Direction-dependent biomechanical effects of transverse process hooks versus pedicle screws at the uppermost instrumented vertebra in adult spinal deformity: a finite element analysis

Proximal junctional complications are often driven by abrupt mechanical transition at the uppermost instrumented vertebra (UIV). This study used patient-specific finite element analysis to compare biomechanical effects of transverse process hooks and pedicle screws at the UIV, specifically examining how hook orientation influences anterior-posterior load sharing. A validated nonlinear three-dimensional T7–L2 spinal model was developed using data from a 60-year-old woman. Three UIV constructs were evaluated at T9: bilateral pedicle screws (BPS) and downward and upward facing transverse process hook (TPH) configurations. Under a 500-N follower load and 8-Nm bending moments, we analyzed facet joint load (FJL), range of motion (ROM), intradiscal pressure, and mechanical failure risk. In flexion, FJL at the UIV+1/UIV level decreased by 6.2% and 19.6% in the downward-facing and upward-facing TPH models, respectively, compared with the BPS model. In extension, FJL decreased by 4.2% in the upward-facing TPH model, but increased by 32.4% in the downward-facing TPH model, relative to the BPS model. Both TPH models preserved greater UIV ROM than the bilateral pedicle screw model, with the upward-facing configuration achieving the highest flexion ROM (2.1°) and the downward-facing configuration achieving the highest extension ROM (3.2°). In the BPS model, UIV screw pull-out risk was 136% in flexion. Conversely, the risks of transverse process breakage for the downward-facing and upward-facing TPH models were substantially lower (50% and 42%, respectively). Hook constructs increased screw pull-out risk at the UIV-1 level compared with the BPS model. Using the TPH at the UIV can facilitate a biomechanical “soft landing” by preserving segmental motion and reducing posterior load at adjacent levels. These effects are orientation-dependent, with the upward-facing TPH configuration potentially offering greater advantage in preserving segmental motion during flexion. Surgical strategies should be tailored to individual bone quality due to potential stress transfer to the UIV-1 level.