Magnetic Resonance Imaging for the Diagnosis and Treatment of Anterior Cruciate Ligament Tears

Abstract

The high sensitivity and specificity of magnetic resonance imaging in diagnosis of anterior cruciate ligament (ACL) tears makes it the “gold standard” imaging modality in the diagnosis of ACL injuries. In this chapter, we discuss the imaging appearances of an intact ACL, as well as the primary and secondary imaging signs of an ACL tear. We also discuss the imaging appearances of partial ACL injuries and associated injuries identified with ACL tears.

Keywords

Anterior cruciate ligament, Magnetic Resonance Imaging

Introduction

Magnetic resonance imaging (MRI) is the most widely used noninvasive investigation for diagnosis of anterior cruciate ligament (ACL) tears. MRI has been shown to have a high sensitivity and specificity for detection of ACL tears. Although clinical evaluation of ACL tears can be highly accurate in experienced hands, MRI can provide valuable information regarding associated injuries, particularly with regard to meniscal and chondral lesions. Such information can be important in preoperative planning and in determining the timing of surgical treatment.

In this chapter, we will discuss the MRI hardware and protocol requirements for imaging the ACL. The MRI appearances of an intact ACL, as well as the imaging appearances of partial and complete tears of the ACL will also be discussed.

Magnetic Resonance Imaging Hardware and Protocol

MRI of the knee is most commonly performed on a 1.5T or 3.0T platform, using a dedicated knee coil. Extremity open bore magnets with varying field strengths have also been used to avoid claustrophobia and as a rapid access evaluation of the knee. Use of a 3.0T platform does not significantly improve diagnostic performance in evaluation of ACL tears when compared with 1.5T.

The specific imaging protocols utilized vary significantly between institutions. However, most institutions use a combination of short echo time (TE) and fat-suppressed fluid sensitive pulse sequences in three orthogonal planes. The sagittal plane may be obtained either as a direct sagittal or with slight obliquity paralleling the ACL. These standard orthogonal planes may be supplemented with additional oblique imaging planes such as coronal oblique and axial oblique planes parallel and perpendicular to the long axis of the ACL respectively. A field-of-view of 14–16 cm and a slice thickness of 3–4 mm are most commonly employed. More recently, three-dimensional (3D) fast spin-echo pulse sequences have been added to the standard pulse sequences at some institutions. Such isotropic acquisitions have the advantage of high spatial resolution and the ability to perform multiplanar reformats.

Keywords

Anterior cruciate ligament, Magnetic Resonance Imaging

Introduction

Magnetic resonance imaging (MRI) is the most widely used noninvasive investigation for diagnosis of anterior cruciate ligament (ACL) tears. MRI has been shown to have a high sensitivity and specificity for detection of ACL tears. Although clinical evaluation of ACL tears can be highly accurate in experienced hands, MRI can provide valuable information regarding associated injuries, particularly with regard to meniscal and chondral lesions. Such information can be important in preoperative planning and in determining the timing of surgical treatment.

In this chapter, we will discuss the MRI hardware and protocol requirements for imaging the ACL. The MRI appearances of an intact ACL, as well as the imaging appearances of partial and complete tears of the ACL will also be discussed.

Magnetic Resonance Imaging Hardware and Protocol

MRI of the knee is most commonly performed on a 1.5T or 3.0T platform, using a dedicated knee coil. Extremity open bore magnets with varying field strengths have also been used to avoid claustrophobia and as a rapid access evaluation of the knee. Use of a 3.0T platform does not significantly improve diagnostic performance in evaluation of ACL tears when compared with 1.5T.

The specific imaging protocols utilized vary significantly between institutions. However, most institutions use a combination of short echo time (TE) and fat-suppressed fluid sensitive pulse sequences in three orthogonal planes. The sagittal plane may be obtained either as a direct sagittal or with slight obliquity paralleling the ACL. These standard orthogonal planes may be supplemented with additional oblique imaging planes such as coronal oblique and axial oblique planes parallel and perpendicular to the long axis of the ACL respectively. A field-of-view of 14–16 cm and a slice thickness of 3–4 mm are most commonly employed. More recently, three-dimensional (3D) fast spin-echo pulse sequences have been added to the standard pulse sequences at some institutions. Such isotropic acquisitions have the advantage of high spatial resolution and the ability to perform multiplanar reformats.

Magnetic Resonance Imaging of Normal Anterior Cruciate Ligament

The ACL extends from the posteromedial aspect of the lateral femoral condyle to the anterior aspect of the tibial eminence. The posterolateral bundle arises more distally than the anteromedial bundle. The origin of the ACL is best evaluated on axial and coronal images and has a uniform low signal intensity appearance on all pulse sequences, with an oval morphology on the most proximal axial images through the intercondylar notch. The normal ACL is taut and parallel to or slightly steeper than the line formed by the roof of the intercondylar notch (Blumensaat line). The tibial insertion of the ACL is flared and wide compared with the femoral origin, due to separation of the anteromedial and posterolateral bundles. The two bundles can be distinguished on axial and coronal images but not in the sagittal plane. The distal ACL demonstrates intermediate signal intensity due to interposition of fat and synovium, as well as the close relationship to the anterior horn of the lateral meniscus. In the sagittal or sagittal oblique plane, this can result in a linear, striated appearance particularly on fluid sensitive images, and this increased signal intensity should not be mistaken for a tear. On axial and coronal images, the bundles are seen to separate near their tibial insertion, with intermediate signal intensity demonstrable between the bundles. The imaging appearances of the normal ACL are demonstrated in Fig. 10.1 .

Though the evaluation of ACL was originally performed predominantly in the sagittal plane, the addition of coronal and axial images increases the sensitivity, specificity, and confidence in diagnosis of tears. The entire length of the ACL may not be visualized in the sagittal plane in 5%–10% of cases. Oblique coronal and sagittal and oblique axial imaging have been advocated by various authors to increase the diagnostic accuracy ( Fig. 10.2 ). However, no improvement in diagnostic efficacy was achieved with simultaneous use of more than one oblique plane imaging for diagnosis of ACL tear.

Magnetic Resonance Imaging Appearances of Anterior Cruciate Ligament Injury

MR imaging is highly accurate in diagnosis of ACL tears with sensitivities ranging from 85% to 95% and specificities of more than 95%. MR signs of ACL injury can be divided into primary signs, which are those related to the changes within the ACL, and secondary signs, which are those related to osseous contusions and biomechanical changes of the knee. Over 20 primary and secondary signs of ACL injury have been described in the literature. In general, with modern high-quality MR imaging, the primary signs of ACL failure are far more useful in diagnosis and grading of ACL tears than the secondary signs, which are typically of limited utility. However, in equivocal cases and partial tears, the secondary signs may be helpful in the diagnosis of an ACL tear.

Primary Signs

The primary signs for diagnosis of ACL tear on MRI have a sensitivity of 93% and specificity of 97%. The most specific primary sign of an acute ACL injury is discontinuity of the ACL fibers. Although this may be appreciated on any of the orthogonal or oblique planes, it is often best visualized on sagittal or axial images. This is most commonly seen in relation to the mid ACL ( Fig. 10.3 ). In addition, the normal orientation of the ACL may be altered. This is best appreciated on sagittal images. The ligament may not be parallel to the Blumensaat line, with the proximal ACL having a more vertical orientation and the distal ACL acquiring a more horizontal orientation ( Fig. 10.4 ). An ACL-tibial plateau angle of 45 degrees or less has a high specificity and sensitivity in diagnosis of ACL tears. In some cases the distal fibers may flip anteriorly in the intercondylar notch.

Diffuse intrinsic changes of the ACL may be encountered on MRI following an acute or subacute injury. The ligament may demonstrate increased signal intensity on short TE and fluid sensitive sequences secondary to edema and hemorrhage, resulting in a mass-like appearance ( Fig. 10.5 ). This is considered to be the most common type of imaging appearance of ACL tear. Diffuse signal change and thickening of the ACL can be seen in the setting of partial or complete tears.

Although the majority of ACL tears occur proximally, identification of distal footprint avulsions, typically in adolescents, is important, as they may be amenable to primary surgical fixation. The distal avulsions are best evaluated on sagittal and coronal planes, whereas proximal tears/peel-off lesions are well visualized on axial and coronal images ( Fig. 10.6 ). Additional imaging with plain radiographs and CT may be of added value for determining the size and nature of any osseous components.