Tapping insertional torque allows prediction for better pedicle screw fixation and optimal screw size selection

Area of Science:

• Spine biomechanics
• Orthopedic surgery
• Surgical instrumentation

Background:

• Current methods for assessing pedicle screw fixation strength intraoperatively are unreliable.
• Pedicle screw insertional torque (IT) correlates with pullout strength but has limited clinical application.
• Tapping IT has not been previously evaluated for its ability to predict pedicle screw pullout strength.

Purpose of the Study:

• To investigate the relationship between tapping IT and pedicle screw pullout strength.
• To determine if tapping IT can predict the optimal pedicle screw size.

Main Methods:

• In vitro biomechanical analysis of 20 human cadaveric thoracic vertebral levels with osteoporotic bone.
• Pilot study to establish optimal tapping IT thresholds (2.5 in-lbs) and tap sizes.
• Paired comparison of pedicle screw instrumentation using two tapping IT thresholds (1.5 vs. 2.5 in-lbs) followed by pullout strength testing.

Main Results:

• A higher tapping IT threshold (2.5 in-lbs) resulted in significantly increased pedicle screw IT (19%), pullout strength (23%), and mean screw diameter (p<.05).
• Optimal pedicle screw size selection (within 1 mm of pedicle width) was more frequent with the higher tapping IT threshold (60% vs. 26.7%).
• Tapping IT showed moderate correlation with screw IT (r=0.54) and pedicle screw pullout strength (r=0.55) (p=.002).

Conclusions:

• Tapping IT directly correlates with pedicle screw IT, pullout strength, and optimal screw size.
• Tapping IT may serve as an adjunct tool for thoracic pedicle screw instrumentation to enhance fixation and optimize screw fit.
• Further in vivo studies are needed to evaluate the intraoperative utility of tapping IT for predicting screw loosening.