Radiologic evaluation of the chronically painful knee in adults

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 as well as a gliding-type joint between the patella and the trochlear groove of the anterior distal femur (figure 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. Other topics review the clinical approach to adults with knee pain, imaging in adults with acute knee pain, imaging of painful joints, ultrasound of the knee, and imaging of knee tumors:

●(See "Approach to the adult with unspecified knee pain".)

●(See "Approach to the adult with knee pain likely of musculoskeletal origin".)

●(See "Radiologic evaluation of the acutely painful knee in adults".)

●(See "Imaging techniques for evaluation of the painful joint".)

●(See "Musculoskeletal ultrasound of the knee".)

●(See "Radiologic evaluation of knee tumors 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 "Approach to the adult with unspecified knee pain", section on 'Referred pain'.)

The assessment of knee pain in children and adolescents, including the use of imaging, is also presented elsewhere. (See "Approach to chronic knee pain or injury in children or skeletally immature adolescents", section on 'Diagnostic imaging' and "Approach to acute knee pain and injury in children and skeletally immature adolescents", section on 'Imaging'.)

OVERVIEW OF IMAGING — 

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. The overall approach to patients with knee pain is discussed elsewhere. (See "Approach to the adult with unspecified knee pain".)

When radiologic evaluation is indicated, the initial study depends on the suspected cause of pain. As an example, plain film radiography (plain x-rays) can show bony changes but is not sensitive for soft tissue pathology. Our general approach to imaging modalities in chronic knee pain is:

●Plain film radiography is used 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. An overview of indications for various radiographic views is summarized in the table (table 1). (See 'Arthritis' below and 'Chronic infection' below and 'Osteonecrosis (avascular necrosis of bone)' below and 'Knee tumors' below.)

●Ultrasonography is used when cystic lesions (eg, Baker's cyst, meniscal cyst, etc) or synovitis are suspected (see 'Cystic lesions' below and 'Arthritis' below and "Musculoskeletal ultrasound of the knee") Ultrasound is sensitive for detecting synovial changes in inflammatory arthritis. When initial radiographs are negative, ultrasound may be used for early detection of synovial inflammation.

●Magnetic resonance imaging (MRI) is used when soft-tissue pathology is suspected (eg, chronic tendon or meniscal tears, focal articular cartilage damage, or plica syndrome). MRI is also valuable for early diagnosis of selected bone pathology, including osteomyelitis, osteonecrosis, and stress fractures. (See 'Tendinitis or chronic tears' below and 'Cartilage damage' below and 'Meniscal damage' below and 'Plica syndrome' below and 'Chronic osteomyelitis' below and 'Osteonecrosis (avascular necrosis of bone)' below and 'Stress and insufficiency fractures' below.)

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

More detailed discussions of imaging techniques to evaluate painful joints and clinical applications of musculoskeletal ultrasound are provided elsewhere. (See "Imaging techniques for evaluation of the painful joint" and "Musculoskeletal ultrasonography: Clinical applications".)

CONSIDERATIONS BASED ON SUSPECTED CAUSE OF KNEE PAIN — 

The optimal type of imaging varies based on the suspected cause of knee pain. Common imaging findings in various conditions that cause chronic knee pain are detailed below.

Arthritis — Arthritis is a common cause of chronic knee pain. 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. The major types of arthritis involving the knee joint include osteoarthritis (OA), calcium pyrophosphate crystal deposition (CPPD) disease, and inflammatory arthritides such as rheumatoid arthritis (RA), psoriatic arthritis, reactive arthritis (formerly Reiter syndrome), and peripheral arthritis in patients with ankylosing spondylitis (AS). Less common arthritides that cause chronic knee pain, such as gout, neuropathic arthropathy, and hemophilic arthropathy, are also discussed.

Plain film radiography is the initial imaging modality of choice for evaluation of all types of arthritis and in most cases the only study required for diagnosis. 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 it 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]. CT and CT arthrography may be alternative imaging studies when MRI is contraindicated.

Osteoarthritis — OA is the most common arthropathy seen in the knee and may be primary or secondary (eg, from prior joint injury or inflammatory arthritis). (See "Clinical manifestations and diagnosis of osteoarthritis", section on 'Knee'.)

Plain film radiography is the initial and often the only imaging modality needed to diagnose OA and exclude other causes of knee pain. The utility of various imaging modalities in diagnosing OA is summarized below and discussed in detail separately. (See "Clinical manifestations and diagnosis of osteoarthritis", section on 'Imaging findings'.)

●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 OA 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 (tendinopathy, followed by ossification of the tendon attachment). 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 crystal deposition disease' below.)

●CT – CT is more sensitive for showing early bony changes of OA; however, it is not routinely indicated. CT can help in detection of radiographically occult loose bodies.

●MRI – MRI is not routinely indicated for diagnosis of knee OA or subsequent evaluation 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 knee OA. In a study of 710 patients over age 50 without radiographic evidence of knee 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.

●Ultrasound – Ultrasound can also detect findings consistent with more advanced OA, including osteophytes and signs of cartilage deterioration. It is also useful for detecting effusions and synovial hypertrophy secondary to OA (see "Musculoskeletal ultrasonography: Clinical applications", section on 'Joints'). Ultrasound is also being used to guide joint aspiration and intraarticular injection in OA [7]. (See "Musculoskeletal ultrasonography: Guided injection, aspiration, and biopsy of joints and related structures".)

Certain imaging findings are associated with OA symptoms and/or prognosis:

●Imaging abnormalities that correlate with severity of knee pain in patients with OA include synovitis, joint effusion, ligament tears such as anterior cruciate ligament (ACL) tear, and subchondral bone marrow edema [6,8-10]. Subchondral bone marrow edema also correlates with progression of OA [9,11,12].

●Changes in cartilage volume also appear to be associated with clinical symptoms [13]. 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 [13-16].

●Degenerative meniscal tears appear to be an early event in development of OA as shown by several studies [17,18]. 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 [19,20].

Inflammatory arthritis

Rheumatoid arthritis — The most common inflammatory arthritis of the knee is RA. Females are more commonly affected than males. RA frequently involves the knees, with up to 13 percent of patients presenting with the knee as initial site of involvement [21]. (See "Clinical manifestations of rheumatoid arthritis", section on 'Lower extremity'.)

The use of various imaging modalities in diagnosing RA is summarized below and discussed in detail separately. (See "Diagnosis and differential diagnosis of rheumatoid arthritis", section on 'Radiologic studies'.)

●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 is not routinely used for evaluation of RA. However, CT shows erosions earlier and demonstrates the degree of joint destruction better as compared with plain film. It also differentiates synovial proliferation from joint effusion.

●MRI – The role of MRI in the diagnosis and monitoring of RA is still uncertain. MRI demonstrates early changes of RA manifested by synovitis before plain film evidence of disease. The extent of synovitis in early RA affecting the knee is the best predictor of development of erosions [22]. 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.

●Ultrasound – Ultrasound is more sensitive than clinical examination in detecting inflammation in the synovial tissues, and it can be useful in the evaluation of early RA [23,24]. It also has utility for the evaluation of the response to therapy in patients with inflammatory arthritis [7]. Semi-quantitative measurements of joint effusion, synovial hypertrophy, and inflammation have been developed, which correlate with disease activity. Ultrasound can also be used to identify tendinopathy and bony erosions when present. These methods, however, need further standardization. Ultrasound can also be used to guide joint aspiration and intraarticular therapeutic injection. (See "Musculoskeletal ultrasonography: Clinical applications", section on 'Joints'.)

Psoriatic arthritis and reactive arthritis — Psoriatic and reactive arthritis (formerly the arthritis of Reiter 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".)

Detailed discussions of imaging findings in psoriatic arthritis and reactive arthritis are provided elsewhere. (See "Clinical manifestations and diagnosis of psoriatic arthritis", section on 'Imaging findings' and "Reactive arthritis", section on 'Imaging abnormalities'.)

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 axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults".)

Plain film radiography is the single most important technique for detection, diagnosis, and follow-up of patients with AS 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. An overview of imaging studies in the diagnosis of AS is provided elsewhere. (See "Diagnosis and differential diagnosis of axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults", section on 'Imaging studies'.)

Of note, patients with AS may develop enthesitis of the knee which would not be seen on plain films. Imaging of enthesitis is reviewed separately. (See "Diagnosis and differential diagnosis of axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults", section on 'Imaging of enthesitis'.)

Crystalline arthropathy

Calcium pyrophosphate crystal deposition disease — CPPD disease is caused by deposition of calcium pyrophosphate dihydrate crystal in and around the joints. CPPD is seen predominantly in adults aged 65 years or older. The knee is the most commonly affected joint in CPPD disease [25]. There are various forms of CPPD disease, including asymptomatic evidence of CPPD on plain film (chondrocalcinosis) and acute and chronic inflammatory arthritis. Subtypes of CPPD disease and the general approach to diagnosis are described elsewhere. (See "Calcium pyrophosphate crystal deposition (CPPD) disease: Clinical manifestations and diagnosis".)

Imaging findings in CPPD disease are discussed in detail elsewhere. (See "Calcium pyrophosphate crystal deposition (CPPD) disease: Clinical manifestations and diagnosis", section on 'Imaging findings'.)

Gouty arthritis — In patients with gout, the knee is the third most commonly affected joint after the foot and ankle. There is usually a latent period of 5 to 10 years between the initial presentation and the appearance of radiographic findings [26]. Urate crystal deposition in the articular cartilage leads to secondary OA. 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 "Gout: Clinical manifestations and diagnosis", section on 'Clinical features'.)

A discussion of imaging studies in the diagnosis of gout is provided elsewhere. (See "Gout: Clinical manifestations and diagnosis", section on 'Imaging findings'.)

Neuropathic arthropathy — Neuropathic arthropathy, also known as Charcot joint, is caused by chronic conditions causing neurosensory deficit, such as diabetes, syphilis, and syringomyelia. In the past, neuropathic arthropathy of the knee was most commonly caused by tabes dorsalis (a "tertiary" form of neurosyphilis); however, this has since been replaced by diabetes mellitus [27]. Charcot joint is characterized by fragmentation of bone and cartilage with increased density and disorganization of the joint.

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.

The general approach to diagnosis of diabetic neuroarthropathy and common imaging findings are described elsewhere. (See "Diabetic neuroarthropathy" and "Diabetic neuroarthropathy", section on 'Imaging'.)

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 affected joint in patients not treated with prophylaxis (45 percent); however, in patients on prophylactic treatment, the ankle joint accounts for the most common site of bleeding [28]. (See "Clinical manifestations and diagnosis of hemophilia A and B", section on 'Joints and muscle' and "Chronic complications and age-related comorbidities in people with hemophilia", section on 'Target joints and hemophilic arthropathy'.)

The radiographic findings in hemophilic arthropathy can mimic juvenile idiopathic arthritis. MRI findings can mimic those of pigmented villonodular synovitis due to intraarticular hemosiderin deposition in both entities. (See "Treatment for tenosynovial giant cell tumor and other benign neoplasms affecting soft tissue and bone", section on 'Tenosynovial giant cell tumor'.)

Radiographic findings in hemophilic arthropathy is discussed in detail separately. (See "Overview of hemarthrosis", section on 'Bleeding disorders' and "Overview of hemarthrosis", section on 'Imaging'.)

Bursitis — Bursae around the knee can become inflamed and mimic intraarticular pathology. The bursae around the knee joint that most commonly cause chronic pain include the popliteal bursa, prepatellar bursa, and pes anserinus bursa. Bursae that less commonly cause chronic pain 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. (See "Knee bursitis".)

Imaging is not always required during the assessment of bursitis. Indications for imaging in the diagnosis of knee bursitis and the optimal imaging modalities may vary based on the location of the bursae and are discussed separately. (See "Knee bursitis".)

Cystic lesions

●Popliteal (Baker's) cyst – Popliteal synovial cysts, also known as a Baker's cyst, cause swelling of the gastrocnemius-semimembranosus bursa. 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 OA and RA. (See "Popliteal (Baker's) cyst".)

Indications for imaging and optimal imaging modalities in the diagnosis of Baker's cyst are discussed elsewhere. (See "Popliteal (Baker's) cyst", section on 'Imaging studies'.)

●Ganglion cyst – A ganglion is a cystic benign tumor of unknown origin. Ganglion cysts may be juxtaarticular, intraarticular, intraosseous, and periosteal. A juxtaarticular 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. Intraarticular ganglions are less common and may be asymptomatic or present with pain, locking, clicking, or popping as well as decreased range of motion [29]. Periosteal ganglions are rare. MRI is the study of choice for detection of ganglion cysts and demonstrates a cystic mass [30]. When present, ganglion cysts may be unilocular or multilocular [29].

●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 [30].

Tendinitis or chronic tears — 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 RA, other inflammatory arthropathies, and chronic infection of tendon sheath), chronic kidney disease, chronic glucocorticoid use, diabetes, and gout. (See "Quadriceps muscle and tendon injuries" and "Patellar tendinopathy".)

The approach to diagnostic imaging for quadriceps and patellar tendinopathies is discussed in detail elsewhere. (See "Quadriceps muscle and tendon injuries", section on 'Imaging studies' and "Patellar tendinopathy", section on 'Diagnostic imaging'.)

Cartilage damage

●Articular cartilage defects – Focal defects in the articular cartilage in the knee can occur with overuse or trauma. (See "Approach to the adult with knee pain likely of musculoskeletal origin", section on 'History of direct trauma or forceful collision'.)

MRI is the modality of choice in demonstrating abnormal articular cartilage in the knee for patients who do not have radiographically advanced OA. (See 'Osteoarthritis' above.)

•MRI – MRI demonstrates the spectrum of cartilage abnormalities, from focal abnormal signal to surface fibrillation or irregularity to partial or full thickness defect. MRI also demonstrates underlying bone signal abnormalities.

•CT arthrography – CT arthrography and MRI are equally accurate in diagnosing cartilage damage. For larger lesions, CT arthrography has higher specificity than MRI [31]. CT arthrography is diagnostic of cartilage tear by showing penetration of contrast material within the cartilage substance. CT arthrography is used when MRI is not available or contraindicated.

●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. (See "Patellofemoral pain".)

•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 OA 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 [32]. MRI shows focal areas of edema of patellar cartilage, surface irregularity, fragmentation, areas of thinning, and areas of cartilage loss with exposure of subchondral bone [33].

Meniscal damage

●Meniscal tear – 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. (See "Meniscus injury of the knee".)

Imaging studies for patients with a suspected meniscal injury are discussed in detail separately. (See "Meniscus injury of the knee", section on 'Imaging'.)

•MRI – MRI may be indicated when the clinician is considering surgical intervention. MRI demonstrates the spectrum of meniscal pathology, from myxoid degeneration to complete tear. Meniscal tear is present when abnormal intrameniscal MRI signal disrupts the articular surface (image 2). 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. Radial, vertical, complex, or displaced meniscal tears are almost always symptomatic.

•MR arthrography – MR arthrography is indicated in the postsurgical evaluation of meniscal retear following prior partial meniscectomy or meniscal repair.

Meniscal tears may also be noted in otherwise healthy persons with no knee pain. Degenerative changes in the fibrocartilaginous menisci increase with age. Thus, radiographic findings, particularly increased signal within the body of the meniscus, must be interpreted with caution.

●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 [34]. MRI diagnoses the abnormal appearance and complications.

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 MRI are diffusely thickened medial plica, which may be associated with synovitis or erosion of the articular cartilage of the patella or femoral condyle [35]. (See "Plica syndrome of the knee".)

Extrasynovial inflammation and impingement syndromes — Extrasynovial inflammation and impingement syndromes involve the fat pads interposed between the synovial membrane and joint capsule. These fat pads protect the knee by changing their shape during knee movement. These syndromes are underdiagnosed and should be included in the differential diagnosis of anterior knee pain. The most commonly encountered inflammation and impingement syndromes about the knee are Hoffa disease, patellofemoral friction syndrome, iliotibial band syndrome, and adhesive capsulitis. Other syndromes include quadriceps fat pad impingement, prefemoral fat pad impingement, and pericruciate fat pad impingement. MRI is the imaging modality of choice for evaluating these syndromes [36].

Hoffa disease — Hoffa disease is caused by acute injury or microtrauma of the infrapatellar fat pad. In the acute phase, MRI shows signal changes consistent with inflammation of the infrapatellar fat pad. Chronic cases show fibrosis, hemorrhage, and, rarely, calcification [36]. (See "Approach to the adult with unspecified knee pain", section on 'Anterior knee pain'.)

Patellofemoral friction syndrome — Patellofemoral friction syndrome presents with anterior knee pain and patellar instability. MRI demonstrates edema of the superolateral portion of the infrapatellar fat pad [36].

Adhesive capsulitis — Adhesive capsulitis is characterized by synovial inflammation and capsular fibrosis. This entity is most commonly seen in the shoulder; however, it has been reported in other joints, including the knee. MRI findings include abnormal signal and enhancement of the pericapsular tissues [36].

Iliotibial band syndrome — Iliotibial band syndrome is the most commonly seen injury in the lateral knee in runners and is also seen in cyclists. It presents with anterolateral knee pain. MRI demonstrates abnormal signal between the iliotibial band and the femur [36]. (See "Iliotibial band syndrome".)

Osteonecrosis (avascular necrosis of bone) — Osteonecrosis, also known as avascular necrosis (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 (eg, systemic lupus erythematosus [SLE]), hemoglobinopathies, and human immunodeficiency virus (HIV). Osteonecrosis is usually asymptomatic in the knee unless epiphyseal collapse occurs. The femoral condyles are more commonly involved, although any area around the knee can be affected. (See "Clinical manifestations and diagnosis of osteonecrosis (avascular necrosis of bone)".)

Spontaneous osteonecrosis of the knee (SONK) is commonly seen in older adults, eg, in the sixth and seventh decade, more often in females [37,38]. SONK may result from trauma, oral or intraarticular steroid administration, systemic disease such as SLE, and meniscal tears. The most common location is the medial aspect of the medial femoral condyle, which is a weightbearing segment. Other locations such as medial tibial plateau and patella have been reported [38].

The use of imaging in diagnosing AVN of the knee and common findings are summarized below and discussed in detail elsewhere. (See "Clinical manifestations and diagnosis of osteonecrosis (avascular necrosis of bone)", section on 'Imaging studies'.)

●Plain film radiography – Plain film radiography findings range from normal to radiolucent subchondral area to epiphyseal collapse. 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 SONK and has prognostic significance by identifying the lesions that are likely to resolve with conservative treatment [39]. MRI typically demonstrates a geographic focus of heterogeneous signal surrounded by low signal on T1-weighted images and a double line on T2-weighted images.

Osteochondritis dissecans — Osteochondritis dissecans (OCD) is a relatively common entity seen in adolescents and young adults, more often in males [40]. OCD is a result of chronic microtrauma and represents a form of AVN that is small and focal. The most common location is the lateral aspect of the medial femoral condyle, which is a non-weightbearing segment. (See "Osteochondritis dissecans (OCD): Clinical manifestations, evaluation, and diagnosis".)

The use of imaging in diagnosing OCD and common findings are discussed in detail elsewhere. (See "Osteochondritis dissecans (OCD): Clinical manifestations, evaluation, and diagnosis", section on 'Imaging'.)

Stress and insufficiency fractures — Stress and insufficiency fractures of the femur, tibia, or fibula can cause knee pain. Stress and insufficiency fractures are defined and discussed in more detail elsewhere. (See "Overview of bone stress injuries and stress fractures" and "Femoral stress fractures in adults" and "Stress fractures of the tibia and fibula".)

Imaging is not always required during the assessment of stress and insufficiency fractures. When imaging is indicated, plain film radiographs are the initial modality of choice but often do not show stress or insufficiency fractures in the first two weeks. MRI, radionuclide imaging, and CT are all more sensitive than plain radiographs for the detection of such fractures. The use of imaging in diagnosing stress and insufficiency fractures and common findings are discussed in detail elsewhere. (See "Overview of bone stress injuries and stress fractures", section on 'Imaging studies' and "Femoral stress fractures in adults", section on 'Diagnostic imaging' and "Stress fractures of the tibia and fibula", section on 'Diagnostic imaging'.)

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

Chronic infectious arthritis — 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. In contrast to Gram-positive or Gram-negative bacterial joint infections, which typically present acutely, mycobacterial and fungal arthritis may have more subtle symptoms. (See "Bone and joint tuberculosis" and "Candida osteoarticular infections" and "Manifestations and treatment of nonmeningeal extrathoracic 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 "Bone and joint 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 [41].

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 "Imaging studies for 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 MRI 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 MRI [42].

●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.

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. (See "Radiologic evaluation of knee tumors in adults".)

Nerve pathology of the knee — Localized neuropathies of the knee may occur due to trauma, tumors, nerve entrapment, or systemic diseases. In addition to clinical and electrophysiologic assessment, MRI and high-resolution ultrasound have a role in evaluation of these entities. Imaging plays a role in localizing the lesion, evaluating the structural integrity of the nerve, and identifying mass lesions. (See "Overview of lower extremity peripheral nerve syndromes".)

●Nerve trauma – MRI has a role in characterizing the morphologic changes of nerve injury, as well as the degree of injury, and in detecting complications of injury such as neuromas.

●Nerve tumors – MRI is the modality of choice for identifying benign peripheral nerve tumors such as schwannoma and neurofibroma and malignant peripheral nerve sheath tumors. Peripheral nerves may be secondarily involved in hematologic malignancies such as lymphoma or leukemia, which may present as a mass.

●Nerve entrapment – Peroneal nerve neuropathy is the most common mononeuropathy of the lower extremity. Tibial nerve neuropathy is less common. MRI shows abnormal signal and morphologic changes of the nerve. Ultrasound shows diffuse thickening and hypoechogenicity of the nerve. High-resolution ultrasound is valuable in evaluation of the peroneal nerve impingement following knee arthroplasty [43].

SUMMARY

●When to image – 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 "Approach to the adult with unspecified knee pain".)

●Type of imaging – When radiologic evaluation is indicated, the initial study depends on the suspected cause of pain:

•Plain film radiography (plain x-rays) is used 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 is used when cystic lesions (eg, Baker's cyst, meniscal cyst, etc) or synovitis are suspected.

•MRI is used when soft-tissue pathology is suspected (eg, chronic tendon or meniscal tears, focal articular cartilage damage, or plica syndrome). MRI is also valuable for early diagnosis of selected bone pathology, including osteomyelitis, osteonecrosis, and stress fractures. CT and/or CT arthrography may be used in lieu of MRI in patients with contraindications to MRI (eg, pacemakers).

●Causes of chronic knee pain – For the purposes of this discussion, chronic knee pain has been defined as being present for at least three months. Chronic knee pain is can be caused by:

•Arthritis (see 'Arthritis' above)

•Bursitis or other cystic lesions (see 'Bursitis' above and 'Cystic lesions' above)

•Tendinitis or chronic tears (see 'Tendinitis or chronic tears' above)

•Cartilage damage (see 'Cartilage damage' above)

•Meniscal damage (see 'Meniscal damage' above)

•Plica syndrome (see 'Plica syndrome' above)

•Extrasynovial inflammation and impingement (see 'Extrasynovial inflammation and impingement syndromes' above)

•Osteonecrosis (see 'Osteonecrosis (avascular necrosis of bone)' above)

•Stress fractures (see 'Stress and insufficiency fractures' above)

•Chronic infection (see 'Chronic infection' above)

•Knee tumors (see 'Knee tumors' above)

•Nerve pathology (see 'Nerve pathology of the knee' above)