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

Table 1 Scoring system for assessing difficulty of laparoscopic radical sphincter-preserving surgery for rectal cancer

Criteria	Score
Duration of postoperative hospital stay > 15 d	2
Surgical duration > 250 min	3
Intraoperative blood loss > 100 mL	1
Conversion to open surgery	3
Intraoperative use of trans-anal approach	2
Postoperative complications classified as CD grade II or above	1

The total score for the difficulty assessment is 12. A score ≥ 5 indicates a high-difficulty surgery, while a score < 5 indicates a low-difficulty surgery. CD: Clavien-Dindo.

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.

Table 3 Soft tissue measurement parameters and definitions

Parameter name	Definition
Rectal area (a)	Area of the rectum at the level of the ischial spine
Mesorectal fat area (b)	Area of the mesorectal fat at the level of the ischial spine
Visceral fat area (c)	Area of visceral fat at the level of the umbilicus
Subcutaneous fat area (d)	Area of subcutaneous fat at the level of the umbilicus
Waist circumference (e)	Circumference of the waist at the level of the umbilicus
Anterior-posterior abdominal diameter (RS)	Measurement distance from the umbilicus to the spine-wall intersection in the abdominal plane
Transverse abdominal diameter (TU)	Measurement distance along the horizontal line of the lumbar vertebrae anterior margin at the level of the umblicus

Table 4 Clinical and pathological characteristics data of 162 patients undergoing laparoscopic radical sphincter-preserving surgery for rectal cancer

	Range	mean ± SD	No. of cases [n (%)]
Age (yr)	(32.00, 89.00)	65 (59, 72)
Gender
Male			105 (64.81)
Female			57 (35.19)
BMI (kg/m2)	(15.94, 31.25)	22.75 ± 3.25
Underlying comorbidities			72 (44.44)
Diabetes			26 (16.05)
Hypertension			58 (35.8)
Heart disease			8 (4.94)
Pulmonary disease			3 (1.85)
Stroke			5 (3.09)
Other			1 (0.62)
ASA grade
I			18 (11.11)
II			137 (84.57)
III			7 (4.32)
History of previous abdominal surgery			15 (9.26)
Preoperative total protein (g/L)	(53.8, 81)	65.45 (62.4, 68.83)
Preoperative albumin (g/L)	(29, 48.5)	38.64 ± 3.32
Preoperative albumin to globulin ratio	(0.9, 2.1)	1.4 (1.3, 1.6)
Preoperative hemoglobin (g/L)	(89, 166)	134.5 (119, 143.75)
PNI	(35, 58)	47.23 ± 5.02
Preoperative CEA (ug/L)	(0.4, 49.9)	2.9 (1.9, 5.18)
Preoperative CA199 (kU/L)	(0.6, 128.1)	8.55 (5.18, 17.98)
Tumor height (cm)	(2, 15)	8 (6.25, 10)
Tumor Location
Upper rectum (10 cm < distance ≤ 15 cm)			30 (18.52)
Midrectum (5 cm < distance ≤ 10 cm)			103 (63.58)
Lower rectum (distance ≤ 5 cm)			29 (17.9)
Maximum tumor diameter (cm)	(0.8, 10)	3.8 (3, 4.88)
Surgical approach
L-AR			20 (12.35)
L-LAR			135 (83.33)
L-ISR			7 (4.32)
Intraoperative preventive ostomy			39
L-AR + preventive ostomy			0
L-LAR + preventive ostomy			32 (82.05)
L-ISR + preventive ostomy			7 (17.95)
pT staging
Tis			4 (2.47)
T1			19 (11.73)
T2			26 (16.05)
T3			109 (67.28)
T4a			4 (2.47)
pN staging
N0			94 (58.02)
N1			36 (22.22)
N2			32 (19.75)
pTNM staging
0			4 (2.47)
I			38 (23.46)
II			52 (32.1)
III			68 (41.98)
Tumor differentiation grade
Well-differentiated			9 (5.56)
Moderately-differentiated to well-differentiated			20 (12.35)
Moderately differentiated			112 (69.14)
Moderately differentiated to poorly differentiated			17 (10.49)
Poorly-differentiated			4 (2.47)
Tumor nodules			16 (9.88)
Vascular tumor emboli			47 (29.01)
Neural invasion			40 (24.69)
Number of lymph nodes removed	(3, 36)	15 (13, 18)
Surgical duration (min)	(78, 465)	216.5 (179.25, 260)
Intraoperative blood loss (ml)	(10, 300)	50 (20, 100)
Postoperative complications (CD grade)
No complications			125 (77.16)
Grade I			7 (4.32)
Grade II			21 (12.96)
Grade III			8(4.94)
Grade IV			1(0.62)
Postoperative anastomotic fistula grading
No anastomotic fistula			152 (93.83)
Grade A			0
Grade B			6 (3.7)
Grade C			4 (2.47)
Duration of postoperative hospital stay (d)	(6, 56)	14 (12, 16)
Intraoperative use of transanal approach (n)			7 (4.32)
Conversion to open surgery			1 (0.62)
Postoperative anal evacuation time (d)	(1, 7)	2 (2, 3)
Postoperative resumption of semi-liquid food time (d)	(3, 25)	8 (5, 9)
Anastomosis method
Manual anastomosis (Hand-sewn anastomosis)			7 (4.32)
Stapler anastomosis			155 (95.68)
Number of staple nail cartridges used in surgery (n)
L-AR	(1, 2)	2 (1, 2)
L-LAR	(1, 4)	2 (1, 2)
L-ISR	0	0
Length of stapler cartridge used in surgery (mm)
L-AR	(45, 120)	90 (56.25, 90)
L-LAR	(45, 180)	90 (60, 90)
L-ISR	0	0

If the data follows a normal distribution: mean ± SD will be used to represent numerical data. Examples (%) will be used to represent count and ordinal data. If the data does not follow a normal distribution: Median (Q1, Q3) will be used to represent numerical data. Examples (%) will be used to represent count and ordinal data. BMI: Body mass index; ASA: American Society of Anesthesiologists; CD: Clavien-Dindo; PNI: Prognostic nutritional index; L-AR: Laparoscopic anterior resection; L-LAR: Laparoscopic low or ultralow anterior resection; L-ISR: Laparoscopic intersphincteric resection.

Table 5 Pelvic bone parameters and soft tissue parameters data of 162 patients undergoing laparoscopic radical sphincter-preserving surgery for rectal cancer

Measurement	mean ± SD or median (Q1, Q3) (range)
Anterior-posterior diameter of pelvic inlet (mm)	115.23 ± 11.79
Transverse diameter of pelvic inlet (mm)	123.52 ± 8.14
Anterior-posterior diameter of mid-pelvis (mm)	111.83 (106.36, 117.74)
Anterior-posterior diameter of pelvic outlet (mm)	88.97 (82.35, 96.86)
Interischial spine diameter (mm)	96.06 (87.3, 104.27)
Interischial tuberosity diameter (mm)	106.63 ± 14.15
Superior-inferior diameter of pubic symphysis (mm)	42.25 (39.31, 45.42)
Sacrococcygeal distance (mm)	123.19 ± 11.95
Superior-inferior diameter of sacrum (mm)	105.77 ± 10.17
Sacrococcygeal curvature height (mm)	39.61 (36.15, 42.71)
Sacral curvature height (mm)	20.82 ± 5.87
Superior pubococcygeal diameter (mm)	120.26 ± 9.56
Anterior-posterior sacropubic distance (mm)	119.84 (113.14, 126.49)
Sacrococcygeal-pubic angle (°)	48.05 ± 7.01
Sacropubic angle (°)	33.04 ± 5.43
Anterior-posterior diameter of pelvic inlet/sacrococcygeal distance	0.94 (0.86, 1)
Rectal area (cm2)	16.67 (12.23, 21.48)
Mesorectal fat area (cm2)	39.26 (27.41, 51.62)
Visceral fat area (cm2)	215.9 (126.21, 274.02)
Subcutaneous fat area (cm2)	258.24 (198.25, 340.77)
Waist circumference (cm)	82.37 ± 9.15
Anterior-posterior abdominal diameter (cm)	18.42 ± 2.91
Transverse abdominal diameter (cm)	28.84 ± 2.79

If the data follows a normal distribution, it is represented as mean ± SD. If the data does not follow a normal distribution, it is represented as median (Q1, Q3).

Table 6 Perioperative outcomes of 162 cases of laparoscopic radical sphincter-preserving surgery for rectal cancer grouped into two groups of high and low surgical difficulty, the postoperative pathological baseline characteristics and univariate analysis, n (%)

	Univariate analysis
	Whole group (n = 162)	Low surgical difficulty group (n = 141)	High surgical difficulty group (n = 21)	P value
pT staging				0.73901
Tis	4 (2.47)	4 (2.84)	0
T1	19 (11.73)	17 (12.06)	2 (9.52)
T2	26 (16.05)	20 (14.18)	6 (28.57)
T3	109 (67.28)	97 (68.79)	12 (57.14)
T4a	4 (2.47)	3 (2.13)	1 (4.76)
pN staging				0.43271
N0	94 (58.02)	83 (58.87)	11 (52.38)
N1	36 (22.22)	32 (22.7)	4 (19.05)
N2	32 (19.75)	26 (18.44)	6 (28.57)
pTNM staging				0.58951
0	4 (2.47)	4 (2.84)	0
Ⅰ	38 (23.46)	33 (23.4)	5 (23.81)
Ⅱ	52 (32.1)	46 (32.62)	6 (28.57)
III	68 (41.98)	58 (41.13)	10 (47.62)
Degree of tumor differentiation				0.07301
Well differentiated	9 (5.56)	9 (6.38)	0
High-moderate differentiation	20 (12.35)	17 (12.06)	3 (14.29)
Moderate differentiation	112 (69.14)	100 (70.92)	12 (57.14)
Moderate-Low differentiation	17 (10.49)	13 (9.22)	4 (19.05)
Low differentiation	4 (2.47)	2 (1.42)	2 (9.52)
Tumor nodules	16 (9.88)	16 (11.35)	0	0.22952
Vascular tumor emboli	47 (29.01)	40 (28.37)	7 (33.33)	0.83372
Neural invasion	40 (24.69)	36 (25.53)	4 (19.05)	0.71012
Number of Lymph Nodes Removed	15 (13, 18)	15 (13, 18)	15 (12, 17)	0.79291
Surgical duration (min)	216.5 (179.25, 260)	205 (176, 238)	324.19 ± 59.62	3.423e-101
Intraoperative Blood Loss (ml)	50 (20, 100)	50 (20, 50)	100 (50, 200)	4.436e-061
Postoperative complications (CD grade)				0.01851
No complications	125 (77.16)	113 (80.14)	12 (57.14)
Grade I	7 (4.32)	5 (3.55)	2 (9.52)
Grade II	21 (12.96)	17 (12.06)	4 (19.05)
Grade III	8 (4.94)	5 (3.55)	3 (14.29)
Grade IV	1 (0.62)	1 (0.71)	0
Postoperative anastomotic fistula grading				0.09071
No anastomotic fistula	152 (93.83)	134 (95.04)	18 (85.71)
Grade B	6 (3.7)	5 (3.55)	1 (4.76)
Grade C	4 (2.47)	2 (1.42)	2 (9.52)
Duration of postoperative hospital stay (d)	14 (12, 16)	13 (12, 15)	23 (22, 27)	6.01e-111
Intraoperative use of trans-anal approach	7 (4.32)	2 (1.42)	5 (23.81)	0.00041
Conversion to open surgery	1 (0.62)	0	1 (4.76)	0.12962
Postoperative anal evacuation time(d)	2 (2, 3)	2 (2, 3)	2 (2, 2)	0.02841
Postoperative resumption of semi-liquid food time (d)	8 (5, 9)	8 (6, 9)	6 (4, 10)	0.17981

¹The non-parametric Mann Whitney U rank sum test was used for unidirectional ordered information on counts.

²The χ² test, continuity-corrected χ² test or Fisher’s exact probability method was used for bidirectional unordered information. Obedience to normal distribution.

If the normal distribution is satisfied, it is expressed as mean ± SD; if not, it is expressed as median (Q1, Q3). Count and rank information is expressed as examples (%). P values denoted by “e” are all less than 0.001.

Table 7 Univariate analysis of risk factors associated with surgical difficulty in 162 cases of laparoscopic radical sphincter-preserving surgery for rectal cancer

Factors	Entire group	Low difficulty group	High difficulty group	P value
Age (yr)	65 (59, 72)	65 (58, 73)	65 (62, 71)	0.992
Gender				0.15712
Male	105 (64.81)	88 (62.41)	17 (80.95)
Female	57 (35.19)	53 (37.59)	4 (19.05)
BMI (kg/m²)	22.75 ± 3.25	22.7 ± 3.2	23.04 ± 3.64	0.6965
Underlying comorbidities	72 (44.44)	65 (46.1)	7 (33.33)	0.38812
ASA grade				0.83321
I	18 (11.11)	15 (10.64)	3 (14.29)
II	137 (84.57)	121 (85.82)	16 (76.19)
III	7 (4.32)	5 (3.55)	2 (9.52)
History of previous abdominal surgery	15 (9.26)	15 (10.64)	0	0.22172
Preoperative total Protein (g/L)	65.45 (62.4, 68.83)	65.1 (62.4, 68.3)	66.4 (62.5, 70.2)	0.32101
Preoperative albumin (g/L)	38.64 ± 3.32	38.54 ± 3.25	39.3 ± 3.79	0.3892
Preoperative albumin to globulin ratio	1.4 (1.3, 1.6)	1.4 (1.3, 1.6)	1.4 (1.3, 1.6)	0.70931
Preoperative Hemoglobin (g/L)	134.5 (119, 143.75)	134 (119, 143)	141 (126, 151)	0.09621
PNI	47.23 ± 5.02	47.02 ± 4.81	48.63 ± 6.2	0.2637
Preoperative CEA (ug/L)	2.9 (1.9, 5.18)	2.9 (1.8, 5.2)	2.9 (1.9, 4.6)	0.22571
Preoperative CA199 (kU/L)	8.55 (5.18, 17.98)	8.3 (5, 18)	10.8 (6.9, 17.9)	0.56631
Tumor height (cm)	8 (6.25, 10)	8 (7, 10)	7 (5, 8)	0.00241
Tumor maximum diameter (cm)	3.8 (3, 4.88)	3.8 (3, 4.5)	3 (2.8, 5)	0.55071
Surgical approach				6.775e-41
L-AR	20 (12.35)	19 (13.48)	1 (4.76)
L-LAR	135 (83.33)	120 (85.11)	15 (71.43)
L-ISR	7 (4.32)	2 (1.42)	5 (23.81)
Intraoperative preventive ostomy	39 (20.07)	26 (18.44)	13 (61.90)	4.644e-052
Intraoperative stapler cartridge count	2 (1, 2)	2 (1, 2)	1 (1, 2)	0.09531
Intraoperative stapler cartridge length (mm)	90 (60, 90)	90 (60, 90)	60 (45, 90)	0.09691
Anterior-posterior diameter of pelvic inlet (mm)	115.23 ± 11.79	115.53 ± 11.85	113.23 ± 11.49	0.4025
Transverse diameter of pelvic inlet (mm)	123.52 ± 8.14	123.98 ± 7.79	120.42 ± 9.83	0.126
Anterior-posterior diameter of mid-pelvis (mm)	111.83 (106.36, 117.74)	112.16 (106.74, 118.12)	110.69 (103.69, 115.17)	0.42211
Anterior-posterior diameter of pelvic outlet (mm)	88.97 (82.35, 96.86)	89.18 (82.52, 96.92)	86.53 (81.69, 94.37)	0.86681
Interischial spine diameter (mm)	96.06 (87.3, 104.27)	96.35 (88.76, 104.58)	90.34 (85.01, 100.56)	0.06211
Interischial tuberosity diameter (mm)	104.3 (96.08, 116.95)	104.09 (96.6, 118.44)	107.77 (95.7, 112.01)	0.61451
Superior-inferior diameter of the pubic symphysis (mm)	42.21 ± 4.75	42.14 ± 4.88	42.71 ± 3.77	0.5341
Sacrococcygeal distance (mm)	123.19 ± 11.95	122.49 ± 12.04	127.91 ± 10.42	0.0378
Superior-inferior diameter of sacrum (mm)	105.77 ± 10.17	105.36 ± 10.18	108.5 ± 9.9	0.1881
Sacrococcygeal curvature height (mm)	39.61 (36.15, 42.71)	39.63 (36.14, 43.43)	38.97 (37.68, 40.96)	0.55961
Sacral curvature height (mm)	20.82 ± 5.87	20.83 ± 6.04	34.61 ± 4.11	0.9529
Superior pubococcygeal diameter (mm)	120.26 ± 9.56	120.01 ± 9.28	121.98 ± 11.35	0.4555
Anterior-posterior sacropubic distance (mm)	119.84 (113.14, 126.49)	120.04 (113.25, 127.11)	119.01 (112.86, 121.95)	0.30551
Anterior-posterior diameter of pelvic inlet/sacrococcygeal distance	0.94 (0.86, 1)	0.94 (0.86, 1.02)	0.87 (0.82, 0.97)	0.01651
Sacrococcygeal-pubic angle (°)	48.05 ± 7.01	47.64 ± 7.21	34.61 ± 4.11	0.0116
Sacropubic angle (°)	33.22 (29.5, 36.66)	32.83 (28.98, 36.56)	34.89 (32.05, 36.79)	0.14131
Rectal area (cm2)	16.67 (12.23, 21.48)	16.64 (12.23, 22.57)	17.07 (10.44, 19.59)	0.47581
Mesorectal fat area (cm2)	39.26 (27.41, 51.62)	38.62 (28.18, 51.35)	46.32 (24.25, 54.02)	0.53641
Visceral fat area (cm2)	215.9 (126.21, 274.02)	213.65 (126.1, 260.2)	240.25 (137.54, 320.74)	0.58341
Subcutaneous fat area (cm2)	258.24 (198.25, 340.77)	264.32 (204.34, 341.38)	240.91 (139.23, 310.51)	0.30911
Waist circumference (cm)	82.37 ± 9.15	82.36 ± 8.92	82.44 ± 10.83	0.9741
Anterior-posterior abdominal diameter (cm)	18.44 (16.38, 20.4)	18.41 (16.48, 20.25)	19.11 (16.2, 20.96)	0.59881
Transverse abdominal diameter (cm)	28.84 ± 2.79	28.83 ± 2.69	28.92 ± 3.48	0.6991

¹The Mann-Whitney U test is employed for single-ordered data, while the chi-square test, continuity correction chi-square test.

²Fisher’s exact test are used for two-way unordered data.

In the case of normal distribution, the data is presented as mean ± SD. For non-normal distribution, median (Q1, Q3) is used. Count and ordinal data are represented as frequency (%). Paired t-test is used for parametric data with equal variances, and nonparametric Mann-Whitney U test is used for non-normally distributed data. P values denoted by “e” are all less than 0.001. BMI: Body mass index; ASA: American Society of Anesthesiologists; CD: Clavien-Dindo; PNI: Prognostic nutritional index; L-AR: Laparoscopic anterior resection; L-LAR: Laparoscopic low or ultralow anterior resection; L-ISR: Laparoscopic intersphincteric resection.

Table 8 Multivariable logistic regression analysis of surgical difficulty in 162 cases of laparoscopic radical sphincter-preserving surgery for rectal cancer

Variable	β	SE	Wald	Odds ratio (95%CI)	P value
Intercept	1.945	6.353	0.094	6.99 (0.000-1787454.283)	0.760
Tumor Height	0.230	0.196	1.387	1.259 (0.858-1.848)	0.239
Surgical approach (Control group = L-ISR)
L-AR	-5.050	2.425	4.336	0.006 (0.000-0.743)	0.037
L-LAR	-3.417	1.311	6.789	0.033 (0.003-0.429)	0.009
Intraoperative preventive ostomy	2.604	0.861	9.147	13.517 (2.500-73.069)	0.002
Sacrococcygeal distance	0.086	0.04	4.567	1.09 (1.007-1.180)	0.033
Sacrococcygeal-pubic angle	-0.115	0.082	1.959	0.891 (0.759-1.047)	0.162
Anterior-posterior diameter of pelvic inlet/sacrococcygeal distance	-9.328	4.151	5.050	0.00009 (0.000-0.304)	0.025

L-AR: Laparoscopic anterior resection; L-LAR: Laparoscopic low or ultralow anterior resection; L-ISR: Laparoscopic intersphincteric resection.