The Benefits of Cement Augmentation of Pedicle Screw Fixation Are Increased in Osteoporotic Bone: A Finite Element Analysis

Study DesignBiomechanical study using a finite element model of a normal and osteoporotic lumbar vertebrae comparing resistance with axial pullout and bending forces on polymethylmethacrylate-augmented and non-augmented pedicle screws.ObjectiveTo compare the effect of cement augmentation of pedicle screw fixation in normal and osteoporotic bone with 2 different techniques of cement delivery.Summary of Background DataVarious clinical and biomechanical studies have addressed the benefits of cement augmentation of pedicle screws, but none have evaluated whether this effect is similar, magnified, or attenuated in osteoporotic bone compared with normal bone. In addition, no study has compared the biomechanical strength of augmented pedicle screws using cement delivery through the pedicle screw with delivery through a pilot hole.MethodsThis study was funded by a grant from DePuy Synthes Spine. Normal and osteoporotic lumbar vertebrae with pedicle screws were simulated. The models were tested for screw pullout strength with and without cement augmentation. Two methods of cement delivery were also tested. Both methods were tested using 1 and 2.5 cm3 volume of cement infiltrated in normal and osteoporotic bone.ResultsThe increase in screw pullout force was proportionally greater in osteoporotic bone with equivalent volumes of cement delivered. The researchers found that 1 and 2.5 cm3 of cement infiltrated bone volume resulted in an increase in pullout force by about 50% and 120% in normal bone, and by about 64% and 156% in osteoporotic bone, respectively. The delivery method had only a minimal effect on pullout force when 2.5 cm3 of cement was injected (<4% difference).ConclusionsCement augmentation increases the fixation strength of pedicle screws, and this effect is proportionately greater in osteoporotic bone. Cement delivery through fenestrated screws and delivery through a pilot hole result in comparable pullout strength at higher cement volumes.