The Superior Articular Process as an External Landmark for Determining the Transverse Plane Angulation of Thoracic Pedicles

Study design: Computed tomographic (CT) study of thoracic spine pedicles.

Objective: To analyze the usefulness of the superior articular process (SAP) as an external landmark for determining the transverse plane angulation of thoracic pedicles.

Summary of background data: The use of thoracic pedicle screws has become commonplace. Although most authors report them to be safe, their use poses a risk to neurovascular structures. Previous studies have provided useful information regarding thoracic pedicle anatomy, but this information is difficult to apply intra-operatively. To avoid neurovascular injury, it is important to determine the correct transverse plane angulation of screw insertion.

Methods: Two separate investigators reviewed thoracic spine CT scans of 53 patients, 26 years of age or younger. Measurements were taken of the angular relationship of the pedicle and the SAP of T4-T11. A 90° angle was subtended from a line parallel to the SAP with a starting point at the midpoint of the lateral half of the SAP. Measurements were then adjusted laterally for medial breeches and medially for lateral breeches, to align the trajectory down the middle of the pedicle. The degree of correction was recorded. Each investigator made 3 sets of measurements. We calculated kappa values to assess intra-observer/interobserver agreement.

Results: Of the 4,008 measurements, 95.2% were contained within bone, leaving 4.8% pedicle violations. The average correction made for medial and lateral breeches was 6.3% and 6.7%, respectively. The first rater had 92.6% agreement, and an intra-observer kappa value of 0.57. The second rater had a 95.3% agreement and an intra-observer kappa value of 0.40.

Conclusions: The results support the hypothesis that the SAP can be a useful external landmark for determining the transverse plane angulation of thoracic pedicle screw insertion.