MRI/MR Arthrogram

MRI is the reference imaging technique for evaluating labral tears, cartilage lesions, and other intra-articular soft-tissue pathology of the hip. There are two distinct approaches to imaging the hip prior to arthroscopy.

Non-contrast MRI is a high-resolution, small field-of-view (15–20 cm) study performed with imaging obtained in axial, coronal, and sagittal planes using proton-density fat-saturated sequences. Many high-volume hip preservation centers now use 3T non-contrast MRI as their default study, since it avoids the morbidity and added cost of intra-articular contrast injection while still adequately characterizing labral tears and identifying alternative extra-articular pathology such as gluteal tendinopathy, ischiofemoral impingement, stress fracture, or neoplasm.

MR arthrography (MRA) involves injecting dilute gadolinium contrast directly into the joint to distend the capsule. Distension improves delineation of the labral-chondral junction and allows differentiation of a true labral tear from normal intrasubstance signal variation. Systematic review evidence supports direct MRA over unenhanced MRI and indirect MRA specifically for detecting labral and cartilage abnormalities, and direct MRA remains the traditional gold standard for labral tear detection.

In practice the use of non-contrast MRI and MRA is dependent on many factors including the availability of both imaging techniques, costs associated with imaging, and surgeon and radiologist preference. Additionally patients will have already had advanced imaging prior to referral to an orthopedic surgeon.

Basics of Reading Hip MRI for Arthroscopy: Reading Hip MRI imaging is an advanced skill that takes practice to master. Below we will summarize the basics of interpreting hip MRIs. It can be beneficial to use a structured approach when beginning to read scans. One such sequence is as follows:

1. Review the coronal images for an overview of the joint, cartilage, femoral head, acetabulum, and surrounding muscles.
2. Review axial or axial-oblique images along the femoral neck for the anterior labrum, head-neck junction, and cam morphology.
3. Review sagittal images for the anterior and posterior labrum and cartilage. Radial images, when available, are useful for following the labrum and femoral head-neck junction circumferentially.
4. Then deliberately check the labrum, cartilage, ligamentum teres, and capsule for structural damages. T2 weighted images tend to be more useful than T1 weighted images for identifying pathology of the labrum, capsule, and other soft-tissues

Labrum: When examining the labrum, it is usually triangular and low signal when undamaged. Tears are often described using clock-face location where 12 o'clock is superior, 3 o'clock anterior, 6 o'clock inferior, and 9 o'clock posterior. Anterosuperior tears are the most common. Labral tears can sometimes occur with a concurrent paralabral cyst which can be identified on MRI.

Czerny classification of labral lesions

While less commonly used in practice, the Czerny classification can be used to describe labral tears on MR arthrograms.

• Stage 0: normal labrum
• Stage 1: intrasubstance degeneration without a tear reaching the labral margin
• Stage 2: partial labral tear with contrast extending into an otherwise attached labrum.
• Stage 3: complete labral tear

Ligamentum teres: The normal ligamentum teres is a low-signal structure extending from the fovea to the acetabular notch. Increased signal, fraying, partial discontinuity, or complete discontinuity suggests injury; thickening with increased signal may represent synovitis. MRI can identify ligamentum teres pathology, but grading the severity is less reliable than for the labrum. The Gray and Villar classification is reviewed in the chapter in ligamentum teres injury.

Capsule: Capsular abnormalities are more subtle and difficult to identify on MRI. Focal defects, thinning, and excessive capsular volume may be identified on imaging, but often the extent of capsule damage is not fully determined until intraoperative examination. T