INTRALESIONAL injection of a corticosteroid suspension is a well-known and accepted treatment of several dermatological conditions. Some clinicians utilize a large bore syringe or an intermediate size syringe with a finger-ring attachment in an attempt to overcome the high degree of resistance characteristic of certain lesions. For example, injection of keloids with such an apparatus is often difficult and attended by considerable frustration. The mathematical interpretation to follow emphasizes that such consequences are unnecessary and explains why a small bore syringe will reduce the effort required.
To explain the principle, two assumptions are prerequisite: (1) The pressure is a constant and (2) fluid flow is small. The system is, therefore, quasistatic and independent of needle size. Pressure is produced by a force applied to the syringe piston. For an applied force (F), the resulting pressure (P) equals the force on the piston divided by the cross
Jetton RL, Hudson HT. Mathematics of Intralesional Injection. Arch Dermatol. 1969;99(4):477. doi:10.1001/archderm.1969.01610220105018