Bone remodelling comparison between normal pedicle screw and expandable pedicle screw instrumented L4-L5 vertebrae: a preclinical study using FE analysis

Lumbar spinal disorder is a chief cause of disability leading to medical intervention, specifically for elderly people worldwide. The present study aims to investigate and evaluate the biomechanical performance of the expandable pedicle screw vis-à-vis normal pedicle screw with a focus on the load transfer and bone remodelling around the lumbar vertebra. A strain energy density (SED)-based bone remodelling algorithm in combination with three-dimensional finite element (FE) models of intact and implanted lumbar spine FSU was used to investigate the changes in bone density around the screws. The loading (flexion, extension, torsion, lateral bending) and boundary conditions influenced strain, and SED distribution within the FSU. In case of normal pedicle screws, bone apposition was primarily concentrated near the screw insertion region in both L4 (60%-80%) and L5 (30%-90%) vertebra. Bone resorption was estimated to be in the posterior region, near screw length in L4 (10%-45%) and in central anterior right side and posteriorly in L5 vertebra (20%-60%). For expandable pedicle screws, bone apposition was predicted in a relatively larger area in central anterior screw insertion region for L4 vertebra (50%-75%) and L5 vertebra (40%-75%). Bone resorption was found in the central anterior right side as well as on posterior side of L4-L5 vertebra although the area was comparatively less. Bone apposition was significantly greater near the cage insertion area for both pedicle types. It was estimated that the average bone density increase was greater in case of expandable pedicle screws than that for normal pedicle screws. Also, the average density in L5 vertebra was higher than the L4 vertebra. The overall result appears to favour expandable pedicles over normal ones so far as bone remodelling is concerned.