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Stabilization of the atlantoaxial complex is a challenging procedure because of the unique anatomy of this region. A variety of techniques for atlantoaxial fixation by anterior, bilateral, and posterior approaches have been described. In recent years, several new posterior techniques and modifications have been described. In year 1979, Magerl and Seemann introduced a new technique using transarticular screws through the C1-C2 articulation. In several studies, this technique showed biomechanical stability superior to that of different wire fixations for lateral bending and/or rotation. The clinical success rates were high, and the biomechanical stability for lateral bending and axial rotation was excellent. But some recent studies show a higher rate of complications, such as malpositioned screws, neurologic deficit, and vascular compromise.In this study, authors did experimental test and clinical application, in order to provide anatomic basis for transarticular screw of atlantoaxialfusion, to evaluate the biomechanical stability of C1-C2 transarticular screw fixation technique for atlantoaxial instability and to study the operative and clinical effectiveness of posterior transarticular screw fixation for atlantoaxial instability.Materials and MethodsPart 1, Six atlas and axis specimens of native adults were used to observe the normal anatomy and to measure pertinent clinical data. The shapes of the axis transverse foramina, the road of vertebral artery was observed. The inferior angulation of the horizontal plane was measured.Part 2, In a nondestructive, repeated-measures in vitro flexibility experiment, Six human cervical specimens were loaded nondestructively with pure moments, and unconstrained motion at C1-C2 was measured. The six specimens were instrumented with each of the following fixation techniques: Pedicle screw and rod fixation, transarticular screws, transarticular screws and Brooks fixation.Part 3, The clinical outcomes of 23 patients treated with posterior C1-C2 transarticular screws and wired posterior C1-C2 autologous bone struts/ free bone particle were evaluated prospectively. The study group was composed of 16 men and 7 women, with a mean age of 38.6 years (range, 17-62 yr). Each patient had documented atlantoaxial instability. Atlantoaxial instability was caused by C2 fractures with dislocation in 14, atlantoaxial dislocation in 5, transverse ligament disruption in 1, osodontoideum in 3.TECHNIQUE (Magerl, Seemann): Position the patient prone. Use lateral image intensification to check the reduction of the C1-2 complex. Perform midline posterior cervical exposure in the routine fashion from C1 to C3. Identify the landmarks for the entry portal of the transarticular screw at the lower inner edge of the inferior articular process of C2. Using a 2-mm bit, drill through the isthmus near its posteromedial surface, exiting from the articular mass of C2 at the posterior aspect of the superior articular surface and entering the lateral mass of the atlas. The drill bit should perforate the cortex of the lateral mass of C1. Determine the appropriate screw length. Use a 3.5-mm cortical tap to cut threads in the drill hole and insert the appropriate 3.5-mm cortical screw across the C1-2 joint. In some cases, as an alternative, cannulated screws may be used after insertion of Kirschner wires under image intensification control. During insertion of the screws, take care to avoid injuring the vertebral arteries. After placing the C1-2 transarticular screws, perform a traditional posterior Cl-2 fusion using posterior interspinous grafting with either structural bone or cancellous bone.Results.In feasiability anatomic measure, the shapes of the axis transverse foramina were variable, the road of vertebral artery was variable too. The inferior angulation of the horizontal plane was (53.90±2.45)° inmaximum, (26.12±0.77)° in the minimum, (42.89±1.25)° in the optimal, and (26.12±0.77)° in the lower limit. The length of screw was ranged from 37.64mm to 39.60mm (averaged 38.74mm±0.44 mm)(se
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