A knee magnetic resonance imaging (MRI) scan provides detailed images of the knee joint’s internal structures, including soft tissues like cartilage, tendons, and ligaments. The meniscus, a C-shaped cartilage in the knee, plays a significant role in joint function. This article explains how to interpret a knee MRI for meniscal tears. The information presented serves an educational purpose and should not replace professional medical advice or a diagnosis from a healthcare provider.
The Meniscus: Anatomy and Function
The knee joint contains two crescent-shaped cartilages called menisci: the medial meniscus on the inner side and the lateral meniscus on the outer side. These structures are primarily composed of fibrocartilage, offering both flexibility and strength. The medial meniscus is more C-shaped and less mobile, which can make it more susceptible to injury. In contrast, the lateral meniscus is more like a closed circle and exhibits greater mobility, providing some protection against tears.
The menisci perform several important functions. They act as shock absorbers, distributing weight evenly across the joint surfaces and reducing stress during activities that load the knee. They also enhance knee joint stability and contribute to load transmission. Menisci aid in lubricating the joint, which helps to reduce friction and protect the articular cartilage on the bone ends.
Understanding Your MRI Scan: Basics and Views
MRI uses a powerful magnetic field and radio waves to generate detailed images of soft tissues. Unlike X-rays, MRI does not use ionizing radiation, making it a common choice for evaluating knee pain, swelling, or weakness. The machine creates multiple “slices” or images viewable from different angles.
Different image “sequences” highlight specific tissues and conditions. T1-weighted images visualize fat and anatomical details, while T2-weighted and proton density (PD) sequences detect inflammation or fluid accumulation by sensing water content. Proton density fat-saturated (PD FS) sequences are commonly used in knee MRIs for excellent soft tissue contrast, clearly showing joint structures, ligaments, and cartilage.
Knee MRIs are acquired in three main planes or “views” for comprehensive assessment. The sagittal view captures images from the side, dividing the knee into left and right halves. The coronal view shows the knee from front to back, segmenting it into anterior and posterior sections. The axial view presents cross-sections from top to bottom, as if looking down the leg. Each view offers unique perspectives, allowing thorough examination of the meniscus and surrounding structures.
Identifying Meniscal Tears on MRI Images
A healthy meniscus appears as a uniformly dark, triangular structure on most MRI sequences. Its dark appearance reflects its dense fibrocartilaginous composition. When examining images, the inner free edge of the meniscus should appear sharp and clear.
Meniscal tears are indicated by specific visual signs on MRI. One indicator is increased signal intensity, appearing as a bright (white) signal within the normally dark meniscus. This bright signal suggests fluid or damage within the cartilage. For a definitive tear diagnosis, this high signal must extend to at least one articular surface of the meniscus, visible on more than one consecutive image.
Another sign is abnormal meniscal morphology, referring to changes in the meniscus’s normal shape or contour. This can include blunted edges, an irregular outline, or displaced fragments. Different tear patterns, such as horizontal, radial, or vertical tears, present with varying appearances, including lines of disruption or abnormal shapes. For instance, a horizontal tear appears as a line parallel to the tibial plateau, dividing the meniscus into upper and lower parts.
Decoding the MRI Report: Key Terms and Findings
The radiologist’s report interprets MRI images, using specific terminology for meniscal tear findings. “Signal abnormalities” are common terms, with “increased signal” or “intrasubstance signal” describing bright areas within the meniscus. These are graded: Grade I indicates a small focal area of increased signal not extending to the surface, while Grade II involves linear areas of increased signal also not reaching the surface. A Grade III signal extends to at least one articular surface, signifying a definite meniscal tear.
Radiologists describe “types of tears” based on their configuration. Common terms include “horizontal tear,” which runs parallel to the tibial plateau, and “radial tear,” which extends perpendicular to the meniscus’s circumference. A “longitudinal tear” appears as a vertical line parallel to the meniscus’s main axis. More complex tears include “complex tear” (a combination of patterns), “bucket handle tear” (a displaced longitudinal tear), or “flap tear” (a displaced horizontal or vertical tear fragment).
The report may also list “associated findings” that accompany meniscal tears. These include “meniscal extrusion” (meniscus pushed out of position) or “parameniscal cyst” (fluid-filled sac near the tear). “Joint effusion” (fluid or swelling within the joint) is another common associated finding. Conversely, “intact meniscus,” “normal signal,” or “no evidence of tear” indicate a healthy meniscus. Discuss the radiologist’s findings with a doctor for comprehensive understanding and appropriate medical guidance.