Anterior CruciateLigament Injury: MRImaging Diagnosis andPatterns of Injury1Erick 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 thatcoronal 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 occurwith 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 INTRODUCTIONThe 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 sagittalimages. 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 onsagittal, 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 ANDAPPEARANCE OF THE NORMAL ACLImaging 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 normal even if

the proximal end is not optimally seen (13).

On a series ofcoronal spin-echo images,

the ACL can be traced from its posterosupe.

nor and lateral origin to its anteroinferior and

medial insertion. Again, the proximal two-

thirds of the ACL has homogeneous low signal

-‘

�

:J5.

906 U RadioGraphics U Remer et a! Volume 12 Number 5

Figures 5, 6. ACL tears. (5) Sagittal MR image

(650/20) demonstrates diffusely increased signal

intensity within a poorly defined ACL (*). A corn-plete tear of the middle substance of the ACL was

identified at arthroscopy. (6) Value of T2-weightedimages in the diagnosis of an ACL tear. (a) Sagittal

Ti-weighted MR image (650/20) reveals poor defini-

tion of the ACL with disordered fibers and increasedsignal intensity within the ACL substance (arrow).

(b) Sagittal T2.weighted MR image (2,200/80) dem-onstrates the disruption ofACL fibers, diffusely ab-

normal signal intensity within the ACL, and a dis-

crete band of high signal intensity within theproximal ACL (arrow). This complete proximal tearwas confirmed at arthroscopy.

U MR IMAGING OF ACL TEARS ANDASSOCIATED INJURIES

The earliest studies applying MR imaging to

the evaluation of the knee demonstrated ACL

injuries (7-9). Subsequent reports defined

the accuracy of MR imaging in demonstratingACL injury. In two series (13,20), the addition

of T2-weighted sagittal MR images was docu-

mented to improve accuracy to 97%. Specific

signs ofACL injury include irregular contour,

discontinuity, focal or diffusely increased in-

trasubstance signal intensity, or poor visual-ization (Fig 5) (13,18,20). T2-weighted MR

images allow high-signal-intensity joint fluid

to be distinguished from a normal ligament

(Fig 6). Areas of interstitial edema or hemor-

rhage are also manifested as increased signal

intensity within the ACL tear on T2-weighted

images. Ifvolume averaging with the lateral

femoral condyle is excluded, the finding of an

intermediate-signal-intensity area on Ti-

weighted images that generally increases in

intensity on T2-weighted images (with or

without discontinuity) is indicative of a tear

(1,18,20).

Figure 8. Bone marrow injury associated with

ACL injury. Sagittal MR image (650/20) through

the lateral compartment reveals low signal inten-

sity within the subarticular marrow of the middle

portion of the lateral femoral condyle (solid ar-

rows). The overlying cartilage appears intact. Geo-

graphic low signal intensity (open arrow) is also

seen within the posterior aspect of the lateral tibia!

plateau. Bone marrow injuries, primarily involving

the lateral compartment, are commonly observedin patients with ACL tears.

steocf�(Jfl( ral injury associated v

ACL injury. Sagittal MR image (650/20) ofan ii-

year-old child with an acute ACL tear shows focal

low signal intensity within the subchondral marrow

(small arrows), an indentation of the cortex, and a

focal area of abnormal signal intensity within the

overlying articular cartilage (large arrow). Arthros-

copy revealed a grade III chondral lesion and a lat-

era! meniscal tear in association with ACL injury.

908 U RadioGrapbics U Remer et a! Volume 12 Number 5

Recent reports have indicated that MR im-

aging can depict a broad spectrum of bone

marrow injury (21-25). Occult bone injuries

have been detected in the lateral compart-

ment in 85%-97% of patients with ACL inju-

ries (22,23). The injuries were primarily bonemarrow contusions or osteochondral lesions.

Bone marrow contusion and edema (Fig 8)

involved the lateral compartment in 72% of

patients with acute ACL tears in one series

(22). In our series, the frequency was some-

what less (68%); however, our patients had

acute, subacute, and some chronic ACL inju-ries. Despite the limitation that bone marrow

injuries would be expected to resolve over

time, lateral compartment bone marrow in-

jury has a much higher predictive value with

respect to ACL status than does meniscal in-

jury (Table).

Osteochondral injuries of the lateral femo-ral condyle occur less frequently (20%) (21)

but are more specific for concurrent ACL in-

jury than lateral compartment contusion. This

finding is the MR imaging equivalent of the

plain radiographic notch sign discussed previ-

ously. It is seen in the lateral femoral condyle

overlying the anterior horn of the lateral me-

niscus as a region of low signal intensity onTi-weighted MR images (Fig 9); the signal

intensity increases on T2-weighted MR images

and is accompanied by deformation or inter-

ruption of the articular cartilage.

a. b.

A B

910 U RadioGraphics U Remer et a! Volume 12 Number 5

Figure i2. MR diagnosis ofACL injury on coronal images. (a) Coronal MR image (2,200/30) obtained

through the intercondylar notch demonstrates nonvisualization of the ACL adjacent to the horizontal seg-

ment of the posterior cruciate ligament. Note the associated medial collateral injury (solid arrows) and lat-era! tibial edema (open arrows). (b) A more anterior image in the same patient helps confirm a lack of ACL

fibers and shows a distal stump (arrow) attached to the anterior tibia! spine. We refer to this appearance as

the empty notch sign, which, in our series, had a 100% positive predictive value for ACL injury.

Figure 15. Internal rotation in valgus stress in-

jury of the ACL. During internal rotation of the tibia

(A), the ACL may be torn. Ifvalgus stress is then

also applied to the ACL-deficient knee, the poste-

rior aspect of the lateral tibia can impact on the

lateral femoral condyle, producing bone marrow or

osteochondral injuries (shaded areas) within thelateral compartment. When the knee returns to a

neutral position (B), the sites ofbone marrow in-

jury are no longer adjacent.

distal segments to equal advantage. In partic-

ular, the accuracy of diagnosis of proximal

ACL injuries is improved with coronal images.

An indistinct ACL on coronal images adjacent

to the horizontal segment of the posterior

cruciate ligament was most common in com-plete ACL tears, producing what we refer to as

the “empty notch” sign (Fig 12). As previ-

ously noted in the section on normal anat-

omy, the distal third of the ACL manifests

increased signal intensity on Ti- and T2-weighted coronal images and should not be

mistaken for injury.

Axial spin-echo sequences enable a cross-

sectional assessment ofACL injury and, at

times, a unique perspective on the status of

the ACL. We have found an indistinct ACL,

increased signal intensity within the ACL, and

nonvisualization of the ACL to be predictive of

ACL injury (Figs 13, 14). Although these find-

ings are usually also observed on sagittal or

coronal images, axial images serve to confirm

initial interpretations and improve diagnostic

confidence in questionable cases.

September 1992 :� V

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swollen ACL wlth abnormal high slgnal tntensity consistent with a cornplett. tt.ar (arrow)

and coronal MR images were also suggestive of an ACL tear�/(13b)�Axial MR image (2,200/ 30) in awith a confirmed ACL tear The middle portion of the ACL isnotvisuahzed adjacent to tht p

ligament This is the axial equivalent of the empty notch sign am in this case sagittal and c

ages demonstrated an ACL injury. (i4a) Axial MR image ‘� �‘ reveals abnormalACL contoi.

and increased intrasubstance signal intensity. (i4b) � �“ � .�-.-.

a swollen ACL sheath with diffusely increased a

found at surgery.

U MECHANISMS OF ACL INJURY �Although ACL injuries can occur during a #{149}� �,

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912 U RadioGraphics U Remer et a! Volume 12 Number 5

Figures 16, 17. (16) Internal rotation and valgusstress injury. (a) Sagittal reconstruction of three-

dimensional gradient-echo data set (500/23, flipangle of 70#{176})demonstrates an ill-defined contusion

within the middle portion of the lateral femoral

condyle (arrows). (b) Adjacent sagittal MR image

(500/23, flip angle of 70#{176})demonstrates contusion

of the posterior tibia! plateau (arrow). These contu-

sions are not adjacent due to rotational forces at the

time ofinjury. (17) Lateral compartment injury as-

sociated with internal rotation in valgus stress

mechanism. Sagittal MR image (650/20) reveals an

osteochondral defect (notch sign) in the middle

portion of the lateral condyle (arrow).

rotation and valgus stress, the medial joint

compartment is distracted, which can pro-

duce medial collateral injury. Medial meniscaltears, which are frequently peripheral, can

also be present, and, along with ACL and me-

dial collateral ligament tears, form the well-

known O’Donoghue triad (25). Injury to the

lateral compartment occurs in the form of im-

pact injuries of the middle portion of the lat-

era! femoral condyle with the posterior lateral

tibia. In our experience, lateral meniscal tears

are also common.

In our series, internal rotation and valgus

stress was the most frequently observed

mechanism ofACL injury. The presence of

meniscal tears and medial collateral injury

was variable, presumably related to the sever-

ity of stress applied and the precise vectors

involved. Bone marrow injury of the lateral

compartment was much more commonly

noted (Fig 16), as has been previously re-

ported (2 1-23). Injury of the articular surface

of the lateral femoral condyle (Fig 17) (bone

marrow signal intensity abnormality or osteo-

chondral injury) was not associated with adja-

cent tibia! contusion. Instead, tibial signal

intensity abnormalities were posterior, consis-

tent with the mechanism of internal rotation.Medial compartment bone marrow injuries

were uncommonly observed.

The second most common mechanism of

ACL injury, hyperextension of the knee, oc-

curs much less frequently than does internal

rotation. Hyperextension can occur during

forward falls while skiing, jumping, and per-

forming high kick maneuvers (26). Schemati-

cally, one can visualize how hyperextension

results in an impact of the femoral condy!es

and the anterior tibia (Fig 18). ACL tears re-

sulting from hyperextension frequently (70%)

occur in isolation from meniscal or collateral

ligament injuries (27). We have observed this

L

914 U RadioGraphics U Remer et a! Volume 12 Number 5

all patients with Segond and Segond-like in-

jury patterns. Variations included more severe

disruption of the lateral complex, including

the iliotibial band (Fig 21).An understanding of mechanisms of ACL

injury can lead to more accurate detection of

injuries associated with ACL tears. In many

cases, the information regarding associated

meniscal or ligamentous injury is of greater

value to the physician than is MR confirmation

ofACL injury. Immediately after the injury,

most ACL tears are treated conservatively with

splinting and arthrocentesis if necessary (28).

Acute ACL reconstructions are not routinely

performed. In this setting, diagnostic informa-

tion obtained with MR imaging can be vital in

clinical decision making and patient counsel-ing. The presence of associated meniscal

tears, particularly unstable tears, or associatedligamentous injury may modify preoperative

planning (29,30). The need for arthroscopic

meniscal repair before ACL reconstruction

can be determined with MR imaging.

U SUMMARYThe short- and long-term seque!ae ofACL in-

juries mandate diagnostic accuracy. MR imag-

ing has been established as a highly useful

method of accurately evaluating the ACL.

However, we believe that a multiplanar ap-

proach can improve interpretive accuracy and

diagnostic confidence. A multiplanar ap-

proach to ACL assessment can result from a

dedicated multiplanar protocol or from the

use of multiplanar reconstructions obtained

with three-dimensional data sets. Although we

supplement our knee imaging protocol with

two-dimensional and three-dimensional gradi-

ent-echo sequences, we prefer a dedicated

multiplanar approach based on Ti- and T2-

weighted spin-echo sequences to evaluate the

ACL. This requires an understanding of nor-

mal ACL anatomy, variation, and injury in all

planes. Some types ofACL injury, most nota-

bly, the “lax” ACL and some healed chronic

tears (31), remain optimally demonstrated on

sagittal images. T2-weighted MR images are

essential for the accurate assessment of the

ACL, independent of the imaging plane.

A multiplanar protocol is useful in docu-

menting associated meniscal, ligamentous,

and bone marrow injuries. Recognition of

Figure 21. Varus stress injury. Coronal MR image

(2,200/80) demonstrates thickening and increased

signal intensity, findings consistent with rupture of

the iiotibial band (arrowhead). No tibia! rim avul-

sion fracture was demonstrated; however, iliotibial

band disruption results from a similar varus stress

mechanism.

characteristic injury patterns on MR images

and an understanding of common mecha-

nisms ofACL injury are helpful in the accuratediagnosis ofACL and associated injuries. In

particular, the detection of bone marrow inju-

ries and an awareness of injury patterns, as

demonstrated with a multiplanar approach,

are helpful in predicting the mechanism of

ACL injury.

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