The anti-loosening characteristics of cortical bone trajectory screws and conventional pedicle screws under different bone quality conditions: a comparative study

Purpose: We aimed to compare the characteristics of anti-loosening and the pullout force between cortical bone trajectory (CBT) and conventional trajectory (CT) screws under different bone qualities. Methods: Finite element models were established for CBT screws and CT screws in a single-segment L4 vertebra. Compression loads were applied to the models, followed by cyclic loading to simulate the screw loosening process resulting from nail channel expansion. The pullout force was then applied along the screw axis to simulate screw pullout, and pull-out force under different bone quality was calculated and compared. Results: In the loosening test, screw channel expansion was significantly smaller in the CBT group. The CT screw channel volume expanded by 20.34% and 20.60% in healthy and osteoporotic bone, whereas the CBT screw channel volume only expanded by 0.20% and 0.19%. The pullout force of CBT screws was higher than that of CT screws in both healthy and osteoporotic bone (CBT, 1457.51 N and 1040.65 N; CT, 761.68 N and 461.36 N). Conclusions: The loosening model combined with pullout testing can better simulate in vivo screw holding capacity. In different bone qualities, especially in osteoporotic bone, CBT screws have better anti-loosening and higher pullout strength than CT screws.