Correlation of joint fluid and internal derangement on knee MRI

Objective. To test the hypothesis that in symptomatic patients, knees in which MRI examinations demonstrate no significant effusion will also be free of internal derangement. Design and patients. One hundred and fifteen knee MRI examinations performed between March 2002 and June 2002 at our institution were retrospectively reviewed, evaluating for both the presence of knee effusions and concurrent internal derangement. The amount of joint fluid was measured quantitatively by obtaining anteroposterior measurements in the midline and lateral aspects of the suprapatellar pouch using sagittal MR images. These measurements were then correlated with presence or absence of MRI-demonstrated internal derangement and statistical analysis was performed. Results. Of the 115 knee MRI examinations evaluated, 47 (41%) were negative for internal derangement and 68 (59%) were positive for internal derangement. Thirty-six of 115 (31%) examinations showed joint fluid with anteroposterior measurement of 10 mm or less in the lateral aspect of the suprapatellar pouch. Of these, 31 (86%) showed no concurrent internal derangement. Statistical analysis was also performed using other lateral suprapatellar pouch cutoff values. Conclusion. While knees in which MRI examinations demonstrate no significant effusion are most often free of internal derangement, there remain a significant number which will exhibit internal derangement. An anteroposterior measurement of 10 mm or less in the lateral aspect of the suprapatellar pouch is a reasonable threshold value for distinguishing a physiologic from a pathologic amount of joint fluid.