Effect of various tapping diameters on insertion of thoracic pedicle screws: A biomechanical analysis

Study Design. A biomechanical cadaver study to assess the effect of various tapping diameters on thoracic pedicle screw insertional torque. Summary of Background Data. Thoracic pedicle screws are now commonly used for deformity and nondeformity cases. The optimal insertion techniques, however, have not been determined. Purpose. To investigate the effect of various tapping techniques before insertion of thoracic pedicle screws in terms of maximal insertional torque (MIT) or screw pullout. Materials and Methods. Thirty-four fresh cadaveric thoracic vertebrae were harvested and evaluated with dual-energy radiograph absorptiometry (DEXA) to assess bone mineral density (BMD). Twenty-three matched, fixed head, 5.0-mm pedicle screws (group 1) were placed using the straight-forward (ST) trajectory (paralleling the endplate) at various thoracic levels after random side selection using either line-to-line tapping (5.0-mm tap) or 1-mm undertapping (4.0-mm tap) under direct and fluoroscopic visualization. After this, 11 matched 5.0-mm pedicle screws (group 2) were placed comparing undertapping by 0.5-mm (4.5-mm tap) with 1 mm undertapping (4.0-mm tap). MIT was recorded for each screw revolution with a digital torque wrench. Results. BMD averaged 0.732 g/cm(2) (0.620-0.884 g cm(2)) for group 1, and 614 g/cm(2) (0.533-0.697 g/cm(2)) for group 2. In group 1, the average MIT was 0.153+/-0.009 (SE) Nm for line-to-line taping and 0.295+/-0.021 (SE) Nm for 1 mm undertapping, a 93% increase in MIT (P<0.0005). In group 2, the average MIT was 0.138±0.009 (SE) Nm for 0.5 mm undertapping and 0.202±0.018 (SE) Nm for undertapping by 1 mm, a 47% increase in MIT (P=0.03). BMD correlated with undertapping by 1 mm in group 1 (P<0.0005), but not with undertapping by 0.5 mm (P=0.087), although there appeared to be a trend in osteoporotic specimens. There were no noted differences in MIT between thoracic regions/levels, despite small differences in thoracic pedicle widths (P=0.193). Discussion and Conclusion. Undertapping the thoracic pedicle by 1-mm increases MIT by 47% (P=0.03) when compared with undertapping by 0.5 mm, and by 93% (P<0.0005) when compared with tapping line-to-line.