Finite Element Study on the Amount of Injection Cement During the Pedicle Screw Augmentation

Study Design: A finite element analysis of the screw pullout procedure for the osteoporotic cancellous bone using screw-bone unit model without cortical layer. Objective: The objective is to determine the region of effect (RoE) during the screw pullout procedure and predict the proper amount of injection cement (AIC) in screw augmentation. Summary of Background Data: For the osteoporotic spine, the AIC is a critical factor for the augmentation screw performance and leakage risk. There are few studies on the proper AIC in literature. Methods: Three finite element models were established, 2 screw-foam models were used for validation study, and 1 screw-bone model was used for investigation of RoE and AIC. The simulations of screw pullout were conducted. A velocity loading of 0.01 mm/s with a maximum displacement of 2.7 mm was applied on the screw. For the validation, the screw-foam models with 2 different densities were used for comparison of pullout force with those published experimental data. After validation, the screw-bone model was used to investigate the RoE and predict the proper AIC during screw augmentation in spine surgery. Results: In validation, the predicted pullout strengths were 2028.8 N for high-density foam model and 607 N for low-density foam model, respectively. They were in good agreement with those of the published experiment. In the screw-bone model, the simulations demonstrated that the RoE changed with the displacement of screw and reached the maximum when the displacement of screw was 1.8 mm. Similar trend was found for the AIC with the displacement. The proper AIC was 2.6 mL when the displacement of screw was 1.8 mm in this study. Conclusions: The RoE and proper AIC for augmentation were evaluated in the osteoporotic spine. This information could provide practical reference for screw augmentation in spinal decompression and instrumentation in the spine surgery.