A new double-blind study at University of Groningen showed no increased effectiveness of Dexamethasone to improve TMJ arthrocentesis.

After carrying out 28 TMJ arthrocentecis' comparing the steroid Dexamethasone to simple saline, the researchers found that the patients did have reduce pain and jaw stiffness, but the steroid did not improve the outcome. 

LINK:
Randomized trial on the effectiveness of dexamethasone in TMJ arthrocentesis.