Numerical study of pedicle screw construction and locking compression plate fixation in posterior pelvic ring injuries: Analyzed by finite element method

Abstract

Background: The aim of this study was to compare the biomechanical performance of pedicle screw construction and locking compression plate fixation in posterior pelvic ring injuries analyzed by finite element method.

Methods: A 3-dimensional finite element model of the spine-pelvis-femur complex with ligaments was reconstructed from computed tomography images. An unstable posterior pelvic ring injury was created, which was fixed with a pedicle screw construction or locking compression plate. A follower load of 400 N was applied to the upper surface of the vertebrae to simulate the upper body weight, while the ends of the proximal femurs were fixed. The construct stiffness, the maximum vertical displacement, the maximum posterior displacement, the maximum right displacement, and the overall maximum displacement of the sacrum, and stress distributions of the implants and pelvises were assessed.

Results: The construct stiffness of the pedicle screw model (435.14 N/mm) was 2 times that of the plate model (217.01 N/mm). The maximum vertical displacement, the maximum posterior displacement, the maximum right displacement, and the overall maximum displacement of the sacrum in the pedicle screw model were smaller than those in the plate model (0.919, 1.299, 0.259, and 1.413 mm in the pedicle screw model, and 1.843, 2.300, 1.053, and 2.895 mm in the plate model, respectively). The peak stresses of the implant and pelvis in the pedicle screw model decreased by 80.4% and 25% when compared with the plate model (44.57 and 34.48 MPa in the pedicle screw model, and 227.47 and 45.97 MPa in the plate model, respectively).

Conclusion: The study suggested that the pedicle screw construction could provide better fixation stability than the locking compression plate and serves as the recommended fixation method for the treatment of posterior pelvic ring injuries.

Figure 1.

Establishment of geometric model of the spine-pelvis-femur.

Figure 2.

Three-dimensional geometric models of 2 implants. (A) Pedicle screw construction. The 2 pedicle screws had a diameter of 7 mm and a length of 55 mm. The rod was modeled as a 6-mm diameter. (B) Locking compression plate. The locking plate was 4 mm in diameter and 147 mm long. The 6 locking screws had a diameter of 3.5 mm and a length of 20 mm.

Figure 3.

Posterior view of the finite element model of the injured pelvis. After reconstruction of the intact spine-pelvis-femur model, the anterior and posterior sacroiliac ligaments, interosseous sacroiliac ligament, and articular cartilage of the left sacroiliac joint were removed to simulate unstable posterior pelvic ring injuries.

Figure 4.

Finite element model of the injured pelvis fixed by 2 implants. (A) Pedicle screw construction. (B) Locking compression plate.

Figure 5.

Von Mises stress distribution of the intact pelvis mode under 500 N vertical loading.

Figure 6.

Displacement distribution (mm) of the 2 models under follower loading of 400 N. (A) Pedicle screw construction. The maximum displacement was located at the top of the sacrum on the intact side, and the value was 1.413 mm. (B) Locking compression plate. The maximum displacement appeared at the top of the sacrum on the injured side, and the value was 2.895 mm.

Figure 7.

Von Mises stress distribution (MPa) of 2 implants under follower loading of 400 N. (A) Pedicle screw construction. The peak stress appeared at the junction of the pedicle screw and the rod on the injured side, and the value was 44.57 MPa. (B) Locking compression plate. The peak stress was concentrated at the junction of the plate and the locking screw around the fracture site, and the value was 227.47 MPa.

Figure 8.

Von Mises stress distribution (MPa) of the pelvis under follower loading of 400 N. (A) Pedicle screw construction. The peak stress was concentrated in the pedicle screw hole on the intact side, and the value was 34.48 MPa. (B) Locking compression plate. The peak stress was concentrated at the middle locking screw hole on the intact side, and the value was 45.97 MPa.