Cannulas and needles have different ways of penetrating the skin for dermal filler injections. Cannulas have a rounded edge and blunt tip, which makes them unable to puncture the skin by themselves. As a result, a needle is usually used to create an entry point for the cannula. 

Needles, on the other hand, have a sharp tip and can easily penetrate the skin, delivering the filler into the targeted area without requiring further manipulation from the practitioner.