The CT scanning and clinical application of atlas pedicle screw placement guided by the morphology of interarticularis

English Keywords:Atlas  Axis  Interarticularis  Pedicle screw

English Abstract:

  【Abstract】 Objective：To evaluate the feasibility and reliability of atlas pedicle screw placement guided by the morphology of interarticularis.Method：Three-dimensional CT reconstruction weas performed on C1-2 of 48 normal adults.The cross sectional images between atlas and medialsuperior edge of axis were used for study.The distance from the middle line to the medial border of atlas pedicle（L1），to the lateral border of C1 pedicle（D1），to the medial border of C2 interarticularis（L2） and to the lateral border of C2 interarticularis（D2） were measured respectively.When the screw was placed with zero introversion，the extreme medial and lateral entry point（termed as A and B） were marked on the posterior arch of atlas，when ignoring the screw diameter of 1.75mm，the horizontal distance between the medial border of C2 interarticularis to point A and B were （L1－L2＋1.75mm） and （D1－L2－1.75mm） respectively.The best entry point M was defined as the maximum introversion angle equal to the maximum extroversion angle.The horizontal distance from the medial border of C2 interarticularis to entry point M was be recorded.From March 2004 to March 2009，29 cases underwent posterior atlantoaxial pedicle screw fixation by this method.Of these，24 cases suffered from old odontoid fracture，and 5 cases had transverse ligment injury.Intraoperative and postoperative complications associated with instrumentation were evaluated.Screw position was also measured under three-dimensional CT scan after operation.Result：When screws were inserted as 0° introversion，the horizontal distance between the medial border of C2 interarticularis to point A and B was 4.22±0.54mm and 8.66±0.73mm respectively.While the distance between the ideal point M and medial border of C2 interarticularis was 5.79±0.63mm.A total of 58 atlas pedicle screws placement were based on the landmark of medial border of C2 interarticularis.There were no neurovascular complications noted.Postoperative three-dimensional CT scan after operation showed no infiltration of screw into spine canal and transverse foramen.The distance between the entry point to the medial border of C2 interarticularis was 5.45±0.82mm，which showed no significant difference with point M.Conclusion：There is a safe zone in C1 posterior arch，and the medialsuperior border of C2 interarticularis can be used as anatomic landmark for screw anchoring.