| BackgroundsThe surgical disorders of the upper cervical spine are common and difficult to treat in the clinic, such as atlantoaxial instability, fracture. A variety of techniques for atlantoaxial fixation by anterior, bilateral and posterior approaches have been described. Gallie described the technique which wires were used in fixation under the posterior arch of the atlas and the sublaminar of the axis with a bone graft in 1939. Fielding-Brooks introduced a technique which double wires were used in fixation for atlantoaxial instability under the vertebral plate with a bone graft in 1977. All of these techniques are associated with potential risks associated with the use of sublaminar wires. In 1979, Magerl and Seemann introduced a new technique using transarticular screws through the C1-C2 articulation, which was called the Magerl technique. Then the Halifax technique and the Apofix technique were introduced soon after in 1984. A new technique was reported by Goel in 1994, which was used in fixation for the atlantoaxial disorders with screws in lateral mass of atlas and screws in pedicle of the axis, and with rods or plates connected. In recent years, the pedicle screws fixation technique, which is used in fixation for atlantoaxial instability associated with trauma, severe degeneration and tumorectomy, has been developed. However, this kind of technique is very difficult to develop because of the complicate of the anatomy as well as the important of the physiological function of the upper cervical spine. The complications are very dangerous, such as malpositional screws, neurologic deficit, and vascular injuries, especially the vertebral artery injury, which is the lethal complication. Nevertheless, the quantitative information is much less regarding the applied anatomy of the upper cervical spine, which prevents this technique developed. This study reports the detailed quantitative information of the external dimensions and the ranges of the lateral mass of the atlas with 30 fresh specimens and the external dimensions and the ranges of the pedicle of the axis with 55 dry specimens. The purpose of the biomechanical testing in vitro aimed to evaluate the stability of atlantoaxial instability fixation with posterior pedicle screws, and compare it with that of Apofix technique and Magerl technique. On the other hand, a finite element model of atlantoaxial instability fixation with posterior pedicle screws was made to analysis the stability and the stress distribution. Then the application perspective of posterior pedicle screws fixation for atlantoaxial instability was to evaluate.ObjectivesThe purpose of this study was undertaken to quantitatively measure the ranges and the external dimensions of lateral mass of the atlas and the external dimensions and the ranges of the pedicle of the axis, and to evaluate the anatomical feasibility of posterior pedicle screws fixation for atlantoaxial instability.The biomechanical testing in vitro aimed to evaluate the stability of posterior pedicle screws fixation for atlantoaxial instability, and then compare it with that of Apofix technique and Magerl technique.A finite element model of atlantoaxial instability fixation with posterior pedicle screws was developed to analysis the stability and the stress distribution.Methods30 isolated atlas specimens of flesh adult cadavers (male 24 and female 6) from the Department of Anatomy were presented for dissection to quantitatively measure the lateral mass of the atlas. Using a digital caliper (Type YA TO Pro-Cal, Tokyo, Japan) to directly measure the width, the thickness, and the height of the lateral mass of the atlas, and the diameter of atlantoaxial joint articular surface, respectively. The obliquity of the facet atlantoaxial joint in coronary plane was calculated. 55 dry axis specimens from the Department of Anatomy were presented to quantitatively measure the pedicle of the axis. Using the same digital caliper (Type YA TO Pro-Cal, Tokyo, Japan) to directly measure the width and the height of the pedicle of the axis. The accuracy of these measurements is within 0.01mm. Each specimen was measured twice respectively so that the data could be precise. All 55 dry axis specimens were taken digital radiographs (DR), while the obliquity of the pedicle of the axis on transverse plane and sagittal plane were surveyed.Eight fresh adult craniocervical specimens (C0-C4) were used for testing the biomechanical instability. All the specimens were tested the three-dimensional ranges of motion (ROM) by Spine 2000 in the following sequence: intact, instability, Apofix fixation, Magerl fixation and posterior pedicle screws fixation. Tested specimens were loaded with 2.0 Nm on flexion-extension, lateral bending and left and right axial rotation. Then the three-dimensional ROM of every specimen was evaluated by Geomagic Studio 5 soft package. All the specimens fixed with Apofix technique, Magerl technique and posterior pedicle screws were loaded on MTS 858 Bionix (Material Testing System, MTS, Minneapolis, MN) with a frequency of 0.5 Hz to test their anti-fatigue characters. And then we performed the test to evaluate the biomechanical fixation intensity of pedicle screws in the atlas and the axis through the experiment on screw pull-out strength on MTS 858 Bionix.The atlantoaxial specimen's geometries were determined from computer tomography (CT) images with 0.9 mm distance in the sagittal and coronal planes. These images of CT were transferred to PC, and the data from the images were then processed so that a finite element mesh of the vertebrae could be constructed by ANSYS 8.0 software. The finite element model was validated for loading in flexion, extension, lateral bending and rotation. It can be use for evaluation the stress changes of the posterior pedicle screws fixation.Statistical analysis of the data was performed using the SPSS 10.0 software package (Department of Biostatistics, FIMMU). Paired-Samples T Test, One-Way ANOVA and Repeated Measures were performed to determine whether significant differences existed.ResultsBased on the dissection of 30 atlases of fresh cadavers and quantitative measurement of the lateral mass of the atlas, the mean and standard deviation of anatomic parameters of the lateral mass of the atlas were calculated separately. The measured dimensions of lateral mass of the atlas showed no significant differences between the two sides. The width of the lateral mass of atlas was 15.51±1.08 mm (12.82-17.54 mm), the thickness was 17.21±0.82 mm (15.51-18.75mm), and the height was 14.10±1.75 mm (11.77-18.45mm). Calculation of the obliquity of the facet atlantoaxial joint in coronary plane showed no statistical significance between the two sides, which was 34.74?±2.90? (28.76?-41.48?).On the base of the quantitative measurement of the pedicle of 55 dry axis specimens, the mean and standard deviation of anatomic parameters of the pedicle of the axis were calculated separately. Statistical analysis using SPSS 10.0 soft package showed no significant differences between the two sides. The width of the pedicle of the axis was 8.22±1.48 mm (4.96-12.58mm), the height was 8.24±0.86 mm (5.56-10.28 mm), and the obliquities of the pedicle of the axis on transverse plane and sagittal plane were 36.57°±3.18°(28.30°-48.21°) and 26.79°±2.10°(3.05°-35.29°), separately.According to the biomechanical testing, it showed that the flexion-extension range of motion (ROM) of atlantoaxial instability fixation with posterior pedicle screws was 1.68?±0.37?, the bending ROM was 0.33?±0.09?, and the rotation ROM was 1.74?±0.16?. There were significant differences between the total ROM of pedicle screws fixation group and those of intact group and instability group, which means posterior pedicle screws fixation for atlantoaxial instability has better stability postoperative immediately. There were significant differences between the total ROM of pedicle screws fixation group and those of Apofix fixation group and Magerl fixation group, too. The stability fixation with posterior pedicle screws is better than that of Apofix technique and Magerl technique. In the posterior pedicle screws fixation group, there was no significant difference between before fatigue and after fatigue in the total ROM. Therefore, atlantoaxial instability fixation with posterior pedicle screws has a good long-term stability.The pull-out strength testing of the screws in the pedicle of the atlas and the axis showed no significant differences between the two sides of their own. The pull-out strength of the screw in the pedicle of the atlas was 1502.39±73.89 N, while the pull-out strength of the screw in the pedicle of the axis was 2385.79±15.13 N. Statistical analysis showed significant differences between the pull-out strength of the atlas and that of the axis.An FE model of human atlantoxial vertebrae fixation with posterior pedicle screws was developed with nonlinear material data to analysis the stress and the stability postoperative. The developed FE model did not include muscles either ligaments. On the base of FEM analysis, it showed that the stress values were different in different location of the atlantoxial vertebrae as well as in different motion states. When the FE model was given anterior flexion motion, the maximum stress value was 0.289×107 Pa on the surface of the atlantoxial joint, 0.519×108 Pa in the middle of the plate, and 0.350×109 Pa in the base of the screw fixation in the axis. The ROM of ante-flexion was 0.7?. When the FE model was given posterior extension motion, the maximum stress value was 0.123×107 Pa at the conjunction of the lateral mass and the posterior arch of the atlas, 0.691×108 Pa in the middle of the plate, and 0.427×109 Pa in the base of the screw fixation in the axis. The ROM of post-extension was 1.2?. In lateral bending condition, the maximum stress value was 0.146×109 Pa at the location of the start point of the screw road of the axis, 0.713×108 Pa in the middle of the plate, and 0.295×109 Pa in the base of the screw fixation in the axis. The ROM of lateral bending was 0.3?. In rotation condition, the maximum stress value was 0.146×109 Pa at the location of the start point of the screw road of the axis, 0.909×108 Pa in the middle of the plate, and 0.635×109 Pa in the base of the screw fixation in the axis. The ROM of rotation was 0.8?. Nevertheless, the maximum stress was located in the base of the screw fixation in the axis in any motion states.ConclusionsThe space of the lateral mass of the atlas is much larger, and there is enough space for a lateral mass screw or a pedicle screw fixation. There is an angle of the facet atlantoaxial joint in coronary plane. Therefore, the direction of the screw fixation in the pedicle of the atlas must be kept an obliquity inwardly and upwardly.The selection of the fixation technique with pedicle screws in the axis depends on the width and the height of the pedicle near the foramen transversarium. On the base of this study, there is enough space for the pedicle screw fixation in the axis in majority. However, the anatomical character of pedicle of the axis is multivariate. X-ray or CT examination on the pedicle of the axis should be done to find whether there is a variation in the pedicle of the axis preoperatively. Based on the digital radiographic (DR) evaluation of the 55 dry axis specimens, it showed that there was an inward obliquity of the pedicle of the axis on transverse plane and a superior obliquity of the pedicle of the axis on sagittal plane. This method of measurement is very useful because it can decrease the anthropogenic error by direct measurement. On the other hand, it can provide visualized references to the doctors because the spinal surgeons usually finish the posterior pedicle screws fixation in atlantoaxial spine on the monitor of C X-ray apparatus or G X-ray apparatus.In this study, we compared the biomechanical stability of the posterior pedicle screw fixation technique with that of intact, instability, Apofix technique and Magerl technique. According to the results of this in vitro study, posterior pedicle screws fixation in atlantoaxial vertebrae showed an outstanding character of biomechanical stability immediately postoperative. The biomechanical stability of posterior pedicle screws fixation is much higher than that of Apofix fixation and Magerl fixation. And the anti-fatigue test of the posterior pedicle screws fixation showed a good character of biomechanical stability in the long-term.Based on the results of pull-out strength test, the pull-out strength of the pedicle screw fixation in atlantoaxial vertebrae was obviously higher than that of lateral mass screw previously described studies. It would be better to choose a unicortical screw fixation in the pedicle while a bicortical screw fixation in the lateral mass because the pull-out strength of unicortical screw fixation in the pedicle was strong enough. Using a unicortical screw fixation in the pedicle can avoid the risks of posterior wall of pharynx and hypoglossal nerve injuries as well as provide strong fixation to maintain the stability.According to FEM analysis with the developed FE model, it showed that the stress values were different in different location of the atlantoxial vertebrae in every motion, including anterior flexion, posterior extension, lateral bending and rotation. However, the maximum stress was located in the base of the screw fixation in the axis in any motion condition. The ranges of the four kinds of motion according to FEM analysis were all less than those of atlantoaxial instability, which displayed posterior pedicle screws fixation for the atlantoaxial instability could provide good strong fixation.
|
PDF Full Text Request