Construction of a nomogram model to predict technical difficulty in performing laparoscopic sphincter-preserving radical resection for rectal cancer

Table 2 Pelvic measurement parameters and definitions

Parameter name	Definition
Anterior-posterior diameter of pelvic inlet (AB)	Distance between the midpoint of the superior pubic symphysis and the midpoint of the anterior margin of the sacral promontory
Transverse diameter of pelvic inlet (PQ)	Maximum distance between the left and right iliopectineal line
Anterior-posterior diameter of mid-pelvis (CD)	Distance between the midpoint of the inferior pubic symphysis and the midpoint of the sacrococcygeal junction
Anterior-posterior diameter of pelvic outlet (CE)	Distance between the midpoint of the inferior pubic symphysis and the tip of the coccyx
Interischial spine diameter (LM)	Shortest distance between the ischial spines
Interischial tuberosity diameter (NO)	Shortest distance between the ischial tuberosities
Superior-inferior diameter of the pubic symphysis (AC)	Distance between the superior and inferior borders of the pubic symphysis
Sacrococcygeal distance (BE)	Distance between the midpoint of the anterior margin of the sacral promontory and the tip of the coccyx
Superior-inferior diameter of sacrum (BD)	Distance between the midpoint of the anterior margin of the sacral promontory and the midpoint of the sacrococcygeal junction
Sacrococcygeal-pubic angle (α)	The angle between the extended lines of the anterior-posterior diameter of pelvic inlet and the anterior-posterior diameter of pelvic outlet
Sacropubic angle (β)	Angle between the extended lines of the anteroposterior diameter of pelvic inlet and the anterior-posterior diameter of mid-pelvis
Sacrococcygeal curvature height (FI)	Vertical distance from the most convex point of the sacrococcygeal curve to the sacrococcygeal distance
Sacral curvature height (FH)	Vertical distance from the most convex point of the sacral curve to the superior-inferior diameter of sacrum
Superior pubococcygeal diameter (AE)	Distance between the midpoint of the superior pubic symphysis and the tip of the coccyx
Anterior-posterior sacropubic distance (FG)	Vertical distance from the most convex point of the sacrococcygeal curve to the superior-inferior diameter of pubic symphysis or its extended line

Among the 15 pelvic diameter and angle parameters, there are 5 parameters reflecting anterior-posterior diameter of pelvic width: anterior-posterior diameter of pelvic inlet, anterior-posterior diameter of mid-pelvis, anterior-posterior diameter of pelvic outlet, superior pubococcygeal diameter, and anterior-posterior sacropubic distance. There are 3 parameters reflecting left and right diameter of pelvic width: Transverse diameter of pelvic inlet, interischial spine diameter, and interischial tuberosity diameter. There are 3 parameters reflecting pelvic depth: superior-inferior diameter of the pubic symphysis, sacrococcygeal distance, and superior-inferior diameter of sacrum. There are 2 parameters reflecting the curvature of the sacrococcygeal bone: sacrococcygeal curvature height and sacral curvature height. There are 2 parameters comprehensively assessing sacrococcygeal (sacral) arc length and bending degree, the superior-inferior diameter of the pubic symphysis, and the distance between the pubic symphysis and sacrococcygeal bone: sacrococcygeal-pubic angle and sacropubic angle.