Deformation of Aflibercept and Ranibizumab Syringes Causes Variation in Intravitreal Injection Volume and Risks Retinal Tear Formation

Purpose: The intravitreal injection volume is known to vary with plunger alignment and the speed of injection. We investigated the role that syringe stopper deformation plays in allowing excess volumes to be injected into the eye and the potential for the vitreous humor to become incarcerated when excess force is released within the eye. Design: Experimental study. Methods: Aflibercept prefilled syringes (PFSs), ranibizumab PFSs, and 1-ml tuberculin (TB) syringes were subjected to increasing injection force to assess the extent to which each design allowed for excess volumes to be expelled after the stopper reached the bottom of the syringe barrel (i.e., after the 50-μl dose was expelled). Main Outcome Measures: Additional volume expelled with stopper deformation. Results: Syringe stoppers are capable of deformation into the dead space when additional force is applied. This allows for progressively greater medication doses to be administered. At an additional force of 3.92 N after the syringe stopper came in contact with the bottom of the syringe barrel, the aflibercept PFSs, ranibizumab PFSs, and 1-ml TB syringes dispensed an additional 17.2%, 11.4%, and 0.8% higher volume than the intended volume of 50 μl, respectively. Upon release of this force, a proportional volume was observed to be drawn back into the needle. Conclusions: The intravitreal injection volume varies with the force applied to fully depressed syringes because of syringe stopper deformation. We advise that performing forceful intravitreal injections be avoided to prevent excessive dosing of medication. We also caution that pressure applied to the plunger during intravitreal injections not be released while the needle is in the vitreous cavity to guard against vitreous incarceration, which could lead to retinal tear formation or detachment.