Magnetic Resonance Imaging (MRI) is the primary non-invasive tool used to visualize the soft tissues of the hip joint, including the acetabular labrum. The hip is a ball-and-socket joint where the head of the femur fits into the acetabulum of the pelvis. When pain suggests a problem with the rim of cartilage surrounding the socket, an MRI is ordered to look for a tear. The appearance of a labral tear on the scan is the focus of the radiologist’s analysis to confirm the diagnosis.
Understanding the Hip Labrum and Tear Types
The acetabular labrum is a ring of fibrocartilage that lines the hip socket (acetabulum). This structure deepens the socket to enhance joint stability and helps seal the joint fluid. It also distributes pressure across the joint surface.
Tears arise from two main mechanisms: trauma or degeneration. Traumatic tears, often seen in athletes, result from an acute event and present as a distinct, linear separation with sharp edges. Degenerative tears are linked to underlying structural issues like femoroacetabular impingement (FAI) or age-related wear. These tears appear on imaging as a frayed or complex area of damage with irregular borders. Most symptomatic tears occur in the anterosuperior (front and top) portion of the labrum.
Standard MRI Versus MR Arthrography
A standard, non-contrast MRI relies on the natural differences in tissue content to create an image. Since the labrum is a small structure that appears dark on all MRI sequences, a standard scan can be challenging for diagnosis. A small tear may not be easily distinguishable from surrounding joint fluid, often leading to inconclusive results.
Magnetic Resonance Arthrography (MRA) is frequently performed to improve diagnostic accuracy. This procedure involves injecting a gadolinium-based contrast agent directly into the hip joint space under guidance before the MRI scan. The contrast material fills the defect created by a tear, making it visible. By surrounding the labrum, the contrast agent makes the tear stand out clearly against the dark cartilage. MRA remains the preferred method, especially for smaller tears.
Key Visual Signs of a Labral Tear
The most definitive sign of a labral tear on MRA is the presence of bright contrast material tracking into the dark labral substance. This injected contrast appears bright white, and its penetration confirms a breach in the tissue. The bright signal may be seen as a linear line extending from the joint space into the labrum or as a wedge-shaped pooling within the defect.
A healthy labrum exhibits a smooth, triangular shape and uniform dark signal intensity. A tear disrupts this normal appearance, showing up as an area of high signal intensity (bright white) within the dark fibrocartilage. Radiologists also look for signs of labral displacement, where the torn fragment is pushed away from the acetabular rim. The tear may also cause the labrum to appear frayed, blunted, or irregular in its contour.
What the Radiologist Looks For
The radiologist reports the precise characteristics of the tear to guide treatment planning. They confirm the tear by identifying contrast material or high signal intensity extending into the labral substance. The location is mapped using an anatomical clock-face analogy, with the top of the hip socket as the 12 o’clock position. Most tears are found between the 10 and 2 o’clock positions.
The report details the tear’s size, extent, and configuration. The radiologist also assesses for associated pathologies. These include paralabral cysts (fluid-filled sacs adjacent to the tear) and signs of femoroacetabular impingement (FAI). FAI signs include bony overgrowth on the femoral head (cam lesion) or excess bone on the acetabular rim (pincer lesion), which are frequent underlying causes of labral injury.