A Magnetic Resonance Imaging (MRI) scan is frequently used to evaluate injuries within the knee joint, particularly involving the meniscus. The meniscus, a pair of C-shaped cartilage pieces, is a common source of pain and mechanical symptoms that require precise diagnosis. Because the meniscus is a soft tissue structure, traditional imaging like X-rays cannot provide the necessary detail to assess its condition accurately. This article will explain how an MRI visualizes the meniscus and identify the specific signs used by radiologists to confirm the presence and characteristics of a tear.
The Meniscus and Why MRI is Used
The meniscus consists of two C-shaped fibrocartilage pads situated between the femur (thigh bone) and the tibia (shin bone) inside the knee joint. These structures function primarily to distribute weight, absorb shock, and stabilize the knee during movement. Damage to this tissue can significantly compromise the joint’s mechanics and lead to pain or locking sensations.
Magnetic Resonance Imaging has become the preferred method for diagnosing meniscal injuries due to its superior ability to create detailed images of soft tissues. The MRI machine uses powerful magnetic fields and radio waves to generate cross-sectional images of the knee without exposing the patient to ionizing radiation. This non-invasive technology allows medical professionals to see the internal structure of the meniscus with high resolution. The diagnostic accuracy of MRI in detecting meniscal tears is high, often falling between 80% and 95%.
Interpreting the Healthy Meniscus Image
To understand a tear, it is necessary to establish the normal appearance of the meniscus on an MRI scan. A healthy meniscus is primarily composed of dense fibrocartilage, which contains a low concentration of mobile protons. Because of this composition, a normal meniscus appears uniformly dark or black on most standard MRI sequences, such as T1-weighted and T2-weighted scans. This lack of bright signal is referred to as low signal intensity.
When viewed in cross-section on sagittal MRI slices, the meniscus exhibits a distinct triangular shape. In the middle section, consecutive sagittal images typically show a “bow-tie” shape, representing the body of the structure. The consistent, uninterrupted dark signal and preservation of this triangular or bow-tie morphology are the defining characteristics of a healthy, intact meniscus. Any presence of internal bright signal within this dark structure suggests an abnormality, though not necessarily a true tear.
Recognizing a Meniscus Tear on MRI
The primary visual cue indicating a meniscal tear is the presence of high signal intensity—appearing as a bright, white line—that clearly extends to the articular surface of the cartilage. This white signal represents synovial fluid from the joint cavity that has entered the defect in the fibrocartilage. The fluid-filled gap disrupts the normal dark, uniform structure of the meniscus.
A tear is confirmed when this bright line reaches the edge of the meniscus that faces the joint space. For a finding to be confidently reported as a tear, the abnormal high signal must be visible communicating with the superior or inferior surface of the meniscus on more than one image slice. The disruption of the normal dark triangle is a clear sign that the structural integrity of the cartilage is compromised.
Classifying Tears by Grade and Morphology
Radiologists use a grading system to describe the severity of signal abnormalities seen within the meniscus, classifying the findings into three main grades. Grade I shows a small, globular area of high signal intensity that remains completely contained within the substance of the meniscus and does not reach the surface. Grade II is characterized by a linear area of high signal that is also contained and does not break the meniscal surface. Both Grade I and Grade II signal changes are considered signs of intrasubstance degeneration or minor injury, rather than an unstable, true tear.
A Grade III signal is the definitive sign of a tear, as the high signal intensity communicates with the superior or inferior articular surface. Beyond this grading, tears are classified by their morphology, or shape, which dictates surgical considerations. Common types include longitudinal tears (parallel to the circumference), horizontal (cleavage) tears (splitting the meniscus), and radial tears (extending from the inner free edge outward).
A particularly significant finding is the bucket-handle tear, which is a type of displaced longitudinal tear. This tear results in a fragment—the “handle”—that has displaced into the intercondylar notch of the knee joint. On MRI, this displacement may result in the “absent bow-tie sign,” where the normal bow-tie shape of the meniscal body is missing or truncated due to the fragment having been pulled away. The specific morphology provides important information about the mechanism of injury and the stability of the remaining meniscal tissue.