Effect of the pilot hole preparation on the anchorage of pedicle screws

OBJECTIVE We evaluated the influence of the diameter and the preparation of the pilot hole on the resistance to the pulling out and the strength when inserting pedicle screws with conical internal diameter. METHODS Mechanical experiments were performed with pedicle conical screws of 4.2 mm and 5.2 mm diameter. They were inserted in the vertebral pedicles of swine. The hole was manufactured with a drill and probes with different diameters. RESULTS While testing the 4.2 mm screw, the perforation of holes with measure equal or inferior to the lesser internal diameter of the screw increased the torque and the resistance to pull-out strength. Perforations with different instruments have presented similar results. Perforations with probes allowed the holes manufactured with dimensions superior to the lesser internal diameter of the screw to show similar resistance to that of the perforations with dimensions equal to the lesser internal diameter of the implant, made with probes and drills. CONCLUSION For 4.2 mm screws, the diameter and the preparation of the hole influence the torque and the resistance. For 5.2 mm screws, there is only influence on the insertion torque. There is no correlation between pulling out strength and insertion torque. Level of Evidence II, Therapeutic Studies - Investigating the Results of Treatment.