Pedicle-screw-based dynamic implants may increase posterior intervertebral disc bulging during flexion

Abstract Posterior disc bulging may lead to nerve root compression and radicular pain, and in extreme cases to a local pressure on the dural sac and thus to back pain. Compared to when standing, posterior disc bulging is increased during extension and decreased during flexion, in an uninstrumented spine. The aim of this study was to determine the effect of a pedicle-screw-based dynamic implant on posterior disc bulging. A finite element model of the lumbosacral spine was used to calculate posterior disc bulging before and after implantation of a dynamic implant for different loading cases. The elastic modulus of the longitudinal rod was varied, and the influence of distraction of the bridged segment on disc bulging was also determined. In addition, the centre of rotation (CoR) was determined. Due to a dynamic implant, the magnitude of posterior disc bulging was reduced compared to that for "standing without an implant" during extension, lateral bending, and axial rotation. During flexion, however, disc bulging was usually increased. With increasing stiffness of the dynamic implant, the CoR moved towards the longitudinal rod. This posterior shift of the CoR led to a compression of the entire intervertebral disc during flexion and thus to an increase of disc bulging.