Version May 2024
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 of a gliding-type joint between the patella and the trochlear groove of the anterior distal femur (figure 1) [1,2]. The major stabilizers of the joint are the extensor tendons (quadriceps and patellar tendons), the medial and lateral collateral ligament complex, and the 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 [3].
Acute knee pain is caused by trauma in the majority of cases. Infection is the next major etiology for acute knee pain, followed by crystal-associated inflammation. Imaging modalities used to evaluate adults with acute knee pain and the appropriateness of particular imaging studies in these more common 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, as well as a general review of imaging tests that are used in the evaluation of bone and joint pain, are presented separately. (See "Approach to the adult with unspecified knee pain" and "Imaging techniques for evaluation of the painful joint".)
Chronic knee pain is 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. Radiologic evaluation of chronic knee pain is presented elsewhere. (See "Radiologic evaluation of the chronically painful knee in adults".)
The use of imaging in the assessment of children and adolescents with knee pain is also presented elsewhere. (See "Approach to acute knee pain and injury in children and skeletally immature adolescents" and "Approach to chronic knee pain or injury in children or skeletally immature adolescents".)
TRAUMA — The knee is vulnerable to direct trauma. Most acute injuries sustained during adolescence and adulthood are caused by athletic activities, motor vehicle accidents, or falls. The most common injuries involve the soft tissues, including ligaments, cartilage, and menisci. Knee fractures are more common than dislocations. Radiological evaluation plays a major role in the diagnosis of knee injuries.
The Ottawa knee rule uses a set of criteria based upon patient age, the presence of local tenderness, limited range of motion, and inability to bear weight. The Ottawa knee rule has a high sensitivity for fractures in the region of the knee but has limited specificity [4]. The Ottawa knee rule is discussed in more detail elsewhere. (See "Approach to the adult with knee pain likely of musculoskeletal origin", section on 'Ottawa Knee Rule (OKR)'.)
When imaging is indicated, acute knee trauma is initially evaluated with plain film radiographs [5]. Patients with a history of trauma, for whom there is a high clinical suspicion of fracture or soft tissue injury that is not apparent on plain radiographs, should be further assessed with computed tomography (CT) or with magnetic resonance imaging (MRI) based upon the type of injury suspected. CT is useful in evaluating bone injuries. Suspected soft tissue injuries involving the cartilage, menisci, ligaments, and tendons are best evaluated by MRI. CT arthrography with intraarticular iodinated contrast may be used in cases when MRI is contraindicated (eg, metallic foreign body around the knee, pacemaker, aneurysm clip).
The role of each imaging modality in evaluation of knee trauma is detailed below. Categories of trauma highlighted include:
●Bone injuries
●Chondral and osteochondral injuries
●Soft tissue injuries
The last category, soft tissue injuries, includes partial and complete tears of the menisci, tendons, ligaments, and muscles as well as nerve and vascular injuries.
Bone injuries — Plain film radiography is the initial study of choice for trauma or suspected fracture involving the knee [5]. However, the high incidence of soft tissue injuries with or without associated fractures often requires additional cross-sectional imaging.
●Plain film radiography – Most fractures of the knee, avulsion injuries, and dislocations will be identified on plain film. The standard initial examination includes anteroposterior and lateral projections. If the patient is unable to stand, a cross table lateral view should be obtained instead of a standing lateral view. Either the cross-table or standing lateral view may allow a fat fluid level to be identified, a radiographic finding indicative of an intraarticular fracture that might otherwise not be apparent on plain films. The tunnel projection is indicated for suspected fracture of the posterior aspect of the distal femur.
Fracture of the patella is common after acute knee injury. If patellar injury is suspected, a sunrise (axial) view of the patella should also be obtained (image 1) [5].
Certain fractures are known to be associated with ligament and meniscal tears, requiring further evaluation by MRI. Examples of such fractures include Segond fracture (avulsion fracture of the lateral tibial condyle) (image 2), fracture of the tibial spine (avulsion fracture of the intercondylar eminence by the cruciate ligaments) (image 3), fibular head avulsion fracture, and posteromedial tibial plateau fracture [5].
Radiographically occult bone injuries include tibial plateau and tibial spine fractures, transient dislocation of the patella, and bone bruises. Tibial plateau fractures may not be obvious on routine radiographs, particularly when there is no depression of the tibial plateau, and may only be detected on the cross table-lateral view. As noted above, the presence of a fat fluid level in the cross-table or standing lateral view is indicative of an intraarticular fracture (image 4) and should be further investigated by CT scan. All other suspected radiographically occult fractures that do not involve the joint should be further investigated by MRI, which is more sensitive for non-cortical fractures and for bone marrow edema due to bone bruise.
●CT – CT is indicated in patients with a history of trauma for whom there is a high clinical suspicion of fracture involving the knee joint not apparent on plain radiographs. Multidetector CT (MDCT) is also recommended in evaluation of all tibial plateau fractures and complex osseous knee injuries [5]. In one study, 409 patients with acute traumatic knee injuries caused by motor vehicle accidents, falls, or sports were evaluated with both plain film and MDCT [6]. This group had an overall rate of fractures of 87 percent. Plain radiographs were falsely negative for the detection of knee fractures in 30 percent of patients, and 70 percent of fracture cases were better characterized by MDCT than by plain film radiography [6]. Part of the difference in sensitivity may be explained by the difficulty in optimally positioning severely injured patients for plain radiographs.
CT demonstrates the position of fracture fragments and the intraarticular extension of fracture and quantifies the extent of the depression in tibial plateau fractures. Three-dimensional reformations are useful in visualization of complex fractures and in surgical planning [7,8]. CT also evaluates nonunion fractures, underlying pathological bone and secondary infection [9-11].
●MRI – MRI is more sensitive than CT in depicting non-displaced trabecular fractures [12]. MRI is the procedure of choice for detection of radiographically occult trabecular fractures and bone contusion (image 5). The pattern of bone contusion on MRI helps to understand the mechanism of trauma and internal derangement injuries [13].
Although CT delineates complex tibial plateau fractures better than MRI and is the imaging procedure of choice for preoperative planning, MRI can also be used in cases of radiation exposure concern (eg, pregnancy) [8]. MRI can detect associated ligamentous and meniscal injuries in cases of tibial plateau fractures, the presence of which may alter management [5]. For example, nondisplaced fractures of the tibial plateau without associated ligamentous injuries can be treated conservatively.
As noted above, MRI is also indicated in the presence of Segond fracture, a small avulsion fracture of the lateral tibial condyle that is highly associated with anterior cruciate ligament (ACL) and lateral meniscus tears and which can result in chronic knee instability [5].
MRI is indicated in the presence of a fibular head avulsion fracture, which can be associated with posterior cruciate ligament (PCL) tears and avulsion of the arcuate complex, resulting in posterior lateral instability [5].
MRI is valuable in diagnosing transient dislocation of the patella, which shows a typical bone contusion pattern from impaction of the medial side of the patella on the anterolateral femoral condyle. MRI can also show the relationship between the patella and the femoral trochlea and evidence of patellofemoral maltracking as a risk factor for patellar subluxation [14]. MRI can also demonstrate the highly associated tear of the medial patellar retinaculum and cartilage injury, which usually requires arthroscopic surgery [5].
●Radionuclide scan – Radionuclide bone scan is indicated in cases with suspected fracture and negative radiographs when MRI or CT is not available.
●Angiography – Angiography is indicated if associated vascular injury is suspected.
Acute osteochondral injuries — Acute osteochondral injuries represent a spectrum of abnormalities that include bone contusions, condyle fractures, subchondral fractures, osteochondral fractures, and chondral fractures [13]. These injuries usually result from athletic activities or direct trauma. The most common injuries in the skeletally immature knee are chondral fractures [15]. Loss of the ability to move the knee smoothly, the presence of a trauma-induced knee effusion, or physical findings suggestive of an associated soft tissue injury (eg, cruciate ligament or meniscal tear) suggest the presence of a chondral fracture or osteochondral injury that is best evaluated by MRI. (See 'Soft tissue injuries' below.)
●Plain film radiography – Chondral fractures are not visualized on plain radiography, since they only involve the articular cartilage, and should be evaluated by MRI. Osteochondral fractures may be detected on plain films, especially if the fragment is displaced in the knee joint.
●MRI and MR arthrography – MRI should be performed in all suspected cases of chondral and osteochondral injuries [5]. MRI detects and classifies the spectrum of these injuries [13]. With arthroscopic assessment as the gold standard, MRI accuracy for detecting articular cartilage defect is greater than 90 percent [16]. MRI demonstrates chondral and osteochondral fractures and osteochondral loose bodies. Focal subchondral edema is commonly associated with full thickness articular cartilage defect and is a marker of cartilage injury [17]. MR arthrography is indicated in assessing the stability of the osteochondral fragment, and is superior to MRI and CT arthrography for detection of chondral lesions of the knee [18]. Evaluation should start with MRI, which is noninvasive; and, in the majority of cases, it will be diagnostic. For equivocal cases, in which a high degree of clinical suspicion remains, MR arthrography has the additional benefit of being more sensitive. If separation of the osteochondral fragment is present, surgical intervention may be necessary. When compared with arthroscopic findings, MRI is sensitive and specific in the qualitative and quantitative follow-up of cartilage repair after treatment of full thickness chondral defects [19].
●CT and CT arthrography – CT and CT arthrography detect osteochondral injuries by identifying a loose intraarticular body and defect of the articular surface.
Soft tissue injuries — Soft tissue injuries include knee joint capsule, meniscal, ligamentous, tendon, and muscle injuries. Plain film radiography is limited in the evaluation of extent of soft tissue injuries; however, according to the American College of Radiology Appropriateness Criteria, initial evaluation of any cause of knee pain should start with a plain film radiograph [5]. Joint effusion, soft tissue swelling, soft tissue foreign body, and avulsion injuries can be easily detected.
MRI is indicated in patients with a history of trauma for whom there is a high clinical suspicion of soft tissue injury, the presence of which, if confirmed, would alter medical management or would warrant surgical intervention. MRI is the procedure of choice for evaluating the menisci, ligaments, and tendons. In addition, MRI detects joint effusions, muscle tears, and hematomas [5]. CT is not optimal for evaluation of the soft tissue, tendons, and ligaments.
Knee joint effusion — Plain film radiography is adequate to detect clinically significant traumatic knee effusion. However, a knee injury sufficient to cause an effusion often causes bone or soft tissue injuries as well [20,21]. As an example, among 114 patients with traumatic knee injury who had plain radiographs that only demonstrated an effusion and whose physical examination revealed no clinically apparent ligamentous injury (ie, who had "stable knees"), further investigation with MRI revealed 144 abnormalities, including bone injury (53 percent), disruption of the ACL (19 percent), sprain of the medial collateral ligament (12 percent), tear of the medial or lateral meniscus (11 and 5 percent, respectively), or, less often, rupture of the posterior cruciate ligament (<1 percent) [21].
●Plain film radiography – Knee joint effusion is demonstrated on the lateral radiograph as an oval-shaped density obscuring the suprapatellar fat pad (image 6). Although no further imaging is needed for detection of an effusion per se, a cross-table or standing lateral view may identify a fat fluid level in cases of lipohemarthrosis (image 4).
●Ultrasound – Ultrasound is sensitive for detection of joint effusion. In the setting of acute trauma, ultrasound can detect lipohemarthrosis, which is a sign of intraarticular extension of fracture [22].
Meniscal injury — As mentioned above, meniscal injuries often accompany serious knee injuries and are well known to occur in association with certain ligamentous injuries (eg, medial meniscal tear and ACL and medial collateral ligament tears). Tears of the medial meniscus are common sports-related injuries. In ACL tears, meniscus injuries are seen in 40 to 80 percent of cases [13]. Tears of the lateral meniscus are less common. Lateral meniscal tears are commonly associated with discoid meniscus, a developmental anomaly. Traumatic meniscus tears are associated with a four- to sixfold increase in risk of knee osteoarthritis [13]. (See "Meniscal injury of the knee".)
●Plain film radiography – The menisci are not visualized on plain radiography; however, considerable narrowing of the joint space seen in severe degenerative joint disease is an indirect sign of chronic meniscal tear.
●MRI and MR arthrography – MRI is the procedure of choice for evaluating the menisci, ligaments, and tendons [5]. When compared with arthroscopic findings, the sensitivity and specificity of MRI for diagnosis of meniscal tears range from 90 to 95 percent and 90 to 98 percent, respectively [23,24]. Several studies performed with higher field 3 Tesla MRI demonstrate sensitivity and specificity of 96 to 97 percent, respectively [25].
MRI demonstrates the spectrum of meniscal pathology, from myxoid degeneration to complete tear [26]. 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. In one study of 74 asymptomatic volunteers without history of knee injury, meniscal tears were seen in 13 percent in individuals under age 45 years and 36 percent in older individuals [27]. In a large study of 991 patients aged 50 to 90 years, prevalence of meniscal abnormalities was 19 percent among women 50 to 59 years of age and 56 percent in men 70 to 90 years of age, with similar values among patients with and without previous knee surgery. In patients with symptoms and radiographic evidence of osteoarthritis, a meniscal tear was seen in 63 percent of cases, compared with 60 percent in patients without symptoms [28].
However, radial, vertical, complex, or displaced meniscal tears are almost always associated with knee pain [29]. Meniscal tear is present when abnormal intrameniscal MRI signal disrupts the articular surface (image 7). MR arthrography is indicated in the postsurgical evaluation of meniscal retear following prior partial meniscectomy or meniscal repair.
Discoid meniscus is a congenital malformation manifested by abnormal shape and size, having discoid rather than "C"-shaped appearance. It most commonly affects the lateral meniscus and is associated with increased incidence of meniscal degeneration and tear. MRI is useful to detect the abnormal appearance and complications.
●CT arthrography – CT arthrography is a reasonable choice for evaluation of suspected meniscal tear if MRI is contraindicated. Compared with arthroscopy, CT arthrography has a sensitivity and specificity for detecting meniscal tears associated with anterior cruciate ligament (ACL) tears of 92 and 88 percent, respectively [30]. Sensitivity and specificity for detecting meniscal tears with intact ACL are 95 and 95 percent, respectively. Sensitivity and specificity for detection of unstable meniscal tears are 97 and 90 percent, respectively [31].
●Ultrasound – Ultrasound is increasingly being used for evaluation of meniscal tears [32,33]. A meta-analysis of 21 studies found a pooled sensitivity and specificity of 78 and 84 percent, respectively, for evaluating meniscal injuries [32]. (See "Musculoskeletal ultrasound of the knee", section on 'Ultrasound examination of the knee'.)
Tendon and ligament tears — Although avulsion fractures that are apparent on plain radiographs may suggest some tendon and ligament tears, MRI is the most useful imaging study to obtain when partial or complete tears are suspected. Injury to the anterior or posterior cruciate ligaments, to the medial collateral ligament, or to lateral stabilizer structures may cause joint laxity.
Disruption of the extensor mechanism, including the quadriceps tendon, patellar tendon, and patellar retinaculum, impair knee extension and/or increase the risk of recurrent patellar dislocation.
●Plain film radiography – Tendon and ligament tears can be suspected on plain radiography when avulsion bone injuries are seen, particularly in young athletes. An example of such a fracture is the Segond fracture (avulsion fracture of the lateral tibial condyle).
Calcification in the medial aspect of the joint, due to a prior or chronic medial collateral ligament tear, is known as the Pellegrini-Stieda lesion.
Tear of the quadriceps tendon is manifested by low position of the patella ("patella baja") (image 8), and tear of the patellar tendon is seen as superior displacement of the patella ("patella alta") (image 9). Patella alta that is not due to patellar tendon tear may also be seen as a developmental anomaly.
●MRI and MR arthrography – MRI evaluates the ligaments and tendons about the knee, including the anterior and posterior cruciate ligaments, the medial collateral ligament, the lateral stabilizer structures, and the extensor mechanism (quadriceps tendon, patellar tendon, and patellar retinaculum) [26]. MRI identifies bone contusion patterns associated with specific injuries [13]. MRI accurately characterizes the degree of injury to the ligaments, which can range from sprain to partial or complete tears [5].
MRI of normal tendons demonstrates low signal (dark appearance) on all MRI sequences (image 10). One exception in the knee is the quadriceps tendon, which normally exhibits a striated appearance of low and intermediate signal intensity.
On MRI, a partial tear demonstrates disruption of some fibers of the tendon (image 11), and a complete tear shows discontinuity of the entire tendon (image 12). Abnormal orientation of the fibers or mass-like tissue in the expected location of the ligament may be seen [34]. MRI demonstrates the quality of the remnants of the tendon in complete tears, the degree of separation of tendon fragments, and the resultant muscle atrophy, which may affect management.
•Anterior cruciate ligament – As confirmed by findings at arthroscopy or surgery, the sensitivity and specificity of MRI for detecting ACL tears range from 87 to 100 percent and from 93 to 100 percent, respectively [35,36]. MRI accuracy rate for detection of partial ACL tears ranges between 25 and 53 percent [34]. MRI detects associated meniscus and collateral ligament injuries [13] as well as occult osseous and cartilage injuries, which are highly associated with ACL tears and occur most commonly in the posterolateral tibial plateau and in the lateral femoral condyle [37,38]. Partial thickness ACL tears may be treated conservatively, while full thickness tears are treated surgically [34]. (See "Anterior cruciate ligament injury".)
•Posterior cruciate ligament – Posterior cruciate ligament (PCL) tears are less common than ACL tears. Isolated PCL injuries are uncommon. PCL injuries are commonly associated with ACL, meniscal, collateral ligament, or posterolateral corner injuries [5]. Most PCL tears are partial thickness, which are usually treated conservatively. PCL injuries associated with bony avulsion, significant knee symptoms with significant PCL laxity, or combined multiple ligamentous injuries are treated surgically [39]. (See "Posterior cruciate ligament injury".)
•Medial collateral ligament – The medial (tibial) collateral ligament (MCL) tear is a commonly encountered injury of the knee. This tear is almost always associated with a tear of the joint capsule. Injuries associated with MCL tears include ACL tear and medial meniscus tear, which together are called the "unhappy triad." MRI can also detect meniscocapsular separation, which requires immobilization or surgical repair. MCL tear is seldom repaired unless multiple other ligaments are torn. (See "Medial (tibial) collateral ligament injury of the knee".)
•Lateral stabilizing structures – Injuries to the lateral knee structures are less common than medial knee injuries but may be more disabling, resulting in chronic instability, pain, and secondary osteoarthritis. Most of these injuries are associated with damage to the cruciate ligaments and to the medial knee structures, most notably meniscus tears. Posterolateral corner injuries are considered a surgical emergency [13]. (See "Lateral collateral ligament injury and related posterolateral corner injuries of the knee".)
The lateral stabilizing structures have anterolateral and posterolateral components. Anterolateral stabilization is provided by the knee capsule and by the iliotibial tract. Posterolateral stabilization is provided by the arcuate ligament complex, which is comprised of the true lateral collateral ligament; the biceps femoris tendon; the popliteus muscle and tendon; the popliteal meniscal and popliteal fibular ligaments; the oblique popliteal, arcuate, and fabellofibular ligaments; and the lateral gastrocnemius muscle [40]. The structures that are assessed routinely by MRI are true lateral collateral ligament, biceps femoris tendon, popliteus muscle and tendon, and lateral gastrocnemius muscle.
Injuries to the iliotibial band are common in acute knee trauma and can be associated with cruciate ligament rupture, posterolateral corner injury, and patellar dislocation [41].
The iliotibial band syndrome is caused by the iliotibial band rubbing on the lateral femoral condyle. It causes anterolateral knee pain and is seen in runners or in those who participate in activities associated with repetitive flexion and extension of the knee. While this condition is readily diagnosed clinically, MRI assessment typically demonstrates fluid along the iliotibial band [42]. (See "Iliotibial band syndrome".)
•Extensor mechanism injuries – The extensor mechanism includes the quadriceps tendon, the patellar tendon, and the patellar retinaculum.
Quadriceps tendon – The quadriceps tendon may rupture in older adults because of preexisting degeneration, even with minimal or no trauma. Quadriceps tendon rupture is infrequent and may be unilateral or bilateral when caused by systemic disease including hyperparathyroidism, chronic renal failure, gout, obesity, diabetes, steroid abuse, and rheumatoid arthritis. MRI is the modality of choice for diagnosis (image 13). (See "Quadriceps muscle and tendon injuries".)
Patellar tendon – Proximal patellar tendon ruptures (or "jumper's knee") typically occur in younger athletes. Jumper's knee can also involve the quadriceps in 25 percent of the cases. MRI is diagnostic, showing thickening and abnormal signal in the proximal patellar tendon [5]. Jumper's knee can be debilitating and usually requires surgery. (See "Patellar tendinopathy".)
In adolescents with suspected patellar tendon avulsion injuries (Sinding-Larsen-Johansson and Osgood-Schlatter disease), MRI is valuable in differentiating normal ossification centers from acute injury. (See "Osgood-Schlatter disease (tibial tuberosity avulsion)" and "Approach to chronic knee pain or injury in children or skeletally immature adolescents", section on 'Traction apophysitis'.)
●CT and CT arthrography – CT arthrography is useful to identify injuries to the articular cartilage, cruciate ligaments, and menisci when MRI is unavailable or is contraindicated. When compared with arthroscopic findings, the sensitivity and specificity of CT arthrography for detection of ACL tears range from 87 to 100 percent and from 93 to 97 percent, respectively [30,43]. Dual-energy CT has also been used to detect cruciate ligament injuries in acute knee trauma [44].
●Ultrasound – Ultrasound allows accurate assessment of anatomical detail of the anterior, medial and lateral supporting structures of the knee [45]. Ultrasound is valuable in evaluating the extensor mechanism, including the quadriceps and patellar tendons, and also identifies the medial collateral ligament [45,46]. Ultrasound identifies tendinosis, partial or full thickness tears, degree of tendon retraction, interposed hematoma, or fluid collection. Ultrasound is helpful in diagnosing iliotibial band syndrome showing thickening and surrounding fluid [22]. (See "Musculoskeletal ultrasound of the knee", section on 'Ultrasound examination of the knee'.)
Musculotendinous injuries — Acute musculotendinous injuries are comprised of muscle contusion or tear, hematoma, myotendinous strain, and tendon avulsion. Muscle injuries usually occur during sports-related activities and commonly involve the myotendinous junction, which is the weakest point of the muscle. The muscles at highest risk for injury around the knee are the rectus femoris, sartorius, semimembranosus, biceps femoris, and gastrocnemius.
●Plain film radiography – Plain film radiography demonstrates soft tissue swelling in muscle tear and hematoma. Muscle detail is not visualized.
●MRI – MRI identifies and grades the extent of these injuries, including partial tear, complete tear, and hematoma [5,47].
●CT – CT is not optimal for evaluation of the soft tissue, tendons, and ligaments; however, muscle tear and hematoma are identified on CT.
●Ultrasound – Ultrasound can demonstrate partial or full thickness tears and hematoma. The quadriceps muscle and tendon and the medial head of gastrocnemius muscle are readily evaluated [22].
Nerve and vascular injuries — Patients with severe knee trauma may benefit from additional evaluation for nerve and vascular injuries. These are discussed in detail elsewhere. (See "Overview of lower extremity peripheral nerve syndromes", section on 'Fibular (peroneal) nerve' and "Overview of lower extremity peripheral nerve syndromes", section on 'Tibial nerve'.)
ACUTE INFECTION — Infection affecting the knee can involve the surrounding soft tissues (cellulitis, fasciitis, pyomyositis, and abscess), the joint or adjacent bursae (septic arthritis and septic bursitis), and the bone (osteomyelitis and Brodie abscess), which can coexist in advanced cases.
Soft tissue infection
Cellulitis — Cellulitis rarely requires imaging for diagnosis. CT is used to differentiate between superficial cellulitis and cellulitis associated with deep infection [48]. MRI or CT can be useful to exclude the presence of myositis, abscess, sinus tract, and fasciitis. Contrast administration is valuable. MRI is the most sensitive for detecting early osteomyelitis, although specificity may be somewhat limited.
Infectious fasciitis — Plain film radiography can detect the presence of soft tissue gas in necrotizing fasciitis. CT can detect the presence of gas and can delineate the presence of a soft tissue abscess, although gas is not present in all cases of necrotizing fasciitis. Lack of enhancement of the fascia after contrast administration may confirm the presence of necrosis while distinguishing non-necrotizing from necrotizing fasciitis [48]. CT also demonstrates the extent of infection. MRI is highly sensitive for the evaluation of fascial inflammation, evaluating the extent of infection and the presence of abscess or osteomyelitis. However, it is not sensitive for necrotizing fasciitis, since it does not easily detect small amounts of fascial air.
Abscess — Abscesses can be easily detected with CT or MRI, particularly with the use of intravenous contrast, which better delineates the thickened wall of the abscess.
Pyomyositis — Pyomyositis can be detected with CT or MRI. Lack of contrast enhancement differentiates between necrotic and viable muscle [48]. MRI is highly sensitive for detection of muscle edema and disease progression to abscess, septic arthritis, or osteomyelitis.
Joint infection
Septic arthritis — Septic arthritis typically presents as an acute, monoarticular arthritis. Joint aspiration is the procedure of choice for diagnosis. Knee arthritis is the most commonly involved joint in both Lyme arthritis and nongonococcal septic arthritis [49]. (See "Septic arthritis in adults".)
●Plain film radiography – Radiographs may show soft tissue swelling, effusion, and periarticular osteopenia. In advanced stages, joint destruction with erosive changes is seen (image 14). In more indolent infections such as tuberculosis or fungal disease, the joint space may be relatively preserved, and marginal erosions may be present [50].
●MRI – MRI is highly sensitive for detecting joint effusion with septations and debris, periarticular soft tissue edema, synovial thickening, and bone marrow edema (which are nonspecific findings), followed by destruction of the articular cartilage and subjacent bone [49]. Subchondral edema is due to hyperemia; however, further extension of bone marrow edema into the medullary cavity and the degree of change in bone marrow signal intensity are helpful to detect progression to osteomyelitis. The presence of a large joint effusion following treatment suggests persistent infection. Lyme arthritis causes nonspecific joint effusion and synovial hypertrophy [49]. The presence of myositis and lack of subcutaneous edema and cellulitis are features suggestive of Lyme arthritis.
●CT – CT is indicated when MRI is not available or is contraindicated for detecting the presence and extent of associated osteomyelitis. CT appearance of septic arthritis includes joint effusion and bone erosions.
●Radionuclide bone scan – Bone scan using Tc-99m MDP with three- or four-phase technique is used in differentiating a septic joint from periarticular infection. Delayed images can detect associated osteomyelitis.
●Ultrasound – As noted above, joint aspiration is the procedure of choice for diagnosis of septic arthritis. Joint aspiration can be performed under ultrasound if the amount of fluid is small or if the procedure is unsuccessful without imaging guidance.
Septic bursitis — Septic bursitis as a cause of knee symptoms most frequently involves the prepatellar bursa. Ultrasound demonstrates a fluid collection in the expected location of the bursa. Ultrasound does not allow reliable differentiation between infected and noninfected bursal fluid collections. Diagnostic fluid aspiration should be performed when infection is suspected. MRI also readily demonstrates the presence of bursal fluid. (See "Septic bursitis".)
Osteomyelitis — Imaging techniques are an important component in the overall approach to the patient with clinically suspected osteomyelitis. An integrated diagnostic approach and the role of imaging studies are discussed in detail separately. (See "Approach to imaging modalities in the setting of suspected nonvertebral osteomyelitis" and "Nonvertebral osteomyelitis in adults: Clinical manifestations and diagnosis", section on 'Clinical approach'.)
The following points regarding the limitations of imaging techniques are worth noting:
●Plain film radiography – Plain film radiographs may not reveal any abnormality in the first few weeks of bone infection. Early diagnosis is enhanced by use of more sensitive imaging, particularly MRI. Radionuclide bone scanning is an alternative when MRI is unavailable or is contraindicated. Findings that may be associated with acute osteomyelitis include soft tissue swelling, joint effusion, periarticular osteopenia, and periosteal reaction. In the proper clinical setting, bone erosion may be virtually diagnostic of osteomyelitis.
●MRI – MRI is both sensitive and specific and is the preferred study for suspected early osteomyelitis. MRI can distinguish between periarticular soft tissue infection and osteomyelitis. Intravenous contrast administration better defines the extent of infection, localizes fluid collections, and differentiates between necrotic and viable tissue; however, it is not required for diagnosing acute osteomyelitis.
●CT – CT is valuable for diagnosis when MRI is not available or is contraindicated. CT demonstrates the extent of soft tissue inflammation, periosteal reaction, and cortical and trabecular bony abnormalities.
●Radionuclide bone scan – Radionuclide bone scan is highly sensitive but nonspecific in the presence of preexisting bone pathology, joint prostheses, recent surgery, or trauma. If radiographs are normal, three-phase bone scan has a sensitivity and specificity of about 95 percent. Imaging with In-111-labeled leukocytes is advocated for detection of osteomyelitis in patients with underlying bone pathology or prosthesis.
Brodie abscess — Brodie abscess is a subacute localized form of osteomyelitis most commonly caused by Staphylococcus aureus and occurring commonly in men in the second decade of life. Common locations for Brodie abscess are in the metaphyses of either the femur or the tibia. Plain film radiography and CT demonstrate an elongated, well-defined radiolucent lesion, surrounded by reactive sclerosis. The lesion may extend to and cross the growth plate. MRI is most sensitive for early detection of Brodie abscess. (See "Nonvertebral osteomyelitis in adults: Clinical manifestations and diagnosis", section on 'Forms of disease'.)
CRYSTAL-ASSOCIATED INFLAMMATION — Acute presentations of crystal disease (eg, gout, pseudogout, basic calcium phosphate deposition) can cause acute knee pain with joint, bursal, or tendon sheath inflammation that can mimic infectious processes.
Calcium pyrophosphate deposition disease — Calcium pyrophosphate deposition disease is the most common crystal arthropathy, caused by deposition of calcium pyrophosphate dihydrate (CPPD) crystals in and around the joints. CPPD crystal disease is the second most common arthropathy of the knee, seen predominantly in the middle and older age groups. CPPD crystal deposition is often asymptomatic and is diagnosed on plain film by articular cartilage or meniscal calcification (chondrocalcinosis). When symptomatic, CPPD deposition disease 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 (CPPD) disease".)
●Plain film radiography – Plain film radiography is the gold standard in the radiologic diagnosis of CPPD. Thin calcification is present in the articular cartilage or menisci (chondrocalcinosis), with preferential involvement of the patellofemoral joint. Calcifications of the synovium, tendon, and ligaments can also be seen. CPPD leads to early development of secondary osteoarthritis. Isolated patellofemoral involvement in the knee is always suggestive of CPPD arthropathy (image 15).
●Joint aspiration (arthrocentesis) – Knee joint aspiration is valuable in the detection of CPPD crystals and serves to exclude joint infection when the clinical manifestation of acute arthritis is present. (See "Joint aspiration or injection in adults: Technique and indications" and "Synovial fluid analysis".)
Gout — The knee is the third most commonly affected joint, after the foot and the ankle, among patients with gout. (See "Clinical manifestations and diagnosis of gout".)
●Plain film radiography – Plain film radiographic changes appear in approximately 45 percent of the patients, six to eight years after the initial attack. Collections of urate crystals (tophi) cause erosion of the adjacent bone with typical sclerotic border.
●Joint aspiration (arthrocentesis) – Knee joint aspiration is valuable in the detection of monosodium urate crystals and serves to exclude joint infection when the clinical manifestation of gout is present and when the plain film is normal or only demonstrates an effusion. (See "Joint aspiration or injection in adults: Technique and indications" and "Synovial fluid analysis".)
SUMMARY AND RECOMMENDATIONS
●Introduction – The knee is one of the largest and most complex joints. Radiologic examination is an important part of the process of assessing patients with acute knee pain due to trauma, suspected infection, or crystal induced inflammation. (See 'Introduction' above.)
●Trauma
•Bone injuries – Plain film radiography is usually the first imaging study for an adult with acute knee pain. The standard initial examination includes an anteroposterior (AP) and either a standing or cross table lateral view of the knee. A tunnel projection is indicated if there is concern about fracture of the posterior aspect of the distal femur, and a sunrise projection of the patella and patellofemoral articulation is indicated if a patellar injury is suspected. (See 'Bone injuries' above.)
CT is used selectively to assess for fractures that are clinically suspected but are not apparent on plain radiographs and to further assess and aid in preoperative planning for complex fractures. MRI also serves as an alternative to CT for detection of radiographically occult fractures when there is a need to avoid exposure to ionizing radiation.
•Acute osteochondral injuries – Loss of the ability to move the knee smoothly, the presence of a trauma-induced knee effusion, or physical findings suggestive of an associated soft tissue injury (eg, cruciate ligament or meniscal tear) suggest the presence of a chondral fracture or osteochondral injury that is best evaluated by MRI. (See 'Acute osteochondral injuries' above.)
•Soft tissue injuries – MRI is used to assess for soft tissue injuries that cannot be detected by plain film radiography or CT, including menisci, ligaments, tendons, and muscle. CT arthrography also has a role in assessing for soft tissue injuries (eg, meniscal tear) in patients who have contraindications to MRI. (See 'Soft tissue injuries' above.)
●Acute infection
•Soft tissue and joint infections – Plain film radiographs may detect gas in the soft tissues due to infection and may also detect most clinically significant knee joint effusions. However, joint aspiration is the procedure of choice for diagnosing septic arthritis, gout, and pseudogout, since radiographic findings in these disorders are nonspecific. (See 'Soft tissue infection' above and 'Joint infection' above.)
•Osteomyelitis – The use of imaging techniques to assist in the diagnosis of osteomyelitis is summarized briefly above (see 'Osteomyelitis' above) and is discussed in more detail separately (see "Approach to imaging modalities in the setting of suspected nonvertebral osteomyelitis"). Although plain film radiographs are the first examination for patients with suspected osteomyelitis, they are insensitive for early disease. If plain films are nondiagnostic, further imaging with MRI, radionuclide scanning, or CT is necessary.
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