An apparatus, comprising: a handpiece comprising a housing, wherein the housing includes a first locking feature; a reservoir having a variable fluid capacity; and a plunger assembly, wherein the plunger assembly is movable relative to the housing to selectively vary the fluid capacity of the reservoir, wherein the plunger assembly includes a second locking feature configured to selectively engage the first locking feature to selectively maintain a location of the plunger assembly at a selected longitudinal position in relation to the housing, wherein the first and second locking features are configured to selectively engage each other by one of the first or second locking features moving linearly along a path that is transverse to a longitudinal axis defined by the plunger assembly, wherein the handpiece comprises a syringe body disposed within the housing, wherein the plunger assembly includes a piston slidably disposed in the syringe body, wherein the piston and the syringe body cooperate to define the reservoir, wherein the first locking feature comprises a helical groove defined by the housing, wherein the second locking feature comprises a first retractable thread member (1230, 1231) configured to selectively engage the helical groove, wherein the first retractable thread member (1230, 1231) is configured to travel linearly along a path that is transverse to a longitudinal axis defined by the plunger assembly to selectively engage the helical groove, and wherein the plunger assembly further comprises: a reciprocating member, and a housing member (1210a, 1210b), wherein the reciprocating member is slidably disposed in relation to the housing member (1210a, 1210b), wherein the reciprocating member is configured to translate relative to the housing member (1210a, 1210b) to selectively engage and disengage the first retractable thread member (1230, 1231) with the helical groove, characterised in that the reciprocating member defines an oblique slot (1234, 1235) and wherein the first retractable thread member (1230, 1231) is secured to a pin (1232, 1233) disposed in the oblique slot (1234, 1235). a handpiece comprising a housing, wherein the housing includes a first locking feature; a reservoir having a variable fluid capacity; and a plunger assembly, wherein the plunger assembly is movable relative to the housing to selectively vary the fluid capacity of the reservoir, wherein the plunger assembly includes a second locking feature configured to selectively engage the first locking feature to selectively maintain a location of the plunger assembly at a selected longitudinal position in relation to the housing, wherein the first and second locking features are configured to selectively engage each other by one of the first or second locking features moving linearly along a path that is transverse to a longitudinal axis defined by the plunger assembly, wherein the handpiece comprises a syringe body disposed within the housing, wherein the plunger assembly includes a piston slidably disposed in the syringe body, wherein the piston and the syringe body cooperate to define the reservoir, wherein the first locking feature comprises a helical groove defined by the housing, wherein the second locking feature comprises a first retractable thread member (1230, 1231) configured to selectively engage the helical groove, wherein the first retractable thread member (1230, 1231) is configured to travel linearly along a path that is transverse to a longitudinal axis defined by the plunger assembly to selectively engage the helical groove, and wherein the plunger assembly further comprises: a reciprocating member, and a housing member (1210a, 1210b), wherein the reciprocating member is slidably disposed in relation to the housing member (1210a, 1210b), wherein the reciprocating member is configured to translate relative to the housing member (1210a, 1210b) to selectively engage and disengage the first retractable thread member (1230, 1231) with the helical groove, characterised in that the reciprocating member defines an oblique slot (1234, 1235) and wherein the first retractable thread member (1230, 1231) is secured to a pin (1232, 1233) disposed in the oblique slot (1234, 1235). a reciprocating member, and a housing member (1210a, 1210b), wherein the reciprocating member is slidably disposed in relation to the housing member (1210a, 1210b), wherein the reciprocating member is configured to translate relative to the housing member (1210a, 1210b) to selectively engage and disengage the first retractable thread member (1230, 1231) with the helical groove, characterised in that the reciprocating member defines an oblique slot (1234, 1235) and wherein the first retractable thread member (1230, 1231) is secured to a pin (1232, 1233) disposed in the oblique slot (1234, 1235).