ABSTRACT
Objective To compare the effects of cortical bone trajectory ( CBT) and traditional trajectory ( TT)pedicle screw internal fixation on the range of motion (ROM) and rod system stress of normal and osteoporotic(OP) spines. Methods The L3-S1 finite element models of normal and OP spines were established. The screwrod system with two kinds of trajectory was used for internal fixation of the L4-5 segment, so as to simulate sixphysiological loads, namely, flexion, extension, left / right bending, left / right rotation. The effects of two internalfixation methods on ROMs and maximum equivalent stress of screws in normal and OP spines were compared.Results For both bone conditions, CBT and TT significantly reduced ROM of the fixed segment (L4-5) and theentire segment of lower lumbar spine ( L3-S1). However, the ROM decline of CBT group was slightly smaller than that of TT group, and their ROMs were similar under flexion and extension, but the ROM differences were significant under lateral bending and axial rotation. In addition, for both the normal and OP spine models, themaximum equivalent stress of screws in CBT group was significantly higher than that in TT group. Compared withTT group, the screw stress of CBT group in normal spine model under flexion and extension, lateral bending,axial rotation was increased by 27% , 268% and 58% , respectively. However, when CBT technique was used atthe same time, the OP spine model had a smaller screw stress distribution than the normal spine model.Conclusions Compared with TT technique, CBT technique can achieve higher screw stress under OP conditionand reduce screw stress concentration under normal bone condition. In addition, CBT slightly increases ROMs of each segment, which is conducive to recovery of spinal physiological function after surgery. Lateral bending and axial rotation can produce negative mechanical effects, and these two physiological loads should be avoided.
ABSTRACT
Objective To study mechanical properties of traditional trajectory (TT) and modified cortical bone trajectory (MCBT) on osteoporotic vertebrae through finite element analysis. Methods The three-dimensional model of L4 segment was established, and pedicle screw (PS) (diameter 6.0 mm, length 45 mm) and MCBT screw (diameter 4.5 mm, length 40 mm) were placed on both sides of the lumbar spine. The pull-out strength and the load-displacement ratio of screws in two different screw trajectories under up, down, left, right working conditions were analyzed, and the stability between the screw and vertebral body under osteoporotic conditions was evaluated. Results Compared with TT, the pull-out strength of MCBT screw was increased by 13.1%. Compared with PS, the load-displacement ratio of MCBT screw under up, down and left working conditions was increased by 57.2%, 32.4%, and 31.6%. Under right working condition, although the load-displacement ratio of MCBT screw was higher than that of PS, no statistical difference was found. The load-displacement ratio of vertebral body in MCBT group under lateral bending and axial rotation was significantly higher than that in TT group. The load-displacement ratio of vertebral body in MCBT group under flexion was lower than that in TT group. Although the load-displacement ratio of vertebral body in MCBT group under extension was higher that that in TT group, no statistical difference was found. Conclusions MCBT is superior to TT in pull-out strength, screw stability and vertebral body stability under lateral bending and axial rotation, but its vertebral body stability under flexion and extension was weaker than that of TT. The research findings demonstrate the superiority of MCBT under osteoporotic conditions and lay the foundation for clinical application of MCBT.
ABSTRACT
This article summarizes recent evidence on the cortical bone trajectory (CBT) obtained from published anatomical, biomechanical, and clinical studies. CBT was proposed by Santoni in 2009 as a new trajectory that can improve the fixation of pedicle screws in response to screw loosening in osteoporotic patients. Recently, research interest has been growing with increasing numbers of published series and frequent reports of new applications. We performed an online database search using the terms “cortical bone trajectory,”“pedicle screw,”“CBT spine,”“CBT fixation,”“MISS CBT,” and “traditional trajectory.” The search included the PubMed, Ovid MEDLINE, Cochrane, and Google Scholar databases, resulting in an analysis of 42 articles in total. These covered three aspects of CBT research: anatomical studies, biomechanical parameters, and clinical cases or series. Compared to the traditional trajectory, CBT improves pullout strength, provides greater stiffness in cephalocaudal and mediolateral loading, and shows superior resistance to flexion/extension; however, it is inferior in lateral bending and axial rotation. CBT seems to provide better immediate implant stability. In clinical studies, CBT has shown better perioperative results for blood loss, length of stay in hospital, and surgery time; similar or better clinical postoperative scores; and similar comorbidity, without any major fixation system complications due to instrumentation failure or screw misplacement. In addition, advantages such as less lateral exposure allow it to be used as a minimally invasive technique. However, most of the clinical studies were retrospective case series or case-control studies; prospective evidence on this technique is scarce, making a definitive comparison with the traditional trajectory difficult. Nevertheless, we can conclude that CBT is a safe technique that offers good clinical results with similar biomechanical and perioperative parameters to those of the traditional trajectory. In addition, new applications can improve its results and make it useful for additional pathologies.