Table 1 Needle entry point determination based on patient stratification using anatomical features offered by ultrasound and illustrated by CT scans

I

Yes

Yes

Skinny-base SP with wide IS;

partially overlapping adjacent laminae

0.5 cm

1 cm

II

Yes

No

Slanted wide-base SP covering part of IS;

partially overlapping adjacent laminae; lateral part of IS open

1 cm

1 cm:

depth of lamina<4 cm;

1.5 cm:

depth of lamina ≥ 4 cm

III

No

No

Steeply slanted wide-base SP obstructing IS;

IS obstructed by closely overlapping lamina

1 cm

1.5 cm