An instrument, comprising: a body having a proximal end and a distal end; a shaft selectively movable relative to the body between a retracted position and an extended position; and a locking mechanism (134, 134b) configured to selectively secure the extendable shaft in the retracted and extended positions, the locking mechanism comprising: one or more compression flanges (166, 166b) that are configured to flex towards and away from the shaft, the one or more compression flanges being configured to restrict movement of the shaft when the one or more compression flanges are flexed towards the shaft; and a locking nut movably connected to and retained on the body, the locking nut being selectively movable between a locked position and an unlocked position, the locking nut being configured to flex the one or more compression flanges towards the shaft when the locking nut is in the locked position, the locking nut having one or more slots, the one or more slots being disposed in a distal end of the locking nut and being configured to reduce a force required to move the locking nut between the locked position and the unlocked position, characterised in that the one or more slots extend through the thickness of the locking nut between interior and exterior surfaces thereof. a body having a proximal end and a distal end; a shaft selectively movable relative to the body between a retracted position and an extended position; and a locking mechanism (134, 134b) configured to selectively secure the extendable shaft in the retracted and extended positions, the locking mechanism comprising: one or more compression flanges (166, 166b) that are configured to flex towards and away from the shaft, the one or more compression flanges being configured to restrict movement of the shaft when the one or more compression flanges are flexed towards the shaft; and a locking nut movably connected to and retained on the body, the locking nut being selectively movable between a locked position and an unlocked position, the locking nut being configured to flex the one or more compression flanges towards the shaft when the locking nut is in the locked position, the locking nut having one or more slots, the one or more slots being disposed in a distal end of the locking nut and being configured to reduce a force required to move the locking nut between the locked position and the unlocked position, characterised in that the one or more slots extend through the thickness of the locking nut between interior and exterior surfaces thereof. one or more compression flanges (166, 166b) that are configured to flex towards and away from the shaft, the one or more compression flanges being configured to restrict movement of the shaft when the one or more compression flanges are flexed towards the shaft; and a locking nut movably connected to and retained on the body, the locking nut being selectively movable between a locked position and an unlocked position, the locking nut being configured to flex the one or more compression flanges towards the shaft when the locking nut is in the locked position, the locking nut having one or more slots, the one or more slots being disposed in a distal end of the locking nut and being configured to reduce a force required to move the locking nut between the locked position and the unlocked position, characterised in that the one or more slots extend through the thickness of the locking nut between interior and exterior surfaces thereof.