When we talk about injecting safely and avoiding major blood vessels it is generally accepted that vessels lay in common planes.  The PATHWAY or location changes regularly, but it is said that the vessel is at least always in a reliable PLANE of depth.   

This anatomical study from Cotofino et al suggests the superior labial artery can either be deep to the orbicularis oris muscle, in the Orbicularis Oris muscle or superior to the OO muscle.  This is quite terrifying when you think about it. 

Cotofana S, et al. Plast Reconstr Surg. 2017. 139(5)_1075-1085. Distribution of superior & inferior labial arteries & impact on safe lip augmentation[2].pdf

• 193 head specimens -  cadavers
• identified 3 different depths of the superior labial artery
• 78.1% of superior labial arteries where DEEP to the orbicularis oris muscle (where it is expected)
• 17.5% of superior labial arteries were in the Orbicularis Oris muscle
• 2.1% of superior labial arteries were located SUPERIOR to the Orbicularis Oris muscle.  Not expected.  This makes injecting upper lip lines (peri-oral rhytids) more dangerous in this group of patients

This is an interesting clinical publication because it disputes a common thought.  Everyone states that arteries will sit in one plane, the PATHWAY of the artery varies in everybody.  But the depth is always the same.  SO understanding the plane in which the artery lies enhances safety when injecting.  

Not according to this paper.  If you are treating one of the 2% of patients that have the superior labial artery above the Orbiculris Oris muscle then you need to take extra care not to occlude the vessel. 

We suggest you read the whole paper. at this link

https://www.ncbi.nlm.nih.gov/pubmed/28092335