An implant (100; 200; 300; 400; 500; 600; 700; 800; 900; 1000; 1100; 1200) for placement between spinous processes (20, 21) of adjacent vertebrae (12, 14) of a spine, the implant comprising: a spacer (102; 202; 302; 402; 502; 602; 702; 802; 1002; 1102; 1202) with a first end, a second end, and a longitudinal axis (110; 512; 814) extending from the first end to the second end, the spacer having a sidewall (112; 708; 1208) generally parallel to and spaced apart from the longitudinal axis and having superior and inferior surfaces (114, 116) operable to abut the spinous processes and maintain the spinous processes in spaced apart relationship, the superior and inferior surfaces being spaced apart a distance corresponding to a predetermined minimum spacing between the spinous processes; and a first extension (126; 204; 404; 504; 604; 704; 804; 904; 1004; 1104; 1204) fixedly coupled to and projecting from the first end of the spacer transverse to the longitudinal axis to lie generally alongside the spinous processes of adjacent vertebrae, the first extension being engageable with the spinous processes to fix the spacer to the spinous processes and limit the maximum spacing between the spinous processes; characterized in a second extension (128; 206; 406; 506; 606; 706; 806; 908; 1006; 1106; 1206) opposing the first extension, the second extension defines an aperture including a plurality of tabs that cooperatively engage a plurality of transverse openings in the sidewall of the spacer such that the second extension directly engages the spacer and is translatable over the spacer from the second end toward the first extension along a length of at least one of the plurality of transverse openings. a spacer (102; 202; 302; 402; 502; 602; 702; 802; 1002; 1102; 1202) with a first end, a second end, and a longitudinal axis (110; 512; 814) extending from the first end to the second end, the spacer having a sidewall (112; 708; 1208) generally parallel to and spaced apart from the longitudinal axis and having superior and inferior surfaces (114, 116) operable to abut the spinous processes and maintain the spinous processes in spaced apart relationship, the superior and inferior surfaces being spaced apart a distance corresponding to a predetermined minimum spacing between the spinous processes; and a first extension (126; 204; 404; 504; 604; 704; 804; 904; 1004; 1104; 1204) fixedly coupled to and projecting from the first end of the spacer transverse to the longitudinal axis to lie generally alongside the spinous processes of adjacent vertebrae, the first extension being engageable with the spinous processes to fix the spacer to the spinous processes and limit the maximum spacing between the spinous processes; characterized in a second extension (128; 206; 406; 506; 606; 706; 806; 908; 1006; 1106; 1206) opposing the first extension, the second extension defines an aperture including a plurality of tabs that cooperatively engage a plurality of transverse openings in the sidewall of the spacer such that the second extension directly engages the spacer and is translatable over the spacer from the second end toward the first extension along a length of at least one of the plurality of transverse openings.