The present disclosure provides a sub-assembly for a medicament delivery device, the sub-assembly extending along a longitudinal axis in a longitudinal direction from a proximal end to a distal end. The sub-assembly comprises a housing and a plunger rod. The plunger rod is arranged at least partly inside the housing. The plunger rod comprises a first surface facing towards the proximal end and a component of the sub-assembly other than the plunger rod comprises a second surface facing towards the distal end, with the first surface and the second surface being arranged so that, when the plunger rod moves towards the proximal end relative to the component during medicament delivery device use, the first surface and the second surface engage to rotate the plunger rod relative to the component and then disengage so that the plunger rod rotates back relative to the component. The sub-assembly comprises a torsion spring attached at one end to the plunger rod and at the other end to the component.

An automatic anesthetic buffering system is provided. The system comprises a carpule having a body that defines an internal chamber, a forward end configured to be pierced by a needle, and a rear end configured to receive a plunger actuation force. A bladder assembly is positioned within the carpule and includes a collapsible bladder containing a buffer solution. An anesthetic is positioned toward the rear end of the carpule. Actuation of the plunger causes the bladder to collapse and discharge the buffer solution through the needle prior to delivery of the anesthetic. In one embodiment, the bladder assembly includes a bladder retention spring, a radial spring clip, and a bladder cushion. The system enables in situ mixing of the buffer and anesthetic solutions immediately prior to or during injection, reducing injection pain and improving anesthetic onset.