensile strength but cause more friction through tissue and a higher potential of inflammation and infection. Common suture materials used in obstetric procedures are noted in Table 1.1.3.
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