In vitro biomechanical study of pedicle screw pull-out strength based on different screw path preparation techniques

Area of Science:

• Spinal surgery
• Orthopedic biomechanics
• Spinal instrumentation

Background:

• Poor screw-to-bone fixation is a common clinical issue leading to screw loosening.
• Under-tapping (UT) pedicle screws has been previously studied biomechanically.
• The study aimed to assess if sequential tapping affects pedicle screw fixation strength.

Purpose of the Study:

• To evaluate the impact of different pedicle preparation techniques on screw-to-bone fixation strength.
• To compare under-tapping (UT), standard-tapping (ST), and sequential-tapping (SQT) methods.
• To utilize a stress relaxation protocol for assessing pull-out strength.

Main Methods:

• Three thoracolumbar calf spines were instrumented with pedicle screws.
• Pedicle preparation included probed, UT, ST, and SQT techniques.
• Pull-out strength was determined using a stress relaxation protocol; insertion torque was also measured.

Main Results:

• Insertion torques varied significantly between techniques (e.g., UT vs. ST/SQT).
• Pull-out strengths were 2443 N (probed), 2353 N (UT), 2474 N (ST), and 2146 N (SQT).
• No significant differences in pull-out strength were observed among the preparation techniques (P ≥ 0.05).

Conclusions:

• Standard-tapping (ST) yielded the highest insertion torque and pull-out strength, while sequential-tapping (SQT) showed the lowest.
• No significant differences in pedicle screw pull-out strength were found across tested preparation techniques.
• Bone density may be a more critical factor influencing screw fixation strength than preparation method.