6. A spinal rod anchoring system comprising: a rod adapted to extend along a spine of a patient; and a first rod anchor adapted to be fixed to a vertebra of the spine, the first rod anchor comprising: a housing forming a receptacle portion having a substantially concave inner surface; and a sleeve having a passage receiving the rod such that the rod is secured against substantial lateral translation relative to the sleeve while being allowed to slide axially relative to the sleeve through a first pivot point, the sleeve having a substantially convex mating surface adapted to mate with the concave surface of the housing such that the sleeve is able to rotate to change in pitch and yaw relative to the housing while being substantially prevented from changing in roll relative to the housing.
6. A spinal rod anchoring system comprising: a rod adapted to extend along a spine of a patient; and a first rod anchor adapted to be fixed to a vertebra of the spine, the first rod anchor comprising: a housing forming a receptacle portion having a substantially concave inner surface; and a sleeve having a passage receiving the rod such that the rod is secured against substantial lateral translation relative to the sleeve while being allowed to slide axially relative to the sleeve through a first pivot point, the sleeve having a substantially convex mating surface adapted to mate with the concave surface of the housing such that the sleeve is able to rotate to change in pitch and yaw relative to the housing while being substantially prevented from changing in roll relative to the housing. 7. The system of claim 6 , wherein the a housing forms a protrusion extending inwardly from the concave inner surface and the sleeve has a circumferential groove in the convex mating surface that mates with the protrusion of the housing such that the sleeve is substantially prevented from changing in roll relative to the housing.