A method for locking an elongate rod within a head of a bone anchor implant assembly that is configured for surgical implantation into a bone of a patient. The method includes providing an elongate rod, providing a separate shank and the head, and providing a closure mechanism configured as a one-part closure or two-part closure. The method further includes receiving the elongate rod in the channel and closing the channel with the closure mechanism. The one-part closure and two-part closure are configured to be interchangeable with each other for closing a channel of the head and locking the assembly and the elongate rod between arms of the head with the closure mechanism.