parts of a suture needle

Surgical suture needles: Materials, parts and aspects to be considered

If the correct choice of suture material according to the tissue to be sutured and its healing time is of vital importance, no less important is the choice of the right needle, as it will allow the professional to pass and direct the thread through the tissues with precision and perform a suture suitable for the efforts that will have to counteract once the tissues are approximated while healing lasts and in the most aesthetic way possible. To suit each specific case there is a great variety of shapes, sizes, tips and forms.

What do we expect from a good suture needle?

  • It should penetrate the tissue(s) in a minimally atraumatic manner.
  • Penetration and passage through the tissue should be done with a minimum of effort. Apart from having a smooth surface, they can have a silicone surface treatment that facilitates sliding.
  • Its dimensions and shape must be adequate to provide the required stitches.
  • It must maintain its shape during the process so it must be sufficiently rigid, but at the same time it must be elastic enough not to split.
  • It must be stable once fixed in the needle holder, with no unwanted twisting or looseness that reduces precision.
  • Its material must be sufficiently resistant, stainless, sterile, non-toxic.
  • Its thickness should be as close as possible to the diameter of the suture thread to which it is attached.

It is not easy to harmonize all the above mentioned requirements in a needle, since some of them are contrary. For example, one way to make the needle resistant is to increase the diameter of the wire that forms it, however, this may mean an excessive thickness for the suture thread that has to pass through, which would cause a larger hole than necessary. The malleability, makes it easier to shape the needle, in its manufacture, while in its use it can be more easily deformed. As always, it is necessary to find a compromise that optimizes both aspects.

 

Materials

In ancient times, the materials that began to be used, before the Metal Age, were plant thorns, wood splinters and animal bones, suitably sharpened and with an "eye" for threading the thread.

Currently, needles are manufactured in medical grade stainless steel, which facilitates many of the requirements mentioned above. All stainless steels contain chromium in their alloy, plus other metals which, in different proportions, improve the properties of rigidity, ductility, hardness, etc.

The different steels used for the production of needles are as follows:

Martensitic steels (AISI420.)

It has a Cr percentage of (11.5-14.5%), a low C content (0.1-0.4%). The addition of S facilitates perforation, which is important for making the hole in the back of the needle where the thread is introduced. Mo improves its anti-corrosion properties. Hardness is achieved with tempering heat treatments.

For years, 420 steel has been the most widely used steel in the manufacture of needles, since, in addition to its acceptable properties at user level, it was easily machinable. Nowadays it is used only in relatively thick needle gauges, as it has a lower bending moment compared to other steels and bends more easily, deforming the needle.

 

Maraging steels (AISI455)

In their basic composition these steels contain very low amounts of C, Cr, (10-17 %) and Ni, (4-12%). Other elements such as Cu, Mo, Al... are also added to improve their mechanical properties. Ni in particular improves their formability and ductility. In general, these steels are more difficult to work, but more resistant to breakage.

 

Austenitic steels (AISI300 302-304)

Contienen C (<0,15%), Ni (6-13%), Cr (2-17%)

Although these steels are the most difficult to work with in terms of perforation and formability, the needles manufactured with them are very resistant to deformation and nowadays the great majority of suture needles used are manufactured with AISI 300 steels. Their high resistance to deformation allows the use of thinner wire diameters close to the diameter of the thread.

 

Parts of the needle

parts of a suture needle

Needle tip

It is in charge of perforating the fabric. Its design must be in accordance with the hardness of the fabric to be perforated.

  • Conical tip: used in soft tissues, such as vascular walls, intestine, stomach.
  • Triangular tip: It usually has three sharp edges that allow penetrating harder tissues such as skin, striated muscle, bone. Posterior to the needle, the body is triangular in section.
  • Diamond point: It is similar to the triangular point, but instead of three, it has four cutting edges. The body of the needle has a square section with more or less blunt edges.
  • Trocar point: it is a triangular point of small dimensions, followed by an ogive of circular section. It is intended to combine the properties of conical and triangular needles. Seen from the front, the shape is a small cutting triangle inscribed in a larger circle. It is used in vessels, in which the needle should not be sharp because the vessel walls are not excessively hard but have atheromas, harder.
  • Spatulate tip: It has 4 edges that, when viewed from the front, have a trapezoidal section, quite flattened. Rather than perforating, it separates different layers of tissue. It is used especially in ophthalmology.
  • Spherical tip: The tip instead of being sharp and very sharp, is a hemisphere. In some cases the hemisphere has the same diameter as the rest of the needle, and it is usually used in friable tissues such as liver or kidney where a conical needle, let alone a triangular one, can cause the organ to break from the puncture. In other cases the tip is of smaller diameter than the needle and is followed by a warhead. It can be said that it "barely punctures". It is used in soft tissues, such as the intestinal wall, in depth, where the surgeon does not "see" the exit of the tip of the needle when giving the point and has to guess it by touching the area with the fingers. Since it is practically not sharp, there is no risk that in this operation the needle may go through the glove and prick the surgeon's hands in patients at risk of transmitting infections.

Types of suture needle tips

 

Needle nose cone

Immediately posterior to the tip is the ogive. It has the shape of two arcs of a circle that would be cut at the tip. It is a transition zone between the tip and the body of the needle, with a section that becomes larger as we move away from the tip until it reaches the same size as the body of the needle. This tapered shape, similar to that of the head of a projectile, allows a better sliding of the needle through the fabric. Except in triangular needles, it is usually circular in cross-section.

suture needle warhead

 

Needle body

It is the central part of the needle, where the thickness is greater. It is in this part where the needle is held by the needle holder to perform the necessary movements for suturing. It is very important that the needle in the needle holder remains fixed, without looseness or twists, so that it can be maneuvered in a predictable and precise way. For this reason, its cross-section is not circular as in the nose cone, but quadrangular. In many cases, the needles even have longitudinal grooves on the upper and lower sides of the body that adapt to the grooves of the needle holder tips and allow a firmer and more precise fixation.

body needle suture

body suture needle 2

 

Crimp zone

At the back of the needle is the area where one end of the suture thread is inserted and fixed.

This area consists of a hole with chamfered edges deep enough to insert the yarn and to compress it by compression.

crimp needle suture holesuture needle crimp

Another option is that this area presents a ribbed shape that is not completely closed and, by means of molds suitable for this shape (and size), it is also closed by compression.

channel suture needle crimpcrimping of suture channel needle

By means of steel molds, mechanically or pneumatically moved, calibrated for each needle diameter, the zone is compressed so that the thread is firmly attached to the needle.

suture needle crimp pressure

 

Needle shape

The suture needles have different shapes designed to facilitate the maneuvers necessary to correctly close the wound, depending on its depth.

When the wound is superficial there is ample space to give the stitches. In this case the movement is straight, since there are usually no obstacles before or after the edge of the wound. However, as deeper planes must be sutured, the space around the wound is smaller, so that the movement to cross the two edges must be the rotational movement of the wrist, since translational movements are not possible. For this reason variable curvature needles are used. Generally the way to express the degree of curvature is instead of in degrees, in parts of circle of the arc of the needle, ¼ , ½, 3/8 and 5/8 of circle. As a general rule, the smaller the maneuvering space and/or the greater the depth, the greater the needle curvature.

suture needle shapes

Parameters to consider

  • Diameter: As previously mentioned, ideally the diameter of the needle should be equal to that of the thread, but since this is impossible, it should be as similar as possible to the diameter of the suture thread so that the needle is really atraumatic and does not make a disproportionate hole. However, taking into account the resistance of the steel that has to be compressed in the crimp area without breaking and that as the length of the needle increases, the diameter must be larger to prevent it from bending during the processing of the suture, the thickness of the needle is usually double or even slightly more than the thickness of the thread.
  • Length: Refers to the length of the arc of the needle, from its tip, to the crimp of the thread, as if the curvature of the needle had been straightened.
  • Crimp strength: During the suturing process, the thread/needle union is subjected to various stresses. It is essential that the thread does not separate from the needle. The Pharmacopoeia has defined resistance values that this joint must withstand, both as an average of ten values and as a minimum individual value.
  • Tip: suitable to overcome the resistance of the tissue in question. It is essential to always choose the right tip, otherwise it is extremely difficult to penetrate the tissue, the needle may bend or even break, or if the tissue is friable, there may be a great deal of trauma from the hole produced.

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