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in the anterosuperior labroacetabular junction is usually considered a small partial detachment, because when probed arthroscopically, it appears unstable. Third, there is, however, a small sublabral sulcus in the posterior inferior aspect of the labrum seen in many individuals that has been reported as normal findings, which our own clinical experience confirms [49]. Fourth, the labrum cartilage interface and zone of transition may have mild increased signal on nearly all sequences and should not be confused for a tear. This "cartilage undercutting" phenomenon is seen in the shoulder involving the glenoid labrum-cartilage interface as well [49,50]. Finally, nonarthro-gram imaging in asymptomatic volunteers described the absence of the ante-rosuperior labrum, particularly in older adults. In our experience in young adult athletes, this finding is markedly abnormal and indicative of a macerated torn labrum.

Classifications for labral tears exists that are based on MR signal intensity, tear morphology, or arthroscopic findings [5,51,52]. The utility of an MR classification scheme has been questioned due to the general acceptance of arthroscopic debridement as the definitive treatment for symptomatic tears. Although de-bridement produces good to excellent results for 85% to 90% of patients, long-term studies are forthcoming [38]. However, there is a strong relationship between acetabular labral tears and arthritis [53]. In the knee and shoulder it is well known that meniscal and glenoid labral resection can cause significant increase in joint contact pressures [54,55]. One should also keep in mind that several decades passed after Fairbank's classic 1948 description of post-meniscectomy arthritis was published before meniscal repair became the standard of care [56]. Therefore, although not proven in the hip, based on past history of injury to fibrocartilage bearing joints, it is reasonable to surgically attempt to restore biomechanical function of the hip using techniques similar to those used to repair menisci and the glenoid labrum to reduce the possibility of late onset arthritis. Consequently, emerging arthroscopic techniques emphasizing tissue preservation and biomechanical function are being developed to repair the labrum [38]. Therefore, to help guide surgical intervention, our MR assessment of tears has progressed from a yes or no evaluation for the presence of a labral tear to a descriptive evaluation emphasizing the amount of residual intact labral tissue, orientation of intrasubstance tears, and the presence of labro-

Fig. 12. Normal MR variants. (A) Oblique axial T2 fat-saturated image showing small cleft or recess (white arrow) under the anterior labroacetabular junction arthroscopically confirmed to be a partially detached unstable tear. (B) Oblique axial T2 fat-saturated image with small cleft between the posteroinferior labrum and acetabulum, which should not be confused with a detached tear (white arrow). (C) Coronal T1 fat-saturated image of mid-hip with normal appearing superior labrum and normal labral-cartilage interface with a mild increased signal that should not be confused with a labral tear (white arrow). (D) Oblique axial T2 fat-saturated images with near complete loss of the anterosuperior labrum (white arrow) consistent with a macerated tear rather than a normal variant.

acetabular detachment. Currently, we categorize tears into three major groups: detached, intrasubstance, and degenerated with combinations thereof.

Detached tears demonstrate separation of the labrum from its acetabular base, which can be complete or partial, and may be nondisplaced. Detached tears with displacement on MR arthrography demonstrate linear fluid or contrast signal gap interposed between the base of the labrum and bony acetabular rim, and are best seen on coronal images for superior predominant tears and oblique axial images for anterior predominant tears (Fig. 13). These tears may exist without displacement, in which case diagnosis can be difficult with the tear manifesting as only a thin line of fluid signal at the labroacetabular junction. Detachment injuries can be treated with suture anchor reattachment much like glenoid labral repair techniques in the shoulder.

Intrasubstance labral tears demonstrate intrasubstance fluid or contrast signal, usually extending to the articular side of the labrum (sometimes capsular side), which is often oblique or curvilinear in shape. However, signal may also be complex extending in multiple directions in the long and short axis of the labrum. These tears can be treated with intrasubstance suture bandings and thermal treatment to restore shape (Fig. 14).

A labrum with abnormal irregular contours and a thin morphology, with or without intrasubstance fluid or contrast signal extending to the free margin, is considered a degenerative type tear. These tears will likely undergo debridement or thermal treatment when necessary. In young athletes, it is not uncommon to have a combination of detached tears or intrasubstance tears with superimposed degenerative components. Treatments include a combina-

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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