An implantable spacer for placement between adjacent spinous processes, the adjacent spinous processes including a superior spinous process and an inferior spinous process, each of the superior and inferior spinous processes having two lateral surfaces, the implantable spacer comprising: a body having a longitudinal axis, a first end and a second end; the first end configured to be positioned inside the interspinous process space proximally to the spinal canal relative to the second end; a wing connected to the body and capable of movement with respect to the body; the wing having at least one caming surface (44, 46); an actuator assembly connected to the body; the actuator assembly comprising: an actuator; and a shaft connected to the actuator; the actuator assembly is configured such that the shaft is accessible at the proximal end of the spacer; and the actuator is configured to move relative to the body to contact the caming surface (44; 460 of the wing to move the wing from a first position to a second position for laterally stabilizing the spacer relative to at least one of the superior or inferior spinous process when in said second position, characterised in that the body comprises a first lateral side, and a second lateral side; the wing comprises a first pair of extension members (38a, 38c) having first longitudinal axes, at the first lateral side of the body, and a second pair of extension members (38b, 38d) having second longitudinal axes, at the second lateral side of the body; wherein the wing is capable of rotational movement with respect to the body to rotate the wing from the first to the second position; wherein the first longitudinal axes of the first pair of extension members (38a, 38c) are coincident with each other in the first and second positions, and the second longitudinal axes of the second pair of extension members (38b, 38d) are coincident with each other in the first and second positions. a body having a longitudinal axis, a first end and a second end; the first end configured to be positioned inside the interspinous process space proximally to the spinal canal relative to the second end; a wing connected to the body and capable of movement with respect to the body; the wing having at least one caming surface (44, 46); an actuator assembly connected to the body; the actuator assembly comprising: an actuator; and a shaft connected to the actuator; an actuator; and a shaft connected to the actuator; the actuator assembly is configured such that the shaft is accessible at the proximal end of the spacer; and the actuator is configured to move relative to the body to contact the caming surface (44; 460 of the wing to move the wing from a first position to a second position for laterally stabilizing the spacer relative to at least one of the superior or inferior spinous process when in said second position, characterised in that the body comprises a first lateral side, and a second lateral side; the wing comprises a first pair of extension members (38a, 38c) having first longitudinal axes, at the first lateral side of the body, and a second pair of extension members (38b, 38d) having second longitudinal axes, at the second lateral side of the body; wherein the wing is capable of rotational movement with respect to the body to rotate the wing from the first to the second position; wherein the first longitudinal axes of the first pair of extension members (38a, 38c) are coincident with each other in the first and second positions, and the second longitudinal axes of the second pair of extension members (38b, 38d) are coincident with each other in the first and second positions.