The meniscus consists of a pair of C-shaped fibrocartilage structures located between the femur and the tibia. These structures function as shock absorbers and stabilizers, distributing weight and reducing friction within the knee joint. When a knee injury, especially one involving twisting trauma, is suspected, Magnetic Resonance Imaging (MRI) is the preferred diagnostic tool. MRI provides detailed cross-sectional images of the soft tissues, allowing clinicians to accurately assess the extent and specific configuration of any meniscal damage. The precise nature of the tear heavily influences the treatment plan, whether surgical or non-surgical.
Understanding the Normal Meniscus on MRI
To identify a tear, understanding the healthy meniscal appearance on an MRI scan is essential. A normal, uninjured meniscus appears uniformly dark or black across standard imaging sequences. This low signal intensity is due to the dense, highly organized fibrocartilage structure, which contains very little mobile water.
When viewed in cross-section, the healthy meniscus maintains a distinct triangular or wedge-shaped profile. This consistent shape and uniform darkness are the standard against which pathology is measured. Any deviation from this dark, triangular morphology suggests either degeneration or a traumatic injury.
The Core Visual Signs of a Tear
The fundamental visual sign of a meniscal tear on an MRI is the presence of an abnormal high signal intensity, appearing bright white, within the dark fibrocartilage. This bright signal represents fluid, typically synovial fluid, that has entered the tissue through a disruption in the cartilage structure.
For an abnormality to be classified as a true, clinically significant tear, the bright signal must unequivocally communicate with at least one articular surface of the meniscus. This means the white line must extend from the interior of the meniscus to either the superior surface (facing the femur) or the inferior surface (facing the tibia). If the bright signal does not breach the surface, it is categorized as internal degeneration, not an acute tear.
Radiologists apply the “two-slice-touch rule” for high diagnostic confidence. This criterion requires the abnormal signal extending to the meniscal surface to be visible on at least two consecutive MRI slices. Visibility across multiple slices confirms the three-dimensional nature of the tissue disruption. In severe trauma, the tear may also manifest as fragmentation or displacement, where a piece of the meniscus shifts away from its normal position.
Grading the Severity of Meniscus Damage
Meniscal abnormalities are categorized into three grades based on the appearance of the internal signal intensity, distinguishing true tears from degenerative changes. Only Grade III is considered a true, full-thickness tear that may require surgical intervention.
Grade I involves a small, focal area of high signal intensity entirely contained within the meniscus, representing early degeneration. Grade II is a linear area of high signal that is more extensive than Grade I but still does not reach the superior or inferior surface.
Grade I and Grade II findings are often degenerative changes, common in aging knees, and rarely cause mechanical symptoms. Grade III is the definitive meniscal tear, characterized by the high signal intensity communicating with and breaking the surface of the meniscus. This disruption correlates strongly with clinical symptoms and guides the decision for surgical treatment.
Common Patterns of Meniscus Tears
Once a Grade III tear is identified, it is classified based on its geometric pattern, as this morphology influences symptoms and surgical repair strategy.
Horizontal (Cleavage) Tears
The horizontal or cleavage tear runs parallel to the tibial plateau, splitting the meniscus into upper and lower halves. These tears are frequently associated with chronic degeneration, particularly in older patients.
Radial Tears
Radial tears are characterized by a vertical split that runs perpendicular to the circumferential axis of the meniscus, extending from the inner free edge outward. These tears are problematic because they disrupt the circumferential “hoop stress” mechanism that allows the meniscus to distribute load, potentially leading to instability.
Bucket-Handle Tears
A bucket-handle tear is a specific type of longitudinal tear where a large, inner fragment separates and displaces into the central part of the knee joint. On MRI, this displaced fragment can create the “double posterior cruciate ligament sign,” appearing as an extra band of tissue next to the normal ligament. This pattern is often associated with the knee physically locking and requires immediate attention.