Air Bubbles In many cartridges, a small air bubble is trapped under the septum as the aluminum cap is spun onto the top of the cartridge during manufacture (fig. 2). The back part of the dental needles protrudes into the cartridge below the cap level and any air bubbles smaller than this distance below the cap (fig. 3) will never pass through the needle; irrespective of which angle the cartridge is held. Cartridges with air bubbles that are visible below the cap when held vertically should be discarded.
	Leakage at the Cap The traditional method of assembling a dental cartridge syringe has been to screw on the needle first, then insert the cartridge. In the majority of instances this will pose no problem. However, with the thinner guage needles there have been some instances of leakage of solution. This happens when the back part of the needle bends to one side when the cartridge is loaded into a syringe with a needle already attached. Instead of piercing the rubber septum cleanly, at right angles, the needle tears the rubber. When pressure is applied, the local anesthetic solution will leak through the torn septum (fig. 4). In such cases, the problem has been prevented by reversing the order of assembly by placing the cartridge in the syringe first and then screwing on the needle. The needle will then pass cleanly through the septum and the hole will not leak when pressure is applied (fig. 5).
	Leakage at the Bung In standard cartridges, when pressure is applied centrally to the bung by the syringe plunger, the distortion that occurs will increase the glass/bung contact and tend to prevent, rather than promote leakage. However, if the pressure is applied to one side only of the standard cartridge bung, it may tip over and the glass/bung seal will then be imperfect (fig. 6). If a syringe plunger in the syringe through which it passes is worn, the end of the plunger may not travel centrally down the cartridge and cause tipping.
Cartridges Jamming in Syringes The use of high quality cartridge syringes that have been finished correctly during manufacture is an important factor in preventing jamming. Any metal burrs inside the syringe can make cartridge loading difficult. In addition, in breech loading style syringes, protruding pins which attach the breech loading mechanism can catch on glass cartridges and lead to jamming (fig. 7).

References:

Malamed, Stanley F. Handbook of Local Anesthesia. 5th ed. St. Louis, MO: Elsevier Mosby, 2004. 109-117.

Evers, Dds, Hans. The Dental Cartridge System. Trosa, Sweden: Trosa Tryckeri AB, 1993.