MRI for Posterior Cruciate Ligament Injuries

Magnetic resonance imaging (MRI) is the preferred examination for evaluating posterior cruciate ligament (PCL) injuries. It is the most sensitive and the most widely used modality for evaluating the PCL and other cartilaginous and ligamentous structures of the knee. ^{[1, 2, 3, 4, 5, 6, 7, 8]}  MRI is superior to physical examination and has replaced computed tomography (CT) and arthrography because it offers superior soft tissue resolution and is noninvasive. ^{[9, 10]} The sensitivity of MRI has obviated the use of arthroscopy as a diagnostic tool for evaluating PCL injuries in almost all patients. ^{[11, 12]}  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 anterior cruciate ligament (ACL), the medial collateral ligament (MCL), the lateral collateral ligament (LCL), and the menisci. ^{[13, 14, 15, 16]}

The posterior cruciate ligament (PCL) derives its name for its attachment to the posterior aspect of the tibia and the “cross” structure formed with the ACL inside the joint capsule of the knee. Similar to other ligaments in the knee, the function of the PCL is to provide stabilization of the knee joint. It is 1 of 2 cruciate ligaments of the knee that acts primarily to restrict posterior tibial translation relative to the femur. The PCL is the functional counterpart of the ACL, which prevents excessive anterior tibial translation relative to the femur. Together, the cruciate ligaments act as static stabilizers that hold the knee joint together throughout its full range of motion. ^[17]

The PCL is the largest and strongest ligament in the knee and consists of 2 bundles: the anterolateral bundle and the posteromedial bundle. The PCL is susceptible to injury by a posterior force to the proximal tibia when the knee is in the flexed position. ^[17]

Research shows that differences in the shape of the knee are associated with the presence of a PCL rupture after injury. A smaller and more sharply angled intercondylar notch and a more flattened tibial eminence are related to PCL rupture. This suggests that the morphology of the knee is a risk factor for sustaining a PCL rupture. ^[18]

(PCL injuries are displayed in the magnetic resonance images below.)

The roles of other radiologic modalities are as follows:

• Plain radiography, including anteroposterior and lateral, is used as 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 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 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 revealing 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 ligaments of the knee.

Clinical MRI of joints is limited to mere morphologic evaluation and fails to directly visualize joint or ligament function. If combined with advanced image post-processing, stress MRI is a powerful diagnostic adjunct for evaluating ligament functionality and joint laxity in multiple dimensions and may have a role in differentiating PCL injury patterns, in therapeutic decision-making, and in treatment monitoring. ^[19]

The following are MRI technique guidelines:

• To characterize PCL abnormalities with MRI, all 3 planes (axial, coronal, and sagittal) should be used. ^[20]

• 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. ^[21]

• 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 ^[22] :

• 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. ^[23]

• Short T1 inversion recovery (STIR) protocols can be generated using a relaxation time (TR) of 4000 milliseconds, a dual-echo time (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 TE sequences of 20 and 80 milliseconds. ^[24]

• T2 or T2 star images (T2*) can be obtained with refocused 2-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 slice thickness less than 1 mm can reduce imaging time.

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

• Axial 3-D FT images can be acquired with a TR of 55 milliseconds, a TE of 15 milliseconds, and an FA of 10°; sagittal 3-D 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 image 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).

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).

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 in 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).

Plain radiographic findings

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

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 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. ^{[25, 26]}

MRI has revolutionized evaluation of the knee for acute and chronic injuries. Its advent has revealed that the PCL is subject to injury more often than was previously believed. Although the significance and the presence of this injury may not be recognized immediately, 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. ^{[27, 28]}

The posterior cruciate ligament (PCL) is the major stabilizer of the knee. It provides most of the restraint against posterior tibial displacement on the femur during flexion. The posterior fibers of the PCL prevent hyperextension. During flexion, the anterior fibers tighten and help prevent hyperflexion. The PCL resists internal tibial rotation during flexion by winding around the anterior cruciate ligament (ACL). In terminal knee extension, the PCL and the ACL, with guidance from the menisci, help externally rotate the tibia to its correct position in relation to the femur (screw home mechanism). The PCL also stabilizes the knee against excess varus or valgus angulation. Biomechanical studies have revealed a strong functional interaction between the PCL and posterolateral structures of the knee. Posterior stability is impaired significantly when both of these complexes are damaged. ^[29]

The meniscofemoral ligaments (MFLs) are associated intimately with the PCL. Their function is to pull the posterior horn of the lateral meniscus anterior and medial during flexion, balancing the action of the popliteus muscle, which, in addition to the MFLs, attaches to the posterior horn of the lateral meniscus. The MFLs also may function as a secondary restraint to posterior tibial translation after complete rupture of the PCL. Meniscotibial ligaments are involved. ^{[30, 31]}

The PCL originates from the lateral surface of the medial femoral condyle. According to Covey et al, the average length of the ligament is 38 mm, and the average width of the ligament is 13 mm at the mid portion. ^[32] The femoral origin consists of 2 parts, including a flat upper portion and a convex lower border, which conforms to the shape of the articular surface of the medial femoral condyle. The tibial attachment site is located on an inclined recessed shelf, posterior and inferior to the articular surface of the tibial plateau. The tibial attachment site is smaller than the femoral origin site (see the images below).

The PCL, like the ACL, is enclosed by a synovial envelope originating from the posterior aspect of the joint capsule. Thus, the PCL and the ACL are intra-articular but extrasynovial. ^[33] The PCL, like the ACL, is composed of individual fasciculi that unite into 2 major bundles—the larger anterolateral and the smaller posterolateral bundles. Both are named for their respective attachment sites on the femur, which are anterior and posterior, and on the tibia, which are lateral and medial. A smaller posterior oblique bundle has also been described.

The anterolateral bundle tightens during flexion and relaxes during extension. The posteromedial bundle acts in a reverse manner; it relaxes during flexion and tightens during extension.

The PCL is twice as strong as the ACL. It contains a larger cross-sectional area and possesses greater tensile strength, explaining its lower rate of injury. The PCL fibers are oriented more vertically than the more oblique fibers of the ACL.

The smaller MFLs are an important component of the PCL complex. They connect the posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle adjacent to the origin of the PCL. They consist of the ligament of Humphrey, which is smaller and courses anterior to the PCL, and the ligament of Wrisberg, which is larger and courses posterior to the PCL.

In the ligament of Wrisberg, 3 types of proximal insertions have been described, as follows:

• Type I is seen with a frequency of 45% and inserts on the medial femoral condyle, above the femoral attachment of the PCL (see the image below).

  Sagittal T2-weighted image shows the normal femoral insertion of the ligament of Wrisberg above the insertion of the posterior cruciate ligament.

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• Type II, seen in 31%, inserts into the proximal portion of the PCL, near the femoral insertion site.

• Type III, seen in 21%, inserts into the distal half of the PCL.

Variations in the insertion site may cause diagnostic pitfalls when PCL tears are evaluated. Either ligament is found in more than 80% of knee specimens. Stoller reports that the 2 are present simultaneously in 6-8% of knees. ^[34] Visualization of knee ligaments may improve in the presence of edema and hemorrhage following PCL tear.

The major vascular supply to the PCL is the middle genicular artery (MGA), a branch of the popliteal artery. The MGA also supplies the synovial sheath, which is a major contributor to the blood supply of the PCL. The MGA originates behind the popliteal surface of the distal femur and passes anteriorly to enter the posterior capsule of the knee joint at the level of the intercondylar notch. The base of the PCL is supplied by some of the capsular vessels arising from the popliteal and inferior genicular arteries. The various vessels enter the PCL at various levels and run within the ligament in a superior and inferior direction. The main innervation of the PCL is provided by the posterior articular nerve, a branch of the posterior tibial nerve. ^{[35, 36]}

Classification

Posterior cruciate ligament (PCL) ruptures can be classified as isolated or combined injuries. ^{[37, 38]} Isolated related injuries can be classified further as partial (grade I and II) or complete (grade III) tears, according to the amount of posterior tibial subluxation determined by the posterior drawer test. ^[39]

Grading is as follows:

• Grade I: Posterior tibial subluxation of 1-5 mm on the posterior drawer test

• Grade II: Posterior subluxation of 5-10 mm

• Grade III: Posterior subluxation greater than 10 mm

Grade III or complete PCL tears must be distinguished from combined injuries because the prognoses are different. ^[40] Isolated PCL injuries are treated conservatively and have an excellent prognosis. Combined injuries involving the PCL have a more guarded prognosis. They are treated by surgical repair or reconstruction within 3 weeks of injury. Surgical results are better than results seen with conservative management; however, it is difficult to distinguish clinically between the 2 types of injury. ^[41]

Mechanism of injury

Hyperflexion of the knee by high-velocity forces acting on the anterior tibia is the most common cause of PCL tears. This results in posterior displacement of the tibia on the femur. It is seen in motor vehicle accidents in which the knee strikes the dashboard and in soccer sliding injuries in which an athlete receives a blow to the anterior tibia from a slide tackle.

Hyperextension of the knee occurs in football players. The posterior capsule is torn initially, after which tearing of the PCL occurs. The ACL is often torn. Rotational injuries with associated varus or valgus stress are a common cause of PCL tears. The medial collateral ligament (MCL) is torn and the ACL may be torn.

Knee hyperflexion in internal rotation with the foot in dorsiflexion or plantar flexion also causes PCL tears. The anterolateral bundle of the PCL comes under increased pressure and tears while the posterior bundle remains intact. Hyperflexion without an associated force on the anterior tibia occurs in freestyle wrestlers.

The PCL is an important source of restraint of posterior tibial translation relative to the femur. In addition, the PCL acts as a secondary restraint to resist varus, valgus, and external rotation moments about the knee. Although less common than ACL injuries, injuries to the PCL can occur from a posterior force directed on the tibia, most commonly with the knee in a flexed position. ^[42]

In a study of 48 MRI examinations of the knee with isolated PCL tears, 69% of the tears occurred in the mid substance, 27% proximally. Meniscal tears were detected in 25% of knees, involving all segments of both menisci except the anterior horn of the medial meniscus. Focal cartilage lesions were seen in 23% and usually affected the central third medial femoral condyle and medial trochlea. Knee fractures were present in 12.5% of knees, and 48% had bone bruises that usually involved the central to anterior tibiofemoral joint. Both the presence of a fracture and the proximal location of the PCL tear were associated with hyperextension injury. ^[43]

Associated injuries

Posterior tibial dislocation resulting from PCL disruption can damage the tibial and peroneal nerves. Peroneal nerve injury is more common in combined injuries involving the arcuate complex of the posterolateral corner of the knee. Most neurapraxia is resolved with conservative therapy within 18 months. The incidence of vascular injury, such as thrombosis and transection of the popliteal artery, can be as high as 14%, regardless of whether the dislocation was reduced spontaneously.

Bone contusions along the inferior aspect of the femoral condyle and the anterior aspect of the tibial plateau can be seen in hyperextension injuries.

PCL injuries rarely occur in isolation and most commonly involve damage to other ligaments of the knee as well. The posterior drawer test is useful to assess the stability of the PCL clinically, and MRI can confirm a PCL injury or tear. ^[17]

Frequency of PCL injuries 

PCL injuries constitute 3-20% of knee injuries. The rate may be higher because acute tears often go undiagnosed. More than one half of PCL injuries occur through traffic and industrial accidents; less than one half occur through sports-related injuries. PCL injuries are rare in children.

According to Mink et al, combined injuries to the PCL and other structures of the knee are much more common (97%) than isolated PCL injuries (3%). ^[44] The ACL is injured most commonly (65%), followed by the MCL (50%), the medial meniscus (30%), the posterior capsule, and the fibular collateral ligament.

Complete tears occur in approximately 40% of cases, partial tears in approximately 55%, and avulsion tears in 7%.

Site of injury

The anatomic site of injury depends on the mechanism of injury and the strain velocity of the ligament. High-velocity injuries produce more mid-substance tears. Low-velocity nonimpact injuries may produce more avulsion fractures.

The mid portion of the PCL is the most frequent site of injury, followed by the proximal portion near the femoral insertion.

The tibial insertion site is strong and is difficult to tear. Avulsion fractures are more common at this site and are more frequent in children. ^[45]

Rotational injuries with associated varus or valgus stress most commonly produce a PCL tear at the femoral attachment site.

Sequelae of PCL tears

Chronic tears of the PCL result in increased stress on the patellar ligaments and quadriceps tendon, resulting in chronic tendinitis. Increased stress at the patellofemoral joint may result in grinding related to bone-on-bone contact and chondromalacia. PCL insufficiency may result in a lateral shift in the center of rotation of the knee joint, leading to articular cartilage degeneration of the medial compartment. Risk of medial meniscus tears and posterolateral instability is increased. These changes may occur within 5 years of injury. Risk of medial meniscus tears and posterolateral instability is increased after a complete PCL tear has been sustained. These changes may occur as early as 5 years from the time of injury.

Knowledge and understanding of the complex anatomy and biomechanical function of the native posterior cruciate ligament (PCL) is vitally important when PCL injury and possible reconstruction are evaluated. The PCL has important relationships with the anterior cruciate ligament, menisci, tibial spines, the ligament of Humphrey, the ligament of Wrisberg, and posterior neurovascular structures. Through various experimental designs, the biomechanical role of the PCL has been elucidated. The PCL has its most well-defined role as a primary restraint/stabilizer to posterior stress, and it seems this role is greatest at higher degrees of knee flexion. ^[46]

History

Posterior cruciate ligament (PCL) evaluation begins with an attempt to determine the mechanism of injury. This may provide important information regarding the potential severity of the injury and possible damage to associated structures within the knee.

Physical examination

Pain is present diffusely or may be located posteriorly at the site of an avulsion fracture to the tibial or femoral insertion. Pain also may be present over the anterior tibia as a result of the flexed knee striking the dashboard. The painful knee can make the physical examination challenging even to the experienced examiner.

Swelling and hemarthrosis may be present. Abrasions from dashboard injuries may be present over the anterior tibial region.

An inability to bear weight on the affected extremity is a sign of severe injury. Some individuals with isolated PCL tears may be able to continue with activity. In chronic PCL injury, the patient may walk with high-heeled shoes, flexing the knee 10-15° to prevent full knee extension because of instability.

Clinical testing to evaluate PCL integrity

The basic function of these tests is to demonstrate posterior proximal tibial subluxation relative to the distal femur with the knee in flexion.

The posterior drawer test is the most accurate clinical test for determining posterior tibial subluxation. PCL injuries have been graded I, II, and III, depending on the amount of subluxation determined by the test. Posterior tibial subluxation of 1-5 mm is considered a grade I injury. The tibia remains anterior to the femoral condyle despite subluxation. Subluxation of 5-10 mm is a grade II injury. The tibia is flush with the femoral condyle. Further posterior tibial subluxation is considered a grade III injury.

In the posterior sag test, the hip and the knee are flexed to 90°. With complete PCL tears, the tibia sags and becomes subluxed posteriorly relative to the femur.

In the quadriceps active test and in the presence of a PCL tear, active contraction of the quadriceps muscle with the knee from 60-90° of flexion causes the tibia to move anteriorly, and normal posterior tibial sag is eliminated.

The reverse pivot test evaluates posterolateral instability related to associated injuries of the posterolateral compartment of the knee (arcuate ligament complex, consisting of arcuate ligament, lateral collateral ligament, popliteus muscle, and lateral head of the gastrocnemius muscle). The patient is supine, with the examiner standing on the side of the injured leg. One hand grasps the foot and externally rotates the tibia. The other hand is placed on the lateral aspect of the knee. Posterolateral instability results in posterior subluxation of the lateral tibial plateau.

MRI findings

Posterior cruciate ligament (PCL) tears are intrasubstance, partial, or complete avulsions. In over 90% of patients with PCL tears, the PCL measures more than 7 mm in anteroposterior diameter on sagittal T2-weighted images. One feature of a torn PCL is a striated appearance, similar to the normal ACL, with longitudinally oriented lines of increased signal intensity. An acutely torn PCL usually maintains its continuity as a single structure. ^[47]

Intrasubstance tears are interstitial (confined within the ligament). They contain hemorrhage and edema that increase in signal intensity on long TR/TE sequences. Abnormal signal within the ligament may be increased inhomogeneously on long TR/TE sequences. This can involve much of the course of the ligament, causing diffuse enlargement but with well-preserved outer margins (see the image below).

Partial tears contain eccentric regions of increased signal intensity within the ligament, extending to interrupt a portion of one of the margins of the ligament on long TR/TE sequences, with preservation of some normal ligament fibers. They may contain a circumferential ring of hemorrhage and edema around the margins of the PCL, with preservation of most of its internal architecture (halo sign), which is dark on short TR/TE sequences and bright on long TR/TE sequences (see images below).

Complete absence of a portion of the ligament is present, with interposition of hemorrhage and edema blurring the margins (see the image below). Focal areas of edema and hemorrhage can replace the ligament at the site of bone attachment. Disruption can involve the tibial insertion, the femoral origin, or the intersubstance. The PCL, with its accompanying bony fragment, is retracted away from the insertion site. Often, bone marrow edema exists at the fracture site. Focal areas of edema and hemorrhage can replace the ligament at the insertion site.

Associated bone findings include hyperextension injuries, which may demonstrate contusion of the tibial plateau and the adjacent femoral condyle, and hyperflexion (dashboard) injuries, which may show contusion of the proximal anterior tibia.

The chronically torn PCL can demonstrate a mild diffuse increase in signal intensity on long TR/TE sequences. The ligament assumes a serpiginous course, becomes irregular in outline, and is no longer taut in flexion

Spontaneous repair may occur after an acute injury. The PCL may parasitize the blood supply of the ACL to propitiate this repair.

The ligament may retain its continuity but may be replaced by a fibrous scar. Because both the scar and the normal ligament are of low signal intensity, the anatomically intact but functionally torn ligament may be a source of interpretive error.

When imaging findings in preparation for arthroscopic knee surgery are reported, evaluation of meniscofemoral ligaments (MFLs), first in the sagittal and then in the coronal plane, will achieve the best results. MFLs and the PCL have distinct morphologic patterns throughout life. These patterns show intimate anatomic relationships and potential biomechanical impact. These patterns and relationships can be quantified with MRI. ^[48]

Arthroscopic findings

Arthroscopic findings in PCL injuries are both direct and indirect. Direct findings include damage to the PCL, similar to that seen on MRI, including mid-substance tears, interstitial tears, and avulsion fractures.

According to Fanelli et al, indirect findings occur secondary to PCL insufficiency and include the sloppy ACL sign and degenerative changes in the patellofemoral joint and the medial compartment ^[49] ; the sloppy ACL sign demonstrates increased laxity of the ACL related to gravity-assisted posterior tibial drop-back; ACL tension returns to normal when the tibia is reduced.

Limitations of techniques

The sensitivity and specificity of MRI in making the diagnosis of PCL tears are high. Sensitivity has been reported to be as high as 100%; reports of specificity have ranged from 84 to 100%.

Unlike injuries to the ACL, PCL injuries may not compel an athlete to stop activity, providing a false sense of security. Tense hemarthrosis may not develop immediately, and frequently an initial lack of soft tissue swelling exists, which may result in a delay in diagnosis. ^[50]

Physical examination in the acute setting may be falsely negative because of hamstring spasm or hemarthrosis. The posterior sag sign in this setting may be normal. Examinations under anesthesia have been falsely negative.

The presence of other associated injuries, such as ACL, MCL, and meniscus injuries, may divert attention away from the PCL.

The anterior drawer test may be misinterpreted as positive because the examiner may not be aware that he or she may be merely reducing the gravity-assisted posterior sag.

Soft tissue swelling and ecchymoses over the anterior tibia are only indirect signs of PCL injury. Further investigation with MRI is needed.

Plain radiograph limitations include the following:

• Joint effusions, although commonly present, are not specific for PCL injuries; they are seen in myriad traumatic, degenerative, and neoplastic conditions.

• Avulsive fractures can be confused with a loose body.

• Poor soft tissue resolution offers no information concerning the status of the PCL or other ligamentous and cartilaginous structures of the knee.

Arthroscopy limitations include the following:

• The PCL is difficult to view arthroscopically; the caudal two thirds is seen poorly from an anterior approach because of synovium and subsynovial fat.

• The ligament is obscured by the ACL.

• The tibial attachment of the PCL is below the articular surface of the tibial plateau.

• An intact ligament of Humphrey may be mistaken for an intact PCL in patients with a PCL tear.

• The PCL can be obscured by a chronic torn ACL adhering to the PCL.

• Capsular tears may prevent joint distention, which is needed to visualize the PCL.

Regions of increased signal intensity within the ligament with normal morphology on T1WI require conventional T2, T2, or FS T2 FSE images to differentiate between eosinophilic degeneration (EG) or the magic angle phenomenon (MAP) and PCL tears. ^[51]

Eosinophilic degeneration may simulate an intrasubstance tear. It consists of focal regions of increased signal intensity within the ligament on short TR/TE sequences that lose signal on long TR/TE sequences. The contour and margins of the ligament are intact. EG is seen more frequently in older persons.

In the magic angle phenomenon, increased signal intensity may be present on the upward sloping portion of the PCL on short TE images, mimicking a tear. It is present in anatomic components of the ligament oriented 55° to the main static magnetic field, along the long axis of the magnetic bore. The phenomenon can be distinguished from a true PCL tear using long TE-weighted imaging sequences. When proton-density imaging is used, the artifact may persist if the TE is 20 milliseconds or less. If the abnormal signal focus persists, a true PCL abnormality is present. Artifacts from MAP disappear when long TE sequences are used (see the image below).

A ganglion cyst may attach to the PCL. It can be differentiated from a true PCL tear by its well-marginated bright echogenicity on T2WI and an intact underlying PCL.

Giant cell tumors (GCTs) of the PCL tendon usually are attached to the periphery of the ligament. The tumor usually is lobulated and multinodular and contains foci of decreased signal related to hemosiderin deposition. Sheppard et al reported that the structural integrity and the configuration of the ligament are preserved in GCTs. ^[52]

Bucket handle tears from the medial meniscus can be displaced laterally to lie beneath the PCL, giving the appearance of a double PCL. ^[53] The "double PCL sign" can be distinguished from a PCL tear by following the course of the normal PCL and by appreciating the meniscal tear (see the image below).

Treatment

Knowledge and understanding of the complex anatomy and the biomechanical function of the native posterior cruciate ligament (PCL) is vitally important when PCL injury and possible reconstruction are evaluated. The PCL has important relationships with the anterior cruciate ligament, the menisci, the tibial spines, the ligament of Humphrey, the ligament of Wrisberg, and posterior neurovascular structures. Through various experimental designs, the biomechanical role of the PCL has been elucidated. The PCL has its most well-defined role as a primary restraint/stabilizer to posterior stress, and it seems this role is greatest at higher degrees of knee flexion. ^[46]

An isolated partial mid-substance tear should be treated nonoperatively with aggressive physical therapy. ^{[54, 55, 56]} Avulsion fractures of the PCL require open repair. The prognosis for patients with these injuries is excellent.

Controversy surrounds the treatment of isolated tears. Although primary repair was advocated initially, results of conservative treatment have been promising. ^[57] Most treated patients have functionally stable and almost completely asymptomatic knees. ^[58]

The dysfunctional PCL may be compensated by quadriceps activity. Contraction of the quadriceps produces an anterior drawer bone that can correct the posterior sag caused by a torn PCL. Complete interstitial tears of the PCL should be treated if other ligamentous injuries of the knee are present, such as ACL, MCL, LCL, or arcuate ligament complex injuries. Chronic PCL tears for which physical rehabilitation has failed should be treated surgically.

A critical length of the distal component of the torn PCL on MRI may predict the ability to perform early proximal femoral repair of the ligament versus reconstruction. Goiney et al studied 27 knees with complete disruption of the PCL that had been evaluated using preoperative MRI and underwent either early reattachment to the femoral insertion or reconstruction. Knees with a distal fragment PCL length of 41 mm or greater underwent early proximal femoral repair, with the distal stump attached to the distal femur. Knees with a distal PCL length of 32 mm or less were not able to undergo repair because of insufficient length; as a result, reconstruction was performed. ^[59]

Prognosis

Poor surgical outcomes after PCL reconstruction have been attributed to many factors, the most common of which include additional intra-articular pathology, poor fixation methods, insufficient knowledge of PCL anatomy, improper tunnel placement, and poor surgical candidates. ^[46]

Neurapraxia resulting from injury to the tibial or peroneal nerves usually heals with conservative therapy within 18 months. Avascular necrosis of the medial femoral condyle can be seen following PCL reconstruction. The cause may be related to the size and location of the surgically created tunnel. ^{[60, 61, 62]}  Future ligament stability is predicted imprecisely solely on MRI findings.

Patients who have isolated PCL tears have a good prognosis because of a high degree of normal revascularization from the inferior genicular artery. Patients with intersubstance tears have a good prognosis. Tearing of the underlying synovium, as in partial or complete tears, exposes the torn PCL to synovial fluid. According to Andrish et al, synovial fluid has a deleterious effect on healing. ^{[63, 64]}