Objective A novel variable-diameter cortical threaded screw used in a modified cortical bone trajectory(MCBT)was designed to verify its mechanical properties using the MCBT technique.Methods According to MCBT technology,the screw pitch was fixed at 2 mm,the total length was 45 mm,the diameter of the thick rod was 5.5 mm,the diameter of the thin rod was 4.0-4.5 mm,and the length of variable-diameter position connecting the thick rod and the thin rod was 2 mm.The parameters were set based on three aspects:variable-diameter position,thread depth,and thread type.Three-factor and three-level L9 tests were conducted and screw models were established.The torsion and the bending and pull-out force of the designed screws were calculated based on the finite element method,the results were analyzed using range analysis,and then the screw models were determined.The three-dimensional(3D)model of L4 vertebral body in osteoporosis specimens was established and screws were placed according to the MCBT technique.The pull-out force of the novel variable-diameter cortical threaded screw was compared with that of a conventional non-variable-diameter cortical threaded screw.Results Range analysis showed that screw No.6(variable-diameter position:24 mm from the screw head,thread depth:0.7 mm,45° symmetrical thread)was the optimal screw.The anti-pull-out force of the No.6 variable-diameter cortical threaded screw was 13.1%higher than that of the 4.5 mm conventional non-variable-diameter cortical threaded screw,and no statistical difference in anti-pull-out force was found between the No.6 variable-diameter cortical threaded screw and the 5.5 mm conventional non-variable-diameter cortical threaded screw.Conclusions The variable-diameter position has the smallest influence on pull-out force of the screw,the thread type has the largest influence on pull-out force,and the thread depth has the largest influence on torsion and bending.Compared with that of the conventional non-variable-diameter cortical threaded screw,the variable-diameter cortical threaded screw had a smaller front end,which prevented splitting at the entrance point of the screw.The screw has a large diameter at rear end,thereby showing improved pull-out performance.The results provide a new theoretical basis for the clinical application of MCBT technology.

BACKGROUND:At present,there are shortcomings and risks in the surgical revision of vertebral bodies that failed to be fixed in clinical practice.To avoid the risks of conventional revision surgery,the cortical bone trajectory technique is used to perform revision surgery on vertebral bodies that failed to be fixed.However,the mechanical properties of cortical bone trajectory technique screws in revision surgery are not clear. OBJECTIVE:The mechanical properties of cortical bone trajectory in lumbar revision surgery were analyzed by the finite element method to provide a theoretical basis for the clinical application of cortical bone trajectory in revision surgery. METHODS:CT scan data of the osteoporotic vertebral body were obtained and the L4 vertebral body model was established.The initial cortical bone trajectory placement and traditional pedicle screw in the L4 vertebral body model were completed,respectively,and their mechanical data were taken as the baseline standard for later evaluation of revision surgical performance.The traditional pedicle screw was removed and the screw path was retained.The cortical bone trajectory screw was used for secondary screw placement on the vertebral body to achieve lumbar refixation.The axial pull-out force,stability,and lumbar motion range of the revised screw were analyzed by the finite element method. RESULTS AND CONCLUSION:(1)The screw axial pull-out force of the cortical bone trajectory revision group was 25.6%higher than that of the traditional pedicle initial group.(2)In the lower,left,and right working conditions,the load-displacement ratio of screws in the cortical bone trajectory revision group increased by 18.5%,41.3%,and 35.0%,respectively,compared with the traditional pedicle initial group.The load-displacement ratio of screws in the cortical bone trajectory revision group was slightly higher than that in the traditional pedicle initial group under the above condition,but there was no statistically significant difference(P>0.05).(3)In anterior and posterior flexion conditions,lumbar motion range in the cortical bone trajectory revision group was increased by 45.5%and 36.1%compared with the traditional pedicle initial group,but there was no statistically significant difference in left bend,right bend,and axial rotation conditions(P>0.05).(4)There were no statistically significant differences in screw axial pull-out force,screw load-displacement ratio,and lumbar motion range between the cortical bone trajectory revision group and cortical bone trajectory initial group(P>0.05).(5)The mechanical data exhibited that although the revised nail track bone was damaged or lost to a certain extent,the mechanical properties of the cortical bone trajectory revision group were still better than those of the traditional pedicle initial group to a certain extent.Moreover,there was no significant difference in the mechanical properties between the cortical bone trajectory revision group and the cortical bone trajectory initial group.It provides a reference for revision surgery of lumbar internal fixation with cortical bone trajectory technique in patients with failed traditional pedicle fixation.

Adjacent segment disease (ASDis) is a common complication of posterior lumbar spine fusion and often requires surgical treatment. In the treatment of ASDis, percutaneous spinal endoscopy can be used for simple decompression without removal of the original internal fixation, or for posterior fixation and fusion under the scope or in combination with other access fixation and fusion techniques, with the advantages of less surgical trauma, less bleeding, and faster postoperative recovery. Traditional trajectory screw technique is one of the risk factors for adjacent segment degeneration because of its tendency to cause damage to the adjacent synovial joint during surgery. In contrast, the cortical tone trajectory (CBT) screw placement technique not only reduces the damage to the articular joint during the screw placement process, but also preserves the original internal fixation in the treatment of ASDis, which significantly reduces the surgical trauma. Secondly, the implantation of CBT screws with the aid of digital technologies such as three-dimentinal printed guides, CT navigation, and robotics allows for more precise "double nailing" of ASDis patients to complete the fusion of adjacent segments, and is a minimally invasive procedure to be considered for patients who meet the clinical indications for fusion. This article reviews the literature on the use of percutaneous spinal endoscopy and CBT in the surgical management of ASDis.