A spacer includes a distal end and a proximal end. The spacer also includes top and bottom surfaces spaced by first and second sides. The top and bottom surfaces define a height, and the sides define a width. The height generally increases distally and from the second side to the first side. The top and bottom surfaces are curved, and include multiple radii of curvature. A lateral curvature is defined as the curvature along the top or bottom surface at a given point along the length of the spacer, taken along a plane extending between the first and second sides and generally perpendicular to the sides. The radius of the lateral curvature for at least one of the top and bottom sides varies along the length of the spacer. For example, the radius of curvature may generally increase distally along the length of the spacer.
