Introduction: Insertion of a pedicle screw that is larger in diameter than that of the native pedicle has been shown to expand the pedicle and also increase biomechanical fixation strength. With this technique, there is a potential concern for medial expansion of the pedicle causing decrease in spinal canal diameter, especially in the concavity of scoliosis, and resulting in spinal cord compression. Also, large pedicle screws that are inserted correctly may still cause undetected medial bony breach during surgery.
Methods: 162 pedicles from 81 thoracic vertebrae (T1-T12) of 7 fresh-frozen adult cadavers were analyzed. After undertapping the pedicle by 1 mm, pedicle screws were inserted in increasing diameter (range, 4.0 mm – 9.5 mm) bilaterally until there was an osseous breach in the pedicle. 938 screws were used in total. Coronal spinal canal diameter and pedicle circumference were measured (in mm) before and after each pedicle screw placement. Photographs and fluoroscopic images of representative specimens were taken for visual assessment.
Results: The average coronal spinal canal diameter was 17.7 mm: T1 20.3, T2 18.4, T3 17.0, T4 16.5, T5 15.9, T6 16.3, T7 16.6, T8 16.3, T9 16.7, T10 16.7, T11 19.7, T12 22.3. The average canal diameter with the largest screw inserted before bony breach was detected was 17.6 mm (p=0.92): T1 21.0, T2 18.0, T3 16.7, T4 16.3, T5 15.4, T6 15.9, T7 16.6, T8 16.7, T9 16.8, T10 16.6, T11 19.2, T12 22.3. The average diameter of the largest screw inserted before breach was 6.9 mm: T1 7.7, T2 6.5, T3 6.5, T4 5.8, T5 6.6, T6 6.5, T7 6.5, T8 5.7, T9 7.3, T10 7.7, T11 7.5, T12 7.9. Pedicle circumference increased from 41.8 mm before screw placement to 43.4 mm at maximal expansion before bony breach with the next sized screw. 28 pedicles did not break with 9.5 mm-diameter screws. There were 133 lateral and 1 medial breaches.
Conclusions: Increasing pedicle screw size caused pedicle expansion laterally but did not significantly alter spinal canal dimensions. When there was an osseous breach, most were lateral (99.3%).
Patient Care: This study will improve patient care through improvement of spinal surgeon's understanding of the pattern of thoracic pedicle expansion leading to the use of larger diameter thoracic pedicle screws achieving increased biomechanical fixation strength of fusion constructs.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) identify concerns with placement of large diameter thoracic pedicle screws, 2) recognize patterns of thoracic pedicle expansion, 3) realize potential of lateral thoracic pedicle expansion