An implantable component of a bone-screw internal spinal fixation system in the form of a thin cylinder intended to provide immobilization and stabilization of spinal segments in the treatment of various spinal instabilities or deformities, and also used as an adjunct to spinal fusion [e.g., for degenerative disc disease (DDD)]. This component is designed as a longitudinal (intervertebral) connecting element passing through the system’s implantable screw heads or hooks. It is available in various forms (e.g., straight, curved, rigid, flexible, with varying diameters); flexible types may allow small movements of the spine.