Since the advent of microcannulas, which revolutionized the field of cosmetic injectables, there has been a large debate over their utility and whether they are safer than needles regarding the risk of intravascular injection. Several studies have shown they can decrease pain, edema, redness, and hematoma from injection^{1, 2}. Over the past 10 years, multiple brands of cannulas have come to the market, though there are not many studies analyzing the differences among them. One animal study by Wu et el. showed there is a significant difference between different cannulas of the same size, with respect to arterial penetration force and the elastic modulus³.

Recently, the safety of smaller size cannulas has been debated⁴. There was a retrospective cohort by Alam et al.⁵ that collected data in 2018-2019 regarding previous procedures performed by board-certified dermatologist, and the chance of vascular occlusion with a needle was 1 in 6,410 per 1 ml, while the chance was 1 in 40,882 with a cannula. Unfortunately, there was no reporting of which brand or even gauge cannula was used. In addition, Goodman et el.⁶ reported 83% of vascular occlusions in their survey were with a needle and 17% with a cannula, however there was no reporting of the absolute numbers of injections with each, though a majority reported using both needles and cannulas. In a previous cadaveric study by Pavicic et al.⁷, they analyzed the penetration force of both needles and cannulas of different gauges, but did not explore the differences between different brands, which could be important to consider given the animal study mentioned above³. Based on these studies, the author wanted to explore whether certain brand 27-gauge cannulas could be safer than others by having decreased arterial penetration force in human cadavers. In addition, a more sensitive force gauge (FS-500) that was calibrated to register as low as 0.1 N was used to detect differences at lower forces than had previously been examined. A secondary site was used to verify the data.

This newly published study confirmed Pavicic et al’s finding⁷ that a decrease penetration force was required in older cadavers. Since cadavers are often older than cosmetic patients, it seems very likely that the arterial force required in most of our clinical patients is higher than in this study, though it also follows that so would be the tissue penetration force as well. While future studies on younger live subjects are needed to find further answers, it makes sense to use less force and be much more cautious in elderly patients knowing their vessels may be prone to easier penetration if the cannula cannot glide around it.

Two sites using a force gauge calibrated to measure as low as 0.1 newtons showed there was a significantly lower penetration force with needles (ranging from 0.1-0.4 newtons) as compared to cannulas (ranging from 0.5-1.0 newtons) of the same gauge at all sizes tested including both the 27 and 30-gauge cannulas, which is a major update to the current literature in this field. This is supported by other published studies, one showing the average force of a 30-gauge needle through skin was 0.2 newtons⁸.

When injecting small delicate areas, such as the infraorbital hollow (IOH), especially with fillers with low friction coefficients, a 27-gauge cannula may be a better choice to decrease trauma to the area and prevent larger amounts of injected filler as the smaller size allows the injector more control (more force needed to inject the same amount). It has been shown that lower volumes injected in the IOH, the lower the side effect profile, especially regarding swelling⁹. In addition, there would be less risk of a more serious occlusion with a smaller filler deposit, and possibly less risk of compression in a tight space such as the nose. In the nose, larger cannulas both have less space to move and are less flexible which limits their ability to glide and bend past vessels. A 27-gauge cannula is .41mm and a 22-gauge cannula is 0.72mm. Given that the distance to maneuver in this space from the skin to the bone is tight, the extra room with a smaller more flexible cannula could be an advantage.

Penetration into an artery could more likely when the force needed to advance through the tissue is higher. Often in younger patients with thicker skin, especially in the nose, and those with denser fibrous interconnections, it can be difficult to find a smooth gliding plane. While the difference between the needle and cannula is smaller with higher gauges, the force needed to penetrate tissue is also less with smaller size cannulas in both living and cadaveric tissue, thereby mitigating the difference in force needed to puncture a vessel and giving certain advantages to all size cannulas.

When comparing 2 different gauges of otherwise identical cannulas, the larger size not only has increased arterial penetration force and increased tissue penetration force, but it also has less flexibility to bend around a vessel. This increased force to maneuver and rigidity, especially through ligaments near vessels, is missed in dissected cadaver studies. The author believes these effects dampen, if not eliminate most of the increased safety of increasing arterial penetration force with larger cannulas, as an increased force with the larger cannula may be required to pass through an area before arriving at a vessel as shown in both our cadaver study and retrospective training data review on live patients. Therefore, the first author proposes that with the correct technique, a small, blunt-end, flexible cannula in small, sensitive, delicate, tight areas could be safer and less traumatic than larger cannulas.

Above all, the study showed that minimal arterial penetration force is not as important as once thought due to the high tissue penetration forces. Yet, in this study, the most striking finding was that penetrating an exposed artery with a cannula is very difficult. It required many attempts and sometimes necessitated an environment not present in live injections with removal of all surrounding fascia and fixation. After completing this research, the author believes the most important factors leading to the increase safety of cannulas is their coefficient of friction – their ability to glide over the vessel without puncturing them and their flexibility. During surgery, this same phenomenon has been corroborated in live patients – that a cannula will glide past vessels multiple times even if intentionally trying to puncture it similar to our study findings at both sites. The fascia around these vessels aide in this process protecting them.

Certain exceptions to the cannula easily gliding past the vessel is nicely described in a paper by Tansatit et al, when a cannula enters a branching point or bend in which the cannula gets trapped¹⁰. In addition, in this experiment, when the vessel was lying in a fixed space, unable to move, it was much easier to puncture while those that were not could take 30 insertion attempts.

While the author uses force gauges now with all trainees to make them aware of their force and feel when their cannula has hit an obstacle, whether traversing through thicker tissue, a ligament or vessel, an injector should be able to detect an increased force needed to advance forward and instead choose to redirect their instrument.

In summary, since the force needed to penetrate tissue is also less with smaller size cannulas in both living and cadaveric tissue, it can mitigate the difference in force needed to puncture a vessel, therefore giving certain advantages to all size cannulas. Most importantly, this study showed that despite minimal tissue penetration force often exceeding minimal arterial penetration force, arteries are very difficult to enter with a cannula due to it rolling over, under, or on the side of the vessel. This signifies that the penetration force of an instrument has very little practical application in preventing entrance into a vessel.

A summary of the main findings of a study utilizing a force gauge that could measure as low as 0.1 newtons, suggests it is 1) easier to penetrate vessels with a needle vs cannula across all gauges, including both 27 and 30-gauge; 2) smaller cannulas and needles have lower arterial penetration force but also lower tissue penetration force which can sometimes be higher; and 3) the gliding of the cannula around the vessel is the main factor contributing to cannula safety and factors contributing to this phenomenon should be further studied.

References
1. Hexsel, D., et al., Double-blind, randomized, controlled clinical trial to compare safety and efficacy of a metallic cannula with that of a standard needle for soft tissue augmentation of the nasolabial folds. Dermatol Surg, 2012. 38(2): p. 207-14 DOI: 10.1111/j.1524-4725.2011.02195.x.
2. Beer, K.R., Safety and effectiveness of injection of calcium hydroxylapatite via blunt cannula compared to injection by needle for correction of nasolabial folds. J Cosmet Dermatol, 2014. 13(4): p. 288-96 DOI: 10.1111/jocd.12115.
3. Wu, K., et al., Comparison of the Microstructures and Properties of Different Microcannulas for Hyaluronic Acid Injection. Plast Reconstr Surg, 2018. 142(2): p. 150e-159e DOI: 10.1097/PRS.0000000000004573.
4. Siperstein, R., The Use of a 27-Gauge Cannula in Aesthetic Medicine. Aesthet Surg J Open Forum, 2022. 4: p. ojac018 DOI: 10.1093/asjof/ojac018.
5. Alam, M., et al., Rates of Vascular Occlusion Associated With Using Needles vs Cannulas for Filler Injection. JAMA Dermatol, 2021. 157(2): p. 174-180 DOI: 10.1001/jamadermatol.2020.5102.
6. Goodman, G.J., Roberts, S., and Callan, P., Experience and Management of Intravascular Injection with Facial Fillers: Results of a Multinational Survey of Experienced Injectors. Aesthetic Plast Surg, 2016. 40(4): p. 549-55 DOI: 10.1007/s00266-016-0658-1.
7. Pavicic T, W.K., Frank K, Gotkin RH, Tamura B, Cotofana S, Arterial Wall Penetration Forces in Needles versus Cannulas. Plast Reconstr Surg., 2019. 143(3): p. 504e-512e DOI: 10.1097/PRS.0000000000005321.
8. Tansatit, T., et al., Effect of Needle Sizes 30 G and 32 G on Skin Penetration Force in Cadavers: Implications for Pain Perception and Needle Change during Botulinum Toxin Injections. Plast Reconstr Surg, 2021. 148(6): p. 1071e-1073e DOI: 10.1097/PRS.0000000000008558.
9. Siperstein, R., Montes, J.R., and Speranza, A., A Retrospective Review of the Safety and Efficacy of Low-dose Triamcinolone Mixed with Hyaluronic Acid Fillers to Reduce Post-injection Infraorbital Swelling. J Clin Aesthet Dermatol, 2022. 15(4): p. 13-19 DOI: Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9017664/.
10. Tansatit, T., Apinuntrum, P., and Phetudom, T., A Dark Side of the Cannula Injections: How Arterial Wall Perforations and Emboli Occur. Aesthetic Plast Surg, 2017. 41(1): p. 221-227 DOI: 10.1007/s00266-016-0725-7.
11. Ugradar S, H.J., Measurement of the Force Required by Blunt-Tipped Microcannulas to Perforate the Facial Artery. Ophthalmic Plast Reconstr Surg. , 2019. 35(5): p. 441-446 DOI: 10.1097/IOP.0000000000001302.