Assemblies for implantation across one or more spinal motion segments to allow for control of the distance between bone anchors. Control of distance between bone anchors may be provided by one inter-anchor element pushing the pair of bone anchors apart and a second inter-anchor element pulling the pair of bone anchors together. Control of distance between bone anchors may be provided through use of dissimilar thread pitch. Compression of intervertebral disc space through controlled movement of a pair of anchored bone anchors towards one another.