Comparison of the pull-out strengths of three different screws in pedicular screw revisions: a biomechanical study

Objectives: We investigated the possible effects of three pedicular screws on axial pull-out strength in pedicular revision surgery. Methods: Two study groups were formed from calf lum¬bar vertebrae. Initially, Alici pedicular screws with an outer diameter of 6.5 mm were applied (with our without tapping) to all the pedicles. All the pedicles were subject¬ed to axial pull-out testing to induce pedicular insuffi¬ciency. Then, Alici pedicular screws with an outer diam¬eter of 7 mm were applied to the left pedicles. The right pedicles in the two study groups were assigned to receive two different types of pedicular screws with an expand¬able (enlargeable) end, respectively. Axial pull-out testing was repeated in both groups and the results were com¬pared with the initial pull-out strength values. Results: In the first group, 65% and 64% of the initial pull-out strengths were obtained with 7-mm Alici pedicu¬lar screws and with expandable pedicular screws, for the left and right pedicles, respectively. The corresponding pull-out strengths in the other study group were 70% and 68.5% of the initial values, respectively. Tapping of the screw hole entrance resulted in a mean decrease of 13% in the pull-out strength compared to screw applications without tapping. Conclusion: Pedicular screw revisions using a 0.5 mm greater screw in diameter did not provide adequate screw- bone inter-face strength and pedicle filling. Similarly, expandable pedicular screws did not contribute to screw stability.