Imaging of the Knee

Background

Imaging plays an important role in diagnosing knee joint pathology. There are a variety of imaging techniques that allow visualization of abnormalities that can affect the bones as well as directly visualizing articular cartilage. In addition, after treatment intervention, imaging can assess interval change. The most common pathologic processes affecting the knee joint are traumatic and degenerative in nature and can be major causes of morbidity.

Imaging

Radiographs are often an important first step in imaging. Fractures, malalignment, joint effusion including lipohemarthrosis, and soft tissue mineralization are some of the important findings that can be identified on radiographs and can also be correlated with findings of advanced imaging. Standard radiographic series includes a frontal projection, with or without a flexed weight-bearing image, lateral view, and a patellofemoral view ( Fig. 91.1 ). The latter can be a merchant view, in which the beam is directed along the patellofemoral articulation with 45 degrees of flexion. Alternatively, the sunrise view can be utilized, for which the patient lies prone and flexes 90 degrees. The beam is directed retrograde toward the patellofemoral articulation. In the setting of acute trauma, a cross table lateral is used in place of the true lateral. Internal and external oblique projections are used to carefully profile the medial and lateral tibial plateau, and are often employed in the setting of trauma. A fracture is seen in only 5% of emergency department radiographs. However, per the American College of Radiology (ACR) Appropriateness Criteria, radiographs should be the first study obtained if there is a twisting injury or fall with focus tenderness, inability to bear weight, or a discernible suprapatellar effusion on exam.

Computed tomography (CT) also plays a role in knee joint imaging. It is employed as a second imaging technique to assess fractures and identify mineralization that is not clearly detected on conventional x-rays. CT without contrast can be performed if a radiographically occult fracture is suspected clinically, such as in the setting of a lipohemarthrosis. Alternatively, known fractures can be further characterized, such as when measuring the degree of displacement of fracture fragments or more accurately determining articular step off of a depressed tibial plateau fracture. Comminuted or complex fractures are often further characterized with CT when planning for open reduction internal fixation (ORIF). Surface rendered sagittal and coronal reformats and three-dimensional reconstruction are helpful to visual complex fracture patterns in an orientation that simulates what will be encountered in surgery. CT angiography can replace conventional angiography in certain cases of posterior dislocation and concern for lower extremity arterial injury. CT can also be useful for evaluation of bone and soft tissue tumor matrix. Matrix assessment is beyond the scope of this chapter. In brief, cartilaginous tumors such as enchondromas have calcified chondroid oriented in rings and arcs matrix. Tumors such as osteosarcoma have osteoid, or fluffy/cloudlike matrix, and lesions like fibrous dysplasia or non-ossifying fibromas have ground glass matrix ( Fig. 91.2 ).

CT with sagittal and coronal reformatting as well as 3D reconstruction is valuable for preoperative planning for total knee arthroplasties. The shape of the glenoid is demonstrated for the surgeon that will allow for presurgical planning and sizing the prosthesis.

CT arthrography can be a useful alternative in patients with contraindications for MRI, such as certain types of pacemakers and debilitating claustrophobia. Iodinated contrast (20 to 30 mL), often diluted to 50% strength with a combination of anesthetic and normal saline, is injected into the knee joint under fluoroscopic guidance. At our institution, the patients are placed supine with 20 to 30 degrees of flexion. A 45-degree caudocranial trajectory is taken to land the 22-gauge, 3.5-inch needle on the mesial aspect of the medial femoral condyle. A lateral or medial approach to the patellofemoral articulation can alternatively be used.

Ultrasound plays an adjunct role in imaging of the knee. A fluid collection in the popliteal recess, a.k.a. Baker cyst, can be a source of posterior knee pain. Ultrasound is also the preferred method for identifying and characterizing deep venous thrombosis (DVT) of the popliteal, peroneal, and posterior tibial veins. Targeted ultrasound can also be used to evaluate high-grade injuries to the Achilles, quadriceps, and patellar tendons, although these are often quite apparent clinically. Ultrasound can determine if intact tendon fibers remain attached to their respective insertions. Recent research has suggested that ultrasound can identify characteristics in the medial meniscus, which can be associated with the onset and progression of osteoarthritis. Ultrasound can be shown in some cases, showing sensitivity and specificity for diagnosing meniscal tear of 85% and 86%, respectively. However, it has been our experience that the evaluation of the free edge as well as the posterior root attachments are extremely limited in even the most experienced hands ( Fig. 91.3 ).

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is the mainstay of knee joint imaging. The ideal MRI is performed on a high field strength magnet (1.5 Tesla or greater), using a dedicated knee surface coil to improve signal to noise ratio (SNR). At least one sequence should be performed in the axial, coronal, and sagittal planes. Some sequences should be performed with fat-suppression (FS) to accentuate abnormal signal from fluid and/or edema. At least one T1-weighted sequence should be used to look for hypointense fracture lines and assess the signal of red and yellow marrow distribution. Proton density (PD) weighting is the workhorse for the knee joint because of its ability to distinguish subchondral bone from cartilage from joint fluid. Emphasis is placed on keeping the echo time (TE) of the sagittal PD FS images at or below 20 ms to ensure that abnormal signal from subtle meniscal tears can be appreciated.

A variety of advanced imaging techniques has been developed for articular cartilage imaging in the knee joint; these are not routinely employed with knee imaging and largely remain areas of research, development, and investigation. Delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC) is based on the increased delayed uptake of the negatively charged gadolinium-based contrast agent (GBCA) in areas of damaged cartilage, which contain abnormally low amounts of negatively changed glycosaminoglycans (GAGs) compared with healthy cartilage. This increased uptake of GBCA shortens the T1 relaxation times, allowing detection of cartilage damage on the structural level before morphologically apparent thinning can be identified by unenhanced MRI. T2 mapping assesses the suprastructure of cartilage. Healthy cartilage has a stratification of the both the organization and water content moving from the subarticular bone plate to the lamina splendens. This organization is lost in chondromalacia. Sodium MRI imaging is another technique that serves to quantify the amount and organization of remaining GAGs. Other techniques including T1 Rho have also been researched.

The remainder of this chapter will be presented by anatomic structure. An overview of anatomy will be followed by useful radiographic signs of injury, variety of pathology, and MR imaging findings of each. Emphasis will be placed on the ideal sequence and imaging plane for each structure.

Menisci

The medial and lateral menisci are curvilinear fibrocartilaginous structures that cover the medial and lateral tibial plateau, respectively. When viewed in the axial plane, the lateral meniscus is a more well-formed “C,” with the anterior and posterior root attachments positioned more centrally along the anteroposterior axis of the tibial plateau. The medial meniscus resembles a semicircle, with the anterior and posterior attachments close to the anterior and posterior margin of the plateau, respectively. The deep fibers of the medial collateral ligament (MCL) attach to the medial meniscal body in the form of the meniscofemoral and meniscotibial attachments. The posterior horn of the medial meniscus is attached to the joint capsule by the meniscocapsular ligaments, while the posterior horn of the lateral meniscus is attached to the adjacent popliteus tendon by the meniscopopliteal fascicles. The meniscus is best evaluated on sagittal images. The body of the meniscus is uniformly low in signal (black) and resembles a rectangle or slab of meniscal tissue. There are typically a couple of body segments if using a 4 mm slice thickness ( Fig. 91.4 ). The anterior and posterior horns are triangular in shape and low in signal when normal ( Fig. 91.5 ). The lateral meniscus has anterior and posterior horns that are similar in size, while the medial meniscus posterior horn is two times the size as the anterior horn. These relationships are important to remember when evaluating the meniscus to identify different types of tears.

Radiographs can play a role in identifying chondrocalcinosis, which can be seen most commonly in the lateral meniscus. Chondrocalcinosis may reflect calcium pyrophosphate arthropathy or sequelae of age.

Meniscal tears are best identified as abnormal signal on PD sequences. Abnormal signal that violates the articular surface on two consecutive images is necessary for diagnosis. Amorphous, ill-defined intrameniscal signal that does not violate an articular surface is often related to meniscal contusion in young patients, and mucoid degeneration/chondrocalcinosis in older individuals. Meniscal tears can be broadly separated into longitudinal and radial types. Longitudinal tears follow the long axis of the meniscus. They can be either vertical or oblique tears. Oblique tears ultimately propagate through the superior or inferior surface, rather than the true free edge. The central half of the cross-sectional area of meniscal tissue is known as the “white zone,” since it is avascular. Tears isolated to this region are unlikely to heal. Treatment is either meniscectomy or nonsurgical. Conversely, tears involving the peripheral half, or “red zone,” are often repaired arthroscopically. Care should be taken to look on MRI for a flap tear in which the inferior/peripheral portion of the body of the meniscus is displaced in the inferior gutter, deep to the MCL, as this can be difficult to identify during arthroscopy, and can lead to persistent pain postoperatively if not addressed. One way to recognize is by assessing the body segments. Typically the meniscus is not rectangular in shape, nor does it resemble a “slab” of meniscal tissue, as a portion of the undersurface is absent ( Fig. 91.6 ).

Vertical, longitudinal tears usually extend from the superior to inferior articular surface of the meniscus. A subset is the bucket handle tear, where a portion of white (or sometimes white and red) zone tissue is flipped away from the body of the meniscus centrally, anteriorly, or posteriorly. Depending on where the tear begins and ends, this can result in the double posterior cruciate ligament (PCL) sign, or the double delta sign of the anterior horn of the meniscus ( Fig. 91.7 ). The double PCL sign is unique to medial meniscus, as the ACL prevents central flipping of the bucket handle when the lateral meniscus demonstrates tears in this fashion.

Radial tears extend through the cross section of the meniscus, orthogonal to the long axis. On MRI these tears are recognized as blunting of the anterior or posterior horn on sagittal imaging and will demonstrate loss of the low signal on coronal images (vertical intermediate signal; Fig. 91.8 ). These can be partial, as in free edge tearing, or a complete radial tear (resembling a fractured meniscus). A posterior root disruption is a subtype of complete radial tear. They are recognized on MRI as loss of the low signal (or slightly increased signal) at the root attachment ( Fig. 91.9 ). Often the meniscus will be extruded into the gutter. When this finding is recognized, attention should be paid to the signal of the root of the meniscus. These tears are unlikely to heal spontaneously. There is also disruption of the hoop stress resistance, one of the most important functions of the meniscus that leads to the extrusion. This often leads to accelerated osteoarthrosis in the affected compartment. Surgical repair is usually necessary.

Meniscal cysts are an often encountered abnormality and can be intrameniscal or parameniscal in location. Intrameniscal cysts cause swelling of the meniscus and are increased in T2 signal. They are most often not fluid signal in their appearance ( Fig. 91.10 ). The origin of this abnormality is not well understood, but many feel it is a result of intrinsic meniscal degeneration that leads to local necrosis accumulation of abnormal signal within the substance of the meniscus. Intrameniscal cysts are not always associated with meniscal tears, and a careful search of the superior and inferior surface of the meniscus should be taken to identify an associated tear.

Parameniscal cysts, on the other hand, do demonstrate fluid signal on T2-weighted imaging. These cysts have a communication to the meniscus and that communication must be identified before the diagnosis of a parameniscal cyst can be made ( Fig. 91.11 ). Parameniscal cysts are most often confused for a bursa on imaging. This error can be avoided if the neck of the cyst is identified originating from the meniscus. The parameniscal cysts can be located anywhere around the periphery of the meniscus and articular surfaced tears are not always present. Formal decompression of a parameniscal cyst is considered unnecessary during partial meniscectomy.

In the setting of advanced osteoarthrosis, complex tears of the meniscus with radial and oblique components are often present. Root tears, extrusion, or flipped fragments should be sought carefully.

Prior partial meniscectomy can be indistinguishable from fraying, and surgical history is valuable in making a proper diagnosis.

Pitfalls

The posterior horn of the lateral meniscus should be examined carefully. Normal anatomy of the meniscofemoral ligaments and the popliteus tendon as it traverses the struts of the posterior horn of the lateral meniscus cause a normal intervening, vertical focus of increased signal in the posterior horn of the lateral meniscus ( Fig. 91.12 ). This is not to be confused with a peripheral tear and can be verified, as the meniscofemoral ligaments and popliteus tendon can be followed on consecutive sagittal images. In addition, prior partial meniscectomy can make identification of a superimposed tear difficult. Comparison with prior imaging is quite useful. Finally, in the setting of an ACL tear, the posterior horn of the lateral meniscus often is associated with a peripheral, radial tear of the posterior horn of the lateral meniscus, but is often overlooked. Abnormal signal in this region in the setting of an ACL should be diagnosed as a tear (making sure that the popliteus tendon and meniscofemoral ligaments have been reconciled).

Musculotendinous Structures

In addition to the ligaments and menisci, multiple muscles and tendons help stabilize the knee. The vastus medialis, vastus lateralis, and vastus intermedius coalesce with the rectus femoral to form the quadriceps and patellar tendon, ultimately inserting on the tibial tubercle. These structures are discussed in the extensor compartment section below. The Sartorius, gracilis, and semitendinosus cross the knee joint medially, and their tendons form the pes anserine, inserting on the tibial tubercle. The adductor tubercle of the medial condyle provides the insertion of the adductor magnus. The semimembranosus attaches at the posterior/medial aspect of the medial femoral condyle. Just lateral to this is the medial head of the gastrocnemius. In between these structures is the popliteal recess, where popliteal cysts can form. The lateral head of the gastrocnemius arises from the lateral femoral condyle posteriorly. The popliteus originates from the popliteal hiatus of the lateral femoral condyle, sweeps laterally across the posterior joint, and is part of the posterolateral corner, discussed later.

Tendon anatomy is best illustrated with T1-weighted MRI, while pathology is diagnosed on T2 WI, particularly when fat suppression is applied. Musculotendinous injury manifests as increased T2 signal within the fibrils of the tendon or the surrounding soft tissues, representing grade I strain. Grade II is partial disruption of hypointense fibers identified as increased T2 signal or fluid signal. The tendon may also demonstrate increased thickness or be attenuated in its appearance. A grade III injury is a complete disruption, often with retraction of tendon ends. T1 images are useful to demonstrate associated hematoma, as well as fatty atrophy of the muscle bellies. Both processes demonstrate T1 hyperintensity. Atrophy is seen in long-standing injury or denervation. Most myotendinous injuries are best identified on coronal or sagittal planes, where the feathery edema tracks along the long axis of the muscle. Measuring tendon gap in full thickness tears is also easiest in those planes and may have implications in treatment.

The plantaris is a thin muscle that arises from the lateral supracondylar ridge of the femur, just superior to the lateral head of the gastrocnemius. Its tendon extends inferomedially to insert on the Achilles tendon. The plantaris assists in foot plantar flexion. “Tennis leg” can be caused by rupture of either the plantaris or the medial head gastrocnemius. MRI shows abnormal increased T2 signal edema and increased T1 signal hematoma tracking deep to the gastrocnemius muscle belly ( Fig. 91.13 ) and often torn, retracted plantaris tendon fibers. Treatment is supportive.

Ligaments

The anterior cruciate ligament (ACL) runs from the mesial aspect of the lateral femoral condyle and inserts on the anterior tibial plateau, blending with the anterior horn of the lateral meniscus. It consists of two bundles: the anteromedial band and the stronger, posterolateral band. The sagittal T2-weighted images are best to evaluate the integrity of the ACL. There is often heterogeneously increased intrasubstance signal because of fiber organization. This can be a source of false positives for partial tear to the untrained eye. Sprains and partial tears are seen as ill-definition and increased signal, often affecting one of the two bands to a greater degree. Laxity or alteration in thickness of the ligament are additional signs of a partial tear or sprain. Full thickness tears can be anywhere along the length of the ligament ( Fig. 91.14 ). The coronal and axial images can be used to confirm integrity or tear of the ACL. Coronal plane provides great visualization of the tibial attachments, while the axial view is helpful to evaluate the femoral origin. Mucoid degeneration within the substance of the ACL can simulate injury. Increased signal is identified throughout the ligament and the ligament is enlarged, but the fibers within the ACL are still identified, and have taken on the appearance likened to a celery stalk ( Fig. 91.15 ).

Several secondary signs can support a diagnosis of ACL injury. The radiologic anterior drawer sign represents anterior translation of the tibia with respect to the femur in the setting of an incompetent ACL. The Segond fracture, an often curvilinear fragment lateral to the lateral tibial plateau, is thought to represent an avulsion fracture from the lateral capsular attachment ( Fig. 91.16 ). A lateral femoral notch greater than 2 mm has been proposed as an additional indirect sign of ACL tear. A curvilinear bone avulsion of the fibular head (arcuate sign) is also a conventional radiographic sign of ACL injury. Bone contusions identified on fluid sensitive sequences are typically in the lateral femoral condyle, with or without the additional involvement of the posterior lateral tibial plateau. These have been referred to as kissing contusions. The presence of these contusions is indicative of an ACL tear. The only exception to that is in adolescents. This population typically has more flexibility to the ligament and can have a pivot shift injury resulting in bone contusion pattern without tearing the ACL.

Contrecoup contusion patterns occur on the medial femoral condyle and medial tibial plateau as a result of relocation of the pivot shift mechanism. The presence of medial contusions indicates a more significant injury. In addition, a careful search for a meniscocapsular separation should be sought. The presence of posterior medical tibial plateau contusion has a high association with meniscocapsular injuries as evidenced as fluid signal between the meniscus and capsule (separation) or contusion (abnormal increased signal at the meniscocapsular interface; Fig. 91.17 ).

The normal appearance of the posterior cruciate ligament on MRI is a curved low signal structure arising from the notch in the medial femoral condyle and inserts on the lateral tibial spine. It is best evaluated on a short TE image (the same images used to evaluate the meniscus). On radiographs and sagittal MRI, the radiographic posterior drawer sign can often be identified and represents posterior translation of the tibia with respect to the femur in the setting of an incompetent PCL ( Fig. 91.18 ).

In contradistinction to the ACL, the findings in PCL tears are much subtler. Any intrasubstance signal should raise concern for injury on the short TE images. A T2-weighted image will often demonstrate low signal, making the observation of a PCL tear easy to overlook. The thickness of the ligament also increases and can be a subtle finding. The subtle increase in signal on the short TE images (PD) and the increase in thickness of the ligament provide MRI evidence of a PCL tear. Even high-grade injury may produce no complete disruption of the ligament. Because of the PCL's intrinsic strength, usually only severe injury produces PCL tears, and additional injury should be sought carefully. Particular attention should be paid to the posterolateral corner (PCL), discussed later. Unlike the contusion pattern noted with ACL tears, there is no consistent contusion pattern associated with PCL tears. However, the presence of a contusion of the anterior tibial plateau should warrant careful inspection of the PCL on the short TE images.

The MCL originates from the adductor tubercle and inserts on the medial tibial metadiaphysis, caudal to the joint line. On MRI it is uniformly low in signal intensity and taught in appearance. The deep layer of the MCL is adherent to the peripheral medical meniscus. The MCL can be injured by direct blow, or more commonly valgus stress. The latter is seen in football clipping injuries and can be associated with the “terrible triad” of ACL and medial meniscal disruption. Radiographs can show focal soft tissue swelling/stranding in the expected region of the MCL. Widening of the medial joint line is a secondary sign of MCL incompetence.

A three-point grading system is often used to classify MCL tears. Grade I is seen as increased signal on fluid-sensitive sequences superficial and deep to the MCL. Grade II is partial disruption, with at least some fibers clearly visualized intact along their length. Grade III is complete disruption, often with retraction ( Fig. 91.19 ). Any associated femoral avulsion fracture can be identified radiographically and by MRI, similar to the Segond fracture discussed above. In the setting of remote injury, dystrophic mineralization can be seen near the femoral attachment—the so-called Pelligrini-Stieda lesion, which may be easier to identify on conventional radiography than on MRI, unless there is signal from the mineralization (ossification; Fig. 91.20 ).

The iliotibial (IT) band, a continuation of the IT tract, crosses the knee laterally to insert on Gerdy tubercle of the anterior, lateral tibia. It is uncommonly torn and has an MR appearance compatible with previously discussed ligamentous injuries. IT band friction syndrome is characterized by abnormal signal deep and superficial to the band at the level of the joint line. Thickening of the band may also be seen with ITB friction syndrome. An adventitial bursa can form superficial to the Gerdy tubercle, appearing as fluid signal collection, often with surrounding inflammation and/or edema.

The posterolateral corner is a complex group of ligaments and tendons that provide posterolateral stability. The fibular collateral ligament originates from the lateral femoral condyle before blending with the biceps femoris tendon to form the conjoined insertion on the fibular head. The popliteus tendon originates from the popliteal hiatus. The popliteus musculotendinous complex consists of the popliteofibular, fabellofibular, and arcuate ligament. The arcuate ligament is a “Y”-shaped structure that connects the fibular styloid to the oblique popliteal ligament and the posterior joint capsule.

PLC injuries are often present in association with ACL and/or PCL injuries and can be subtle and difficult to identify on physical exam due to pain and swelling of the knee. Axial and coronal fluid-sensitive sequences are often best to demonstrate these injuries. Generalized increased T2 signal in the region of the PLC should raise concern for injury. The fibular collateral ligament and conjoined tendon should be plainly visible on T1 and fluid-weighted sequences. If a bare area is identified in the hiatus, a popliteal origin injury should be suspected. A fibular head avulsion can simulate a PLC injury on radiograph. Careful correlation should be performed with T1-weighted sequences and, if available, radiographs ( Fig. 91.21 ).