Biomechanical stability and pedicle screw loosening

Pedicle screws are widely utilized in spinal surgeries, yet postoperative loosening remains a major complication in clinical practice. A better understanding of the screw loosening mechanisms will aid the development of proper approaches to predict screw loosening and to reduce the loosening rate. From a unique perspective of biomechanical stability, this review is motivated to provide insights into the mechanisms of pedicle screw loosening, methods to predict screw loosening risk, and strategies to reduce screw loosening risk. The biomechanical stability of screw includes primary stability and secondary stability, which are closely associated with screw loosening and affected by multiple factors, such as bone mineral density, screw parameters, osseointegration, and stress shielding. To assess the risk of pedicle screw loosening, radiographic evaluation and several in vivo (e.g., acoustic modal analysis and resonance frequency analysis), in vitro (e.g., biomechanical testing), and in silico (e.g., finite element analysis) approaches are available. Regarding the strategies to reduce the screw loosening rate, advancements in screw material, surface modification, structural design, and bone cement augmentation have demonstrated promising potential, while optimization of preoperative planning as well as robot- and augmented reality-assisted surgical techniques have significantly improved the implantation accuracy. It can be found that current research has achieved notable progress in improving the primary biomechanical stability of the pedicle screw. However, how to enhance the mechanobiological osseointegration of the bone-screw interface to achieve an improved secondary biomechanical stability for the screws, ultimately reducing the risk of long-term screw loosening, remains a question requiring further studies.