Trocars are used to make small incisions in the outer tissue layers. These incisions allow surgeons to insert cannulas, which are used to introduce surgical instruments. 

Trocars are also used to treat ascites and hydrothorax, and to implant slow-release hormone pellets beneath the skin. 

A trocar consists of an obturator for the initial puncture, a cannula for instrument insertion, and a valve mechanism to maintain pneumoperitoneum and prevent gas leakage. 

Trocars and cannulas come in different sizes and diameters depending on the instrument they are used with. The most common cannula sizes are 5 mm and 10 mm.