The cruciate ligaments of the knee: correlation between MR appearance and gross and histologic findings in cadaveric specimens.

OBJECTIVE: Using anatomic and histologic correlation, we investigated the reasons for differences in the MR signal intensity between the normal anterior and posterior cruciate ligaments and for the focally increased signal intensity within these ligaments occasionally found in elderly patients.

MATERIALS AND METHODS: The MR images of 18 anterior and 20 posterior cruciate ligaments obtained from cadavers (age at death, 56-88 years old; mean, 74 years) were evaluated blindly. Their overall signal intensity and focal regions of increased signal were noted. The findings then were correlated with corresponding anatomic and histologic sections.

RESULTS: The anterior cruciate ligament had greater signal intensity than did the posterior cruciate ligament in 15 of 18 knees and had comparable signal intensity in three knees. No histologic basis to explain the different MR appearances of the anterior and posterior cruciate ligaments was found. Rather, differences in macroscopic anatomy appeared to be the best explanation. In the posterior cruciate ligament, the fibers were parallel, whereas they were diverging and twisted in the anterior cruciate ligament, causing volume-averaging artifacts. A focal area of signal increase was found in 29 of the 38 ligaments. In 17 of these 29 ligaments, mucoid and/or eosinophilic degeneration was found that appeared to have caused the focal MR signal change. In nine ligaments without focally increased signal on the MR images, only one had histologic evidence of degeneration. The predictive value of an abnormal finding on MR was 59%, and the predictive value of a normal finding on MR was 89%. The relationship between increased signal on MR and histologic degeneration was statistically significant (chi 2 test, p = .0126).

CONCLUSION: The differences in the MR appearances of the anterior and posterior cruciate ligaments probably are due to differences in gross architecture. Increased MR signal within the cruciate ligaments occurring in elderly patients often can be attributed to degenerative changes.