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

Abstract

We investigated the possible effects of three pedicular screws on axial pull-out strength in pedicular revision surgery. Two study groups were formed from calf lumbar vertebrae. Initially, Alici pedicular screws with an outer diameter of 6.5 mm were applied (with or without tapping) to all the pedicles. All the pedicles were subjected to axial pull-out testing to induce pedicular insufficiency. Then, Alici pedicular screws with an outer diameter 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 expandable (enlargeable) end, respectively. Axial pull-out testing was repeated in both groups and the results were compared with the initial pull-out strength values. In the first group, 65% and 64% of the initial pull-out strengths were obtained with 7-mm Alici pedicular 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. 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.