Subchondral stress fractures of the femoral head associated with military training: Clinical features and analysis

Table 1 Staging and imaging features of subchondral stress fracture of the femoral head

	Plain radiographs	Magnetic resonance images
Stage 0	No definite findings	Bony edema limited to the femoral head area
Stage I	Mild depression of articular margin (Crescent sign)	Low signal intensity lesion on T1 image and joint effusion
Stage II	Severe depression of articular margin	Subchondral fracture and a large joint effusion with edema extending to below the trochanter
Stage III	Arthritic changes (bone spurs or cysts)	Osteoarthritis with receding bone marrow edema

Table 2 Comparison of clinical and imaging characteristics across subchondral stress fracture of the femoral head stages at the patient level (one index hip per patient; n = 127), mean ± SD/n (%)

Parameter	Stage I (n = 13 patients)	Stage II (n = 52 patients)	Stage III (n = 62 patients)	P value
Age (year)	21.15 ± 2.76	19.96 ± 1.96	20.56 ± 1.92	0.105
Gender (male/female)	8/5	51/1	61/1	0.058
BMI (kg/m2)	21.13 ± 1.55	21.34 ± 1.94	21.78 ± 2.72	0.455
Time interval (month)	5.74 ± 3.19	7.71 ± 4.78	10.13 ± 6.41	0.005a
Bilateral involvement	5/13 (38.5)	5/52 (9.6)	2/62 (3.2)	0.001a
VAS score	5.84 ± 1.42	7.63 ± 1.42	6.57 ± 1.49	0.001a
Harris score	73.37 ± 7.21	64.15 ± 8.03	70.34 ± 8.26	0.001a
Hip flexion-extension ROM (º)	137.84 ± 9.76	117.78 ± 18.78	130.13 ± 15.98	0.001a
Hip inner-external rotation ROM (º)	68.47 ± 16.69	38.34 ± 21.65	56.43 ± 22.82	0.001a
BME
Grade 0	0	0	0
Grade 1	9	6	26
Grade2	2	17	14
Grade 3	2	29	22	0.001a
Effusion
Grade 0	9	0	0
Grade 1	3	2	30
Grade 2	1	14	26
Grade 3	0	36	6	0.001a
Tönnis Classification
Grade 0	11	0	0
Grade 1	2	42	20
Grade 2	0	10	41
Grade 3	0	0	1	0.001a
Laboratory findings
ALP (U/L)	82.38	77.31	71.84	0.225
CK (U/L)	122.23	107.77	115.39	0.608
TC (mmol/L)	3.9877	3.9071	3.7860	0.528
UA (μmol/L)	424.92	411.81	390.23	0.244

^aP < 0.05.

One index hip per patient (the more severely affected side in bilateral cases) was used for cross-stage comparisons, yielding n = 13, 52, and 62 patients in Stages I, II, and III, respectively. Bilateral involvement is reported at the patient level. ALP: Alkaline phosphatase; BMI: Body mass index; CK: Creatine kinase; ROM: Range of motion; TC: Total cholesterol; UA: Uric acid; VAS: Visual Analog Scale.

Table 3 Clinical and laboratory characteristics of subchondral stress fracture of the femoral head and osteonecrosis of the femoral head

Parameter	SSFFH (n = 127)	ONFH (n = 47)	P value
Age (years)	20.0 (19.0-21.0)	23.0 (20.5-29.0)	< 0.001a
BMI (kg/m2)	21.28 (20.07-22.53)	23.17 (21.20-25.79)	< 0.001a
Sex			0.116
Male	120 (94.5)	41 (87.2)
Female	7 (5.5)	6 (12.8)
Time from symptom onset to hospitalization (months)	6.0 (3.0-10.5)	6.0 (4.0-8.0)	0.736
Single-trauma history			0.703
None	29 (22.8)	10 (21.3)
Low-energy	50 (39.4)	22 (46.8)
Moderate-energy	44 (34.6)	15 (31.9)
High-energy	4 (3.1)	0 (0.0)
Alcohol exposure			< 0.001a
None	74 (58.3)	7 (14.9)
Light	34 (26.8)	12 (25.5)
Moderate	18 (14.2)	16 (34.0)
Heavy	1 (0.8)	12 (25.5)
Corticosteroid exposure			< 0.001a
None	125 (98.4)	6 (12.8)
Low	2 (1.6)	3 (6.4)
Moderate	0 (0.0)	12 (25.5)
High	0 (0.0)	26 (55.3)
Training intensity			< 0.001a
Low	2 (1.6)	18 (38.3)
Moderate	10 (7.9)	19 (40.4)
High	115 (90.6)	10 (21.3)
Laterality			< 0.001a
Unilateral	115 (90.6)	13 (27.7)
Bilateral	12 (9.4)	34 (72.3)
Tönnis grade			< 0.001a
Grade 0	2 (1.6)	3 (6.4)
Grade 1	19 (15.0)	17 (36.2)
Grade 2	44 (34.6)	20 (42.6)
Grade 3	62 (48.8)	7 (14.9)
VAS score	7.0 (6.0-8.0)	7.0 (6.0-8.0)	0.863
Harris Hip Score	68.90 ± 8.82	68.38 ± 7.68	0.724
Hip flexion-extension ROM (°)	133.0 (120.0-140.0)	130.0 (121.0-136.0)	0.457
Hip internal-external rotation ROM (°)	50.0 (30.5-70.5)	50.0 (40.0-70.0)	0.560
ALP (U/L)	71.0 (63.0-85.0)	69.0 (56.5-79.0)	0.225
CK (U/L)	95.0 (79.0-136.5)	87.0 (54.0-109.5)	0.002a
TC (mmol/L)	3.86 ± 0.72	4.33 ± 1.01	0.004a
UA (μmol/L)	402.61 ± 84.83	385.13 ± 81.77	0.225

^aP < 0.05. Data are presented as n (%), mean ± SD, or median (IQR). Median (IQR) and Mann-Whitney U tests were used for age, BMI, time interval, Visual Analog Scale, range of motion, alkaline phosphatase, and creatine kinase. Harris Hip Score and uric acid were compared with Student’s t tests; total cholesterol was compared with Welch’s t test. Categorical variables were compared with Fisher’s exact or Fisher-Freeman-Halton exact tests. Training intensity: Low ≤ 30 hour/week, moderate 31-45 hour/week, high > 45 hour/week. All analyses were patient-level. ALP: Alkaline phosphatase; BMI: Body mass index; CK: Creatine kinase; IQR: Interquartile range; ONFH: Osteonecrosis of the femoral head; ROM: Range of motion; SSFFH: Subchondral stress fracture of the femoral head; TC: Total cholesterol; UA: Uric acid; VAS: Visual Analog Scale.

Table 4 Multivariable logistic regression analysis of factors associated with diagnostic group

Variable	B	SE	Adjusted OR	95%CI	P value
Training intensity (per category increase)	-2.117	0.498	0.120	0.045-0.319	< 0.001a
Bilateral vs unilateral	3.053	0.630	21.188	6.168-72.784	< 0.001a
Alcohol exposure (overall)					0.019a
Alcohol exposure: Light vs none	1.181	0.749	3.259	0.751-14.137	0.115
Alcohol exposure: Moderate vs none	1.846	0.795	6.336	1.334-30.100	0.020a
Alcohol exposure: Heavy vs none	3.013	1.274	20.353	1.675-247.233	0.018a

^aP < 0.05. Outcome coding: ONFH = 1 and SSFFH = 0. Forward likelihood-ratio binary logistic regression was used (entry P < 0.05; removal P > 0.10). Training intensity was coded 1-3 and its odds ratio represents a one-category increase. Alcohol exposure used none as the reference. Corticosteroid exposure was excluded because quasi-complete separation produced unstable maximum-likelihood estimates. Overall alcohol P value is from the likelihood-ratio test; contrast P values are Wald tests. Model n = 174; omnibus likelihood-ratio χ²(5) = 124.18, P < 0.001; Hosmer-Lemeshow χ²(4) = 3.91, P = 0.418. OR: Odds ratio; SE: Standard error.