Biomechanical Study of Revision Screw Strategies for Failed Pedicle Screw Fixation

This study aims to systematically explore the strategies for salvage pedicle screws in two ways: modified PMMA augmentation technique and cortical bone trajectory.Pedicle screw fixation has been widely accepted for treatment of spinal disorders, including degeneration, trauma, infection, and other pathological conditions of thoracolumbar spine. To date, following screw loosening, salvage surgery for failed pedicle screw in an enlarged problematic hole remains challenge. A frequently used method is to inject cement into the problematic hole, and then install the screw to increase the screw fixation strength. However, failure of pedicle screws results from incidences of cement leakage, insufficient screw fixation strength is not uncommon.Previous reports have demonstrated that screw fixation strength can be effectively improved when revision pedicle screw is augmented with PMMA cement. However, large amount of PMMA increases the risk of cement leakage, which evaluates complication rates. To solve this problem, with less amount of PMMA, we propose to devise a modified PMMA-augmented technique, to achieve a more safety and more robust fixation for revision pedicle screw. This modified technique may be considered as the transpedicular PMMA expandable anchor screw technique. In our preliminary idea, following the filling of the space of the problematic screw hole with a small amount of PMMA, a small-diameter screw (cylindrical or conical) is then inserted. After the PMMA has hardened, the screw is then carefully removed, leaving a screw hole with PMMA inner thread. A larger-diameter screw (with identical thread type) is then inserted to “expand” the smaller PMMA-threaded hole to achieve a more robust fixation. Recently, cortical bone trajectory (CBT) screws were developed as an alternative to traditional pedicle trajectory (TPT) in the lumbar spine. Previous reports have shown that CBT fixation provided similar stability as traditional pedicle screw fixation. However, literatures examining the feasibility of CBT screws salvage failed pedicle screws or pedicle screws salvage failed CBT screws is lacking. The contents of this two-year study are summarized as follows:The first-year study: Modified PMMA augmentation techniqueSinge-level porcine lumbar vertebrae (L5) are used to evaluate the efficacy of the modified technique. Following the filling of the space of the enlarged screw hole using a small amount of PMMA, a small-diameter (cylindrical or conical) screw is then inserted. After the PMMA has hardened, the screw is then carefully removed, leaving a high-stiffness threaded PMMA hole. A larger-diameter screw (with identical thread type) is then inserted to “expand” the smaller PMMA-threaded hole to achieve a more robust fixation.The second-year study: Cortical bone trajectoryL4-L5 lumbar motion units (LMUs) of the porcine spines will be used to mimic human spines. For the TPT, both L4 and L5 are bilaterally instrumented with pedicle screws; whereas in CBT, they are instrumented with cortical screws. In case that bilateral screw loosening occurs on superior (L4) level, the possibility of TPT and CBT screws when each is conducted to salvage the other will be assessed.

Project ID:PB10907-2488
External Project ID:MOST109-2221-E182-006-MY2