Incomplete insertion of pedicle screws in a standard construct reduces the fatigue life: A biomechanical analysis

Abstract

Pedicle screws are commonly used for posterior stabilization of the spine. When used in deformed or degenerated segments, the pedicle screws are often not fully inserted into the bone, but instead the threaded portion is exposed by 1 or 2 threads to accommodate rod placement and ensure alignment between the tulip of the screw and the rod. However, broken pedicle screws have been reported with the use of this method. The aim of this study was to determine how the fatigue life of the screw is affected by not fully inserting the screw into the bone. Spinal constructs were evaluated in accordance with ASTM F1717. The following three screw positions were subjected to compression bending fatigue loading; (i) pedicle screw fully inserted in the test block with no threads exposed (EXP-T0), (ii) pedicle screw inserted with one thread exposed outside the test block (EXP-T1), (iii) pedicle screw inserted with two threads exposed outside the test block (EXP-T2). Corresponding finite element models FEM-T0, FEM-T1 and FEM-T2 were also constructed and subjected to the same axial loading as the experimental groups to analyze the stress distribution in the pedicle screws and rods. The results showed that under a 190 N axial load, the EXP-T0 group survived the full 5 million cycles, the EXP-T1 group failed at 3.7 million cycles on average and the EXP-T2 groups failed at 1.0 million cycles on average, while the fatigue strength of both the EXP-T1 and EXP-T2 groups was 170 N. The constructs failed through fracture of the pedicle screw. In comparison to the FEM-T0 model, the maximum von Mises stress on the pedicle screw in the FEM-T1 and FEM-T2 models increased by 39% and 58%, respectively. In conclusion, this study demonstrated a drastic decrease in the fatigue life of pedicle screws when they are not full inserted into the plastic block.

Partial Threading of Pedicle Screws in a Standard Construct Increases Fatigue Life: A Biomechanical Analysis

“…Four finite element models (FEM-FT-01, FEM-FT-02, FEM-PU-01, and FEM-PU-02) were created using the same boundary and loading conditions as the experimental fatigue test setup detailed above (Figure 2a,b). A vertical load was applied to the analytically rigid A previous study by our institute [16] determined the critical condition for pedicle screw insertion as having the threaded portion exposed by 1 or 2 threads to accommodate rod placement and ensure alignment between the tulip of the screw and the rod. Two different setups were evaluated ( Figure 1b): (i) pedicle screw fully inserted into the test block with an exposed length of 3.6 mm [16] (EXP-FT-01 and EXP-PU-01) and (ii) pedicle screw inserted leaving 7.6 mm [16] of the screw shaft exposed (EXP-FT-02 and EXP-PU-02).…”

“…Loading was applied in a cyclic sine wave at a frequency of 5 Hz with a load ratio of 0.1 (minimum load divided by maximum load). Static testing was first used to determine the ultimate load for the EXP-FT-01 model as 340 N [16]. In accordance with ASTM F1717, loading for fatigue testing should begin at 50% of the ultimate load, which is 170 N for the EXP-FT-01 construct.…”

“…Four finite element models (FEM-FT-01, FEM-FT-02, FEM-PU-01, and FEM-PU-02) were created using the same boundary and loading conditions as the experimental fatigue test setup detailed above (Figure 2a,b). A vertical load was applied to the analytically rigid surface, which was inserted within the horizontal hole of the UHMWPE test block; the lower rigid surface was fixed [16]. These two rigid surfaces were assumed to have a frictionless contact with the test block.…”

“…All meshing and simulations were conducted using ANSYS 16.0 (ANSYS Inc., Park City, UT, USA). The pedicle screws, support rods, and UHMWPE test blocks were modeled as linearly elastic materials with the properties detailed in Table 1 [16]. The rods were meshed using eight-node hexahedral elements, and the screws used four-node tetrahedral elements.…”

“…A previous study by the authors demonstrated that having the proximal 1/3 of the pedicle screw left unthreaded significantly improves the pull-out strength and withdrawal force in comparison to a fully threaded screw [15]. The authors considered that this novel screw design could also improve the fatigue life of the pedicle screw in cases where only partial screw insertion is required [16]. Hence, this study aimed to evaluate the fatigue life and stress distribution of proximally unthreaded screws in accordance with American Society for Testing and Materials (ASTM) F1717 [17] and using finite element analysis.…”