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Imaging the knee: Ligaments by ProQuest

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The anterior cruciate ligament The ACL extends in an inferior, anterior, and medial direction from its origin on the inner surface of the posterior, lateral femoral condyle to its insertion on the anterior tibial plateau anterior to the tibial spines between the attachments of the medial and lateral menisci and be - neath the transverse ligament.7 It consists of 2 distinct bands-the anteromedial and posterolateral bundles-according to their distal attachment's relationship to the tibial spine. Secondary signs of an ACL tear include anterior tibial translation and abnormal curvature of the posterior cruciate ligament (PCL), which are related findings, with the latter secondary to the former.

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									Imaging the knee: Ligaments
Daniel B. Nissman, MD, R. Hal Hobbs, MD, Thomas L. Pope, MD,
C. David Geier, Jr., MD, and William F. Conway, MD




I
    njuries of the knee are common. At        participate in the magnetic moment of        The anterior cruciate ligament
    our institution, the Medical Univer-      MRI. Therefore, under normal circum-            The ACL extends in an inferior, ante-
    sity of South Carolina, trauma and        stances, ligaments show low signal           rior, and medial direction from its origin
sports-related activities are the most fre-   intensity on all pulse sequences. Injury     on the inner surface of the posterior, lat-
quent causes of knee injuries. Second-        allows loosely bound hydrogen atoms          eral femoral condyle to its insertion on
ary to their role in maintaining stability,   as well as infiltrating edema and hemor-      the anterior tibial plateau anterior to the
the ligaments of the knee are commonly        rhage to produce signal on the various       tibial spines between the attachments of
involved in these injuries. To prevent        pulse sequences used to evaluate these       the medial and lateral menisci and be-
long-term sequelae, early diagnosis and       structures.6                                 neath the transverse ligament.7 It consists
treatment—whether conservative or sur-           MRI protocols for the knee vary by        of 2 distinct bands—the anteromedial and
gical—are key in planning management          magnet and interpreter preference. Pro-      posterolateral bundles—according to
of these injuries. Because of its excellent   tocols should include sequences ob-          their distal attachment’s relationship to
soft-tissue contrast, magnetic resonance      tained in the axial, coronal, and sagittal   the tibial spine. These bands function to
imaging (MRI) has proven very useful for      planes, with at least one fluid-sensitive     resist anterior displacement of the tibia
identifying these important structures.1-4    sequence. In general, a high field-           and hyperextension, respectively. Also,
In the immediate postinjury period, clini-    strength magnet and a dedicated knee or      because of these two separate compo-
cal assessment of the knee is unreliable,     extremity coil is preferred, but adequate    nents, the normal ACL is taut throughout
which accentuates the importance of MRI       evaluation can be attained with mid-         the full range of knee motion.8 Further,
as a diagnostic tool.5 This article reviews   and low-field magnets. Patients are           the posterolateral bundle provides an ele-
the MRI appearance of the knee liga-          imaged supine with the knee in slight        ment of rotational stability.
ments in their normal and injured states.     external rotation both for better visual-       The normal ACL is a low-signal-
                                              ization of the anterior cruciate ligament    intensity band that roughly parallels the
Basic imaging principles                      (ACL) and patient comfort. The authors       intercondylar roof (Blumensaat’s line).
   Because of the biochemical composi-        do not use intravenous contrast unless       Normal interspersed fat and connective
tion of ligaments, the tightly bound hy-      evaluating for neoplasm or infection.        tissue give the ACL a striated appear-
drogen molecules are unavailable to           Intra-articular contrast is used primarily   ance that should not be mistaken for
                                              in the patient who has had prior surgical    pathology. Usually, even with optimal
 Dr. Nissman is a Resident and Dr. Hobbs
                                              meniscal repair.                             positioning, the ACL is visualized on
 was a Musculoskeletal Fellow in the
 Department of Radiology and Radiologi-          In general, the collateral ligaments      ≥2 contiguous sagittal images rather
 cal Science, Medical University of South     are best evaluated in the coronal plane,     than on a single image (Figures 1 and 2).
 Carolina. Dr. Hobbs is now in private        and the cruciate ligaments and exten-           The ACL is commonly injured from
 practice in Augusta, GA. Dr. Pope is a       sor mechanism are best evaluated in          excessive valgus stress, also called the
 Professor of Radiology and Orthopedics
                                              the sagittal plane. The coronal plane is     “pivot-shift” mechanism. The classic ex-
 and the Director of the Hollings Cancer
 Center Breast Imaging Program;               also an important projection for the         ample of this is the clipping type of inju
								
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