A spacer separates bones of a joint using a driver tool having a threaded shaft. The spacer has a superior endplate with inferior facing ramps, and an inferior endplate with superior facing ramps. Two bearings are positioned between the endplates, each bearing has superior facing ramps which mate with the inferior facing ramps of the superior endplate, and inferior facing ramps which mate with the superior facing ramps of the inferior endplate. One bearing has a threaded aperture, and the other a thrust surface. A threaded shaft is threaded into the threaded aperture to push against the thrust surface to drive the bearings apart. As the bearings move apart, the mated ramps slide against each other to drive the superior and inferior endplates apart.