Sutures 06 How sutures work

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Sutures 06 How sutures work

This subsection has text and images from an interactive multimedia training program on basic 
surgical skills called PrimeSkills in Surgery.
You can use this subsection on its own or follow the whole program (further details at the end of 
this subsection).

How sutures work.





Memory tip


How sutures work

Sutures work in two ways.

First, the suture surrounds the tissue and compresses it to the required degree.

Second, the suture is knotted so that the pressure on the tissues will be maintained.

ie A suture should not break and the knot should not slip.

The surgeon uses the different properties of sutures described below to avoid these mishaps,


The length of the suture in the packets is designed to be:

Long enough to give a useful number of stitches per length.

Long enough to reach deep suture sites easily.

Short enough to prevent tangling.

Short enough to run through the tissues without cutting or bunching.


It would seem sensible to give a suture a gauge according to its diameter.

This is done with the metric system using tenths of a millimetre.

The metric sizes go down as the suture diameter decreases.

However, as the diameter of the stitches gets smaller, the difference between sizes also gets


Measuring the diameter in tenths of a millimetre becomes rather clumsy.

e.g. 4, 3.5, 3.2, 2, 1.5, 1, 0.7, and 0.4 tenths of a millimetre.

The older O (oh) gauge system remains popular because the downward steps in sizes are very convenient, and the numbering is

simple, although the numbers go up as the diameter decreases.

e.g. 1, 0, 2/0(called two-oh), 3/0, 4/0, 5/0, 6/0, and 7/0 for the metric sizes above.



3/0 is a metric 2, and a 2/0 is a metric 3.

This is a difference in diameter of one metric step (0.1mm.).

The other O sizes go up or down in half metric steps in this range of diameters.

NB. All catgut used to be a larger O gauge size than other types of suture, to allow for the weakness of catgut compared with the

suture materials used previously such as silk or linen.

To avoid confusion, get used to a small range of suture/needles.

Use the code numbers issued by the manufacturers for the suture/needle combination you want, rather than a description of the



Despite the efforts of the manufacturers, the sutures are folded or kinked to some degree in their packets.

This makes the sutures a bit springy and difficult to handle.

Pull on the suture to stretch the suture so that the kinking disappears or improves.

The force needed to straighten the suture is very close to the breaking strength of the swage.

Therefore, protect the swage from this pull by holding the suture instead of the needle.


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An important aim in surgery is to know how much push and pull the tissues will stand from the stitches without being damaged.

However, you need to learn the strengths of the sutures before you start stitching the tissues, so that you choose sutures that are

strong enough.

The strength of a suture depends on:

The material (i.e. its molecular structure).

The diameter of the suture.

Whether it is braided or monofilament. (Braided is stronger)

The strength of any particular suture will be reduced by:

Any knots in it.

The swaging onto the needle.

Damage from dissecting forceps, needle holders, needles, and diathermy.

In surgery, the strengths of sutures and the amount of pull on sutures and tissues is usually given non-numerical terms such as:

Strong. Very strong. Gentle. Snug. Firm. Not too hard. Too much. Sister, this suture is rubbish! etc.

These are difficult to learn and are prone to wide variations between surgeons, leading to argument, frustration and blame.

Measuring the pull and strengths in absolute terms would seem to be sensible.

However, the official unit of strength used is a Newton.

This is unfamiliar to surgical staff, although it is almost exactly the same as a 10 gram weight.

Also the basic work on measuring the forces used by surgeons when stitching has not been done.

In this program, you will be learning to:

Estimate the amount of pull in grams.


Familiar weights: 1 litre of coke 1000gm./1 packet of butter 500gm./1 bar of soap 125gm.

Learn breaking strengths and ideal forces on tissues.

This is based on the measurements and experience of the author using a spring balance.

Research in progress should confirm whether other surgeons do about the same.

All suture strengths will apply to a suture with an overhand knot in it.

This is because a suture is weakened by a knot, and is only at risk of breaking when a knot is being tightened.

The strongest pull that a surgeon can give without getting severe pain in the hand is about 5000g.

Most sutures will feel slightly springy just before they break, but there is virtually no warning.


                Needle	           Suture		 Length 	   Knotted        Swage                                          
                                                                           Strength gm.   Strength gm.	
    Ethicon 	26mm. 3/8 curve	  3/0 or 2 metric	 150cm.	           2000  	  1000
    W9890  	reverse cutting   coated polyglactin (Vicryl)
    Ethicon	50mm. 1/2 curve	  1 or  4 metric 	 100cm.		   3800           4100 
    W749	round bodied	  polyamide(nylon)
    Ethicon	50mm. 1/2 curve	  1 or 4 metric		 75cm.	 	   5000+          2500 
    W9251	round bodied	  polyglactin(Vicryl)		           To straighten: 2500 
    Ethicon	30mm. 1/2 curve   2/0 or 3 metric	 75cm.		   3000	          1400
    W9136	round bodied	  polyglactin(Vicryl)		           To straighten: 1500	
    Ethicon 	90mm. 3/8 curve   1 or 4 metric	silk	 50cm.		   3900	          5000+
    W2793	cutting			

Next subsection, click on Sutures 07 How to use sutures
Last subsection, click on  Sutures 05 Parts
Surgical Education and Training page, click on Surgical Education and Training
Whole program on WikiSurgery, click on PrimeSkills in Surgery
Whole program on CD-ROM, click on Michael Edwards