Fig. 2

Measurement and application of the needle angle. a An ultrasound image from the paramedian sagittal oblique (PSO) view, which shows the L2-L3 interlaminar space, posterior complex (PC), and anterior complex (AC). ∠α is the measured maximum cephalad angle of the needle. A to B is the distance from the skin to the posterior complex. b Measuring the medial angle with a 180° protractor during the paramedian approach, which is the tilting angle of the probe to the median plane. c An aseptic 180° protractor was used to assist the cephalad needle insertion angle. d After a successful puncture, the actual medial angle was measured with an aseptic 180° protractor