Disclosed is a syringe assembly which includes a barrel with a cylindrical side wall having an inside surface defining a chamber for retaining fluid, an open proximal end and a distal end including a distal wall having a passageway therethrough in fluid communication with the chamber. The syringe assembly includes an elongate hollow plunger rod having a proximal end, an open distal end, an interior surface, and a distal lip on the distal end of the plunger rod. The syringe assembly includes a stopper at the distal end of the plunger rod. The stopper has a sealing element with a peripheral surface forming a seal with the inside surface of the barrel, and a projection extending proximally from the sealing element and forming a longitudinal recess including a distal portion. The longitudinal recess has a plurality of ramps disposed on the distal portion of the longitudinal recess and a plurality of proximal recess detents inclined outwardly in a distal direction. The syringe assembly includes a locking element with a proximal portion having a proximal edge and a distal portion having an outwardly facing barb. The locking element is positioned in the longitudinal recess so that the barb contacts the interior surface of the plunger rod and the proximal edge contacts the proximal recess detent. The plurality of ramps and the plurality of proximal recess detents on the longitudinal recess cooperate so that the distal portion of the locking element rides up the plurality of ramps to move the distal portion of the locking element outwardly towards the interior surface of the plunger rod during distal sliding movement of the plunger rod and stopper. Proximal sliding movement followed by distal sliding movement of the plunger rod and stopper within the chamber causes the barb to extend past the distal lip on the plunger rod and toward the inside surface of the barrel, preventing further proximal movement of the plunger rod with respect to the chamber.
