How is pedicle screw instrumentation accuracy measured?
Accuracy rates of pedicle screw placement are commonly reported based on whether pedicle screws are located entirely within the pedicle. The amount of pedicle breach is commonly measured in millimeters and graded from A to E according to the Gertzbein-Robbins classification: Grade A (0 mm breach), B (<2 mm), C (<4 mm), D (<6 mm), E (≥6 mm). The clinical significance of a pedicle breach depends on several factors, including breach direction, breach magnitude, spinal level, presence of a positive electromyography (EMG) response or neurologic deficit or other clinical sequalae. Medial breach is a concern due to the proximity of neural structures, while anterior breach of the vertebral body cortex endangers vascular and visceral structures. However, lateral breach in the thoracic spinal region may be contained within the posterior rib, and thoracic screw trajectories that involve intentional lateral pedicle penetration (the “in-out-in” technique) have been recommended to optimize thoracic fixation. It has been reported that less than 5% of misplaced pedicle screws are associated with symptoms. Medial pedicle breach >2 mm is considered significant in various literature reports. However, it is challenging to compare breach rates between studies due to variable verification methods and different definitions of breaches.