What Is a Surgical Suture?

The device, not just the thread

A suture is usually pictured as a length of thread, but the device that arrives in theatre is three things at once: the strand, the needle it is swaged to, and the sterile barrier that has kept both sterile since manufacture. Each is specified separately. The strand has a material and a gauge; the needle has a geometry, a curvature and a point type; the pack has a sterilisation method and an expiry. A suture selected on material alone has only been specified in one of three dimensions.

The anatomy of a suture

The strand does one thing mechanically: it holds tissue in apposition. It is the tissue that closes the wound. That distinction is worth keeping in view, because it is what separates the properties of a suture — which belong to the material and can be described here — from the course a given wound takes, which belongs to the patient and cannot.

The axes a suture varies on

Suture materials differ along a handful of independent axes, and nearly every term in a specification sheet is a position on one of them.

Absorbable or non-absorbable. An absorbable strand is broken down in tissue over time. A non-absorbable strand is not: it is removed once a surface wound has closed, or it stays. This is the first axis because it changes what every subsequent figure means — for an absorbable material, strength retention and absorption period are two separate numbers on two different timelines, both stated in the product’s instructions for use.

Monofilament or multifilament. A monofilament is a single extruded filament. A multifilament is many fine filaments braided or twisted into one strand. This is a construction difference rather than a chemical one, and most of what a surgeon notices in the hand traces back to it — pliability, how a knot seats, how the strand passes. Braid construction also creates interstices, which is where capillarity comes from and why braided strands are the ones that get coated.

Natural or synthetic. Natural materials are derived from a living organism: purified collagen, or silk. Synthetic materials are polymers produced by chemical synthesis. The distinction is not merely one of provenance, because it largely determines the absorption mechanism — collagen is broken down by proteolytic enzymes, whereas synthetic absorbables are cleaved by water. One mechanism is coupled to the patient’s inflammatory response; the other is not.

Attached or not attached to a needle. Most sutures are swaged: the strand is joined to the needle during manufacture, so the strand emerges from the needle’s end rather than doubling back through an eye. A strand supplied without a needle is a ligature, used to tie a structure rather than approximate tissue.

The vocabulary

Two terms carry more weight than their length suggests. Capillarity is the tendency of a multifilament strand to draw fluid along its length between the filaments; it is a property of construction, so it follows the braid and not the polymer. Gauge is the strand’s diameter, designated by the USP scale and by a parallel metric figure — and because the USP fixes both a diameter range and a tensile-strength requirement at each size, gauge is never only a measurement.

Everything else in a suture specification hangs off these axes. A page describing a single material — polyglycolic acid, say, or polypropylene — is describing where that material sits on each of them, and then pointing at the instructions for use for the figures that only the controlled document can carry.