Figure 1.

Measurement standards and Moro zones. (left): Disc midpoint (O); disc sagittal diameter, DSD (OA); operating window, OW (BC); psoas major transverse diameter, PMTD (DG); psoas major sagittal diameter, PMSD (EF). (right): The disc was divided into Zones A, I, II, III, IV, and P s. The psoas major muscles were divided according to the positions of their anterior borders. The left psoas major with a teardrop shape was located in Moro Zone A. The right psoas major with a normal shape was located in Moro Zone I.