Avaliação radiológica para genuartralgia

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IMAGING — Radiographs and other procedures are used to confirm a diagnosis and to determine the severity of the condition. The suspected clinical diagnosis determines the need for and type of imaging study. The radiographic evaluation of knee pain virtually always begins with plain films. The Ottawa knee rules can help to determine when knee x-rays are necessary following acute trauma.

MRI is the best imaging technique for diagnosis of soft tissue injuries of the knee (eg, ligament and meniscus injury). Radiologic assessment of patients with acute knee pain is reviewed in detail separately. (See "Radiologic evaluation of the acutely painful knee in adults".)

Knee trauma and Ottawa knee rule — The Ottawa knee rule states that radiographs of the knee should be obtained after acute injury ONLY in patients who meet one or more of the following criteria [15-17]:

●Age ≥55 years

●Isolated tenderness of patella (with no other bony tenderness of the knee)

●Tenderness at the head of the fibula

●Inability to flex the knee to 90 degrees

●Inability to bear weight both immediately and in the emergency department for four steps, regardless of limp (ie, unable to transfer weight onto each lower limb two times)

In a meta-analysis, the Ottawa knee rule was almost 100 percent sensitive for identifying patients who have knee fractures (99 percent, 95% CI 93-100 percent) and 49 percent specific (95% CI 43-51 percent) [16].

CONFIRMATORY PROCEDURES

●Joint aspiration – All patients with a knee effusion in whom infection cannot be ruled out should have a diagnostic aspiration performed (picture 1). The presence of leukocytosis may help to distinguish an inflammatory from a noninflammatory process (table 4). (See "Synovial fluid analysis".)

●Bursal aspiration – Bursal aspiration is performed to distinguish bursitis due to trauma, crystal deposition disease (gout), or infection (picture 2).

●Local anesthetic block – Local anesthetic block of the anserine bursa may be used to determine the contribution of arthritis to the patient's overall pain level (picture 3). Anserine bursitis can accompany osteoarthritis, joint effusion, or other primary diagnoses that cause an impaired gait.

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INTRODUCTION — The knee is one of the largest and most complex joints in the body. It is lined by synovium and consists of two hinge-type joints between the femoral condyles and the medial and lateral tibial plateaus and a gliding-type joint between the patella and the trochlear groove of the anterior distal femur (picture 1) [1]. The major stabilizers of the joint are the extensor tendons (quadriceps and patellar tendons), medial and lateral collateral ligament complex, and cruciate ligaments. The menisci are fibrocartilaginous structures that distribute the stress over the articular cartilage, absorb the shock in axial loading, stabilize the joint in flexion and extension, and have a role in joint lubrication [2].

For the purposes of this discussion, “chronic knee pain” is defined as pain of at least three months’ duration. Imaging modalities used to evaluate adults with chronic knee pain and the appropriateness of particular studies in different clinical scenarios will be reviewed here. The history and physical examination, which are necessary to develop a differential diagnosis prior to the selection of imaging tests, a general review of imaging tests that are used in the evaluation of bone and joint pain, and radiologic evaluation of adults with acute knee pain are presented separately. (See "General evaluation of the adult with knee pain" and "Imaging techniques for evaluation of the painful joint" and "Radiologic evaluation of the acutely painful knee in adults".)

Chronic knee pain is most commonly caused by arthritis, bursitis and cystic lesions around the joint, tendon pathology, cartilage pathology, chronic infection, osteonecrosis (avascular necrosis), stress fractures, and avulsion injuries.

Referred pain is an important clinical consideration in the patient with chronic knee pain; however, knee pain that is referred from other anatomic sites is beyond the scope of our discussion and is discussed elsewhere. (See "General evaluation of the adult with knee pain", section on 'Referred pain syndromes'.)

The use of imaging in the assessment of children and adolescents with knee pain is also presented elsewhere. (See "Approach to the young athlete with acute knee pain or injury", section on 'Radiographic evaluation' and "Approach to the young athlete with chronic knee pain or injury", section on 'Radiography'.)

ARTHRITIDES — The major arthritides involving the knee joint include osteoarthritis, calcium pyrophosphate deposition disease, and inflammatory arthritides such as rheumatoid arthritis, psoriatic arthritis, reactive arthritis (formerly Reiter’s syndrome), and peripheral arthritis in patients with ankylosing spondylitis. Less common arthritides that cause chronic knee pain such as neuropathic arthropathy, gout, and hemophilic arthropathy are discussed below.

As noted above, the knee joint consists of three compartments: the medial tibiofemoral, the lateral tibiofemoral and the patellofemoral compartment. These compartments are affected differently by various arthropathies.

Plain film radiography is the initial imaging modality of choice for evaluation of all arthritides and in most cases the only study required for diagnosis. Magnetic resonance imaging (MRI) has some value for evaluation of arthritides, which will be discussed below under individual entities. MRI can differentiate between proliferating synovium and joint effusion, and quantifies synovial volume. The latter is valuable in assessing severity of the disease and response to therapy in clinical trials. MRI and ultrasonography may be useful in directing synovial biopsy [1,3]. Computer-aided tomography (CT) and CT arthrography may be alternative imaging studies when MRI is contraindicated.

Osteoarthritis — Osteoarthritis (OA) is the most common arthropathy seen in the knee. OA can be primary or secondary to prior joint injury or infection, osteonecrosis, inflammatory and crystal-induced arthropathies, metabolic disorders, or hemophilia. (See "Diagnosis and classification of osteoarthritis".)

Plain film radiography is the initial, and is often the only, imaging modality needed to diagnose OA and exclude other causes of knee pain. The presence of articular calcification may be a clue to the presence of OA due to underlying endocrine, metabolic, or inherited disorder (see 'Calcium pyrophosphate deposition disease' below).

   Plain film radiography — Plain film radiographs are not sensitive for detection of early cartilage loss associated with arthritis. Hallmarks of OA of the knee seen in plain film include osteophyte formation, joint space narrowing, subchondral sclerosis, and subchondral cyst formation (image 1). Osteophyte formation is the most specific sign for detection of OA [4]. Joint effusion can also be present. Advanced manifestations of osteoarthritis include complete loss of joint space, osteochondral loose bodies, and Baker's cyst formation. Other degenerative changes in the knee can occur at the insertion of the quadriceps or patellar tendon on the patella, leading to enthesopathy (ossification of the tendon attachment).
   CT — CT is more sensitive for showing early bony changes of osteoarthritis, however, it is not routinely indicated. CT can help in detection of radiographically occult loose bodies.
   MRI — MRI is not recommended in the presence of advanced OA seen on plain radiographs. In early OA, MRI may provide additional information about the extent of bone and soft tissue involvement, such as subchondral bone marrow edema, extent of cartilage damage, meniscal and ligament tears, synovitis, osteochondral loose bodies, and Baker’s cyst [5].

   However, MRI often reveals abnormal findings even in asymptomatic patients without radiographic OA. In a study of 710 patients over age 50 without radiographic evidence of OA, the most common changes on MRI examination of the tibiofemoral joint were osteophytes, cartilage damage, and bone marrow abnormalities (in 74, 69, and 52 percent, respectively) [6]. Moreover, at least one finding was present in similar numbers of patients with or without pain (90 to 97 percent and 86 to 88 percent, depending upon the pain definition used). Changes were more common with advancing age. Thus, MRI is generally not useful in routine clinical practice for the evaluation of OA.

Several of the above findings correlate with severity of knee pain in patients with OA. These include synovitis, joint effusion, ligament tears such as anterior cruciate ligament (ACL) tear, and subchondral bone marrow edema [6-9]. Subchondral bone marrow edema also correlates with progression of OA [8,10,11].

Degenerative meniscal tears appear to be an early event in development of OA as shown by several studies [12,13]. However, degenerative meniscal tears are highly prevalent in both symptomatic and asymptomatic osteoarthritic knees and MRI is not indicated in the evaluation and management of these tears in patients with OA [14,15].

Changes in cartilage volume also appear to be associated with clinical symptoms [16]. Studies using quantitative techniques emphasize the ability of MRI to accurately measure the progression of knee OA by assessing changes in cartilage volume. This in turn could be helpful in following the effect of treatment with structure-modifying agents in clinical trials, or in selecting patients at high risk for requiring joint replacement [16-19].

Calcium pyrophosphate deposition disease — Calcium pyrophosphate deposition disease (CPPD) is caused by deposition of calcium pyrophosphate dihydrate crystal in and around the joints. CPPD is the second most common arthropathy of the knee, seen predominantly in the middle and older age group. CPPD is often asymptomatic and diagnosed on plain film by articular cartilage or meniscal calcification (chondrocalcinosis). When symptomatic, CPPD is also called pseudogout. Joint aspiration may be necessary to obtain synovial fluid to confirm the presence of CPPD crystals and to exclude infection. (See "Clinical manifestations and diagnosis of calcium pyrophosphate crystal deposition disease".)

   Plain film radiography — Thin calcification (chondrocalcinosis) is present in the articular cartilage or menisci (image 2), with preferential involvement of the patellofemoral joint. Calcifications of the synovium, tendon, and ligaments can also be seen. CPPD may lead to the early development of secondary osteoarthritis. Isolated patellofemoral involvement in the knee should suggest the possibility of CPPD arthropathy. Definitive diagnosis of CPPD arthropathy also requires the demonstration of positively (but weakly) birefringent CPPD crystals in tissue or synovial fluid.

Rheumatoid arthritis — The most common inflammatory arthritis of the knee is rheumatoid arthritis (RA). Women are more commonly affected than men. The knees are involved in 80 percent of the patients with RA [20]. The presence of a popliteal (Baker's) cyst (see 'Popliteal bursitis' below) is a common occurrence in RA.

   Plain film radiography — Characteristic findings of RA on plain film are bilaterally symmetric and uniform joint space narrowing involving all three compartments (medial and lateral tibiofemoral and patellofemoral), periarticular osteoporosis, soft tissue swelling and joint effusion. Large synovial cysts may be present. Erosion is not a predominant characteristic in the knee and there is relative lack of sclerosis and osteophyte formation.
   CT — CT demonstrates erosions earlier than plain film; however, CT is not routinely used for evaluation of RA. CT better demonstrates the degree of joint destruction as compared to plain films. CT differentiates synovial proliferation from joint effusion.
   MRI — MRI demonstrates early changes of RA manifested by synovitis before plain film evidence of disease. The extent of synovitis in early RA is the best predictor of development of erosions [21]. MRI shows the degree of synovial hypertrophy (pannus formation), which correlates with disease activity. MRI identifies involvement of the cartilage, tendon, and ligaments. MRI is useful in demonstrating the presence of loose bodies and Baker's cyst, which may complicate RA. Despite these advantages, the role of MRI in the diagnosis and monitoring of RA is still uncertain. (See "Diagnosis and differential diagnosis of rheumatoid arthritis", section on 'Evaluation for suspected RA' and "General principles of management of rheumatoid arthritis in adults".)

Psoriatic arthritis and reactive arthritis — Psoriatic and reactive arthritis (formerly the arthritis of Reiter's syndrome) have similar radiographic findings in the knee. Asymmetric involvement is a common feature. (See "Clinical manifestations and diagnosis of psoriatic arthritis" and "Reactive arthritis (formerly Reiter syndrome)".)

   Plain film radiography — Plain film radiography is the single most important technique for detection, diagnosis, and follow-up of reactive arthritis. Typical findings include osteophyte and enthesophyte formation (bone proliferation at the ligamentous and tendinous attachments) and periostitis.

Ankylosing spondylitis — Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the axial skeleton (spine and sacroiliac joints). It is more commonly seen in young men. Knee involvement is uncommon. (See "Diagnosis and differential diagnosis of ankylosing spondylitis in adults".)

   Plain film radiography — Plain film radiography is the single most important technique for detection, diagnosis, and follow-up of patients with ankylosing spondylitis and knee involvement. Uniform joint space narrowing, small erosions, and bone sclerosis are seen early in the disease, which may lead to ankylosis of the entire joint.

Neuropathic arthropathy — Neuropathic arthropathy, also known as Charcot joint, is caused by chronic conditions causing neurosensory deficit such as diabetes, syphilis, and syringomyelia. The lower extremity, including the hip and knee, is involved in 60 to 70 percent of patients with tabes dorsalis [22]. Charcot joint is characterized by fragmentation of bone and cartilage with increased density and disorganization of the joint.

   Plain film radiography — Plain film radiographic findings of neuropathic arthropathy include large recurrent joint effusions, joint subluxations, pathological fractures and bone debris in the joint, leading to complete derangement of the joint.

Gouty arthritis — Gout affects men significantly more than women. The knee is the third most commonly affected joint, after the foot and ankle. Plain film radiography changes in gout appear in approximately 45 percent of the patients, six to eight years after the initial attack [23].

Urate crystal deposition in the articular cartilage leads to secondary osteoarthritis. Urate crystals can also deposit in the soft tissues in the form of tophi, which in turn cause erosion of the adjacent bone with typical sclerotic border and overhanging edges. (See "Clinical manifestations and diagnosis of gout".)

   Plain film radiography — Plain film radiography of gouty arthritis or tophaceous gout can show periarticular soft tissue masses with occasional internal calcifications and possible adjacent erosions. When the knee is affected, osteoarthritic changes predominate [23].
   MRI — MRI shows gouty erosions earlier than plain film radiography, demonstrates presence of tophi and differentiates suprapatellar soft tissue masses from joint effusion. Tophi characteristically have relatively homogeneous intermediate or low-signal intensity on T1-weighted images and variable signal intensity on T2-weighted images [24].

Hemophilic arthropathy — Joint changes in hemophilia are secondary to repeated hemarthrosis (joint hemorrhage) and intraosseous bleeding, which occur in 75 to 90 percent of patients. Articular changes occur most often in the first and second decade of life. The knee is the most commonly involved joint, with 50 percent of the patients developing permanent bone changes around the joint [25]. The radiographic findings can mimic juvenile idiopathic arthritis. MRI findings can mimic those of pigmented villonodular synovitis (PVNS) due to intraarticular hemosiderin deposition in both entities. (See "Clinical manifestations and diagnosis of hemophilia", section on 'Hemophilic arthropathy' and "Antineoplastic therapy for miscellaneous benign diseases affecting soft tissue and bone", section on 'Pigmented villonodular synovitis/tenosynovial giant cell tumor'.)

   Plain film radiography — In acute hemarthrosis, joint effusion and osteoporosis are present. Chronic hyperemia in the joint leads to overgrowth of the tibial and femoral epiphyses, causing widening of the intercondylar notch. Overgrowth and squaring of the patella is present. Subchondral cyst formation is seen secondary to intraosseous bleeding. In the chronic phase, there is secondary osteoarthritis with uniform joint space narrowing.
   MRI — MRI is indicated for determining the extent of cartilage destruction and synovial thickening, which affect management. Hemosiderin deposition due to chronic joint hemorrhage in hemophilia may result in clumps of low T1-weighted and low T2-weighted signal lining the synovium. The presence of hemosiderin is associated with advanced joint destruction. MRI is important in assessment of response to therapy and disease progression [1].

BURSITIS AND CYSTS — The bursae around the knee joint most commonly involved by pathology include the popliteal bursa, prepatellar bursa, and pes anserinus bursa. Less commonly involved bursae are the supra- and infrapatellar bursae, semimembranosus tibial collateral ligament bursa, tibial collateral ligament bursa, iliotibial band bursa, and fibular collateral ligament-biceps femoris bursae.

Bursitis — Bursae around the knee can become inflamed and mimic intraarticular pathology. The bursae around the joint are not seen arthroscopically, and ultrasound (US) is useful in evaluation of pes anserinus bursitis and Baker's cyst. MRI is the modality of choice for imaging evaluation of suspected bursal pathology in other knee bursae. (See "Knee bursitis".)

Popliteal bursitis — The most commonly inflamed bursa of the knee is the popliteal bursa, also known as Baker's cyst. Baker's cyst is commonly associated with meniscal tears and other causes of internal derangement of the joint. Baker's cyst is also associated with arthropathies, including osteoarthritis and rheumatoid arthritis. (See "Popliteal (Baker's) cyst".)

Baker's cyst may extend superiorly or inferiorly and can cause compartment syndrome. It can cause pressure on the venous system and lead to edema or thrombophlebitis. MRI (image 3) or ultrasound (US) (image 4) is diagnostic. Rupture of the cyst with extravasation in the surrounding muscle may mimic thrombophlebitis. Thus, if there is suspicion of venous thrombosis the use of US to both assess for proximal clot and for the presence of an intact or ruptured Baker's cyst is appropriate. (See "Diagnosis of suspected deep vein thrombosis of the lower extremity", section on 'Compression ultrasonography'.)

Prepatellar bursitis — Prepatellar bursitis is a common etiology of anterior knee pain that can be caused by repetitive trauma from kneeling, also referred to as "housemaid's knee". Although MRI can be diagnostic (image 5), clinical examination alone augmented with bursal aspiration usually suffices for diagnosis of this superficial bursitis.

Pes anserinus bursitis — Pes anserinus bursitis is commonly seen in runners, can clinically mimic medial meniscal tear, and present as anteromedial knee pain. The pes tendon is formed by the sartorius, gracilis, and semitendinosus tendons, which insert medially in the upper tibia. While MRI is diagnostic and ultrasound can also be useful, the diagnosis can usually be established clinically, without imaging studies.

Cysts — Popliteal bursal swelling (Baker's cyst) is one type of cystic swelling that occurs near the knee joint (see 'Popliteal bursitis' above). The other two types that are commonly found in this location are ganglion cysts and meniscal cysts.

Ganglion cyst — A ganglion is a cystic benign tumor of unknown origin. Ganglion cysts may be juxta-articular, intra-articular, and periosteal. A juxta-articular ganglion cyst is a common entity within the soft tissue around the knee, which can be asymptomatic or present with pain or nerve entrapment symptoms. Intra-articular ganglions are less common and may be asymptomatic or present with pain, locking, clicking, or popping, and decreased range of motion [26]. Periosteal ganglions are rare. Magnetic resonance imaging is the study of choice for detection of ganglion cysts and demonstrates a cystic mass [27]. When present, ganglion cysts may be unilocular or multilocular [26].

Meniscal cyst — A meniscal cyst is a collection of synovial fluid within or adjacent to the meniscus, and is most often associated with meniscal tear. Symptoms include swelling, palpable mass, pain and decreased range of motion. MRI demonstrates a cystic mass and detects associated meniscal tear [27].

TENDON PATHOLOGY — The two knee tendons most frequently involved by tendinosis are the quadriceps and patellar tendons. The spectrum of tendon abnormalities includes degeneration, tendinosis, tenosynovitis, and partial or complete tears. Factors that predispose to tendon tears are chronic repetitive stress, tendon degeneration, inflammatory process of the tendon (such as rheumatoid arthritis, other inflammatory arthropathies, and chronic infection of tendon sheath), chronic renal disease, chronic glucocorticoid use, diabetes, and gout.

   MRI — The imaging modality of choice for evaluating tendon pathology is MRI. MRI of normal tendons demonstrates low signal (dark appearance) on all MRI sequences (image 6). One exception in the knee is the quadriceps tendon, which normally exhibits a striated appearance of low and intermediate signal intensity.

   On MRI, tendinosis appears as intermediate increased signal (bright) on both T1-weighted and T2-weighted images. Tendinosis can lead to either partial tear with disruption of some fibers of the tendon, (image 7) or to complete tear with discontinuity of the entire tendon (image . MRI demonstrates the quality of the remnants of tendon in complete tear, degree of separation of tendon fragments, and resultant muscle atrophy, which affect management.

ARTICULAR CARTILAGE PATHOLOGY — In settings other than radiographically advanced OA (see 'Osteoarthritis' above), MRI is the modality of choice in demonstrating abnormal articular cartilage in the knee. Focal defects in the articular cartilage surface may result from trauma to the knee. (See "Radiologic evaluation of the acutely painful knee in adults", section on 'Acute chondral and osteochondral injuries'.)

   CT arthrography — CT arthrography and MRI are equally accurate in diagnosing cartilage damage. For larger lesions, CT arthrography has higher specificity than MRI [28]. CT arthrography is diagnostic of cartilage tear by showing penetration of contrast material within the cartilage substance.
   MRI — MRI demonstrates the spectrum of cartilage abnormalities, from focal abnormal signal, to surface fibrillation or irregularity, partial thickness defect or full thickness defect. MRI also demonstrates underlying bone signal abnormalities.

MENISCAL CARTILAGE PATHOLOGY — Degenerative changes in the fibrocartilagenous menisci increase with age. Thus, radiographic findings, particularly increased signal within the body of the meniscus must be interpreted with caution. Clinically important meniscal tears may be found in patients with knee pain without a history of trauma. However, meniscal tears may also be noted in otherwise healthy persons with no knee pain.

Chronic knee pain with mechanical symptoms such as locking or an inability to fully extend the knee, popping, giving way, or "not moving properly" may indicate a clinically significant meniscal tear. Patients for whom surgical intervention would be indicated if a meniscal tear is present are best evaluated with MRI. (See "Meniscal injury of the knee", section on 'Imaging'.)

   MRI — MRI demonstrates the spectrum of meniscal pathology, from myxoid degeneration to complete tear. Tears of the medial meniscus are common sports-related injuries. Tears of the lateral meniscus are less common. Lateral meniscal tears are commonly associated with discoid meniscus, a developmental anomaly. Horizontal or oblique meniscal tears are frequently seen in both symptomatic and asymptomatic knees, and the prevalence of MRI-detected tears in asymptomatic knees increases markedly with age.

   However, radial, vertical, complex, or displaced meniscal tears are almost always symptomatic. Meniscal tear is present when abnormal intrameniscal MRI signal disrupts the articular surface (image 9).
   MR arthrography — MR arthrography is indicated in the postsurgical evaluation of meniscal retear following prior partial meniscectomy or meniscal repair.

CHRONIC INFECTION — Chronic infection may occur in the knee joint or in the juxta-articular bone. Mycobacterial and fungal infections in either site may be indolent but can cause progressive joint damage.

Chronic infectious arthritis — In contrast to gram positive or gram negative bacterial joint infections, which typically present acutely, mycobacterial and fungal arthritis may have more subtle symptoms and patients with arthritis of the knee due to organisms such as mycobacterium tuberculosis, Candida species, and Coccidioides immitis can present with chronic monoarthritis of the knee. (See "Skeletal tuberculosis" and "Candida osteoarticular infections" and "Manifestations and treatment of extrapulmonary coccidioidomycosis".)

   Plain film radiography — There are no findings on plain films of the knee that are specific for mycobacterial or fungal infection. Relative preservation of the joint space in the presence of adjacent bone destruction has been suggested as a clue to articular tuberculosis. (See "Skeletal tuberculosis", section on 'Arthritis'.)
   MRI — MRI features of tuberculous arthritis of the knee include synovial proliferation with intermediate to low T2-weighted signal and synovial enhancement following intravenous contrast [29].

Chronic osteomyelitis — The advantages and limitations of various imaging studies in the assessment of suspected chronic osteomyelitis are summarized below and are discussed in more detail separately. (See "Approach to imaging modalities in the setting of suspected osteomyelitis".)

   Plain film radiography — Chronic osteomyelitis presents with a mixed pattern of lysis, sclerosis, and cortical thickening. Sequestrum formation (necrotic bone), involucrum (thick, irregular periosteal reaction), cloacae (cortical defects), and sinus tract can also be present.
   CT — CT is indicated in chronic osteomyelitis to evaluate the extent of the disease such as sequestrum, involucrum, cloacae, sinus tract, and soft tissue swelling. CT is considered superior to MR imaging in the setting of chronic active osteomyelitis for the demonstration of cortical destruction and gas. The most specific sign of active chronic osteomyelitis is the presence of a sequestrum, which is best demonstrated on CT, although the viability of sequestered bone is more reliably determined with MR imaging [30].
   MRI — MRI is more sensitive than CT in demonstrating activity of the disease in chronic osteomyelitis. MRI demonstrates most features of chronic active osteomyelitis, including the presence of sequestrum, intramedullary abscess, cloaca, subperiosteal fluid collection, and sinus tract formation. Serial MRI demonstrates the progression or regression of chronic infection.
   Radionuclide scintigraphy — Nuclear imaging may be useful if CT and MRI are contraindicated. Bone scanning, gallium scanning, and tagged white blood cell scanning are among the options available.

OSTEONECROSIS (AVASCULAR NECROSIS OF BONE) — Osteonecrosis, also known as avascular necrosis of bone (AVN), frequently affects the knee, which is the third most commonly involved joint after the hip and shoulder. Osteonecrosis of the knee develops in adults secondary to underlying disease or predisposing factors such as steroid use, alcoholism, connective tissue disorders (systemic lupus erythematosus), hemoglobinopathies, and HIV. Osteonecrosis is usually asymptomatic in the knee, unless epiphyseal collapse occurs. (See "Osteonecrosis (avascular necrosis of bone)".)

The femoral condyles are more commonly involved, although any area around the knee can be affected. Plain film radiography findings range from normal to radiolucent subchondral area to epiphyseal collapse. MRI demonstrates a geographic focus of heterogeneous signal surrounded by low signal on T1-weighted images and by a double line on T2-weighted images. (See "Osteonecrosis (avascular necrosis of bone)", section on 'Imaging'.)

Spontaneous osteonecrosis of the knee (SONK) is commonly seen in older adults, eg, in the sixth and seventh decade, more often in women [31,32]. Spontaneous osteonecrosis may result from trauma, oral or intra-articular steroid administration, systemic disease such as lupus erythematosus, and meniscal tears. The most common location is the medial aspect of the medial femoral condyle, which is a weight-bearing segment. Other locations such as medial tibial plateau and patella have been reported [32].

   Plain film radiography — The radiographic appearance of osteonecrosis of the medial femoral condyle ranges from mild flattening of the condyle to subchondral lucency, later surrounded by sclerosis.
   MRI — MRI should always be performed to exclude predisposing meniscal tears. MRI is useful for the early detection of spontaneous osteonecrosis of the knee and has prognostic significance by identifying the lesions that are likely to resolve with conservative treatment [33].

Osteochondritis dissecans — Osteochondritis dissecans is a relatively common entity seen in adolescents and young adults, more often in men [34]. Osteochondritis dissecans is a result of chronic microtrauma and represents a form of avascular necrosis that is small and focal. The most common location is the lateral aspect of the medial femoral condyle, which is a non-weightbearing segment. The clinical manifestations, diagnosis, and management of osteochondritis dissecans affecting the knee (and other joints) are reviewed in detail separately. (See "Clinical manifestations and diagnosis of osteochondritis dissecans (OCD)" and "Management of osteochondritis dissecans (OCD)".)

   Plain film radiography — Joint effusion may be the only initial radiographic finding. The classic finding is a radiolucent line separating the osteochondral fragment from the femoral condyle. This can lead to complete separation of the fragment into an intraarticular loose body.
   MR imaging and MR arthrography — MR imaging demonstrates abnormal signal in the lateral aspect of the medial femoral condyle due to osteochondral fracture (image 10). MR arthrography may be helpful in assessing the stability of the osteochondral fragment.
   CT arthrography — CT arthrography with intraarticular iodinated contrast may be useful in cases when MRI is contraindicated (eg, metallic foreign body around the knee, pacemaker, aneurysm clip). CT arthrography detects chondral injuries and can be helpful in assessing the stability of the osteochondral fragment in osteochondritis dissecans.

STRESS AND INSUFFICIENCY FRACTURES — Stress and insufficiency fractures are defined and discussed in more detail elsewhere. (See "Overview of stress fractures".)

Plain film radiographs are the initial modality of choice for the evaluation of suspected stress and insufficiency fractures. However, MRI, radionuclide imaging, and CT are all more sensitive than plain radiographs for the detection of such fractures, particularly during the first few weeks following the onset of pain.

   Plain film radiography — Stress and insufficiency fractures are often not visualized in the first two weeks on plain radiographs. The sensitivity of early radiographs is as low as 15 percent, increasing to 50 percent in follow-up radiographs [35]. When stress or insufficiency fracture is clinically suspected, further evaluation with MRI is warranted. Development of radiographic findings can be prevented by cessation of physical activity. Stress and insufficiency fractures appear as periosteal reaction and/or horizontal band of sclerotic density in the tibial metaphysis.
   MRI — MRI is the modality of choice for early detection of stress reaction and stress fracture [36]. MRI shows the earliest changes of stress reaction by detecting bone marrow edema before development of the fracture line. MRI is more sensitive than radionuclide bone scan in early detection of bone marrow edema and has high specificity.
   Radionuclide bone scan — Radionuclide bone scanning with single photon emission computed tomography (SPECT) is close to 100 percent sensitive in early detection of stress fractures. Positive findings can be seen within 6 to 72 hours of development of stress fracture. Specificity of bone scan is low.
   CT — CT has a limited role in diagnosis of stress injuries [36]. CT is more sensitive than plain film radiography for detecting certain stress fractures, but the sensitivity is lower than MRI.

KNEE TUMORS — Tumors and tumor-like processes within the knee or in the surrounding bone or soft tissues can cause chronic knee pain. Imaging techniques play an important role in the evaluation of such lesions and, when biopsy or excision is being considered, in the preoperative planning process.

PLICA SYNDROME — Plica syndrome is an uncommon entity caused by a thickened and stiff medial patellar plica trapped between the patella and femur, causing symptoms of pain, locking, and clicking similar to those due to a torn meniscus. The plicae are embryological remnants that may be found in the suprapatellar, infrapatellar and medial compartments of the patellofemoral joint. The suprapatellar and infrapatellar plica do not cause plica syndrome. Features of the plica syndrome identified on magnetic resonance imaging (MRI) are diffusely thickened medial plica, which may be associated with synovitis or erosion of the articular cartilage of the patella or femoral condyle [37].

CHONDROMALACIA PATELLAE — Chondromalacia patellae is a syndrome of patellofemoral pain seen primarily in adolescents and young adults secondary to repeated microtrauma or single traumatic injury to the posterior patellar articular cartilage. Patellofemoral maltracking is a risk factor, resulting in recurrent patellar subluxation and trauma to the patellar cartilage. Although the diagnosis can usually be made clinically, MRI is the modality of choice when chondromalacia patellae is suspected but the diagnosis is uncertain.

   Plain film radiography — Plain film radiography is limited in diagnosis of early chondromalacia patellae. In more advanced stages, joint space narrowing and signs of secondary patellofemoral osteoarthritis are seen.
   MRI — Reported sensitivity and specificity of MRI for detection of articular cartilage lesions of the patella and femoral trochlea are 91 and 98 percent, respectively [38]. MRI shows focal areas of swelling of patellar cartilage, surface irregularity, areas of thinning and areas of cartilage loss with exposure of subchondral bone [39].

DISCOID MENISCUS — Discoid meniscus is a congenital malformation manifested by abnormal shape and size, having discoid rather than "C" shape appearance. It most commonly affects the lateral meniscus and is associated with increased incidence of meniscal degeneration and tear, which can in turn predispose to the development of OA and may contribute to chronic knee pain [40]. MRI diagnoses the abnormal appearance and complications.

SUMMARY — For the purposes of this discussion, chronic knee pain has been defined as being present for at least three months.

Chronic knee pain is most commonly caused by:

   Arthritis (see 'Arthritides' above)
   Bursitis or other cystic lesions (see 'Bursitis and cysts' above)
   Tendinitis or chronic tears (see 'Tendon pathology' above)
   Cartilage damage (see 'Articular cartilage pathology' above)
   Meniscal damage (see 'Meniscal cartilage pathology' above)
   Chronic infection (see 'Chronic infection' above)
   Osteonecrosis (see 'Osteonecrosis (avascular necrosis of bone)' above)
   Stress fractures (see 'Stress and insufficiency fractures' above)
   Knee tumors (see 'Knee tumors' above)
   Plica syndrome (see 'Plica syndrome' above)
   Chondromalacia patellae (see 'Chondromalacia patellae' above)

Whether or not radiologic evaluation is necessary for an adult patient with chronic knee pain is a decision that is based on the results of the medical history and physical examination. (See "General evaluation of the adult with knee pain".)

When radiologic evaluation is indicated, the initial study is generally:

   Plain film radiography (plain x rays) when arthritis, chronic infection, osteonecrosis, or tumors are suspected or when the cause of chronic knee pain is obscure despite a careful history and physical examination.
   Ultrasonography when cystic lesions (eg, Baker's cyst, meniscal cyst, etc) are suspected.
   Magnetic resonance imaging (MRI) when soft tissue pathology is suspected (eg, chronic tendon or meniscal tears, focal articular cartilage damage, or plica syndrome).

Computed tomography (CT) and/or CT arthrography may be used in lieu of MRI in patients with contraindications to MRI (eg, pacemakers)