266 - Pedicle Screw Pullout Strength of Four Different Screw Hole Preparatio...

#266 Pedicle Screw Pullout Strength of Four Different Screw Hole Preparation Techniques

Basic Sciences-Research

Poster Presented by: M. Moldavsky

Author(s):

J. Mehta (1)
M. Moldavsky (2)
K. Salloum (2)
B. Bucklen (2)
S. Khalil (2)

(1) ABMU Health Board Morriston Hospital, Swansea, United Kingdom
(2) Globus Medical, Audubon, PA, USA

Abstract

Introduction: Maximizing pedicle screw-to-bone strength is challenging in osteoporotic patients. Preparation of the pedicle screw trajectory through the pedicle and into the vertebral body can affect the pullout strength. Four different preparation techniques were used in calf vertebral bodies. Insertion torque and pullout strength were compared.

Methods: A 6.5mm diameter, 40mm length screw, REVERE® (Globus Medical; Audubon, PA) was used for all testing. The preparation techniques were as follows after an awl was used to perforate the cortex; 1) a pedicle probe is inserted to a depth of 40mm (probed), 2) a probe is inserted to a depth of 40mm then a 5.5mm diameter tap is inserted 40mm (under-tapped), 3) a probe is inserted to a depth of 40mm then a 6.5mm diameter tap is inserted 40mm (standard tapped), 4) a probe is inserted to a depth of 40mm then taps are inserted to a depth of 40mm in the following order: 4.5mm, 5.5mm, and 6.5mm (sequentially tapped). Insertion torque was measured during final screw insertion.

A stress relaxation loading protocol was used to simulate a physiological environment. An axial pull-out load rate of 5mm/min was applied using an 858 Mini Bionix® II (MTS Corporation, Minneapolis, MN). Tensile loading was paused for 1,000 seconds for every 0.5mm of advancement. A one-way ANOVA and Tukeys post hoc test was used to determine significance.

Results: The pedicle screw insertion torques for the probed, under-tapped, standard tapped and sequentially tapped techniques are 5.09(±1.08) Nm, 5.39(±1.61) Nm, 2.93(±0.43) Nm, and 3.54(±0.67) Nm, respectively. There is a significant difference between probed compared to standard-tapped (p≤0.05), as well as under-tapped compared to standard tapped and sequentially tapped (p≤0.05).

The pull-out strength for pedicle screws for the probed, under-tapped, standard tapped and sequentially tapped techniques is 2443(±782) N, 2353(±918) N, 2474(±521) N, and 2146(±582) N, respectively, with no significant difference (p≥0.05).

Conclusion: The screw insertion torque significantly increased based on the pedicle preparation technique used. The probed technique resulted in the highest pull-out strength, while the sequentially tapped technique resulted in the lowest. However, there is no significant difference in the pull-out strength in pedicle screw pullout based on preparation technique. These findings suggest that while the insertional technique has an impact on the screw fixation at the time of placement, other factors such as bone density may have a greater influence on pull-out strength.

Figure 1: Pedicle screw insertion torque

Figure 2: Pedicle screw pull-out strength