A fastener and a bone fixation assembly for the internal fixation of vertebral bodies such as a pedicle screw is provided which allows a detachable tulip to be provisionally locked to a pedicle screw in a first position, while securely locking the tulip assembly to the pedicle screw when in a second position. According to one exemplary embodiment, the tulip assembly includes a tulip body, a coupling saddle, and an expansion ring serving as a fastener. The fastener secures the tulip body to the pedicle screw, while the coupling saddle provides a means to provisionally lock the tulip body at an angle relative to the pedicle screw via an interference fit. Incline planes on the walls of the tulip body provide means to rotationally lock a rod within the tulip assembly, and fully lock the tulip assembly to at an angle relative to the pedicle screw.