Review Question - QID 5448

Straightforward trajectory when placing pedicle screws in addition to prior tapping 1mm smaller than the screw diameter increase the maximal insertional torque and resistance to screw pullout.

Contemporary segmental pedicle screw placement used in the treatment of scoliosis deformity offer significantly higher screw pullout and deformity correction than prior hook and wire constructs. Additionally, screw insertional torque has been found in numerous studies to correlate with resistance to screw pullout. Several factors have been found to increase maximum screw insertional torque, including tapping 1mm smaller than the screw diameter and using the straightforward trajectory. It is important to note that while undertapping makes for a stronger screw, there are some studies that suggest not tapping at all makes for an even stronger screw.

Lehman et al. performed a biomechanical study evaluating maximum insertional torque when tapping line to line, undertapping by 0.5mm, and undertapping by 1mm in 34 fresh frozen cadavers. They found undertapping the thoracic pedicle by 1mm increased maximum insertional torque by 47% when compared to undertapping by 0.5mm and by 93% when compared to line to line tapping.

Kuklo et al. performed a biomechanical study on thirty cadavers using the straightforward technique (sagittal trajectory of the screws parallels the superior endplate of the vertebral body) versus anatomic trajectory (22 degrees in the cephalo-caudad direction in the sagittal plane). They found maximum insertional torque to be 2.58 pounds using the straightforward technique versus 1.86 pounds using the anatomic trajectory (p=0.0005).

Incorrect Answers:
Answer 1: These techniques do not allow for anatomic placement of the screws. An anatomic trajectory would also decrease the maximum insertional torque.
Answer 2: Depth insertion is not based upon tapping or using the straightforward technique.
Answer 4: Resistance to screw pullout is directly correlated with maximum insertional torque, and would be increased by undertapping 1mm and by using the straightforward technique.
Answer 5: Stability of the construct is increased by using the techniques described in the question as the maximum insertional torque and resistance to screw pullout would increase.