Expandable interspinous process spacer implant

A method of implanting an expandable spacer implant into a vertebral space defined between a superior vertebral body and an inferior vertebral body that are spaced from one another along a first direction, the method comprising:

• inserting the implant into the vertebral space in a collapsed configuration such that (i) an outer surface of a first housing of the implant engages one of the superior and inferior vertebral bodies, (ii) an outer surface of a second housing of the implant engages another of the superior and inferior vertebral bodies, (iii) inner surfaces of the first and second housings face one another and are spaced from the outer surfaces of the first and second housings along the first direction, and (iv) a first pair of internal surfaces of the second housing are spaced from one another along a second direction, perpendicular to the first direction, and a second pair of internal surfaces of the second housing are spaced from one another along a third direction, perpendicular to the first and second directions, so as to at least partially define a void that extends into the inner surface of the second housing;
• and
• expanding the implant by rotating a distracting member about an axis of rotation that extends along the second direction so as to move the distracting member within the void from a first position to a second position, wherein (1) in the first position, a first pair of opposed surfaces of the distracting member that define a first distance therebetween engage the first and second housings, (2) in moving to the second position, a second pair of opposed surfaces of the distracting member that are angularly offset with respect to the first pair and define a second distance therebetween, greater than the first distance, engage the first and second housings so as to cause the first and second housings to move away from one another along the first direction, and (3) the first and second pairs of opposed surfaces engage the first and second housings at least midway between a proximal-most end of the implant and a distal-most end of the implant.