PEDICLE SCREW PULLOUT STRENGTH - CORRELATION WITH INSERTIONAL TORQUE

This study was designed to correlate several parameters regarding pedi cle screw bone/metal interface strength. The insertional torque measur ed during tapping and placement of pedicle screws was correlated with the bone mineral density of the vertebral body, the dimensions of the pedicle, the method of preparation of the pedicle, and the amount of l oad and number of cycles to failure of the bone/metal interface. Thirt y human cadaveric lumbar vertebrae were instrumented with 6.5-mm pedic le screws. The maximum torque achieved during insertion was digitally recorded. A cyclic pedicle screw pullout test was performed. A linear correlation existed between both the insertional torque when tapping o r when inserting a screw and the number of cycles to ultimate pedicle screw pullout. An inverse linear relationship was found between the pe dicle width and cycles to failure. There was no linear correlation fou nd when comparing the number of cycles to failure to bone mineral dens ity. These findings suggest that insertional torque is a good predicto r of bone-metal interface failure. Bone mineral density of the vertebr al body was less effective as a predictor of failure. Smaller pedicle width correlated with increased insertional torque and cycles to failu re. This may explain why patients with osteoporosis on radiography may still obtain stable fixation with pedicle screws. Other factors, such as pedicle dimensions and shape, affect screw purchase as much as ver tebral body bone density. Insertional torque less than 4.0 inch-pounds led to early pedicle screw pullout. This study forms the basis for th e authors' clinical use of an instrumented torque screwdriver to measu re insertional torque in the operating room.