Patients with ONFH staged as ARCO 2/3 from January 2015 to August 2021 were retrospectively analyzed. The diagnosis standard was Chinese Experts' Consensus on the Diagnosis and Treatment of Osteonecrosis of the Femoral Head in Adults (2016), and the cases were staged according to ARCO 2019. The criterion of ARCO stage 2 is osteosclerosis, focal osteoporosis, or cystic changes are seen in the femoral head on plain radiographs or CT scan, and there is no evidence of subchondral fracture, fracture in the necrotic portion, or flattening of the femoral head; and criterion of ARCO stage 3 is subchondral fracture, fracture in the necrotic portion, and/or flattening of the femoral head is seen on plain radiography or CT scan, without manifestation of hip osteoarthritis, stage 3 is subdivided into early and late sub-stages according to the amount of femoral head depression (≤ 2 mm versus > 2 mm)12. As ARCO stage 1 is clinically rare and does not contain bone resorption lesion. ARCO stage 4 is associated with osteoarthritis and may contain subchondral bone cyst, which may be confused with the bone resorption, so ARCO stages 1 and 4 were not included. The ARCO staging was determined by two orthopedics based on hip CT image with negotiation.
Inclusion criteria: ARCO stage 2/3, with bone resorption lesion on hip CT image, the interval of MR and CT examinations within 1 month. Because patients underwent hip MR examination are relatively less than CT in our hospital, screening is based on MR.
Exclusion criteria: Without CT examination, the interval between CT and MR examinations exceeded 1 month, no bone resorption lesion on CT, ARCO stage 1/4, history of hip surgery or trauma, and connective tissue diseases.
There were 359 ONFH cases underwent MR examination, after excluding all kinds of unqualified patients, 41 cases were finally collected, of which 5 cases were bilateral ONFH, so there were 46 cases. Among them, 16 cases were ARCO stage 2, 14 cases were ARCO stage 3A, and 16 cases were ARCO stage 3B. M/F = 29/12. See Fig. 1 for details.
This study was approved by the Medical Ethics Committee and patient consent was waived.
MRI was performed at 3 T (Skyra, Siemens AG, Germany) using a phased array body coil. Scanning protocols included: axial T1W Turbo Spin Echo (TSE) sequence, axial T2W fat-saturated (FS) TSE sequence, and coronal proton density weighted imaging (PDWI)-FS-Dixon fat/water image. Among the enrolled patients, 8 cases underwent additional axial DWI sequence examination, b-values: 0, 500, and 1000 s/mm2. (The sequence parameters are summarized in Table 1).
CT was performed with SOMATOM Definition Flash, Siemens. The scan parameters were: tube voltage:140 kVp; automatic tube current modulation (90–130 mA); DFOV: 35.6×35.6 cm; slice thickness:0.75 mm. Both hip joints were scanned simultaneously. The scanning range was from the top of the acetabulum to the femoral lesser trochanter. Coronal views were reconstructed with bone windows using the B60s kernel, sharp,3 mm for all hip joints. Besides, supplemental coronal views reconstructions were performed in the PACS system (Tianjian RIS 6.0) with the slice thickness of 1mm.
All subjects were placed in supine position with feet in the neutral position.
Ct Image Evaluation
The size and density of bone resorption were blindly measured by three radiologists (SS, 9 years working experience; LS, 7 years working experience; HB T, 25 years working experience) and take the average as result. The number and shape of bone resorption lesions, and whether they were connected to subchondral fractures were determined by two radiologists (SS, 9 years working experience; HB T, 25 years working experience) with negotiation. All the CT signs were measured and evaluated on the coronal reconstructed image (slice thickness 3mm), with referring to the original axial image and supplemental coronal reconstruction image.
1. The shape of bone resorption: evaluation was performed at the largest slice of the bone resorption lesion on the coronal CT, if the shape was approximately round or oval, regarded as regular and recorded as 1; other shapes were regarded as irregular, recorded as 2.
2. The maximum area in coronal position (MAC) and density of bone resorption lesion were measured by the methods used in the previous research3.
3. Whether the bone resorption is connected to the subchondral fracture: if the low-density bone resorption lesion was directly connected to the subchondral fracture, recorded as 1, otherwise was recorded as 0.
Mr Image Evaluation
Before the formal evaluation, 10 enrolled patients were randomly selected, and the MR signs of bone resorption were observed and studied by three radiologists by referring to the CT images.
1. The number and size of bone resorption:
Two radiologists blindly evaluated the number of bone resorption in the coronal PDWI-FS-Dixon water image, and they reached consensus through consultation if inconsistent. The measurement method of MAC of bone resorption is the same as on CT3.
2. MR T1WI and T2WI signal characteristics of bone resorption lesion:
The T1 and T2 signal characteristics of the maximum bone resorption in each enrolled patient were evaluated by referring to the CT image.
On the axial T2W FS TSE sequence, the signal of bone resorption was divided into two types, type I, with equal or slightly higher signal (equal signal: similar to bone marrow signal intensity, slightly higher-equivalent to bone marrow edema signal intensity); type II, with high signal (slightly lower than or equal to the signal intensity of joint effusion).
Besides, the heterogeneous of T2 signal was evaluated, heterogeneous was recorded as 1, and homogeneous was recorded as 0.
On the axial T1W TSE sequence, the signals of bone resorption were evaluated and recorded (iso-signal: equivalent to muscle signal; slightly higher signal - higher than muscle signal but still lower than bone marrow signal; heterogeneous signal with iso and slightly higher signal).
3. Bone marrow edema (BME).
BME was assessed on the axial T1W TSE and axial T2W FS TSE sequence, the extent of BME was scored from 1 to 3 according to its scope.
For 8 cases underwent DWI examination, the ADC values were measured by referring to the axial T2W FS TSE or CT images. On the Siemens workstation(SYNGO VFA-SOF), select the largest layer of the axial bone resorption lesion and draw a largest circular or elliptical range of interest to measure.
In image evaluation, the number, shape, and MR signal characteristics of bone resorption, BME, and whether it is connected to subchondral fracture was determined by two radiologists in consultation (SS, HB T). The MAC, density, and ADC value of bone resorption were blindly measured by three radiologists (SS, LS, HB T).
The cases were divided into two groups according to the T2 signal type of bone resorption on FS T2WI. Group 1- T2WI signal type I, Group 2- T2WI signal type II.
Continuous variables were summarized as medians with interquartile ranges. Categorical variables were presented as counts and percentages. Age, BME, MAC, intensity and ADC value of bone resorption between the two groups were compared by Mann-Whitney test. (SPSS 22.0). The Chi-square test was used to evaluate the differences of gender, shape and heterogeneous T2 signal of bone resorption, and whether the bone resorption is connected to the subchondral fracture between the two groups. The correlation between the MAC of bone resorption lesion on MR and CT was analyzed by Pearson’s correlation. The consistency of the data from the 3 radiologists was analyzed by intraclass correlation coefficient (ICC). P < 0.05 is statistically significant.