The Quality of MRI in Meniscal Tears

This article summarizes my research and clinical experience in the diagnosis of meniscal tears using knee MRI. I have referenced the recent literature and give my opinions as well as unpublished clinical observations when definitive research is not available on specific topics. When evaluating a knee MR examination, I study the medial and lateral menisci first on the sagittal images and then on the coronal images because sagittal images are the most useful in diagnosing meniscal tears. In one study, 95 % of medial and 90 % of lateral meniscal tears could be identified on sagittal MR images. In a later study, 82 % of meniscal tears were definitively diagnosed on sagittal images alone. Menisci should have low signal intensity on MR images because of their fibrocartilage composition, but they may have central globular or linear increased signal intensity secondary to internal mucinous degeneration. In addition, the menisci in asymptomatic children have a 66 % frequency of internal signal on MRI and that signal is presumed to reflect normal vascularity. Another proposed cause of increased intrameniscal signal is a meniscal contusion after acute trauma. I reserve the diagnosis of meniscal contusion for menisci with internal signal equal to fluid on T2-weighted images in patients with a recent episode of trauma. Knowing the distribution of meniscal tears is helpful in assessing the menisci on MRI. In an arthroscopic series of 1086 medial meniscal tears, the posterior horn was involved in 98 % of the torn medial menisci. Because of this tear distribution, I am cautious in diagnosing a medial meniscal tear that does not involve the posterior horn. However, lateral meniscal tears are more varied in location: Investigators who conducted an arthroscopic series of 399 lateral meniscal tears reported that tears involved the posterior horn in 55 %, the body or the body and anterior horn in 30 %, and the anterior horn alone in 15 %. The Quality of MRI In Meniscal Tears: Despite the improvement in the quality of knee MR images in the past 25 years, the two primary MR criteria for the diagnosis of meniscal tears have not changed since the late 1980s. These criteria are, first, contact of intrameniscal signal with the superior or the inferior surface of a meniscus (or with both surfaces) and, second, distortion of the normal appearance of a meniscus. To diagnose a meniscal tear using these criteria, it is essential to understand how normal variations in the shape of the menisci and their attachments compare with the MR appearance of a meniscal tear.