Posterior spinal stabilization: A biomechanical comparison of Laminar Hook Fusion to a Pedicle Screw System

Background: Several spine instrumentation techniques have been introduced to correct inter-segmental alignment, or provide long-term stability. Whilst pedicle screws are considered the intervention of reference, we hypothesize that the week hold of osteoporotic bone, might be a clinical indicator for an alternative surgical approach.

Methods: To put this to the test, a non-linear Finite Element model, of a ligamentous lumbosacral spine, was employed to examine a stabilization spanning over L3-L5. Two different immobilization techniques (a Pedicle Screw System and Laminar Hook Fusion) are compared as to their biomechanical response during 7.5 Nm flexion, lateral flexion and torsion, while considering a 280 N follower load. Fifteen analyses performed in total, simulating patients of healthy and osteoporotic Bone Mineral Density.

Findings: Range of Motion was significantly reduced after instrumentation for both implant systems. This trend was more pronounced in the Pedicle Screw models, which were stressed to a higher degree. To evaluate implant loosening risk, we introduce the consideration of strain energy patterns around the screw tract. The notably higher intensity of these, for the osteoporotic model, taken into consideration with the weaker strength of the tissue and inconsistencies in the stress allocation between implant and bone, affirmed an increased risk for loosening of the Pedicle Screws in osteoporotic patients.

Interpretation: The analysis provided refined insight as to the treatment of osteoporotic patients as well as to their postoperative care, as restriction of specific movements (e.g. through bracing), could significantly restrict the stress values in the bone-implant interface and thus, reduce implant failure.