Anterior Cruciate Ligament Injury: Anterior Cruciate Ligament Injury: MR Imaging Diagnosis and Patterns

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  • Anterior Cruciate Ligament Injury: MR Imaging Diagnosis and Patterns of Injury1 Erick M. Remer, MD

    Steven W. Fitzgerald, MD

    Harold Friedman, MD

    Lee F. Rogers, MD

    Ronald W. Hendrix, MD

    Michael F. Schafer, MD

    The anterior cruciate ligament (ACL) is an important stabilizer of knee

    motion. Injury of the ACL can lead to substantial disability; an accu- rate diagnosis ofACL injury is vital in both short-term and long-term

    patient care. Magnetic resonance (MR) imaging has emerged as the

    study of choice to evaluate the status of the ACL and other associ- ated structures in the knee. Sagittal MR images have been commonly

    used in the evaluation of the ACL. However, the authors believe that coronal and axial imaging planes can add useful information about

    ACL injury and, thus, lead to improved accuracy and confidence re-

    garding diagnosis. Multiplanar imaging can readily demonstrate me- niscal, ligamentous, and bone marrow injuries that commonly occur with the most frequent mechanisms ofACL injury. These mechanisms,

    in order of frequency, include internal rotation and valgus stress, hy-

    perextension, and varus stress with external rotation. An understand-

    ing ofthese mechanisms is helpful in the MR diagnosis ofACL injury.

    U INTRODUCTION The anterior cruciate ligament (ACL) stabilizes the knee by preventing anterior

    translation and internal rotation of the tibia with respect to the femur. Loss of these

    restraints leads to substantial morbidity and can result in secondary dysfunction of

    other structures in the knee. Thus, early and accurate diagnosis ofACL injuries is

    Abbreviation: ACL anterior cruciatc ligament

    Index terms: Knee, ligaments and menisci, 452.40, 452.4852, 4526.4857 #{149}Knee, MR. 452.1214 #{149}Ligaments. injuries,

    4526.4857

    RadloGraphics 1992; 12:901-915

    I From the Departments of Diagnostic Radiology (E.M.R., S.W.F., L.F.R., R.W.H.) and Orthopaedic Surgery (M.F.S.), 01-

    son Pavilion 3536. Northwestern University Medical School and Northwestern Memorial Hospital. 710 N Fairbanks, Chi-

    cago, IL 6061 1; and Northeast Illinois Magnetic Resonance Imaging, Prairieview, Illinois (HF.). From the 1991 RSNA

    scientific assembly. Received February 19, 1992; revision requested March 19 and received May 26; accepted May 26.

    Address reprint requests to S.W.F.

    C RSNA. 1992

    901

  • 902 U RadioGraphic-s U Remer et a! Volume 12 Number 5

    important. However, despite being the most

    common ligamentous injury ofthe knee (1),

    most ACL tears are missed in initial clinical

    encounters (2). The ACL is injured in field sports, skiing,

    motor vehicle accidents, and falls (2,3). Inju-

    ries most commonly occur from actions in-

    volving deceleration, twisting, or jumping.

    Because of the morbidity associated with the

    ACL-deficient knee, accurate diagnosis of ACL

    injury is essential for appropriate patient care.

    Making an accurate diagnosis, however, is

    often difficult because symptoms can mimic

    meniscal and other ligamentous injury. The

    clinical examination depends on the ability to

    demonstrate anterior tibial motion relative to

    a fixed femur (such as with the Lachman and

    pivot shift tests) (3). In the acutely swollen

    knee, these tests are difficult to perform be-

    cause of pain and guarding; accuracy is thus

    diminished. Hemarthrosis seen at arthrocen-

    tesis is suggestive of a cruciate injury but is

    nonspecific (3).

    Conventional radiographs have limited

    value in the diagnosis of acute ACL injury.

    Findings are indirect and limited to bone or

    soft-tissue abnormalities (4). Anterior tibial

    spine avulsion is a rare but specific finding (5). An exaggeration of the normal indenta-

    tion in the middle third of the lateral femoral

    condyle (lateral “notch” sign) has been re- ported to be suggestive of an ACL tear (5).

    Arthrography has been used but is invasive

    and requires careful technique and interpre-

    tation (5). Its accuracy has varied in reported

    series (60%-97%) (6).

    Magnetic resonance (MR) imaging has

    markedly expanded the role of radiology in

    the evaluation of acute knee injuries. Along

    with meniscal tears, injuries of the ACL were

    described in early reports of MR imaging of

    the knee (7-9). Although early studies dem-

    onstrated ACL tears, the reliability ofACL in- jury detection was not well established (10).

    Difficulties in diagnosing ACL injuries soon

    became apparent with use of straight sagittal images. Subsequent reports have emphasized

    the utility of angled sagittal images (1 1), dou-

    ble-oblique sagittal images (12), and T2-

    weighted sagittal images (13) to improve the

    accuracy of MR imaging in the diagnosis of

    ACL tears.

    This article reviews the normal appearance

    ofthe ACL and signs ofACL injury as seen on sagittal, coronal, and axial MR images. We

    demonstrate the utility of coronal and axial

    images in the diagnosis ofACL injury and il-

    lustrate how reliance on the sagittal plane

    alone may lead to pitfalls in interpretation.

    Common mechanisms ofACL injury and their

    associated meniscal, ligamentous, and bone

    marrow abnormalities are discussed.

    U NORMAL ANATOMY Knowledge of anatomic features of the ACL

    on MR images is essential to the detection of

    injury. The ACL is composed of collagen

    fibrils grouped to form 1-20-�m fibers. These

    ligamentous fibers merge to form larger sub-

    fascicular units that fan out into functional

    bands (14). Discrete anteromedial and pos-

    terolateral bands have been identified. The

    anteromedial band becomes taut in flexion,

    when most of the ligament is relaxed. In ex-

    tension, the larger, posterolateral portion is

    under tension (15). These collagen-based

    bands are attached to bone by a transitional

    zone of fibrocartilage and mineralized carti-

    lage. This transitional zone affords a gradual

    change in stiffness, thus helping prevent

    stress concentration at the two attachment

    sites of the ACL (16).

    The ACL originates from the posterior me-

    dial surface of the lateral femoral condyle, where it attaches in a semicircle. Its larger and

    stronger tibial insertion is broad and fanlike

    and is in the anterior intercondylar area,

    slightly lateral and anterior to the anterior

    tibial spine (15). In this location, it is inti-

    mately associated with the anterior horn of

    the medial meniscus. The ACL is, on average,

    4 cm long and 1 cm thick (17). Because of the

    orientation of its attachments to bone, the

    ACL turns on itself in an outward spiral as it

    courses anteriorly, medially, and inferiorly

    across the joint as it passes from the femur to

    the tibia. Along with the posterior cruciate

    ligament, the ACL is completely covered by

    synovium and is thus extrasynovial but intra-

    articular. The middle genicular artery, arising

    from the popliteal artery, is the primary sup-

    ply to the ACL, with additional supply from

    the medial and lateral genicular arteries (17).

    Nerve fibers are supplied by branches of the

    tibial nerve (17).

    U PROTOCOL FOR MR IMAGING AND APPEARANCE OF THE NORMAL ACL Imaging in 205 patients with arthroscopic cor-

    relation was performed with a dedicated knee

    coil on one of three units: Philips 55 (0.5 1) and 515 (1 .5 1) (Philips Medical Systems

    North America, Shelton, Conn) or Hitachi 0.2

    T (Hitachi Medical Corporation of America,

    Tarrytown, NY) . Our standard protocol con-

  • -.�,,

  • a. b.

    C.

    904 U RadioGrapbic.s U Remer et a! Volume 12 Number 5

    Figure 3. Normal ACL anatomy with a constant appearance on coronal MR images (2,200/30), with

    a section thickness of 4 mm. (a) Image shows the

    proximal ACL at the femoral origin as a thin, low- signal-intensity band paralleling the inner aspect of

    the lateral femoral condyle (arrow). (b) Image ob-

    tamed through the central ACL adjacent to the hori-

    zontal portion of the posterior cruciate ligament

    again shows the ACL as a low-signal-intensity band

    as it begins to curve toward the tibial spine (arrow).

    (c) A more anterior image of the distal ACL demon- strates the normally expected increased signal in-

    tensity of its fanlike insertion (arrow). This signal

    intensity is thought to result from nonfibrous tissue

    as the distal ACL unwinds to insert itself on the an-

    terior tibial spine.

    Most ACL tears involve the middle sub-

    stance of the ligament (90%) and, infre-

    quently, the femoral (7%) or tibial (3%) at-

    tachments (1). The angle at which the image

    is obtained determines whether the femoral

    or tibia! attachments are well seen. The distal

    end of the ACL is generally better seen than

    the proximal end because of partial volume

    averaging of the proximal ligament with the

    medial aspect of the lateral femoral condyle. If

    the distal and middle ACL is seen coursing in

    the proper direction and unassociated with a

    soft-tissue mass, it is usually nor