The optimal introversion angle and length of pedicle screw to avoid L1-S1 vascular damage.

Background: posterior pedicle screw fixation is common method, one of the most severe complications is iatrogenic vascular damage, no report investigated association of different introversion angles (INTAs) and length of pedicle screw. The aims were to investigate the optimal introversion angle and length of pedicle screw for improving the safety of the operation, and to analyze the differences of vascular damage types at L1-S1.

Methods: Lumbar CT imaging data from110 patients were analyzed by DICOM software, and all parameters were measured by new Cartesian coordinate system, INTAs (L1-L5:5°,10°,15°,S1: 0°, 5°,10°,15°), D (the distance between the origin (O) with anterior vertebral cortex (AVC)), D (the distance between AVC and the prevertebral great vessels (PGVs)), D (the distance between the O and PGVs). At different INTAs, D were divided into four grades: Grade III: D ≤ 3 mm, Grade II: 3 mm < D ≤ 5 mm, Grade I: D > 5 mm, and N: the not touching PGVs.

Results: The optimal INTA was 5° at L1-L3, the left was 5° and the right was 15° at L4, and screw length was less than 50 mm at L1-L4. At L5, the left optimal INTA was 5° and the right was 10°, and screw length was less than 45 mm. The optimal INTA was 15° at S1, and screw length was less than 50 mm. However, screw length was less than 40 mm when the INTA was 0° or 5° at S1.

Conclusions: At L5-S1, the risk of vascular injury is the highest. INTA and length of the pedicle screw in lumbar operation are closely related. 3 mm interval of screw length may be more preferable to reduce vascular damage.