KNEE EXTENSION AND FLEXION - MR DELINEATION OF NORMAL AND TORN ANTERIOR CRUCIATE LIGAMENTS

Objective: Our goal was to assess the effect of joint position of semi flexed and extended knees in MR delineation of the anterior cruciate l igament (ACL). Methods: With a mobile knee brace and a flexible surfac e coil, the knee joint was either fully extended or bent to a semiflex ed position (average 45 degrees of flexion) within the magnet bore. Se ts of oblique sagittal MR images were obtained for both extended and f lexed knee positions. Thirty-two knees with intact ACLs and 43 knees w ith arthroscopically proven ACL tears were evaluated. Two observers co mpared paired MR images of both extended and flexed positions and rate d them by a relative three point scale. Anatomic correlation in MR ima ges was obtained by a cadaveric knee with incremental flexion. Results : The MR images of flexed knees were more useful than of extended knee s in 53% of the case reviews of femoral attachments and 36% of reviews of midportions of normal ACLs. Compared with knee extensions, the MR images for knee flexion provided better clarity in 48% of reviews of d isrupted sites and 52% of residual bundles of torn ACLs. Normal ACL ap peared taut in the knee extension and lax in semiflexion. Conclusion: Compared with MR images of knees in extension, MR images of knees in f lexion more clearly delineate the femoral side of the ligament with wi der space under the intercondylar roof and with decreased volume-avera ging artifacts, providing superior visualization of normal and torn AC Ls.