The Effect of Tapping Versus Undertapping Before Placement of Thoracic Pedicle Screws: A Biomechanical Analysis

Purpose: To investigate the effect of various tapping techniques prior to insertion of thoracic pedicle screws in terms of maximal insertional torque (MIT) or screw pullout.

Methods: Thirty-four fresh cadaveric thoracic vertebrae were harvested and evaluated with dual energy X-ray 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. Following 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 732 g/cm2 (620-884 g/cm2) for Group 1, and 614 g/cm2 (533-697 g/cm2) for Group 2. In Group 1, the average MIT was 1.37+0.08(SE) in-lbs. for line-to-line tapping and 2.61+0.19(SE) in-lbs. for 1 mm undertapping, a 93% increase in MIT (p<0 .0005). In Group 2, the average MIT was 1.22+0.08(SE) in-lbs. for 0.5 mm undertapping and 1.79+0.16(SE) by 1 mm, a 47% increase in (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="0.193)." although there appeared to be a trend in osteoporotic specimens. There were no noted differences MIT between thoracic regions/levels, despite small pedicle widths

Discussion and Conclusion: Undertapping the thoracic pedicle by 1 mm increases MIT by 47% (p=0.03) when compared to undertapping by 0.5 mm, and by 93% (p<0 .0005) when compared to tapping line-to-line.