The Optimum Design Of New Fenestrated Pedicle Screw And Reasonable Bone Cement Distribution In The Osteoporotic Vertebral Body

Osteoporosis is common in the aging population, and once spine surgery is indicated due to spondylolisthesis, infection, trauma, or ma lignancy in the elderly, instrumentation is inevitable. It is challenging for a spine surgeon to place instruments into a spine with severe osteoporosis. Many studies have revealed that complications such as loosening, backout, or migration occur frequently after instrumentation in pa tients with severe osteoporosis, resulting in a poor clinical outcome. Recently, some studies have proven that the polymethylmethacrylate(PMMA) augmentation of the pedicle screw can significantly increase stiffness and strength of spinal fixation, which is a hot spot today, especially in fenestrated pedicle screw(FPS). However, there isn’t a standard in designing FPS. We designed this novel FPS, and discussed its security, reasonable bone cement injection dosage and biomechanical stability in this article, in order to provide theoretical basis for the design of the optimum FPS and clinical application.1.An experimental study of bone cement distribution for the safety and biomechanics of fenestrated pedicle screwObjective:This study aims to clarify cement distribution and its morphology in the osteoporotic vertebral body, which is influential in the analysis of the biomechanical stability of the two different design fenestrated pedicle screw (FPS) and the risk of cement leakage after pedicle screw augmentation. Methods:Eighteen vertebrae (T11～L5) obtained from3fresh cadavers (1males and2females; mean age76years)were used. The mean bone mineral density was0.542±0.13g/cm2. They were random delivided into three groups:fenestration interval one thread group(FPS-1)、fenestration interval two thread group(FPS-2) and the conventional pedicle screw group(CPS). Polymethylmethacrylate (PMMA) cement(1.5ml) was perfused through the screws into each vertebra in FPS-1group and FPS-2group.Twenty four hours later, CT and three-dimensional reconstruction examination was performed to all samples. Then, axial pull-out tests was performed to evaluate the screw stability. Results:Three-dimensional reconstruction examination showed that cement all distributed in vertebral body in FPS-1group, while in FPS-2group part of bone cement distributed in vertebral pedicle. The maxmum axial pull-out was respectively (573.2±136.30)N in FPS-1group;(769.2±92.30)N in FPS-2group;(361.8±58.84)N in CPS group. The screw stability in FPS-1group and FPS-2group were both significantly superior to CPS group(P<0.05), FPS-2group was superior to FPS-1group(P<0.05). Conclusion:FPS at the distal third of the screw thread following cement augmentation are safety zone,which is effective in the control of cement leakage. The screw stability after augmentation can be due to the cement distribution and morphology in the vertebral body.2. The optimum design of fenestrated pedicle screw and reasonable bone cement perfusional dosagesObjective:This study aims to discuss the optimum fenestration design and reasonable bone cement dosages by comparing cement distribution morphology and biomechanical stability in two different design perfusional pedicle screw.Methods:We designed two different FPS, the group with the same fenestration diameter (FPS-A)、the group with different fenestration diameter(FPS-B) and the conventional pedicle screw group(CPS) as the control group. Polymethylmethacrylate(PMMA) cement1.0ml、1.5ml、2.0ml was separately perfused through the screws into each osteoporotic vertebra in FPS-A group and FPS-B group. CPS group had no cement perfused. Twenty four hours later, CT and three-dimensional reconstruction examination was performed to all samples. Then, axial pull-out tests was performed to evaluate the screw stability. Results: Three-dimensional reconstruction examination showed that bone cement distribution is conical in vertebral body in FPS-A group, while in FPS-B group bone cement distribution is cylindrical in vertebral body. When the bone cement dosages reached2.0ml, the risk of cement leakage began to increase. FPS-B group showed the lower risk compared with FPS-A group. The maxmum axial pull-out showed that the screw stability in FPS-A group and FPS-B group were both significantly superior to CPS group(P<0.05), FPS-B group was superior to FPS-A group(P<0.05), that is to say cylindrical bone cement distribution was superior to conical on the biomechanics strength. Conclusion:The optimum bone cement dosages in perfusional pedicle screw are1.5ml-2.0ml. The fenestration design can control the cement distribution morphology in vertebral body. Cylindrical bone cement distribution (FPS-B group) has advantages in the safety and biomechanics.