Longer Screws Can Reduce the Stress on the Upper Instrumented Vertebra With Long Spinal Fusion Surgery: A Finite Element Analysis Study

Abstract

Study Design: A finite element analysis study. Objective: Of proximal junctional failure, upper instrumented vertebra (UIV) fracture can causes severe spinal cord injury. Previously, we reported that higher occupancy rate of pedicle screw (ORPS) at UIV prevented UIV fracture in adult spinal deformity surgery; we had not yet tested this finding using a biomechanical study. The purpose of present study was to measure the differences in loads on the UIV according to the length of PS and ORPS. Methods: We designed an FE model of a lumbar spine (L1-S1) using FE software. The PS was set from L2 to S1 and connected the rod. The FE model simulated flexion (8 Nm) to investigate the loads at UIV (L2) according to the length of the PS. There were 5 screw lengths examined: 40 (ORPS 36.4%), 45 (48.5%), 50 (66.7%), 55 (81.8%), and 60 mm (93.9%). Results: Stress with bending motion was likely to occur at the upper front edge of the vertebral body, the pedicles, and the screw insertion point. The maximum equivalent stress according to screw lengths of 40, 45, 50, 55, and 60 mm were 45.6, 37.2, 21.6, 13.3, and 14.8 MPa, respectively. The longer screw, the less stress was applied to UIV. No remarkable change was observed between the screw lengths of 55 and 60 mm. Conclusions: Increasing ORPS to 81.8% or more reduced the load on the UIV. To prevent UIV fracture, the PS length in the UIV should be more than ORPS 81.8%.

Biomechanical evaluation of multiple pelvic screws and multirod construct for the augmentation of lumbosacral junction in long spinal fusion surgery

“…Therefore, the abnormal spinal loading caused by ASD and the individual variation of degeneration were not considered. This methodology was consistent with the previous FE studies focusing on the performance of different instrumentations and instrumentation-related problems in ASD ( Buell et al, 2019 ; He et al, 2021 ; Oe et al, 2021 ; Leszczynski et al, 2022 ; Son et al, 2022 ). Further studies should construct and use ASD patient-specific FE models to elucidate the clinical significance of the various instrumentations.…”

Occupancy Rate of Pedicle Screw Below 80% is a Risk Factor for Upper Instrumented Vertebral Fracture After Adult Spinal Deformity Surgery

“…However, aiming for ORPS > 80% and avoiding bicortical screw placement can be challenging. Oe et al 12 reported that a 5 mm change in screw length can change ORPS by more than 10%. Therefore, the use of an intraoperative navigation system may be useful to achieve optimal ORPS.…”

“…Strengthening bone density is considered an important measure,8,9 but some reports indicate that bone density and PJF are not related 5,10,11. Recently, Oe et al 12 named the ratio of pedicle screw (PS) length to the anterior-posterior diameter of the vertebral body at the UIV as the occupancy rate of a pedicle screw (ORPS) and, using finite element analysis, reported that the stress on the UIV decreased the most when the ORPS was 81.8% or more. However, if the screw is too long, it can penetrate the anterior wall of the UIV.…”

Effect of the intermediate pedicle screws and their insertion depth on sagittal balance and functional outcomes of lumbar fracture

“…Matsukawa et al suggest that longer screws increase the degree of bone contact, and the use of deeper screw insertion and larger diameter screws is justified for better stability ( 26 ). Oe et al pointed out that to some extent, the longer the pedicle screw, the greater the biomechanical stability, and the stability decreases after a certain length ( 27 ). However, it has been suggested that intermediate short-pedicle screw fixation can provide a similar level of stability to intermediate long-pedicle screw fixation, with no significant difference in the stress associated with bending, extension, and left–right axial rotation ( 28 ).…”