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Background
The acetabular labrum of the hip is a fibrocartilaginous ring of tissue continuous with the distal edge of the acetabulum of the hip and completed inferiorly in its arc by the transverse acetabular ligament. Wide variability in the shape and MR signal intensity of the acetabular labrum has been observed across patients and age groups. The labrum performs many functions at the hip joint, such as increasing spherical joint contact area, sealing the pressurized synovial fluid of the hip joint to evenly distribute forces, and increasing the joint's resistance to distraction via the fidelity of its vacuum-like seal.

Educational Goals / Teaching Points
Tears of the labrum must be properly distinguished from non-pathologic anatomic variants for correct diagnosis. The purpose of this exhibit is to describe labral tears, labral sulci, and the differentiating features between the two.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Tears of the labrum occur when either the labrum externally detaches from the acetabulum or internally degenerates. Most labral tears occur in the anterior labrum, and can be associated with perilabral abnormalities such as paralabral cysts. Clinically, labral tears may present with hip pain, clicking, locking, and decreased range of motion. Diagnosis can be purely clinical, or confirmed with MR imaging findings of a hyperintense, irregular labral signal, labrum signal heterogeneity, abnormal labral shape, or adjacent paralabral cysts. Treatment of labral tears is performed by arthroscopic debridement of frayed labral tissue and reattachment of detached labral tissue with sutures. Two important non-pathologic anatomic variants of the hip acetabular labrum are sublabral sulci and transverse ligament-labral junction sulci. Sublabral sulci are formed at the transition of the labrum and articular cartilage, and are present in as many as 22% of hip surgical patients. They can occur in any of the labral quadrants, though they occur most commonly at the posterior labrum. Sublabral sulci tend to be defined by the appearance of a high intensity signal extending along the labral edge with smoothly defined borders, a normal shape and signal of the adjacent labrum, and the absence of adjacent paralabral cysts. Transverse ligament-labral junction sulci are sulci formed at the transition of the labrum and transverse ligament, and occur in up to 33% of hip surgical patients. On imaging, they appear as an anteroinferior hyperintensity extending along the labral edge. Transverse ligament-labral sulci may appear similar on imaging to anteroinferior labral tears, while sublabral sulci may appear similar to labral tears in any quadrant.

Conclusion
Special care must be taken in diagnosing labral tears, as many recent MRA studies have shown less than an 80% specificity in differentiating labral tears from non-pathologic anatomic variants such as sulci. Properly differentiating between labral sulci and labral tears increases the specificity of labral tear diagnosis on imaging and is thus integral in delivering excellent clinical outcomes and care to potential hip surgical patients.