An implantable spacer for placement between adjacent spinous processes in a spinal motion segment is provided. The spacer includes a body defining a longitudinal axis and passageway. A first arm and a second arm are connected to the body. Each arm has a pair of extensions and a saddle defining a U-shaped configuration for seating a spinous process therein. Each arm has a proximal earning surface and is capable of rotation with respect to the body. An actuator assembly is disposed inside the passageway and connected to the body. When advanced, a threaded shaft of the actuator assembly contacts the earning surfaces of arms to rotate them from an undeployed configuration to a deployed configuration. In the deployed configuration, the distracted adjacent spinous processes are seated in the U-shaped portion of the arms.