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Dr Tim Pearce
Like a member of an orchestra about to play a symphony, one of the keys to success with lip filler treatment lies in your choice of instrument – between needle or cannula. Understanding the science and mode of action of these tools is vital to making the right decision
In this blog, Dr Tim Pearce will share some introductory expert tips on aspects such as needle gauge and the advantages and disadvantages of using cannulas over needles.
This educational piece is based on content from the world’s first online lip training with interactive 3D anatomy and injection animation – introducing the 8D Lip Design training course.
What is needle gauge and why is it important?
One of the most important metrics to understand when choosing your instrument for injection is gauge. The gauge is a number that refers to the diameter of the lumen (opening) in a needle.
Dr Tim notes that this measurement is now somewhat archaic, as gauge units for hypodermic needles are based on the thickness of a wire first defined in the early 19th Century, as the Birmingham gauge. Smaller wires are pulled sequentially from larger wires; thus, each gauge needle is relative to the sequence in which it is made; the largest gauge, 1G, starts with the first thickness of wire and the unit numbers increase as wires, or needles get thinner, with the smallest or thinnest at 36G. Each step between a gauge (and an increase in the gauge number) represents an 11% reduction in thickness of the wire or needle.
Most aesthetic clinicians will use the needle gauge supplied with the dermal filler product, as packaged by the manufacturer. They may perhaps switch occasionally, out of curiosity, or to choose a different instrument, like a cannula, whereupon they will notice the dramatic difference such small changes can make.
Dr Tim explains that were it not for the laws of physics, the smaller or thinner the needle, the less trauma would be caused during the procedure, but unfortunately, this logic does not hold. Particularly with high viscosity fluids like dermal filler, smaller needles can increase the procedure time due to the pressure required to extrude the product, which would be quicker with a slightly thicker needle due to reduced resistance.
The supplied instruments are usually chosen to allow for easy, controlled injection, that avoids putting too much product into the tissue too easily. By changing gauge, you will experience a significant difference in flow and extrusion force, which may be more difficult to control. Poiseuille’s law explains the physics behind passing a liquid through a tube.
In summary, choosing a thinner needle (than the one supplied) may make it too difficult, and uncomfortable for the practitioner to apply the right pressure to correctly deliver product without increasing trauma and treatment time for the patient.
Conversely, a thicker needle may lead to uncontrolled product delivery, especially with a low viscosity filler flowing through a wide bore lumen, leading to over treatment or lumps. In general, a thicker needle will increase injection trauma over a thinner one, simply due to size, but the viscosity of the product through the tube is the primary factor – a fine balance is required. Increasing or decreasing the length of the needle also has an affect on extrusion force, but not as significantly as the diameter of the needle.
The take home message from Dr Tim is to use caution when choosing to change from the needle gauge supplied with products as needle diameter will affect your procedure; you should aim for a balance between control and ease of placement of the product.
Cannulas tend to be significantly longer than needles, but do not need to be proportionally wider. Therefore, as a guide, Dr Tim notes that product will usually flow well through a cannula that is one step up from the needle size supplied by the manufacturer.
A full conversion table for needle to cannula gauges to use with different viscosity dermal fillers is available in Dr Tim’s 8D Lip Design training course.
The pros and cons of using cannulas for lip filler treatments
The core difference between a needle and a cannula is the blunt tip which decreases the pressure against the tissue, asserts Dr Tim. The connective tissue often repels the cannula, and it passes through the non-connective tissue instead, primarily fat, causing less trauma and with the instrument remaining in one compartment. Cannulas therefore move away from blood vessels, dermis, bone, and muscles but towards fatty tissue and mucosa. This does mean less trauma, but also means less control because you are guided by the tissue and less by your desire to place product in different layers of the skin.
Breaking things down into pros and cons, we can see the following advantages in the use of cannulas.
- Blunt cannulas decrease the chance of damaging key structures like blood vessels through trauma, including bruising and vascular occlusion, when compared to needles.
- Cannulas are longer; therefore, more work can be done through one entry point, decreasing the length of time required for some procedures. Similarly, the number of penetrations through the dermis is decreased with this approach.
- They may be less painful for patients.
- Cannulas can be used (with caution) in so-called danger points where you would not place a sharp needle, for example, replacing volume beneath the orbicularis oris muscle.
However, there are some disadvantages to consider when deciding whether a cannula is the correct choice of instrument to use.
- Due to their bluntness, there is less control over the layer or depth of injection, which relies on penetration and the ability to cross tissue boundaries. Once placed, a cannula will tend to stay in a fatty or mucosal layer.
- It can be difficult to achieve definition or detail in the lips using a cannula. Such features that benefit from this aspect of lip treatment tend to be in areas of higher density connective tissue which repel cannulas.
- There is a potential risk, although very rare, that severe or larger vascular occlusions may be associated with cannula-based injections. If a cannula does make its way into a vessel, the resistance to crossing tissue boundaries (so it can exit) can pose a disadvantage, thus, there is a risk the cannula would remain in the vessel whilst a larger quantity of filler product is dispensed.
- Smaller cannulas may allow for more control but increase the risk of trauma. They become impossible to use with high viscosity products due to the length and diameter of the cannula increasing resistance to extrusion, thus they tend to be limited to medium and low G-Prime products
According to Dr Tim, the idea cannula for lips is a 25G x 38mm instrument, allowing the injector to reach each quadrant of the lip through a single-entry point.
For more on this topic, read another of Dr Tim’s blogs where he asks, is needle or cannula safer for dermal fillers?
8D Lip Design
With all the conflicting advice out there about lip filler treatments – vertical or horizontal? needle or cannula? – it can be difficult to know how to inject to create the lips your patient desires.
If you are suffering from technique overwhelm, worrying about causing a vascular occlusion (VO), or panicking about injecting thin lips, then Dr Tim Pearce’s brand-new ultimate lip course is going to teach you the different techniques, anatomy, and skills you need to create medically beautiful lips.
Dr Tim Pearce eLearning
Dr Tim Pearce MBChB BSc (Hons) MRCGP founded his eLearning concept in 2016 in order to provide readily accessible BOTOX® and dermal filler online courses for fellow Medical Aesthetics practitioners. His objective was to raise standards within the industry – a principle which remains just as relevant today.
Our exclusive video-led courses are designed to build confidence, knowledge and technique at every stage, working from foundation level to advanced treatments and management of complications.
Thousands of delegates have benefited from the courses and we’re highly rated on Trustpilot. For more information or to discuss which course is right for you, please get in touch with our friendly team.
