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The filler must be injected under sufficient pressure.  

 

Sufficient pressure means a higher pressure than the ophthalmic artery pressure inside the eye. However, the pressure applied when the filler is injected into the tissue is often higher than this pressure.

 

The ophthalmic artery pressure is 90mmhg for SBP and 60mmhg for DBP. Due to the ophthalmic artery pressure, the blood flow is directed from the inside of the eyeball to the outer skin of the eyeball.

 

Thus, in order to move against the direction, the filler must move forward in the form of a column filled in the blood vessel while being connected to the ophthalmic artery and the branch of the central retinal artery.

 

If the pressure of injection is high but the volume of the filler injected is smaller than required to maintain the column form, the filler may cause embolism as it travel to another skin area. 

 

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A sufficient volume of filler must be injected.   

 

The author considers a sufficient volume of filler to be closely related to the risk of blindness. The filler injected into the blood vessel does not necessarily lead to blindness. If the volume of the filler to be injected at one time is small, it can never move from the ophthalmic artery to the branch of the central retinal artery.

 

The blood pressure of the eyeball is maintained at 90/60mmhg thanks to the strong pumping of the heart from the internal carotid artery (ICA) inside the ophthalmic artery. The filler needs sufficient pressure and volume to move against the direction of this blood flow and invade the branch of the central retinal artery. The closest extraocular blood vessel to the ophthalmic artery and the branch of the retinal artery is the supra-trochlear artery (STA).

 

This blood vessel located in the glabella is the most dangerous. Since STA is mainly located in the subcutaneous layer, the filler is highly likely to be injected into the subcutaneous layer in order to smoothing out glabellar frown lines, which may result in blindness, as shown in the previous case.

 

Assuming that the distance from this artery to the branch of the ophthalmic artery is 4.5cm; and the thickness of the blood vessel is 0.05cm, the volume of this blood vessel is approximately 0.035ml.

 

Theoretically, blindness may be caused if a thin needle penetrates the blood vessel and 0.035ml or more of filler material is injected at one time in order to smooth out glabellar frown lines. Indeed, there is a case where blindness occurred when the bolus of filler to be injected was 0.1ml, greater than 0.035ml.

 

The quantity of filler injected at one time is proportional to the risk of blindness. The 23G cannula (needle) has an internal diameter twice as large as a 30G needle, but the quantity of filler injected at one time is proportional to the 4th power of the internal diameter.

 

Therefore, if the pressure of injection is the same, a larger-diameter cannula (needle) may lead to the greater risk of blindness as 16-fold amount of filler may be injected at one time. 

 

- To be continued