MR findings of meniscal extrusion correlated with arthroscopy.

PURPOSE: To correlate MR exams showing meniscal extrusions >3 mm beyond the tibial margin with arthroscopic findings. Meniscal extrusion (> 3 mm extension beyond the tibial margin) has been described on previous studies as having a high index of association with meniscal root tears. These previous studies were limited by a lack of exact arthroscopic correlation. We also assessed the prevalence of meniscal root tears on MR examinations without meniscal extrusions.
MATERIALS AND METHODS: A retrospective review was performed of 300 consecutive knee MR exams of patients who went on to arthroscopy. All patients had a complete MR exam performed on a 3T GE MR scanner. MR exams were reviewed for medial meniscal extrusion > 3 mm from the tibial margin. The exams with medial meniscal extrusion were reviewed for meniscal tears on MR exam. The exams without meniscal extrusion were reviewed for meniscal root tears. Results were subsequently correlated with arthroscopy.
RESULTS: Of the 300 MR exams, 42 demonstrated medial meniscal extrusion >3 mm from the tibial margin. Of these 42 patients, 34 had meniscal degeneration, complex tear, or a large radial tear near to or involving the meniscal root on MR examination. A total of 33 of these tears described on MR exam were seen at arthroscopy. A total of 24 of these tears were root tears, seven were complex tears, and two had severe meniscal degeneration. There was one root tear described on MR exam that was not seen on arthroscopy. Eight of the 42 patients had no meniscal tear demonstrated on MR examination or arthroscopy. All eight of these patients were > or = 50 years old. There were two meniscal root tears on both MR exam and at arthroscopy in the 258 patients without meniscal extrusion on MR exam.
CONCLUSION: There is a high prevalence of meniscal root tears in patients with meniscal extrusion on MR exam. Meniscal root tears are uncommon in patients without meniscal extrusion on MR exam. There may be a subset of patients in which the meniscal root is stretched rather than torn. Medial meniscal extrusion in patients > 50 years old may be associated with a meniscal "stretch" injury due to degeneration of the meniscus without a meniscal tear detectable on arthroscopy. These menisci may have increased laxity due to compromised meniscal collagen fibers. This may predispose the patient to premature osteoarthritis. (c) 2008 Wiley-Liss, Inc.