Supplementary delta-rod configurations provide superior stiffness and reduced rod stress compared to traditional multiple-rod configurations after pedicle subtraction osteotomy: a finite element study.

PURPOSE: The biomechanical performance of conventional multi-rod configurations (satellite rods and accessory rods) in pedicle subtraction osteotomies has been previously studied in vitro and using finite element models (FEM). Delta and delta-cross rods are innovative multi-rod configurations where the rod bends were placed only in its proximal and distal extremities in order to obtain a dorsal translation of the central part of the rod respect to the most angulated area of the main rods. However, the biomechanical properties of the delta and delta-cross rods have not been investigated. This study used FEM to analyze the effect of delta-rod configurations on the stiffness and primary rod stress reduction in multiple-rod constructs after pedicle subtraction osteotomy.
METHODS: The global range of motion in the spine and the magnitude and distribution of the von Mises stress in the rods were studied using a spine finite element model described previously. A follower load of 400 N along with moments of 7.5 N in flexion/extension, lateral bending, and axial rotation were tested on the spine model. Initial breakage was created on the rod based on the maximum stress location. The post-breakage models were tested under flexion.
RESULTS: Delta and delta-cross rods reduced more range of motion (up to 45% more reduction) and reduced more primary rod stress than other previously tested rod configurations (up to 48% more reduction). After initial rod fracture occurred, delta and delta-cross rods also had less range of motion (up to 23.6% less) and less rod von Mises stress (up to 81.2% less) than other rod configurations did.
CONCLUSIONS: Delta and delta-cross rods have better biomechanical performance than satellite rods and accessory rods in pedicle subtraction osteotomies in terms of construct stiffness and rod stress reduction. After the initial rod breakage occurred, the delta and delta-cross rods could minimize the loss of fixation, which have less rod stress and greater residual stiffness than other rod configurations do. Based on this FEA study, delta-rod configurations show more favorable biomechanical behavior than previously described multi-rod configurations. These slides can be retrieved under Electronic Supplementary Material.