Introduction: Proximal junctional kyphosis (PJK) is a common complication following long instrumented posterior spinal fusions. Biomechanical studies suggest that one of the leading causes is the sudden mobility change between the instrumented and healthy spinal segments.

Aims: The aim of this study is to compare the biomechanical impact of two semirigid fixation techniques (SFTs) with conventional rigid fixation in terms of spinal mobility and pedicle screw loading.

Method: Four T7-L5 finite element (FE) models were developed: 1) intact spine; 2) ⌀5.5mm titanium rods between T8 and L5 (TRF); 3) five ⌀1.9mm titanium rods between T8 and T9 connected with ⌀5.5mm titanium rods between T9 and L5 (MRF); 4) ⌀5.5mm PEEK rods between T8 and T9 connected with ⌀5.5mm titanium rods between T9 and L5 (PRF). A modified multidirectional hybrid test protocol was employed with two successive loading steps. The first step simulated flexion, extension, lateral bending, and axial rotation, and the intervertebral rotation (IVR) angles were recorded. The second step evaluated the von Mises stress values in the pedicle screws at the upper instrumented vertebra (UIV) using the motion of the TRF technique obtained from the first loading step.

Results: In the load-controlled step, relative to TRF, at the uppermost instrumented segment, the IVR values increased by 46.8% and 99.2% for flexion, by 43.2% and 87.7% for extension, by 90.1% and 137% for lateral bending, and by 407.1% and 585.2% for axial rotation, in MRF and PRF, respectively. In the motion-controlled step, the maximum pedicle screw stress values at the UIV level were highest for TRF with 37.26 MPa, 42.13 MPa, 44.4 MPa, and 44.59 MPa for flexion, extension, lateral bending, and axial rotation, respectively. Compared to TRF, in the case of MRF and PRF, the screw stress values were reduced by 17.3% and 27.7% for flexion, by 26.6% and 36.7% for extension, by 6.8% and 34.3% for lateral bending, and by 49.1% and 59.8% for axial rotation.

Conclusion: The findings of this study suggest that semirigid fixations increase the mobility at the upper instrumented segment, providing a more gradual transition in motion between the instrumented and healthy spinal segments. SFTs decrease the pedicle screw loads at the UIV level, and hence could help reduce the risk of PJK.