The Anatomical Study And Biomechanical Evaluation Of The Transarticular Screw Placement In Upper Thoracic Spine

BackgroundSurgery of the upper thoracic spine presents a challenge to orthopedic surgeons. A complicating feature of this transition is the disparate anatomy between the cervical and thoracic posterior elements. The differences in anatomy require a variety of internal fixation devices utilized for stabilization and fusion. Meanwhile, the location between the mobile cervical and the fixed thoracic vertebrae requires adequate fixation strength to bear a complex stress. So the creation of a fusion construct that is both safe and biomechanically reliable is extremely important.In the subaxial cervical spine, the most common type of posterior fixation is the lateral mass screw coupled with top-loading rod or plate. The large volume of the cervical lateral mass allows safe placement of these type of screw. However, morphology of C7 lateral mass is different with the other cervical spine.It is difficult to put screws into the elongated lateral mass and often causing lateral mass burst or damage of surrounding tissue.Posterior fixation remains the main technique in the upper thoracic spine for stabilization and fusion, for which several methods are available, including interspinous or sublaminar wires, hook plates and pedicle screws. The wire technique is easy to perform but requires the integrity of the posterior elements. Hooks suffer from similar drawbacks and additionally require entry into the spinal canal, potentially leading to canal stenosis or iatrogenic spinal cord injury. Compared with these systems, pedicle screws are increasingly applied in clinical practice by affording the advantage of three column control. However, pedicle screw insertion in the upper thoracic spine is technically demanding and runs potential risks of causing spinal cord and nerve root injuries. The three-dimensional orientation of the pedicles is complex and demonstrates a high variability. Considering the rate of pedicle violation during intraoperative placement of thoracic pedicle screws has been reported to be as high as 38.9%, patients must be carefully selected and those with narrow pedicles, great variability, or curvature must be excluded. Given these issues, we tried to seek alternative methods for posterior instrumentation that would be as stiff yet avoid the potential risks of pedicle screws.During the past several years, transarticular screws have been used as a fusion construct at the cervical and lumbar spine. Trauma, inflammation, cancer and congenital malformations can cause instability or dislocation of the upper cervical spine, and most of which need the treatment of atlantoaxial fusion. Posterior atlantoaxial transarticular screw fixation has been widely used as a technique that secures a rigid fixation for atlantoaxial instability because of its easy exposure and fixation; for the other hand, the muscle bulk in the neck is notably more massive posteriorly and is related to the tendency of the head to be poisoned in such a way. Biomechanically, transarticular screws exhibited greater pullout strength than lateral mass screws in the lower cervical spine. Holte et al demonstrated an increase in the fusion rate in ALIF from 75% to 98% by using transfacet screws. The use of transarticular screws offers numerous potential benefits including superior pullout strength, load-to-failure ration, decreased risk of damage to the spinal cord, as well as easier technology. Meanwhile, transarticular screw trajectory located a little far from the neural canal and minimized the risks of neurological complications. However, few studies looking at transarticular screw fixation are available in the upper thoracic spine.To our knowledge, however, there is no transarticular screw fixation related study in the upper thoracic spine. In this study, we evaluated the dimensions of upper thoracic facet joints by CT images.Then we simulated inserting into the transarticular screw and determined proper starting point and safe screw length for this technique. According to the recommended method of screw inserted after imaging study, transarticular screws were inserted in eight human cervical spine specimens. After measured the distance from the point of screw piercing out to the upper and lower nerve roots, all the specimens take CT examination to determine the effect of screw placement.Subsequently, an in vitro biomechanical study using cadaveric model was performed to investigate if the range of motion and biomechanical stability was comparable by the transarticular screw and traditional pedicle screw. It provided technical guidance and to be meaningfulfor further clinical application. We believe that the data from this study will substantially increase the spinal surgeon's understanding of the upper thoracic spine fixation.Objectives1. To measure linear and parameters of the upper thoracic facet joints. Then we simulated inserting into the transarticular screw and determined proper starting point and safe screw length for this technique.2. To measured the distance from the point of screw piercing out to the upper and lower nerve roots after transarticular screws were inserted in eight human upper thoracic spine specimens according to the recommended method after imaging study. Then all the specimens take CT examination to determine the effect of screw placement.3. To investigate the the stress distributions of the destabilized models with the transarticular technique in the upper thoracic spine and compare it with the commonly used construct of pedicle screws.Materials and Methods1. Morphometric analysis was performed on computed tomography scans of the upper thoracic zygapophysial joints of C7,T1,T2,T3 in 20 male and 20 female patients in the axial and sagittal planes. The degrees of the screw angulations were recorded in the sagittal and axial plane and the screw lengths were measured at the spinal level from C7-T3.2. In this study, transarticular screws were inserted in eight human candaveric upper thoracic spine specimens according to the recommended method of screw inserted after imaging study. After measured the distance from the point of screw piercing out to the upper and lower nerve roots, all the specimens take CT examination to determine the effect of screw placement.3. Eight fresh human cadaveric spine specimens (T1-T3) were harvested and tested for six cycles in flexion, extension, lateral bending and torsion in their intact condition. Each specimen was then destabilized and restabilized with three fixation methods: the pedicle screw/rod construct, the transarticular screw/rod construct, and transarticular screws alone. The instrumented specimens were retested using the same protocol.Results1. The smallest medial lateral diameter and anterior posterior diameter of IAP was found at T3 among the female and C7 among the male. The screw trajectory length ranged from 14.9 mm to 20.5mm in all patients. All measurements above was significantly different between male and female patients at all levels (P﹤0.05). The mean value of screw trajectory angle in axial plane was 19.3°-20.1°and 44.3°-45.7°in sagittal plane. There were no statistically significantly dependent (P > 0.05) between males and females in axial and sagittal angle.2. All the specimens underwent X-ray examination after inserted screw. and then CT reconstruction in coronal and sagittal reconstruction. It is showed that all the screws were successfully inserted into the facet joins and no screws penetrate into the spinal canal or facet broken. For the suprior articular process of C7-T3, the width was 9.4 ~ 10.1mm and the height was 8.1 ~ 8.8mm. For the inferior articular process, the width was 9.4~9.9mm and the height was 8.1~8.7mm. The length of screw trajectory was 15.9mm to18.1mm. The distance was 5.7±0.9mm from the point of screw piercing out to the upper nerve roots and 4.1±0.6mm to the lower nerve roots. 3. All fixation systems reduced the range of motion significantly with respect to flexion, extension, lateral bending and axial rotation (P<0.01). However, no significant difference was observed between the three instrumented groups.Conclusions1. A transarticular screw was attempted based on radiographic measures of upper thoracic zygapophysial joints. The data indicate that a diameter 4.0-4.5mm and length 16-18mm screw was suitable for this technique and the screw should be inserted inferior and anterolateral.2. The transarticular screw technique in upper thoracic spine was described after an anatomical study. It is a safe trajectory for inserting screw and all important structure were avoided injury. It is feasible to put this technique into clinic.3. This biomechanical study demonstrates in vitro that transarticular screw and pedicle screw have statistically similar biomechanical stability in a non-corpectomy model. Posterior transarticular screw may afford an alternative for internal fixation in upper thoracic spine.