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Fig. 13. (A) Coronal T1 fat-saturated image with typical finding of minimally displaced detached labral tear (white arrow) from the acetabular margin without an intrasubstance or degenerated component. (B) Axial image of a complex tear of anterosuperior labrum with more subtle detachment. Note the fraying and thinning of the free edge with small vertical intrasubstance tear (black arrow).

Fig. 14. Oblique axial T2 fat-saturated images of an intrasubstance tear of the antero-superior labrum extending transversely from the acetabular base to the apex of the free edge (white arrowheads).

tion of reattachment, suture banding, debridement, and thermal contouring (Fig. 15).

We also note the estimated length of tears as well as location (anterior, anterosuperior, superior, posterosuperior, posterior). We include an additional clock face modifier to help convey beginning and endpoints of the tear based on arthroscopic appearance to aid the surgeon (Fig. 16). For example, a professional golfer may have a left hip 3 cm-long labral tear with an intrasubstance oblique articular sided tear extending from the 10 to 11 o'clock position, a detached component extending from the 11 to 1 o'clock position, and margins of the anterosuperior labrum demonstrating thin and frayed morphology consistent with acute on chronic injury.

Fig. 15. Oblique axial T2 fat-saturated image with degenerated labral tear. Note loss of sharp triangular appearance and normal dark signal (white arrows). This finding is more commonly seen in older individuals, but one must also search for superimposed acute detachments and intrasubstance tears.

Fig. 16. Oblique sagittal T1-weighted image of the left hip showing clock face descriptors as seen if viewed lateral to medially. The acetabulum is horseshoe shaped with the iliopsoas tendon anteriorly at the 9 o'clock position (black arrow). There is partial visualization of ligamentum teres merging with transverse acetabular ligament at the 6 o'clock position (white arrows). It is important to avoid confusion when describing tears and include the name of the quadrant (ie, anterosuperior) along with clock face description, as some may switch the orientation of the clock face depending if it is a left or a right hip.

Fig. 16. Oblique sagittal T1-weighted image of the left hip showing clock face descriptors as seen if viewed lateral to medially. The acetabulum is horseshoe shaped with the iliopsoas tendon anteriorly at the 9 o'clock position (black arrow). There is partial visualization of ligamentum teres merging with transverse acetabular ligament at the 6 o'clock position (white arrows). It is important to avoid confusion when describing tears and include the name of the quadrant (ie, anterosuperior) along with clock face description, as some may switch the orientation of the clock face depending if it is a left or a right hip.

Femoroacetabular Impingement

Plain film findings of FAI have been well described. The MR appearance of FAI has been recently described, and corroborates surgical and radiographic findings [25]. A recent study described a triad of MR findings of FAI included loss of femoral head-neck junction offset, anterosuperior labral tears, and adjacent chondrosis [57]. The alpha angle measurement is used to quantify cam type impingement on MR images (Fig. 17) [58]. MRI quantification of pincer type impingement has not been described to our knowledge, but cross-sectional analysis of axial CT findings of acetabular retroversion have been described and emphasize the importance of evaluating the superior aspect of the acetabu-lum rather than mid-portion to accurately measure version and avoid a false negative finding [20].

Cartilage Injury

Cartilage injury is often associated with labral tears and femoroacetabu-lar impingement. Accurate assessment of articular cartilage of the hip can be difficult due to its thinness and spherical contours unlike the knee [59]. Principles of cartilage evaluation in other parts of the body are applied to the hip and include assessment of size, location, defect thickness, subchondral bone interface, and subjacent marrow signal (Fig. 18). Although difficult, cartilage assessment is critical, as arthroscopic labral debridement outcomes are linked to the degree of underlying cartilage abnormality [2]. Plain film findings of cartilage injury due to labral tears and type 1 FAI involving the anterosuperior rim likely does not result in joint space narrowing on AP radiographs. One

Fig. 17. (A) Oblique axial T2 fat-saturated images in the plane with the femoral neck demonstrating normal head-neck offset with an alpha angle measuring about 45° (normal less than 50-55°). This angle arises from two rays originating at the center of a best-fit circle of the femoral head. The first is along the axis of the femoral neck, and the other intersects the point where the cortex of the anterior femoral head-neck junction separates from the best-fit circle. (B) Comparison image shows MR appearance of FAI in a professional golfer. Note mild loss of normal head-neck offset measuring 60° along with a focal fibrocystic change at the area of impingement consistent with radiographic finding of a synovial herniation pit (black arrow).

Fig. 17. (A) Oblique axial T2 fat-saturated images in the plane with the femoral neck demonstrating normal head-neck offset with an alpha angle measuring about 45° (normal less than 50-55°). This angle arises from two rays originating at the center of a best-fit circle of the femoral head. The first is along the axis of the femoral neck, and the other intersects the point where the cortex of the anterior femoral head-neck junction separates from the best-fit circle. (B) Comparison image shows MR appearance of FAI in a professional golfer. Note mild loss of normal head-neck offset measuring 60° along with a focal fibrocystic change at the area of impingement consistent with radiographic finding of a synovial herniation pit (black arrow).

recent study describes different cartilage pattern losses with different types of impingement. Specifically type 2 or pincer type results in diffuse circumferential cartilage injury, whereas type 1 had anterior superior injury primarily [60]. Acetabular delamination injuries have been reported in cases of type 1 FAI that were identified with direct MR arthrography [61].

Therefore, from an imaging standpoint, cross-sectional imaging is needed to evaluate cartilage unless plain film findings are advanced. MR arthrography has been found to offer moderate sensitivities and specificities between 47% to 79%

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