A syringe assembly comprising: a barrel including a cylindrical sidewall having an interior surface with a first cross-sectional width defining a chamber for retaining fluid, an open proximal end and a distal end including a distal wall having an opening therethrough in fluid communication with said chamber, said sidewall including a rib adjacent said proximal end defining a second cross-sectional width that is less than the first cross-sectional width; an elongate plunger rod including a proximal end, a distal end, and a main body extending between the proximal and distal end (549, 541), the plunger rod being distally and proximally movable within said chamber, the proximal end including a thumb press, the distal end including a stopper-engaging portion, the plunger rod further including a flexible protrusion having a cross-sectional width greater than the cross-sectional width of the barrel at the rib and the plunger rod further including a first support member disposed perpendicularly to the plunger rod that includes an annular projection having a distally facing surface, a proximally facing surface and an outer edge; a stopper having a proximal end and a distal end, the stopper attached to the stopper-engaging portion of the plunger rod such that when the distal end of the stopper is in contact with the distal wall of the barrel, the flexible protrusion is permitted to advance distally past the rib in the barrel and lock the plunger rod in the barrel to prevent reuse of the syringe assembly; characterized in that a strut element is disposed on the distally facing surface between the flexible protrusion and the annular projection the strut element includes a first beam and a second beam which extend in a distal direction from the distally facing surface along a first axis wherein the first beam and the second beam intersect one another at a midpoint between the first beam and the second beam to form an intersection; the first beam and the second beam have a rounded edge which has a height that increases from the outer edge of the annular projection to the intersection. a barrel including a cylindrical sidewall having an interior surface with a first cross-sectional width defining a chamber for retaining fluid, an open proximal end and a distal end including a distal wall having an opening therethrough in fluid communication with said chamber, said sidewall including a rib adjacent said proximal end defining a second cross-sectional width that is less than the first cross-sectional width; an elongate plunger rod including a proximal end, a distal end, and a main body extending between the proximal and distal end (549, 541), the plunger rod being distally and proximally movable within said chamber, the proximal end including a thumb press, the distal end including a stopper-engaging portion, the plunger rod further including a flexible protrusion having a cross-sectional width greater than the cross-sectional width of the barrel at the rib and the plunger rod further including a first support member disposed perpendicularly to the plunger rod that includes an annular projection having a distally facing surface, a proximally facing surface and an outer edge; a stopper having a proximal end and a distal end, the stopper attached to the stopper-engaging portion of the plunger rod such that when the distal end of the stopper is in contact with the distal wall of the barrel, the flexible protrusion is permitted to advance distally past the rib in the barrel and lock the plunger rod in the barrel to prevent reuse of the syringe assembly;