A spinal fixation system comprising at least one fastener having a fastener head at a proximal end and at least one body. The body comprises a rod receiving channel for receiving a fixation rod, a pressure cap for engaging the fastener head, and a fastener head receiving chamber for retaining the fastener head within the body. The fastener head receiving chamber allows for multi-axial movement of the body in relation to the fastener when the pressure cap is disengaged from the fastener head. The body is secured in a mono-axial position in relation to the fastener when the pressure cap is biased against the fastener head.