Basic And Primary Clinical Study Of A Modified Posterior Atlantoaxial Transarticular Screw Fixation For Atlantoaxial Instability

ObjectStabilization of the atlantoaxial segment through posterior approach is the most common procedure for treatment of atlantoaxial instability. A series of techniques had been involved: (1) cable fixation techniques such as Gallie or Brooks-Jenkins cable fixation. (2) Lamina clamp techniques such as Halifax or Apofix lamina clamp. (3) screw/plate and screw/rod construct such as C1 lateral mass screw+C2 lateral mass screw or C1 lateral mass screw+C2 pedicle screw. (4) Occipitocervical fusion technique, such as Cervifix system. (5) Atlantoaxial transarticular screw fixation such as Magerl's technique. Among all these fixation system, Magerl's technique provides immediate rigid atlanto-axial fixation and it has biomechanical advantages when compared to other techniques. Meanwhile, it brings about other benefits including high fusion rate, easy manipulation and low cost. It has been proved by previous research that the ideal entry point of Magerl technique was located at 3.0mm superior and 2.0mm medial to C2 inferior facet, and the ideal exit points located at midpoint or upper one-third point of ventral cortex of C1 lateral mass. In this study, we modified the entry point and screw pathway of Magerl screw to provide a more accurate anatomic landmark of entry point, improve the biomechanical strength and enhance the post-operative segmental stability. In our modified technique, the entry point was located at the midpoint of C2 inferior facet with exit point located at superior edge of ventral cortex of C1 lateral mass. Morphometric measurements, Radiological measurement, biomechanical test and primary clinical experiment were involved in this study based on the modified Magerl technique. Our research aimed to set up a much simple and reliable technique for atlantoaxial transarticular screw placement, and evaluated it by anatomy, biomechanics and clinics. 1. To evaluate the feasibility of modified Magerl technique and compare morphometric parameters of these two techniques by anatomical measurements on dry specimens of C1 and C2.2. To evaluate the feasibility of modified Magerl technique and compare morphometric parameters of Magerl technique and modified technique by anatomical measurements and Surgical Simulation on helical CT three- dimensional reconstruction of C1 and C2.3. To compare the three-dimensional biomechanical segemental stability of these two techniques.4. To compare biomechanical segemental stability of these two techniques by three-dimensional biomechanical analysis after Fatigue test.5. To evaluate, verify and summarize features and outcomes of modified Magerl technique by clinical assessment.Materials and Methods1. Sixty paired and dry first and second cervical vertebrae for anatomic measurements were obtain from the Department of Anatomy, and the following parameters were measured or evaluated including: (1) height of C2 pedicle (2) width of C2 pedicle (3) inner height of C2 lateral mass (4) sagittal angle of C2 pedicle (5) medial angle of C2 pedicle (6) depth of the vertebral artery groove (7) optimal screw length, optimal screw sagittal and medial angle of Magerl technique (8) optimal screw length, optimal screw sagittal and medial angle of modified Magerl technique.2. Sixty helical computerized tomography scans of healthy volunteers were obtained for surgical simulation on three- dimensional reconstruction images. Morphometric measurement were carried on two-dimensional image of optimal screw trajectory, including: (1) Height of C2 pedicle. (2) Width of C2 pedicle. (3) Optimal screw length of these two techniques. (4) Optimal screw sagittal angle of these two techniques. (5) Optimal screw medial angle of these two techniques. (6) Distance between the groove and optimal screw path. The Morphometric parameters measured on reconstructed CT image were compared and analyzed to find the difference of the two techniques and evaluate the feasibility of modified Magerl technique. Surgical risk such as vertebral artery injury was also assessed.3. Twelve fresh C1 to C3 specimens which were divided randomly 2 groups, were tested in status of normal, injured, fixed with Magerl technique, and fixed with modified Magerl technique. The three dimensional data were measured and compared.4. The Aforesaid specimens which underwent biomechanical tests were used for fatigue tests utilizing MTS - 858 Bionix test system (MTS, Mini BionixR). Using torque of 1.5Nm, three types of fatigue load were involved: flexion/extension, lateral bending, and axial rotation, each type for 1000 times. Three-dimensional biomechanical measurement were performed on the the twelve specimens after fatigue test compared the biomechanical stability of the two techniques.5. 12 patients who were diagnosed definitely as atlantoaxial instability were treated with atlantoaxial transarticular screw fixation by modified Magerl technique. The radiological materials and clinical outcomes were analyzed to evaluate the feasibility and safety of this modified technique.Results:1. The results of dried vertebrae measurements: The mean height of C2 pedicle was 8.50mm, the mean width of C2 pedicle was 5.95mm, the mean inner height of C2 lateral mass was 3.46mm, the mean sagittal angle of C2 pedicle was 37.9°,the mean medial angle of C2 pedicle was 24.9°,the mean depth of the vertebral artery groove was 5.66mm, the mean optimal screw length of Magerl technique was 35.60mm, the mean optimal screw length of modified Magerl technique was 40.67mm, the mean optimal screw sagittal angle of Magerl technique was 46.8°, the mean optimal screw sagittal angle of modified Magerl technique was 51.9°, the mean optimal screw medial angle of Magerl technique was 11.2°and the mean optimal screw medial angle of modified Magerl technique was 16.3°.2. The results of morphometric measurement on multi planar reconstruction on CT image of optimal screw trajectory were list as following: the mean width of C2 pedicle was 6.01mm, the mean height of C2 pedicle was 8.21mm, the mean optimal screw length of Magerl techniques was 36.42mm, the mean optimal screw length of modified Magerl techniques was 41.27mm, the mean optimal screw sagittal angle of Magerl techniques was 49.87°, the mean optimal screw sagittal angle of modified Magerl techniques was 53.87°,the mean optimal screw medial angle of Magerl techniques was 8.80°, the mean optimal screw medial angle of modified Magerl techniques was 14.81°, the mean distance between the groove and optimal screw path of Magerl techniques was 4.07mm, the mean distance between the groove and optimal screw path of modified Magerl techniques was 3.99mm. Based on the the multi-planar reconstructed image, it was considered that a distance less than 2.5 mm between the groove and the screw path would make transarticular fixation technically difficult and with great risk of vertebral artery injury. In our study, C2 articular masses without anatomic variations predisposing to vertebral artery injury were considered as low risk type which constituted 88%.The cases with isthmus narrowing, considering isthmus height or width measuring less than 5 mm as the criteria constitute 3%, cases with distance less than 2.5 mm between the groove and the screw path constitute 7%. Cases showing both two criteria constitute 2%. In other words, 12% of the total cases were not suitable for transarticular screw fixation.3. The results of three dimensional biomechanical measurements: Significant increased range of movements of flexion, lateral bending and axial rotation were observed in specimens with odontoid fracture. However,range of movements of extension were similar. Range of movements in three types of fatigue load were significantly reduced after transarticular screw fixation comparing with the normal and injured specimens, with no significant statistical difference between the two techniques.4. Results of biomechanical measurements after fatigue tests indicated that Range movements in of three types of fatigue load (three dimensional: flexion/extension, lateral bending, and axial rotation) were increased in both two group (Magerl group and modified Magerl group),before or after fatigue tests. Increasing of range of movements in three dimension were significantly higher in Magerl group than modified Magerl group with markedly statistical difference.5. Twelve patients were treated modified Magerl procedures with 24 transarticular screws placed and granulated bone graft over C1 posterior arch and C2 lamina. Immediate rigid atlanto-axial stability was obtained in all 12patients without injuries of spinal cord, hypoglossal nerve or vertebral artery. Postoperative X-ray and CT image indicated satisfactory position of the screws. The length of screw ranged from 38 to 42 mm with a mean length of 42.0mm. All the patients were followed up for 3 to 10 months and gain solid bony fusion without screw loosing or breakage.Conclusion:1. The modified Magerl technique we designed is technically feasible according to the results of anatomic measurement on specimens. The entry point of transarticular screw in modified Magerl technique located ventrally, laterally or superiorly to the axis of C2 pedicle, which guaranteed the length of the screw path. While, the exit point located at the superior edge of C1 lateral mass with steeper inclination angle which may provide more biomechanical stability.2. Morphometric measurements based on three dimensional reconstruction helical CT image of healthy volunteers showed that the value of optimal screw length and optimal medial angle and sagittal screw angle are bigger in modified Magerl group with significant statistical defference(P<0.05). No statistical deference was found in distance between vertebral artery groove and optimal screw trajectory between the two groups. It's worth noting that 12% of the total cases may not suitable for posterior transarticular screw placement as the vertebral artery lies too close to the screw trajectory. Preoperative helical CT scans with 3 three dimensional construction is strongly recommended to assess the anatomic parameters and avoid the risks related with anatomical variation.3. It was proved by biomechanical tests that modified Magerl technique provide stastifactory atlantoaxial stability as Magerl technique. The modified procedure showed better biomechanical performance after fatigue tests.4. Clinical outcomes indicated that modified posterior atlantoaxial transarticular screw fixation is effective and safe for the treatment of atlantoaxial instability. The modified Magerl technique bring about satisfactory results with less intra-operative blooding, operation time and trauma, as exposure of C1/C2 facet joint is not necessary.