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Acetabular labral tears have become a commonly recognized source of intra-articular hip pain that affects athletes and nonathletes alike. Although strongly associated with athletes performing twisting pelvic motions and rotations of the hip that occur in sports like soccer, golf, football, ballet, and hockey; athletes in all major sports (and even minor ones such as skateboarding and Olympic yachting) have been affected [38]. Many tour-level professional golfers have undergone successful hip surgery for labral pathology with return to previous level of play and sometimes beyond prior performances (Marc J. Philipponm, personal communication). As stated earlier, direct MR arthrography is the best imaging modality for evaluation of underlying intra-articular disorders. Interpretation should not only include labral evaluation, but also evaluation of chondral, capsular, bony, ligamentum teres, and adjacent extra-articular (iliopsoas, rectus femoris, pubic symphysis) abnormalities (Fig. 10). However, it is important to also realize that the clinical situation ultimately dictates the need for surgical intervention, as a negative MR arthrogram does not currently obviate arthroscopic evaluation [4].

Fig. 10. Oblique axial T2 fat-saturated image of an intact labrum, but there is a partial tear of the undersurface of gluteus minimus tendon insertion (white arrow) with surrounding lateral edema and inflammation (black arrow). It is essential to search for surrounding extra-articular abnormalities.

The labrum is generally considered a triangular-shaped structure with its medial base firmly anchored to the rim of the acetabulum with the apex extending laterally. It extends nearly circumferentially around the horseshoe-shaped acetabulum but blends with the transverse acetabular ligament inferi-orly (Fig. 11).

On the articular side, the labrum merges with the acetabular cartilage over a 1- to 2-mm transition zone [39]. On the capsular side, this transition does not exist. The labrum (like the meniscus) has been shown to contain nerve endings (presumable related to nocioceptive and proprioreceptive function), and is thought to have low intrinsic healing ability due to low vascularity primarily obtained from the capsule [40,41]. Biomechanically, the labrum increases the depth of the acetabular socket and helps maintains negative intra-articular pressure that increases static stability [42,43]. When the labrum is torn, forces on adjacent cartilage increase, suggesting a role in the development of cartilage injury and arthritis [44].

The labrum demonstrates typical MR imaging features of organized collagen elsewhere in the body with decreased low signal intensity on T1- and T2-weighted images. However morphologic (rounded or irregular) and increased intrasubstance signal intensity changes have been seen in asymptomatic individuals with increasing age based on nonarthrogram MR imaging and likely represent areas of degeneration [45-47]. However, in the young athlete undergoing evaluation for labral tear these findings are considered abnormal.

Several confusing issues regarding the MR appearance of the labrum should be addressed and understood (Fig. 12). First, on MR arthrography there is a normal perilabral recess between the capsule of the hip (particularly superiorly on coronal images) and the capsular side of the labrum (Fig. 11B). This recess may not be seen in a nonarthrogram MRI due to lack of capsular distention,

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