ABSTRACT
The spine is comprised of multiple segments, each one defined by White and Panjabi as a “functional spinal unit” (FSU). The FSU consists of a three-joint complex including the disc (endplate, annulus fibrosis, and nucleus pulposus) and the dorsal facet joints. More recently, it has been redefined to include the passive and active constraints that are provided by the attached ligaments and muscles. The FSU performs as a continuous, semi-constrained joint, where complex three-dimensional movements take place. These movements, which occur through six degrees of freedom, include axial compression/distraction, anterior, posterior and lateral bending, translation, and rotation. These movements are constrained by the need to protect neurological elements and yet maintain the head balanced over the pelvis to facilitate interaction with the surrounding environment.
