The Buford complex in the shoulder is a well-described normal variant, where there is hypoplasia of the anterosuperior glenoid labrum with an associated enlarged middle glenohumeral ligament. Thus, one should be cautious when relying exclusively on intrasubstance signal alterations to detect labral tears. Although many of these abnormalities resolved on more heavily T2-weighted sequences, intermediate signal persisted in 12% of labra on both T2-weighted conventional spin echo and fast spin echo images. In their study, the abnormal signal was sometimes seen extending to the capsular and/or articular surfaces of the labrum, thus mimicking labral tears in these asymptomatic patients. Intralabral signal was globular, linear, or curvilinear and was located in the superior (87%), posterior (21%), and anterior (8%) labrum. In a study of 52 hips in 46 asymptomatic volunteers, Cotten and colleagues found areas of intermediate or high labral signal intensity in 58% of hips on T1- and proton density–weighted spin echo images in nonarthrographic studies.

Hodler and colleagues compared MR images of the hip with histologic findings in cadavers and found that abnormal signal in the acetabular labrum correlated poorly with histologic signs of degeneration. Increased signal within the labrum can be a normal finding, particularly on shorter TE pulse sequences such as T1-weighted or proton density techniques. Often there is a paralabral cyst with a neck arising either from the labrum itself or from the labro-osseous interface that is commonly seen in association with labral tears. In more advanced cases, there may be blunting of the labrum, with loss of the normal triangular morphology. Tears can be diagnosed on MR arthrography by visualizing injected contrast material extending into the labrum or through the labrum/acetabulum junction. Many tears have intrasubstance high signal on T2-weighted or proton density sequences, with surface irregularity and irregularity of the adjacent hyaline cartilage. Labral tears or detachments can be seen on nonarthrographic MR studies with linear fluid signal tracking into the labral substance or undermining the labral attachment to the acetabular rim. Several variations in the morphology and imaging appearance of the acetabular labrum can mimic pathologic conditions and lead to ineffective treatment. The labrum usually has a low signal intensity on all routinely used pulse sequences, particularly on more heavily T2-weighted sequences with longer echo times (TE). On imaging, the labrum is usually triangular and slightly thicker posterosuperiorly than anteriorly. Simultaneous intra-articular local anesthetic injection may also be useful to help distinguish between symptomatic and asymptomatic labral tears or developmental variations. These patients may benefit from MR arthrography, which distends joint recesses and increases intra-articular pressure, thereby allowing detection of smaller labral tears. Occasionally, patients may have asymptomatic labral tears or developmental variations that mimic labral pathology. As a result, evaluation of the labrum to detect abnormalities before the onset of cartilage loss is among the most common indications for imaging of the hip. Tears occur most commonly in the anterior superior quadrant of the labrum. However, repetitive impingement and labral tears are a frequent cause of hip pain.

Unlike the knee menisci, which have a load-bearing and cushioning function, and the glenoid labrum, which helps to deepen the glenoid fossa and improve glenohumeral contiguity, the acetabular labrum primarily seals the margins of the articular hyaline cartilage to prevent premature cartilage loss. The acetabular labrum is a fibrocartilaginous horseshoe-shaped structure along the anterior, superior, and posterior margins of the acetabular rim. This article describes the numerous variants of the hip that are frequently seen on arthrographic and nonarthrographic MR imaging examinations. As in any imaging study, it is important to be cognizant of these variations as well as associated findings that help distinguish between true abnormality and developmental variation when interpreting an MR image of the hip. Several developmental variations are seen in the hip, which can be mistaken for disease or potentially even contribute to the development of a pathologic condition. MR imaging of the hip is one of the most common musculoskeletal MR imaging studies performed today to assess for occult fractures, acetabular labral tears, hyaline cartilage loss, and musculotendinous injuries.

Xray of normal hip