Imaging In Practice

The painful knee: Choosing the right imaging test

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PLAIN RADIOGRAPHY STILL THE FIRST STEP IN KNEE EVALUATION

Radiography is the first step in the evaluation of knee pain. It is quick and inexpensive and can yield many diagnostic clues. It can readily reveal fractures, osteochondral defects, bony lesions, joint effusions, joint space narrowing, and bone misalignment.

In patients with knee trauma, supine anteroposterior and cross-table lateral radiographic images are generally obtained. In patients whose knee pain is not due to trauma, standing projections are done, as well as dedicated projection of the patellofemoral articulation. A standing series is most helpful for evaluating joint space and alignment.

Applying the Ottawa rules

When a patient presents to the emergency room with acute knee pain, the immediate concern is whether he or she has a fracture. The Ottawa knee rules9 for when to order radiography in adults with knee pain are highly sensitive for detecting a clinically important fracture. If any one of the five Ottawa criteria applies—ie, the patient is age 55 or older, has tenderness at the head of the fibula, has patellar tenderness, is unable to flex the knee to 90°, or is unable to bear weight—then radiography is indicated.

While studies have validated the ability of the Ottawa rules to detect important fractures in acute knee injury,2,10 fracture is the cause of only a small percentage of knee complaints in the primary care setting. More common causes include osteoarthritis, meniscal injury, ligamental injury, and crystal arthropathy, and these account for approximately half of all diagnoses. Sprain and strain account for most of the rest of knee injuries.1

Acute exacerbations of osteoarthritis

Osteoarthritis is a chronic problem, yet it is not unusual for a patient to present to the primary care physician with an acute exacerbation of joint pain. The clinical hallmarks include age over 50, stiffness lasting less than 30 minutes, bony enlargement and tenderness, and crepitus. The radiographic hallmarks, according to the Kellgren-Lawrence grading scale, are joint space narrowing, osteophytes, subchondral cysts, and sclerosis. These radiographic findings correlate well with clinical findings in these patients.11

Situations in which radiography is less helpful

In some cases the radiographic findings may not explain the patient’s clinical signs and symptoms. For example, in suspected crystalline and septic arthritis, the clinical presentation may include warmth, erythema, and effusion. Arthrocentesis would be indicated in such a patient. Indeed, in the case of suspected pseudogout, chondrocalcinosis may be radiographically evident. However, it is also present in many patients without symptoms or with osteoarthritis, so radiographic evidence does not provide a definite diagnosis.

While radiography may not always identify the cause of knee pain, it is useful in excluding serious problems such as fractures, advanced degenerative changes, and neoplasms, and it may help direct further management. Radiography is not useful in the evaluation of the cruciate and collateral ligaments, the menisci, and the hyaline cartilage of the knee and may fail to show an insufficiency or stress fracture. To evaluate these structures and associated soft tissues, MRI is preferable.

COMPUTED TOMOGRAPHY IN ACUTE KNEE PAIN

Figure 2. A 56-year-old woman with left knee pain after a fall. (A) Anteroposterior radiograph shows fracture of the tibial eminence (short arrow). The subchondral bone of the lateral tibial plateau is indistinct (long arrow), leading to suspicion of tibial plateau fracture. (B) A lateral radiograph shows joint effusion (short arrow). The lateral tibial plateau is depressed posteriorly (long arrow). (C) Sagittal computed tomography (CT) shows fracture of the tibial eminence (arrows) extending to the lateral tibial plateau. (D) Sagittal CT shows depression of the lateral tibial plateau (arrow). (E) Coronal CT shows fracture of the tibial eminence (short arrow) and tibial plateau (long arrow). The fibular head (arrowhead) is intact.

CT is the imaging method of choice when patients have knee trauma but radiographs are negative for fracture. CT can detect and help analyze fracture better, and it can better define fractures seen on conventional radiographs (Figure 2). CT is fast, the procedure lasting only a few minutes. It costs less than MRI and provides a better picture of bony detail. Because of this, CT is generally recommended in patients with knee trauma, since it can show fractures too subtle for radiography.

CURRENT USES OF MRI TO EVALUATE ACUTE KNEE PAIN

As mentioned above, MRI is useful in evaluating suspected meniscal and ligamentous injuries.

Figure 3. T2-weighted MRI of the left knee of the 47-year-old woman who was in a motor vehicle accident. (A) A coronal image reveals a fragment of the lateral meniscus displaced into the notch (long arrow). The medial meniscus is also shortened (short arrow). Edema (arrowhead) of the tibial plateau is consistent with bone bruise. (B) A sagittal image through the intercondylar notch shows absence of the anterior cruciate ligament (red arrow). The patella (short white arrow) and intact extensor mechanism (long white arrows) are also seen. (C) A sagittal image through the medial compartment shows the medial tibial plateau (short arrow) and the medial femoral condyle (long arrow). There is a tear of the posterior aspect of the medial meniscus, which appears shortened (red arrow). (D) A sagittal image of the lateral compartment shows the lateral tibial plateau (short white arrow) and the lateral femoral condyle (long white arrow). The posterior horn of the lateral meniscus (red arrow) is missing. The fibula (white arrowhead) is intact.

Patients with meniscal injury may report a history of twisting injury while bearing weight. Symptoms may include locking or catching, with loss of motion related to a mechanical block. Ligamentous injury may be due to a direct blow or forceful stress while the patient is bearing weight. In tearing of the anterior cruciate ligament, the patient may report having heard a pop at the time of injury, followed by swelling. Valgus and varus stresses may lead to collateral ligament injury. Often, more than one injury coexists: more than a third of meniscal tears are associated with anterior cruciate ligament injury.12

Figure 3 shows how T2-weighted MRI was used to evaluate for suspected meniscal injury in our 47-year-old female patient with left knee pain after a motor vehicle accident.

Figure 4. A 35-year-old man with intermittent locking of the right knee following a snowboarding injury underwent sagittal T2-weighted MRI of the right knee. (A) An image through the intercondylar notch shows the femur (long white arrow), proximal tibia (short white arrow), and patella (white arrowhead), as well as a “double posterior cruciate ligament” sign (red arrows), representing a “bucket-handle” tear of the medial meniscus, which is displaced into the intercondylar notch. The normal posterior cruciate ligament is the most posterior structure, denoted by the arrow on the right, and the smaller, inferior structure that resembles the ligament is the displaced meniscus. (B) An image of the medial compartment shows the medial tibial plateau (long white arrow), with a focal region of full-thickness loss of the articular cartilage (short red arrow) on the weight-bearing surface of the medial femoral condyle, likely representing an acute chondral fracture. Note the large knee effusion (long red arrow) and shortening of the medial meniscus (short white arrow). An anterior cruciate ligament tear, not shown, was also identified.
Figure 4 shows how sagittal T2-weighted MRI was used in a 35-year-old man with intermittent locking of the right knee following a snowboarding injury. In this patient, MRI was able to uncover coexisting injuries.

Still a matter of debate

MRI’s role in the diagnosis of knee pain is still a contentious issue.

Advantages of MRI are that it is noninvasive, it does not use ionizing radiation, it gives multiplanar images, and it provides images of soft-tissue structures, which other imaging methods cannot.12 It is a well-proven and widely accepted test. Its sensitivity for detecting meniscal and cruciate ligament injury ranges from 75% to 88%,1 and it can help in the evaluation of other injuries for which radiography is not useful, including synovitis, bone bruise, stress or insufficiency fracture, osteochondral defects, and osteonecrosis.

In addition, several studies show that using MRI to establish the diagnosis in acute knee pain can mean that 22% to 42% fewer arthroscopic procedures need to be performed.4–8 Authors of a prospective double-blind study8 recommended that MRI be used in patients with acute knee injury when the findings of the clinical history and examination by orthopedic surgeons prove equivocal.8 MRI evaluation is especially desirable for young, active patients who wish to resume activity as soon as possible.

A routine MRI examination consists of T1- and T2-weighted images in three planes, although the number of sequences and planes varies from hospital to hospital. The use of gadolinium contrast is indicated only when osteomyelitis, septic arthritis, or a mass is suspected.

Disadvantages of MRI include its cost: Medicare reimbursement for knee MRI is around $400, compared with $200 for knee CT and $50 for knee radiography with four views. Also, while studies have shown MRI to have a high sensitivity and specificity in the diagnosis of acute knee injury, some have reported a high false-positive rate for the detection of meniscal tear.13,14 MRI has also been shown to have a lower sensitivity than arthroscopy for lesions of the articular cartilage.13 Furthermore, MRI has been shown to reveal cartilage lesions, osteophytes, and meniscal abnormalities in asymptomatic study volunteers with no history of pain, trauma or knee disease.14 Therefore, findings on MRI must closely correlate with findings on the history and physical examination.

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