A medicament delivery device defining a longitudinal axis and having a distal end, the device comprising: a proximal knob ; a plunger rod having a longitudinally extending structure ; a collar disposed coaxial with the longitudinal axis, the collar including: an anti-rotation projection extending radially inwardly into the interior of the collar , the anti-rotation projection being configured to slidingly engage the longitudinally extending structure of the rod ; characterized in that the anti-rotation projection is further configured to rotationally retain the rod during a rotation of the knob about the longitudinal axis, the rotation thereby driving the rod toward the distal end; in that the plunger rod is threadingly engaged, coaxially within the collar , with the proximal knob , the proximal knob being coaxially disposed within the collar and having a track that includes a protrusion ; and in that the collar includes a flexible panel disposed on an interior portion of the collar and extending into the interior of the collar , wherein: the panel is configured to: move in deflection relative to the axis, responsive to interaction with the protrusion ; and slidingly engage the track to longitudinally retain the knob during the rotation of the knob about the longitudinal axis. a proximal knob ; a plunger rod having a longitudinally extending structure ; a collar disposed coaxial with the longitudinal axis, the collar including: an anti-rotation projection extending radially inwardly into the interior of the collar , the anti-rotation projection being configured to slidingly engage the longitudinally extending structure of the rod ; characterized in that the anti-rotation projection is further configured to rotationally retain the rod during a rotation of the knob about the longitudinal axis, the rotation thereby driving the rod toward the distal end; in that the plunger rod is threadingly engaged, coaxially within the collar , with the proximal knob , the proximal knob being coaxially disposed within the collar and having a track that includes a protrusion ; and in that the collar includes a flexible panel disposed on an interior portion of the collar and extending into the interior of the collar , wherein: the panel is configured to: move in deflection relative to the axis, responsive to interaction with the protrusion ; and slidingly engage the track to longitudinally retain the knob during the rotation of the knob about the longitudinal axis. an anti-rotation projection extending radially inwardly into the interior of the collar , the anti-rotation projection being configured to slidingly engage the longitudinally extending structure of the rod ; characterized in that the anti-rotation projection is further configured to rotationally retain the rod during a rotation of the knob about the longitudinal axis, the rotation thereby driving the rod toward the distal end; in that the plunger rod is threadingly engaged, coaxially within the collar , with the proximal knob , the proximal knob being coaxially disposed within the collar and having a track that includes a protrusion ; and in that the collar includes a flexible panel disposed on an interior portion of the collar and extending into the interior of the collar , wherein: the panel is configured to: move in deflection relative to the axis, responsive to interaction with the protrusion ; and slidingly engage the track to longitudinally retain the knob during the rotation of the knob about the longitudinal axis. move in deflection relative to the axis, responsive to interaction with the protrusion ; and slidingly engage the track to longitudinally retain the knob during the rotation of the knob about the longitudinal axis. <EOS>