Soft tissue augmentation is a common, office-based procedure in which a filler is delivered into the dermis or subcutaneous tissue. The traditional delivery tool for this procedure used to be a hypodermic needle, most often supplied with the product. However, recent years have seen a major shift from using sharp needles for dermal filler injections to using blunt cannulas to deliver product in this procedure.
Similar cannulas have been used extensively for years in cosmetic surgery; predominantly for fat injections. However, these tools have been modified to make them suitable for use in dermal filler placement procedures (Niamtu, 2009). The increasing popularity of this new tool is related to a number of reasons, including:
Traditionally in this procedure, the typical injection technique included use of hypodermic needles to deliver the product sub- or intra-dermally by linear threading, fanning or bridging. Most of these techniques were relevant in the era in which dermal fillers were used as wrinkle ‘correctors’. However, in recent years, there has been a natural evolution to 3-D volumisation of the face, rather than simply filling the lines of the face. As such, the use of cannulas has increased in popularity, owing to the fact that this tool is suitable when filling a large area of the face in the subdermal or supra-periosteal plane. Regions such as tear troughs, midface, temples, forehead, cheeks and marionettes, require even ‘inflation’ by the filler, rather than the more focused approach to filling that is required to address individual rhytides.
Minimising side effects
When precision of supraperiosteal placement of a filler with a sharp needle was compared with a non-traumatic cannula in cadaver specimens, the cannula was shown to be more precise in placement of the product (van Loghem et al, 2017). With the sharp needle, the material injected on the periosteum migrated in a retrograde direction along the trajectory of the needle path, ending up in multiple anatomic layers (van Loghem et al, 2017).
Most of the immediate adverse effects of dermal filler procedures are related to the trauma induced by the injection process itself, rather than the product delivered (Glogau and Kane, 2008). A needle's sharp tip causes a sensation of pain, traumatises the tissue and may lacerate blood vessels and nerves. However, when using a cannula, the side effects of pain, bruising, oedema and redness—all associated with the use of a needle—are minimised (van Loghem et al, 2017). Nevertheless, some patients do feel discomfort related to the subdermal movement of the cannula; however, this discomfort is fundamentally different from the sensation of pain induced by a sharp needle penetrating the skin.
Safety
No doubt the most important advantage of using a device with a blunt tip is that this reduces the risk of perforating blood vessels and causing intra-arterial embolus of the filler with subsequent ischemic event (van Loghem et al, 2017). Indeed, it was reported that in 83% of cases in which an intra-arterial injection of the filler occurred, the injection was performed using a needle, as compared with 17% when the injection was performed using a cannula (Goodman et al, 2016). Nevertheless, in some areas, such as the lips, it is the author's opinion that a combination of needles and cannulas are sometimes required to achieve the optimal cosmetic outcome. To inject safely in areas such as these requires thorough anatomical knowledge (Tansatit et al, 2017a).
When forces required to penetrate an artery using different sized needles and cannulas were measured, 22- and 25-gauge cannulas required significantly higher force to penetrate the arterial wall than the same size needles. However, comparing the forces needed to penetrate the facial arterial vasculature with smaller needles and cannulas, no significant difference was detected, indicating that small cannulas are not safer than needles (Pavicic et al, 2019).
In spite of the evidence discussed previously, it has been experimentally shown that rarely, even bigger cannulas can cause an arterial wall perforation, especially when an artery is fixed in place by a fibrous band (Tansatit et al, 2017b). While use of blunt cannulas is mostly suggestive, there are zones in which cannulas are nearly mandatory. This includes anatomical locations in which there is a particularly high risk of remote embolisation, causing devastating consequences such as blindness (Signorini et al, 2016). Among these locations are the central forehead, temples, intraorbital zone and nose.
From a practical aspect, when working with cannulas, an opening has to be created first for cannula introduction, by a sharp needle. The direction and depth of penetration of the needle define the gliding plane of the cannula. If the injection is intended to be supraperiosteal, the ‘pilot’ needle has to be introduced perpendicular to the skin's surface and deeply to the bone. For superficial subdermal injection, the needle should be inserted in 30–45° subcutaneously in the direction in which the practitioner wishes the cannula to travel.
Size and shape advantages
Cannulas are longer than the majority of traditional sharp needles. Therefore, the extrusion force needed to deliver the filler through the ‘dead space’ of the cannula is significantly higher when using comparative sizes. In the author's opinion, current recommendations using a cannula of 25 gauge or bigger makes the action of injection through the cannula no more challenging than through a needle.
The tip of the cannula is usually rounded and atraumatic, and therefore does not cut through the tissue. This is why any fibrotic tissue on the way will resist and block a cannula's progress with a subsequent need of a path change. To overcome this problem, some cannulas have a tapered tip, comprising a hybrid between a completely blunt and sharp ending. In addition, in the author's experience, wiggling the cannula during its movement through the tissues can help to overcome the resistance points.
The location of the opening port of the cannula is also an important consideration. With a sharp needle, the product leaves the syringe and enters the tissue at the tip of the needle; however, in cannula, the injection port is located proximal to the tip. When the injection port is located far from its tip, the injector has to consider the exact deposition point and progress the cannula distally to the intended product deposition area. If the opening port is located closer to the tip, the deposition of the product is more correlative with the tip location. This aspect might be of special importance in areas such as tear troughs, in which minor nuances create major differences in the outcomes of the procedure.
The best performing cannulas present the right balance between their flexibility and rigidity. These features are partially dependent on the manufacturing technology (Wu et al, 2018). Bigger cannulas are always more rigid, while smaller ones are more flexible. The smallest cannulas available are of 30 gauge—these cannulas are very flexible, require high extrusion force and are almost as sharp as needles. Therefore, it is the author's opinion that their usage should be abandoned. Cannulas of 22 and 25 gauge are currently considered to be the ‘gold standard’ for most facial and extra-facial indications.
Conclusion
With the popularity of injectable procedures still on the rise, and the increasing number of injection tools now available on the market, it is imperative that aesthetic practitioners have the knowledge to select the best injection tool for a given procedure. Cannulas have been increasingly used to administer dermal fillers due to the changing goals of treatment, consideration of the patient experience and ability to prevent the rare but devastating complications that can arise as a result of injectable procedures. Aesthetic practitioners should always consider their choice of injection tool before commencing any treatment in order to guarantee the safety of their patients.