Patient-Specific Finite Element Models of Posterior Pedicle Screw Fixation: Effect of Screw’s Size and Geometry

Abstract

Pedicle screw fixation is extensively performed to treat spine injuries or diseases and it is common for thoracolumbar fractures. Post-operative complications may arise from this surgery leading to back pain or revisions. Finite element (FE) models could be used to predict the outcomes of surgeries but should be verified when both simplified and realistic designs of screws are used. The aim of this study was to generate patient-specific Computed Tomography (CT)-based FE models of human vertebrae with two pedicle screws, verify the models, and use them to evaluate the effect of the screws’ size and geometry on the mechanical properties of the screws-vertebra structure. FE models of the lumbar vertebra implanted with two pedicle screws were created from anonymized CT-scans of three patients. Compressive loads were applied to the head of the screws. The mesh size was optimized for realistic and simplified geometry of the screws with a mesh refinement study. Finally, the optimal mesh size was used to evaluate the sensitivity of the model to changes in screw’s size (diameter and length) and geometry (realistic or simplified). For both simplified and realistic models, element sizes of 0.6 mm in the screw and 1.0 mm in the bone allowed to obtain relative differences of approximately 5% or lower. Changes in screw’s length resulted in 4–10% differences in maximum deflection, 1–6% differences in peak stress in the screws, 10–22% differences in mean strain in the bone around the screw; changes in screw’s diameter resulted in 28–36% differences in maximum deflection, 6–27% differences in peak stress in the screws, and 30–47% differences in mean strain in the bone around the screw. The maximum deflection predicted with realistic or simplified screws correlated very well (R2 = 0.99). The peak stress in screws with realistic or simplified design correlated well (R2 = 0.82) but simplified models underestimated the peak stress. In conclusion, the results showed that the diameter of the screw has a major role on the mechanics of the screw-vertebral structure for each patient. Simplified screws can be used to estimate the mechanical properties of the implanted vertebrae, but the systematic underestimation of the peak stress should be considered when interpreting the results from the FE analyses.

Clinical Effect of Transverse Process Hook with K-Means Clustering-Based Stratification of Computed Tomography Hounsfield Unit at Upper Instrumented Vertebra Level in Adult Spinal Deformity Patients

“…In biochemical study, the bone density of the intrapedicular segment has a significantly stronger correlation with pedicle screw pull-out strength than other segments, 24) and the finite element method (FEM) study also reported more von Mises stress at the pedicle than vertebra body. 20,26) Although the use of TP hook as a UIV implant was associated with a lower incidence of proximal junctional kyphosis (PJK) and better functional outcomes than using pedicle screws, 5,9,12) some researches raise questions about its clinical effectiveness. 10,16) Although the importance of pedicle HU has been reported in the spine surgical field, consideration of the UIV bone environment of patients with spinal deformity corrective surgery was insufficient.…”

“…In biochemical study, the bone density of the intrapedicular segment has a significantly stronger correlation with pedicle screw pull-out strength than other segments [ 24 ], and the finite element method study also reported more von Mises stress at the pedicle than vertebra body [ 20 , 26 ].…”

Effect of Minimally Invasive Internal Arch Nailing Surgery on Tissue Traumatic Stress Response in Patients with Vertebral Fractures

“…After surgical trauma patients will have a significant stress response, and the degree of the stress response can be reflected clinically by the serum CRP index [ 20 ]. In the case of tissue trauma and inflammatory reaction, serum TNF- α expression is enhanced, inflammatory factor activity is increased, and a series of inflammatory cascade reactions will be accompanied, and the body will experience elevated levels of stress hormones in response to external stimuli [ 21 ]. Serum Cor and NE indicators can also reflect the status of the body's stress response with a high sensitivity.…”

Evaluation of instrumentation and pedicle screw design for posterior lumbar fixation: A pre‐clinical in vivo/ex vivo ovine model

“…Another study independently demonstrated that the screw's macrogeometry, specifically screw diameter, significantly contributes to primary stability. 30 Additionally, research has corroborated that cylindrical singlelead thread screws yield superior biomechanical anchorage relative to conical dual-lead thread screws. 31,32 The present study was designed to evaluate both osteotomy instrumentation and pedicle screw design (specifically the individual threads as shown in Figure 1) effect on osseointegration and soft tissue infiltration after 6-and 24-weeks in vivo using a large, pre-clinical animal (sheep) model.…”