Augmentation of anterior transvertebral screws using threaded teflon anchoring

This is a biomechanical study to investigate the effect of augmentation of single anterior transvertebral screws. Two subsequent experiments (pullout and caudal loading) were performed in 78 porcine vertebral bodies of equal bone mineral density using 6.5-mm transvertebral screws augmented or not with specially designed Teflon anchoring. Three different types of 35- and 45-mm unaugmented screws (Kaneda, TSRH, and Zielke) were inserted at 90 degrees laterally in the vertebral body and were tested for pullout force. The pullout load to loosen the three different unaugmented screws did not significantly differ. The pullout force to loosen the 35- and 45-mm Zielke screws was 507 +/- 22 and 860 +/- 50 N, respectively. The corresponding pullout load to loosen the construct Zielke screw-Teflon anchoring was 1,005 +/- 148 N and 1,306 +/- 135 N for the 35- and 45-mm screws, respectively. When comparing the pullout force needed for unaugmented versus the augmented Zielke screw of the same length, there was a statistically significant (p < 0.0001) difference. During the caudal loading test, the unaugmented 45-mm Zielke screw showed that a uniform slope up to the yield point occurred at 0.35 +/- 0.12 mm of displacement, with an average tilting force for the tested screw of 1,362 +/- 151 N, when the screw became loose. The caudal loading test for the augmented Zielke screw was interrupted at 2,000 N because the load applied exceeded the load capacity of the testing machine, and thus no loosening occurred. The findings of this in vitro study showed that the Teflon anchoring provides superior anchorage and stability of single transvertebral Zielke screws. However, further biomechanical and clinical studies are required before using this device or its modification.