Strategies of posterior screw fixation on C1 pedicle with the variation of posterior arch of altas

English Keywords:Atlas  Pedicle screw  Image anatomy

English Abstract:

  【Abstract】 Objectives: To investigate the strategy and safety of posterior screw fixaiton on C1 pedical with variation. Methods: 28 cases from January 2005 to January 2011 suffering from upper cervical spine instability and with C1 pedicle variation treated surgically in our hospital were reviewed retrospectively. There were 11 males and 17 females, with the ages ranging from 6 to 75 years old（average, 36.2±15.5 years）. Three cross sections(surface of posterior arch, vertebral artery groove and junction of pedicle and lateral mass) and their two sides(lateral and medial side) were divided on CT reconstruction image, and the height of posterior arch was measured thereby. Three types of the atlas with variation could be defined accordingly. Depended on these types, different screw placements were performed as follows: type Ⅰ(height of C1 posterior arch less than 3.5mm and vertebral artery groove more than 3.5mm) with the screw placed under posterior arch; type Ⅱ(both of them less than 3.5mm) with the fixation at the boundary between pedicle and lateral mass; type Ⅲ(height of C1 posterior arch more than 3.5mm and vertebral artery groove less than 3.5mm) with in-out-in screw placement technique. The accuracy of screw placement and associated complications were evaluated. Results: 56 atlas pedicles were placed which included 34 type Ⅰ, 18 type Ⅱ and 4 type Ⅲ. A total of 54 screws was successfully placed into the atlas(96.4%), while the other 2 screws failed and changed into laminar hook due to small pedicle diameter from type II variation. After operation, CT scan showed ideal or acceptable(96.3%) screw position in 52 and unacceptable(3.7%)in 2 with screws penetrating into spinal canal. There were no obvious complications in the present series, while two patients complained of greater occipital nerve irritation which was alleviated after appropriate treatment. Conclusions: Pedicle screw can be inserted safely in the C1 with posterior arch variation. Different screw placement strategies can be employed depending on the C1 variation types.