Biomechanical effects of cortical bone trajectory and hybrid CBT-TT fixation on fusion-site mechanics and adjacent segment loading: a finite element study.

BACKGROUND

Posterior lumbar interbody fusion (PLIF) with pedicle screw fixation is a standard treatment for degenerative spinal disorders. However, rigid fixation may accelerate adjacent segment degeneration (ASD). Cortical bone trajectory (CBT) fixation has been proposed as an alternative to traditional trajectory (TT) fixation in lumbar fusion surgery. However, the biomechanical characteristics of CBT, particularly in comparison with hybrid CBT-TT fixation, remain incompletely understood.

METHODS

A validated L4-L5 finite element model was developed to compare CBT, TT, and hybrid CBT-TT fixation constructs. Two simulation phases were performed. The immediate postoperative model was used to evaluate fusion-site micromotion and strain distribution, whereas the post-fusion model was used to assess adjacent-segment range of motion (ROM), intradiscal pressure (IDP), and facet joint force.

RESULTS

All fixation constructs provided sufficient segmental stability. Compared with TT fixation, CBT fixation produced greater but controlled fusion-site micromotion and a broader strain-stimulated region. The hybrid CBT-TT construct demonstrated intermediate biomechanical characteristics between CBT and TT fixation. Following simulated solid fusion, CBT and hybrid CBT-TT fixation reduced superior adjacent-segment ROM, IDP, and facet joint forces relative to TT fixation, indicating lower adjacent-segment biomechanical loading.

CONCLUSION

CBT fixation modified fusion-site mechanical conditions while maintaining overall construct stability. Both CBT and hybrid CBT-TT fixation reduced adjacent-segment biomechanical loading compared with TT fixation. These findings represent comparative biomechanical observations under simulated conditions and should not be interpreted as direct evidence of enhanced osseous fusion or prevention of ASD.