Biomechanical analysis of interbody and posterolateral fusion with transpedicular screw fixation for spondylolisthesis: A finite element study

This study investigates and compares the mechanical response of interbody and posterolateral fusion along with the transpedicular screw fixation for the degenerative spondylolisthesis under different load conditions using finite element (FE) analysis. Image processing, computer aided design (CAD), and computer aided engineering techniques were applied to build a three-dimensional model of a functional spinal unit (L4-L5) with transpedicular screw fixation for the posterolateral fusion FE model. Additionally, the intervertebral disc was replaced by two cages to represent the interbody fusion FE model. A unit moment of 1 Nm was applied on the top of L4 in different directions to simulate the flexion, extension, lateral bending, and axial rotation, respectively. The lower of L5 was fixed in all directions for constraint. The simulated results revealed that using cages obviously decreased (13%-58%) the stress imposed upon the instrumentations. The stress concentration occurred at the locking nut on the transpedicular screw head, the middle part of the bone plate, and the thread of transpedicular screw near the head. These findings were comparable to clinical observations. With the limited data, our results suggested interbody fusion in combination with transpedicular screw fixation demonstrated less stress on the instrumentations than the posterolateral fusion with only transpedicular screw fixation.