Pedicle screw fixation strength: a biomechanical comparison between 4.5-mm and 5.5-mm diameter screws in osteoporotic upper thoracic vertebrae.

The purpose of this study was to evaluate the difference between 4.5-mm and 5.5-mm diameter pedicle screws inserted into the pedicles of upper thoracic vertebrae (T2 to T5). Seven fresh human spines were obtained. The bone mineral density was measured by dual-energy radiograph absorptiometry. The 4.5-mm and 5.5-mm diameter screws were inserted alternately in the right or left pedicle of each vertebra. The insertion torque and force (applied in the cephalic direction) to produce loosening of the screw were measured. The average bone mineral density for the seven thoracic spines was 0.710 g/cm(2). All of the vertebrae were classified as osteoporotic. The torque of insertion for the 5.5-mm diameter screws was significantly greater (59% greater on average) than that for the 4.5-mm diameter screws (p =. 001). Although the average force to loosening for the 5.5-mm diameter screws was higher than the average force to loosening for the 4.5-mm diameter screws (14%), the difference was not significant (p =. 33).