Needle

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QuotationMarkLeft.png Law VI: There is no body cavity that cannot be reached with a #14 needle and a good strong arm. QuotationMarkRight.pngSamuel Shem, The House of God

A needle is a slender, cylindrical piece of solid material with a tapered tip. They are designed to be sharp, but in some applications the tapered end is intentionally blunt. Most needles are made of metal and can either be solid, e.g. in a modern suture needle, or hollow, when the needle is required to to aspirate or to administer a substance. The applications for needles in medical practice are legion.

Contents

Types of Needle and Their Applications

Standard double-bevelled needle tip
Standard needle 
With Luer hub attached (see picture). Several uses, e.g. venepuncture, administration of i.m. and s.c. injections, ABG, fine needle aspiration

Aspiration of various cavities

'Broken' needle 
Used in paediatric venepuncture. The name is historical from when needles used to have the hub broken off. Commercially-available equivalents are now available without the need to break the needle. They have a free end which allows blood to drip out of small, fragile veins through capillary action.
Butterfly needle 
Venepuncture, one-off i.v. administration, can be kept in situ for s.c. infusions
Cannula over needle
Cannula over needle 
Various types of needles used for introducing cannulae, e.g. for peripheral venous access, central venous access or arterial cannulation.
Insulin syringe
Insulin needle 
Very fine gauge needle, often 29G or 31G, often with syringe attached (see insulin syringe). For drawing up and administering insulin. Can be useful to give small, relatively pain-free blebs of local anaesthetic
Intraosseous needle 
Firm, rigid needle to gain intraosseous access
Suture needle 
for suturing
Tuohy needle
Tuohy needle 
for epidural catheter insertion
Veress needle
Veress needle 
for creation of pneumoperitoneum.
Diamond pointed needles 
which lack the cutting bevel - have some advantages for local anaesthesia. Spinal tap needles are of this sort.
Others
  • Radiological percutaneous drainage
  • Biopsy needles (e.g. Trucut) - these are spring-loaded cutting needles, sometimes also called core biopsy needles.

Technical Aspects

Parts of a Needle

Parts of a needle
Bevel 
The sloped surface that creates the sharpened edge. There are various shapes of tip and bevel. Some needles have a double bevel: the first to create the tapered end and the second to produce the sharpness of the tip.
Hub 
This allows connection to syringe. There are standardised connections, e.g. the Luer system (either push-on or Luer lock).
Shaft or Barrel
The main body of the needle.
Stylet/obturator 
(e.g. in spinal needle)

Needle Gauge

It is traditional to refer to needle size by reference to their gauge(G). This is a throwback to the Victorian technique of manufacturing wire.

Ductility is that property of metals which allows them to be drawn out into long wires. In the manufacture of wire, a solid cylinder of metal is pulled through a form which is a thick metal plate with a hole in it which is just large enough to accommodate the cylinder. However, the sides of the hole are not parallel but taper inwards slightly, which forces the diameter downwards slightly (increasing the length). This happens a number of times, with each successive form being slightly narrower than the one before. The larger the number of forms, therefore, the smaller the gauge of the wire. Thus a 20G wire (having passed through 20 forms) is smaller than a 16G wire.

Unfortunately, different manufacturers used differently-sized forms. This was standardised by the introduction of the Imperial Standard Wire Gauge (abbreviated SWG or just G), which is the standard which we use today.

In the manufacture of needles, a similar process is used, except that the original cylinder is hollow, and the "wire" which is produced is a hollow tube which can then be cut to length and sharpened into needles.

The manufacture of wire and needles no longer conforms to the original technique, but the gauge nomenclature is so widespread that it is very difficult to change (although all needle packaging now also gives the diameter and length of the needle in standard units (e.g. millimetres)).

There is no convenient relationship between SWG and diameter in millimetres, since the sizing of the original forms was somewhat arbitrary.

In the Samuel Shem quotation above, the "#14 needle" is sized according to the American Wire Gauge, a related (but not interchangeable) system.

Urinary catheters, chest drains, and so forth, are sized according to the French gauge, which has no relation to wire gauge.

Selection of needle gauge

As technology has progressed the diameter (or gauge) of needle which can be manufactured economically and relied upon to be strong enough in use has steadily decreased. At the same time the wall thickness required - the thinness of walls which can be achieved - has also steadily decreased. The minimum external diameter for a needle of a sufficient internal lumen and a particular length and cost has thus steadily been reduced. Where thickness is of great importance - for spinal anaesthesia for instance - greater expense for a narrower gauge is reasonable, and where needles are used in large numbers - for immunisations and local anaesthesia the gauge may remain wider.

Selection of needle length

Patients may assume that the longer the needle, the more painful the injection will be.

This is not necessarily the case. When it comes to intra-muscular (IM) injections - for example vaccinations given IM - longer needles are likely to be less painful.[1][2] While this may appear to be counter-intuitive, this is not so surprising: muscle contains relatively few pain sensors compared to the more skin and more superficial tissues.

References

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