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MRI for Posterior Cruciate Ligament Injuries

Overview

MRI is the preferred examination for evaluating posterior cruciate ligament (PCL) injuries.
It is the most sensitive and widely used modality for evaluating the PCL and the other cartilaginous and ligamentous structures of the knee.

MRI is superior to physical examination and has replaced CT and arthrography because it offers superior soft tissue resolution and is noninvasive.
The sensitivity of MRI has obviated the use of arthroscopy as a diagnostic tool for evaluating PCL injuries in almost all patients.

MRI should be obtained in all patients with suspected PCL tears because of the high incidence of injury to other structures of the knee, such as the ACL, MCL, lateral collateral ligament (LCL), and menisci.

PCL injuries are displayed in the magnetic resonance images below.

The normal femoral origin of the posterior cruciat

The normal femoral origin of the posterior cruciate ligament is shown on this T1-weighted coronal image. Only a portion of the ligament is seen because of the normal oblique course of the ligament. The posterior cruciate ligament (black arrow) is of lower signal intensity than the anterior cruciate ligament (open arrow).

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Combined anterior and posterior cruciate ligament

Combined anterior and posterior cruciate ligament (PCL) tear. Proton-dense sagittal image demonstrates straightening of the orientation of the course of the PCL (straight black arrow in A) resulting from the loss of the normal restraining function of the anterior cruciate ligament secondary to a severe tear. The proton-dense coronal view shows almost all of the substance of the PCL within one image (C), reflecting the abnormal accentuated vertical orientation of the ligament.

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Proton-dense sagittal image (A) and T2-weighted sa

Proton-dense sagittal image (A) and T2-weighted sagittal image (B) show an extensive tear involving the proximal and distal portion of the PCL. The margins of the PCL are well delineated indicating the ligament is initial.

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The roles of other radiologic modalities are as follows:

Plain radiography, including anteroposterior and lateral, is used an initial screening examination for evaluation of avulsion fracture (see the image below), dislocation, joint effusion, (lipo) hemarthrosis, and associated soft tissue injuries

Anteroposterior radiograph of the right knee demon

Anteroposterior radiograph of the right knee demonstrates interruption and discontinuity (black arrow) at the femoral origin of the posterior cruciate ligament, representing an avulsion fracture.

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Stress radiography in the lateral projection using gravitational assistance or muscle contraction can be performed to evaluate posterior displacement of the tibia

Arthroscopy is no longer needed to make the diagnosis of PCL tears as a result of the development of MRI with its excellent sensitivity and specificity; arthroscopy is indicated only when conditions, such as patient size and motion, degrade the quality of MRI or when intraorbital metallic foreign bodies, intracranial metallic surgical clips, or pacemaker wires preclude the performance of an MRI examination

CT scan is excellent for identifying underlying fractures, including small fractures produced by avulsion of the PCL from either attachment site; however, it provides suboptimal contrast for determining ligamentous injuries; thus, for other uses, CT has been supplanted by MRI

Arthrography can visualize the PCL only indirectly; it is not indicated for evaluating the ligaments of the knee

The following are MRI technique guidelines:

To characterize PCL abnormalities with MRI, all 3 planes (axial, coronal, sagittal) should be used

The use of a dedicated knee coil improves the signal-to-noise ratio

A small field of view (10-14 cm) helps improve spatial resolution but generally requires higher field strength magnets

An acquisition matrix of 256 and 1-2 excitations (NEX) is routinely used

Slice thickness of 3-4 mm with a 10-20% interslice gap is adequate for good resolution of the PCL

Both high-field (1.5 tesla [T]) and low-field (0.2 T) MRI systems are accurate in making the diagnosis of PCL tears

The following are imaging protocols
:

The sagittal oblique plane (performed 10-14° off the triangular line) is most sensitive for evaluating the PCL

The standard protocol for assessing the PCL has been the spin-echo sequence, including T2 fast spin-echo imaging (T2FSE) with or without fat suppression in all 3 planes; T2-weighted images (T2WI), with and without fat suppression, accentuate edema and hemorrhage within and around the PCL

Short T1 inversion recovery (STIR) protocols can be generated using a relaxation time (TR) of 4000 milliseconds, a TE of 18 milliseconds, a T1 of 140 milliseconds, and an echo-train length of 4

T1-weighted images (T1WI) can be generated using a TR of 500 milliseconds and a TE of 15 milliseconds

Conventional T2-weighted double echo images can be generated using a TR of 2000 milliseconds and dual-echo time (TE) sequences of 20 and 80 milliseconds

T2 or T2 star images (T2*) can be obtained with refocused 20-dimensional Fourier transformation (2-DFT) gradient-echo images using a TR of 400 milliseconds, a TE of 20 milliseconds, and a flip angle (FA) of 20-30°

Gradient-echo volume imaging using a slice thickness of less than 1 mm can reduce imaging time

3-Dimensional (3D) FT images can be acquired with a TR of 55 milliseconds, a TE of 15 milliseconds, and an FA of 10°

Axial 3D FT images can be acquired with a TR of 55 milliseconds, a TE of 15 milliseconds, and an FA of 10°; sagittal 3D FT T2WI can be generated using a TR of 33 milliseconds and a TE of 13 milliseconds

Normal MRI anatomy

On properly performed MRI examinations, the entire course of the PCL is visualized easily on a single or in the composite of 2 consecutive sagittal images but is not seen on a single coronal image. Such an appearance implies a more vertical course of the ligament related to buckling and foreshortening secondary to a PCL or ACL tear (see the images below).

Only a small portion of the posterior cruciate lig

Only a small portion of the posterior cruciate ligament is seen on a coronal image. Increased signal intensity within the posterior cruciate ligament on this proton density image is the result of an interstitial tear.

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Combined anterior and posterior cruciate ligament

Combined anterior and posterior cruciate ligament (PCL) tear. Proton-dense sagittal image demonstrates straightening of the orientation of the course of the PCL (straight black arrow in A) resulting from the loss of the normal restraining function of the anterior cruciate ligament secondary to a severe tear. The proton-dense coronal view shows almost all of the substance of the PCL within one image (C), reflecting the abnormal accentuated vertical orientation of the ligament.

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The normal PCL has a uniform low signal intensity and lacks the striations seen in the normal ACL except near the femoral insertion, where some signal may be seen, especially on T2 images. The normal PCL also has less signal intensity than the ACL (see the image below).

Proton-dense coronal image shows the relative inte

Proton-dense coronal image shows the relative intensity of the anterior cruciate ligament (white arrow) and the posterior cruciate ligament (black arrow). The posterior cruciate ligament has less signal intensity because its fibers are more linearly organized than the helically arranged ACL.

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Morphology of the normal PCL depends on the integrity of the ACL and depends on the degree of knee flexion. The PCL has a gentle convex posterior margin in extension or minimal flexion. When more flexion is applied, the ligament becomes taut and slightly thinner. Minimal buckling is normal.

When present, the ligament of Wrisberg is seen as an oblique fibrous band of low signal intensity extending from the posterior horn of the lateral meniscus to the medial femoral condyle. The ligament of Humphrey is a similar band of decreased signal intensity anterior to the PCL and is oriented in a plane similar to the ligament of Wrisberg (see the images below).

Proton-dense sagittal images of the knee. The cour

Proton-dense sagittal images of the knee. The course of the normal ligament of Humphrey is indicated (black arrow) as it progresses from its origin at the posterior medial portion of the lateral meniscus (A), in front of the lower portion of the posterior cruciate ligament (B), the mid portion (C), and the proximal portion (D).

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Proton-dense coronal images demonstrate the normal

Proton-dense coronal images demonstrate the normal ligament of Wrisberg originating from the medial horn of the lateral meniscus and inserting at the lateral aspect of the medial femoral condyle near the femoral origin of the posterior cruciate ligament.

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Plain radiograph findings

Plain film radiography demonstrates avulsion fractures to the tibial or femoral insertion site (see the image below).

Anteroposterior radiograph of the right knee demon

Anteroposterior radiograph of the right knee demonstrates interruption and discontinuity (black arrow) at the femoral origin of the posterior cruciate ligament, representing an avulsion fracture.

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Lateral radiographs may demonstrate posterior subluxation of the tibia on the femur. When dramatic or obvious, this is pathognomonic for a PCL tear. Soft tissue swelling may be seen anterior to the proximal tibial plateau on lateral radiographs. Knee effusions/hemarthroses may be seen on lateral radiographs.

Injury to the posterior cruciate ligament (PCL) has been overlooked as a cause of internal derangement of the knee. Improved basic science knowledge of the anatomy and biomechanics of the PCL has provided the orthopedic surgeon with new information on which to base treatment decisions.

MRI has revolutionized the evaluation of the knee for acute and chronic injuries. Its advent has revealed that the PCL is subject to injuries more often than previously believed. Although the significance and presence of this injury may not be recognized immediately, the late onset of instability and arthritis resulting from injury may herald irreversible limitations in activity, leading to debilitation. Correct diagnosis of PCL tears can be challenging but rewarding for both physician and patient.

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