Anatomical Study And Clinical Application Of Atlantoaxial Pedicle Screw Fixation

Objective: Provide accurate anatomic foundation for a new convenient and exactapproaches to determine the entry point and screw path in posterior atlantoaxial pediclescrew fixation via anatomic research and measurement on adult dry samples of atlas andaxis.Method: A total of20paired human adult atlas and axis specimens weremeasured bilaterally to decided the pedicle screw entry point and the direction of the screwtrajectory. On the bases of anatomic research,56patients with C1-C2fracture dislocationor atlantoaxial instability were treated with trans-posterior arch lateral mass screw fixationof the atlas, combined with axis pedicle screw fixation. An autologous piece of iliacspongy bone was inserted between the decorticated C1-C2inter-laminar space. CT scans ofsuch parameters as the width and height of pedicle, the longest trajectory distance, theangle of screw direction relative to coronal and transverse plane were to survey beforeoperation aiming at the reference of screw fixation. Postoperative CT scanning wereperformed to assess if pedicle screw placement was successful. All of the methods above toevaluate the surgical feasibility and screw safety.Result: The screw entry piont and path had been decided as follows. The C1pointof screw penetration in present technique was defined as the method of making a verticalline through the midpoint of distance from the junction of C1posterior arch medial andlateral border to lateral mass, and the entry point was2mm superior to the inferior borderof posterior arch. The screw trajectory was2°~5°to the coronal plane and3°~6°cephaladto the transverse plane.The C2pedicle screw was inserted at the entry piont determinedusing the following method: making a vertical line through the midpoint of distance fromthe junction of C2pedicle medial and lateral border to lateral mass, making a horizontalline through the junction between the lateral border of inferior articular process and theposterior branch of transverse process. The entry point was1~2mm lateral to the crossing point of two lines. The screw trajectory was25°~30°to the coronal plane and20°~25°cephalad to the transverse plane.224screws were inserted in all patients. The incidence ofscrew malposition was4.46%(5/112) and5.35%(6/112) in atlas and axis respectively.There were5screws violation into vertebroarterial foramen partly (3in C1and2in C2),1screw was perforated into the internal wall of C1lateral mass and1screw into the internalwall of C2pedicle.4screws excess over the length of pedicle. No patients appearingclinical sign related to vertebral artery injury.Conclusion: According to the anatomic research and initial clinic application,C1-C2transpedicular screw fixation was confirmed to be effectively and safely performedusing the entry point and screw angle of the present study.