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Effect of various tapping diameters on insertion of thoracic pedicle screws: a biomechanical analysis
Timothy R Kuklo1, Ronald A Lehman
1Department of Orthopaedic Surgery and Rehabilitation, Walter Reed Army Medical Center, Washington, DC, USA. kuklot@aol.com
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/cm2 (0.620-0.884 g/cm2) for group 1, and 614 g/cm2 (0.533-0.697 g/cm2) for group 2. In group 1, the average MIT was 0.153 +/- 0.009 (SE) Nm for line-to-line tapping 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.
