Cannula gauge and length: what changes in practice

The conversation about cannulas often turns into a slogan: "a cannula is safer." In practice, safety depends on a set of variables, and gauge and length are two of them. This page explains, at a review level, what each parameter changes from a physical and clinical standpoint, so the choice can be made with care. The choice of material, technique and protocol follows the package insert, the training and the decision of the qualified professional, in line with the rules of their professional board in Brazil.

What is gauge and how is it read?

Gauge is measured in G. The higher the number, the smaller the outer diameter. A 25G cannula is thinner than a 22G, for example. The outer diameter matters for the trauma along the path, and the inner diameter matters for flow and for the pressure needed to inject the product. Two instruments with the same G can have different walls and inner diameters, which is why manufacturers and lots should be taken into account.

For the conceptual difference between cannula and needle, see cannula and needle. For the debate on choosing, see cannula or needle and syringes and needles in HOF.

What does gauge change in practice?

The pressure needed is a point that matters. When a lot of force is required to push the product through a very thin cannula, there is a risk of sudden changes in pressure and injected volume, and the professional's tactile feedback changes. This is relevant in areas with vascular risk.

What does length change?

Length determines reach from an entry point. Longer cannulas allow wide areas to be treated with few punctures, but they require control of the path and attention to the tip position in tissue the professional cannot see. Shorter cannulas offer more direct control, but may require more entry points. In general, the longer and thinner the instrument, the greater the flexibility and the lower the ability to "steer" the tip precisely.

The choice of length depends on the area, the extent to be treated and the professional's training. There is no universal length, and instruments must be used according to the manufacturer's instructions and the product's indication.

Why doesn't a cannula eliminate vascular risk?

A blunt-tip cannula tends to push vessels aside rather than puncture them, which can reduce the risk of direct entry into the lumen. That is an advantage, but not a guarantee. Vessels can be injured, the tip can pass through tissue under some conditions, and the product can embolize under pressure. In addition, a cannula does not remove the need to know the course of the arteries or to be prepared for complications. See vascular danger zones of the face, frequency of vascular complications and vascular occlusion.

Gauge and length by area, at the level of principle

Without prescribing an instrument for each point, it helps to understand the logic:

• Delicate areas with thin skin, such as the lips and the periorbital region, tend to favor finer instruments, for volume control and less trauma. See lip filler.
• Areas with more volume and thicker tissue, such as the cheek, usually tolerate larger gauges. See cheekbone filler.
• Areas of high vascular risk, such as the glabella, the nose and the nasolabial fold, call for extra care regardless of the instrument. See nasolabial fold filler.
• Firmer products require more pressure and may call for gauges compatible with the package insert.

The product's package insert usually lists compatible needles and cannulas, and it is worth following those recommendations. See how to read an injectable's instructions for use.

Tip, port and design: what the G does not tell you

Two cannulas of the same gauge can behave differently because of the tip design and the position of the side port. A well-rounded, polished tip tends to glide through tissue with less damage. A port very close to the tip or with rough edges can compromise product distribution or cause friction. The material and stiffness of the shaft also change the tactile feel and control. That is why switching manufacturer or model takes an adaptation period, and it is prudent to adjust volume and speed while you gain familiarity.

Another point is compatibility with the syringe and the product. The fit, the viscosity and the force needed to push the plunger vary, and the product's package insert usually gives recommendations. Ignoring that compatibility can cause the connector to disconnect, leakage or excessive pressure. The overview is in syringes and needles in HOF.

Speed and pressure: the factor the instrument does not decide alone

Even the best instrument depends on who is using it. Injection speed and the pressure applied to the plunger influence the risk of embolization and the distribution of the product. Injecting slowly and in small amounts, watching the response of the tissue and the patient, is a widely recognized general principle, and it applies to needles and cannulas alike. The instrument is one element of a safe technique, not a substitute for it.

Caring for the instrument

• Use only single-use, registered materials, as required by regulation. See biosafety in HOF.
• Check the integrity of the tip and the connector before use.
• Dispose of it in a proper container. See sharps disposal and PGRSS.
• Record the type, gauge and lot of the instrument when the clinic adopts that traceability.

Where does Harmy fit in?