A bi-directional fixating transvertebral (BDFT) screw/cage apparatus is provided. The BDFT apparatus includes an intervertebral cage including a plurality of internal angled screw guides, a plurality of screw members, and a novel screw locking mechanism which consists of leaf springs which mechanically interact with BDFT screws which have ratcheted screw heads. The small leaf springs allow the ratchet teeth of the screw heads to rotate only in the penetrating direction. Due to the geometric orientation of the ratchet teeth vis-a-vis the adjacent spring leaf, rotation of the screw head in the opposite direction is prevented by the insertion of the string leaf in the space between the ratchet teeth (trough) of its final rotation. The uni-rotational interaction between the screw head-ratchet teeth/troughs and adjacent leaf spring is the mechanical basis for this novel locking mechanism. The internal angled screw guides orient a first screw member superiorly and a second screw member inferiorly in some embodiments, and orient a second screw member and a third screw member superiorly, and a first screw member and fourth screw member inferiorly in other embodiments. The intervertebral cage is adapted for posterior lumbar intervertebral placement, anterior lumbar intervertebral placement, anterio-lateral thoracic intervertebral placement, or anterior cervical intervertebral placement.
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