An active bias circuit having a combined configuration of the Wilson and Widlar current source configurations is provided, which makes it possible to set the output bias voltage at approximately zero even if a reference voltage applied to generate a reference current does not reach 0V. The access door pivots between a closed positioned and an open position. More particularly, the present invention is directed to screws for use in systems and methods of treating the spine, which eliminate pain and enable spinal motion, which effectively mimics that of a normally functioning spine.