Patients
We designed a non-randomized control study for patients clinically suspected to have FAI. We met 50 patients presenting by chronic hip or groin pain and/or clinically suspected to have FAI, referred to the Radiology department, during the time span: March 2015 to July 2016. The study included 31 (chosen according to the age group) FAI patients (17 male and 14 female), age range 17–52 years, and mean age 31.9 years. The included patients underwent conventional MRI and MR arthrography to identify labral and chondral abnormalities, and the results were correlated to arthroscopy as the gold standard. We excluded patients whose clinical data or conventional MR imaging revealed bone or joint diseases other than FAI (e.g., avascular necrosis, fractures, and bone tumor) or not confirmed as FAI by conventional MR measurements, and patients who missed arthroscopy.
Magnetic resonance imaging
The patients were examined using MRI machine: GYROSCAN INTERA 1.5T MAGNET (PHILPS). The patients lay supine with the leg in slight internal rotation. For all the patients, conventional MRI sequences were obtained (Table 1). Images were reviewed to exclude other pathologies; US-guided intra-articular contrast injection was done.
Technique of ultrasound (US)-guided intra-articular contrast injection (Fig. 1)
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The device used is Sonoline Si-250 Elega Imaging System Siemens (Germany) ultrasound device that is equipped by 18-MHz linear array transducer for the musculoskeletal part probe.
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The area is prepared and draped in a sterile fashion, then the subcutaneous tissue is locally anesthetized.
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The needle tip 22 gauge is advanced in an anterior, posterior direction until it reaches the femoral head neck junction then injection of contrast mixture starts (the mixture used is composed of 1mmol gadopentatedimeglumine, 5 ml iodinated contrast, 3 ml Xylocaine, and completed to 20 ml with sterile saline).
Magnetic resonance arthrography
After intra-articular contrast injection, the patient was sent to MRI scanner in a period not exceeding 30 min. The post-contrast scan included different imaging sequences and planes (Table 1).
Images interpretation and analysis
The produced MR conventional and post intra-articular contrast injection images were transferred to a workstation. For each examined joint, the following were assessed and reported.
Bone changes
Alpha angle is measured in the axial oblique plane by (Fig. 2) placing a circle around the oblique axial circumference of the femoral head, drawing a line passing through the narrowest portion of the femur neck to the center of the femur head, and then placing a second line that extends from the point of intersection of the first line and the center of the femoral head to a point where the osseous anterior femoral head intersects the circle. Alpha angle is abnormal if the measured value is > 55°, showing cam type.
Acetabular depth (Fig. 3) is measured in the axial oblique plane by placing a line connecting the anterior and posterior acetabular rim then another line passing through the center of the femoral head and running parallel to the first line.
The value is classified as “positive” if the center of the femoral head is lateral to the line that connects the anterior and the posterior acetabular rim, and value is negative if the center of the femoral head is medial to the line which indicates deep acetabulum, a sign of pincer type.
Bone marrow edema, subchondral erosions and cysts, bone bumps at the head/neck junction, and bone spurs at the acetabulum margins were evaluated from conventional MR images
Labrum abnormalities
A labral tear was diagnosed by a high signal contrast medium entered through the inner of surface of acetabular labrum or located between the acetabular labrum and acetabular edge or presence of an associated labral cyst. Confirmation was done by the presence of any of these findings in other imaging planes. Labral tears were classified by their sites into antero-superior, antero-inferior, postero-superior, and postero-inferior. Anterior and posterior tears were detected using axial, sagittal, and radial planes while superior and inferior tears were detected on coronal and radial planes.
Chondral abnormalities
Defects were identified by noting fluid signal replacing the normal intermediate signal intensity of the cartilage.
Other findings
These include herniation pits, paralabral cyst formation, synovial effusion, and bursa.
Arthroscopy
All the included patients had done arthroscopy, and the results were used as a gold standard.
Statistical analysis
Data was analyzed using SPSS win statistical package version 21 (SPSS Inc., Chicago, IL). Numerical data were expressed as mean, and standard deviation or median and range as appropriate. Qualitative data were expressed as frequency and percentage. Chi-square test (Fisher’s exact test) was used to examine the relation between qualitative variables. For quantitative data, a comparison between two groups was done using either student t test or Mann-Whitney test (non-parametric t test) as appropriate. A P value of less than 0.05 was considered significant. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of MR arthrography for diagnosing the acetabular labral tears and chondral abnormalities were calculated, using the results of hip arthroscopy as the gold standard.