Correlation of subchondral edema to degree of cartilage loss in the hip

Statistical analysis

Categorical variables are summarized by counts and percentages, and continuous variables are summarized by means and standard deviations. The association between categorical variables is tested using Fisher’s exact test, and the association between categorical and continuous variables is tested using the Kruskal-Wallis rank sum test. The receiver operating characteristic (ROC) curve is used to evaluate the predictive ability of cartilage loss degree for subchondral edema/cystic changes and arthroscopic grade 2-4. P-values < 0.05 are considered statistically significant. All analyses were performed using R version 4.1.1.

Patient characteristics

Patient-level characteristics are summarized by sex in Table 1. Cartilage loss, edema/cystic changes, and arthroscopic grade 1-4 and 2-4 are binary variables indicating the presence of the respective findings in any of the quadrants of the hip. The number of days from MRI to surgery is calculated as the difference between the date of surgery and the date of MRI.

Table 1: Patient-level characteristics by sex.

Characteristic	F, N = 143¹	M, N = 81¹	Overall, N = 224¹
Age (years)	38 (24, 48)	37 (25, 42)	38 (25, 45)
Laterality
Left	67 (47%)	41 (51%)	108 (48%)
Right	76 (53%)	40 (49%)	116 (52%)
Cartilage loss	62 (44%)	49 (60%)	111 (50%)
Unknown	1	0	1
Edema/cystic changes	16 (11%)	22 (27%)	38 (17%)
Arthroscopic grade 1-4	77 (54%)	69 (85%)	146 (65%)
Arthroscopic grade 2-4	60 (42%)	64 (79%)	124 (55%)
Days from MRI to surgery	144 (78, 228)	167 (92, 274)	147 (88, 249)
¹ Median (IQR); n (%)

Subchondral edema/cystic vs cartilage loss

Cross-tabulation of subchondral edema/cystic changes and cartilage loss is shown in Table 2. There is a strong and significant association between the two variables.

Table 2: Counts of hip quadrants by cartilage loss and edema/cystic changes.

Characteristic	None, N = 1,750¹	Edema, N = 14¹	Cystic, N = 22¹	Mixed, N = 6¹	p-value²
Cartilage loss					<0.001
Normal	1,619 (93%)	3 (21%)	0 (0%)	0 (0%)
Superficial	84 (4.8%)	4 (29%)	2 (9.1%)	2 (33%)
Deep	36 (2.1%)	5 (36%)	11 (50%)	2 (33%)
Full	8 (0.5%)	2 (14%)	9 (41%)	2 (33%)
¹ n (%)
² Fisher’s exact test

The receiver operating characteristic (ROC) curve of edema/cystic/mixed as predicted by cartilage loss degree (normal < superficial < deep < full) is shown in Figure 1.

Figure 1: ROC curve of edema/cystic/mixed against cartilage loss degree.

In particular, the cutoff normal vs superficial/deep/full leads to sensitivity 0.929 (0.794, 0.981) and specificity 0.925 (0.912, 0.937).

Size of edema/cystic change vs cartilage loss degree

There are 42 quadrants with edema/cystic changes, most of which have non-normal cartilage loss (Table 2). As shown in Table 3 and Figure 2, the size of edema/cystic changes does not seem significantly associated with the degree of cartilage loss.

Table 3: Summary of edema/cystic change size by cartilage loss degree.

Characteristic	Normal, N = 3¹	Superficial, N = 8¹	Deep, N = 18¹	Full, N = 13¹	p-value²
Depth (mm)	8.00 (8.00, 10.00)	5.00 (5.00, 7.00)	4.50 (4.00, 6.00)	7.00 (5.00, 9.00)	0.015
Area (mm^2)	144 (133, 144)	84 (19, 233)	41 (21, 92)	108 (42, 120)	0.13
¹ Median (IQR)
² Kruskal-Wallis rank sum test

Figure 2: Boxplots of edema depth and area by cartilage loss degree.

Size of edema/cystic change vs articular cartilage 0-4

A similar analysis is performed for the association between the size of edema/cystic changes and arthroscopic grade 0-4. As shown in Table 4 and Figure 3, the size of edema/cystic changes does not seem significantly associated with the arthroscopic grade.

Table 4: Summary of edema/cystic change size by arthroscopic grade 0-4.

Characteristic	0, N = 4¹	Wave sign, N = 3¹	1, N = 1¹	2, N = 9¹	3, N = 8¹	4, N = 17¹	p-value²
Depth (mm)	5.50 (3.50, 7.25)	8.00 (6.50, 10.00)	3.00 (3.00, 3.00)	5.00 (4.00, 7.00)	6.00 (5.00, 7.00)	6.00 (5.00, 9.00)	0.5
Area (mm^2)	66 (11, 126)	121 (71, 205)	30 (30, 30)	63 (18, 195)	45 (37, 101)	77 (40, 121)	0.8
¹ Median (IQR)
² Kruskal-Wallis rank sum test

Figure 3: Boxplots of edema depth and area by arthroscopic grade 0-4.

Articular cartilage 0-4 vs cartilage loss degree

Cross-tabulation of arthroscopic grade 0-4 and cartilage loss is shown in Table 5. There is a strong and significant association between the two variables.

Table 5: Summary of arthroscopic grade 0-4 by cartilage loss degree.

Characteristic	0, N = 1,536¹	Wave sign, N = 62¹	1, N = 36¹	2, N = 72¹	3, N = 45¹	4, N = 41¹	p-value²
Cartilage loss							<0.001
Normal	1,480 (97%)	44 (71%)	27 (75%)	34 (47%)	24 (53%)	13 (32%)
Superficial	31 (2.0%)	15 (24%)	8 (22%)	27 (38%)	7 (16%)	4 (9.8%)
Deep	19 (1.2%)	3 (4.8%)	1 (2.8%)	9 (13%)	10 (22%)	12 (29%)
Full	3 (0.2%)	0 (0%)	0 (0%)	2 (2.8%)	4 (8.9%)	12 (29%)
¹ n (%)
² Fisher’s Exact Test for Count Data with simulated p-value (based on 2000 replicates)

Superficial partial = Grade 2
Deep partial = Grade 3
Full thickness = Grade 4

The receiver operating characteristic (ROC) curve of grade 2-4 as predicted by cartilage loss degree (normal < superficial < deep < full) is shown in Figure 4.

Figure 4: ROC curve of arthroscopic grade 2-4 against cartilage loss degree.

In particular, the cutoff normal vs superficial/deep/full leads to sensitivity 0.551 (0.47, 0.629) and specificity 0.949 (0.937, 0.959).

Subchondral edema/cystic vs cartilage loss at surgery (articular cartilage 0-4)

Table 6: Summary of subchondral edema/cystic changes by arthroscopic grade 0-4.

Characteristic	None, N = 1,750¹	Edema, N = 14¹	Cystic, N = 22¹	Mixed, N = 6¹	p-value²
Articular cartilage 0-4					<0.001
0-2, wave	1,689 (97%)	8 (57%)	8 (36%)	1 (17%)
3-4	61 (3.5%)	6 (43%)	14 (64%)	5 (83%)
¹ n (%)
² Fisher’s exact test

Table 7: Summary of subchondral edema/cystic changes by arthroscopic grade 0-4 (expanded).

Characteristic	None, N = 1,750¹	Edema, N = 14¹	Cystic, N = 22¹	Mixed, N = 6¹	p-value²
Articular cartilage 0-4					<0.001
0	1,532 (88%)	2 (14%)	2 (9.1%)	0 (0%)
Wave sign	59 (3.4%)	2 (14%)	1 (4.5%)	0 (0%)
1	35 (2.0%)	1 (7.1%)	0 (0%)	0 (0%)
2	63 (3.6%)	3 (21%)	5 (23%)	1 (17%)
3	37 (2.1%)	1 (7.1%)	6 (27%)	1 (17%)
4	24 (1.4%)	5 (36%)	8 (36%)	4 (67%)
¹ n (%)
² Fisher’s Exact Test for Count Data with simulated p-value (based on 1e+05 replicates)

Table 8: Summary of subchondral edema/cystic changes by arthroscopic grade 3-4.

Characteristic	None, N = 61¹	Edema, N = 6¹	Cystic, N = 14¹	Mixed, N = 5¹	p-value²
Articular cartilage 0-4					0.065
3	37 (61%)	1 (17%)	6 (43%)	1 (20%)
4	24 (39%)	5 (83%)	8 (57%)	4 (80%)
¹ n (%)
² Fisher’s exact test