Finite Element Analysis Of Pedicle Screw Extraction Force Under Different Bone Mineral Densities

The pedicle screw fixation system is the standard internal fixation method in clinical treatment of spinal diseases,which can treat degenerative spinal diseases,traumatic fractures,tumors and spinal deformities.These system provides not only solid support,but also a pull-down reduction that creates an environment for intervertebral bone graft and fusion,Its stability depends on the biomechanical properties of the "screw-bone" interface.In osteoporotic vertebrae,due to the decrease of bone mineral density,absorption of bone trabecula and thinning of bone cortex,the strength of the "screw-bone" interface is decreased,and the screws are easy to loosen and pull out,leading to the failure of internal fixation.However,the specific range of bone mineral density for pedicle screw fixation is still not clear at present,that is to say,the extent of the decrease in the strength of pedicle screw fixation under the condition of gradual decrease in bone mineral density still needs to be further studied.Therefore,the main purpose of this study was to determine the axial biomechanical fixation strength of pedicle screws under different vertebral bone densities.In this study,five vertebral bodies with different bone densities were selected as the research objects.This setting is related to bone loss and can simulate the clinical environment to the greatest extent.However,the strength of the "screw-bone" interface binding of vertebral bodies with different bone densities is not clear yet.Therefore,the method of "simulated axial screw pull-out test" was used in this study to compare the pull-out force of pedicle screws inserted into the vertebral body with five different BMD,and to analyze the influence of changes in BMD on the pull-out force and stability of pedicle screws,so as to provide experimental basis for the selection of surgical screw placement for patients with different BMD in clinical practice.Objective: Finite element method was used to analyze and evaluate the stress distribution and maximum extractive force during pedicle screw extraction under different bone mineral density conditions,and to explore the minimum bone mineral density for safe implantation of pedicle screw internal fixation.Methods: Based on the lumbar CT scan data of A normal male volunteer,A three-dimensional model of lumbar 1 vertebral body and pedicle screws was established.The vertebral bodies were divided into group A(BMD=120mg/cm 3),group B(BMD=110mg/cm 3),group C(BMD=100mg/cm 3),group D(BMD=90mg/cm 3),and group E(BMD=80mg/cm3)according to the bone mineral density(QCT).Then,according to the relationship between bone mineral density and elastic modulus,the material properties of cortical bone and cancellous bone in each group of vertebrae were assigned,and five vertebral body-screw finite element models with different bone mineral density were established.The axial pull-out force experiment was simulated to the established finite element model respectively.The stress distribution of screws in each group was observed and the maximum axial pull-out force was measured.The measured indexes were compared and analyzed among different bone densities.One-way analysis of variance was used for statistical analysis.Results: The maximum pull-out force of the finite element model of A-E group was(1834±78),(1626±65),(1441±55),(1141±48)and(1039±42)N,respectively,and there was significant statistical difference between the two groups(F=3.413,P<0.05).Bone mineral density decreased by 10mg/cm3,the maximum axial pull-out force decreased by 11%,23%,43%,51%,and the maximum screw stress decreased by 9%,16%,33%,53%,respectively.The maximum principal strain of the screw appears at the end of the screw,and the maximum stress of the screw moves gradually from the end to the head during pullout.Conclusions:1.The maximum pull-out force(axial stability)of pedicle screws is highly significantly correlated with BMD,and the stability of pedicle screws decreases with the decrease of bone mineral density.2.When bone mineral density≤90mg/cm3(QCT),female age≥60 years,male age≥70 years,simple application of common pedicle screw for spinal internal fixation is not recommended.