P6. Mechanical stress change of intervertebral disc in rod fracture model after posterior spinal fusion: finite element analysis

Background Context

Rod fracture is the one of postoperative complications in posterior spinal fusion surgery for adult spinal deformity and its prevalence has been reported as 6.8-38.8%. The reoperation rate of rod fracture after spinal fusion is reported to be 41.2-87%, which is higher in patients with bilateral rod fracture (75-91%) than those with unilateral rod fracture (21-43%). Despite high reoperation rate in patients with rod fracture, especially bilateral rod fracture, the pathological mechanisms including mechanical stress change of intervertebral disc in patients with rod fracture have not been well-studied.

Purpose

To clarify mechanical stress change of intervertebral disc in rod fracture model after posterior spinal fusion using 3D-CT finite element analysis (CT/FEA).

Study Design/Setting

A comparative biomechanical study.

Patient Sample

Seven patients ≥ 20 years old (3 males and 4 females) who underwent lumbar spinal surgery.

Outcome Measures

Patients’ characteristics such as age and BMI, and global spinal alignment (eg, sagittal vertical axis [SVA] and pelvic incidence minus lumbar lordosis [PI-LL]) were examined. Posterior spinal fusion models from L3 to S1 with non-rod fracture (NRF, a), unilateral rod fracture (URF, b), and bilateral rod fracture (BRF, c) at L4/5 were created using three-dimensional finite element analysis software. Forward bending loads were applied to each model, and the minimum principal stresses (MPa) of the intervertebral discs at the adjacent level (L2/3) and the rod fracture level (L4/5) were measured in each model using CT-FEA.

Methods

The minimum principal stresses of the intervertebral discs at L2/3 and L4/5 were compared among the NRF, URF, and BRF models using the Friedman's test and Bonferroni correction.

Results

The mean age was 38.0 ± 11.2 years old and BMI was 22.4 ± 2.9 kg/m². The mean SVA was 30.4 ± 60.9 mm and the PI-LL was 6.7 ± 12.5°. No significant differences in the minimum principal stresses at L2/3 level among three models were observed (NRF, 11.0 MPa; URF, 7.5 MPa; BRF, 6.7 MPa, p=.651. At L4/5 level, no significant differences in the minimum principal stresses between the NRF and URF models or the URF and BRF models were observed; however, the minimum principal stress of the intervertebral disc at L4/5 was significantly higher in the BRF model compared to the NRF model (NRF, 0.7 MPa; BRF, 5.4 MPa, p=.001).

Conclusions

The mechanical stress at rod fracture level was significantly higher in bilateral rod fracture model by CT-FEA, which may contribute to the higher reoperation rate in patients with bilateral rod fracture after posterior spinal fusion surgery.

FDA Device/Drug Status

This abstract does not discuss or include any applicable devices or drugs.