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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.
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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