Pedicle screw systems used for fastening spinal rod systems to the pedicle region of two or more vertebral bodies exist in a variety of forms. Successive designs have strived to attain optimal levels of performance, costs, and ease of use. Some known pedicle screw systems provide a locking cap that is threadingly received into a holding sleeve. To lock the pedicle screw relative to the rod the cap is placed into the sleeve (the rod positioned therebetween), and the cap is tightened. This task is difficult since the surgeon must manipulate and tighten the cap while holding the pedicle screw and rod at a particular desired angle. Associated problems are difficulty of installation and cross-threading.
Alternative designs include cams or circular ramps to reduce the number of turns required to lock the cap. Such designs require costly and precise tolerances and are difficult to use during surgery.