Imaging In Practice

The painful knee: Choosing the right imaging test

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Additional indications for knee MRI

Cartilage can be assessed on routine MRI sequences of the knee. Since closed MRI systems have more powerful magnets than open systems, closed MRI systems provide greater anatomic detail.

MRI can identify other lesions, such as spontaneous osteonecrosis of the knee, usually seen in elderly women who may present with sudden knee pain. In such patients, MRI findings of focal replacement of the bone marrow and surrounding edema are specific for osteonecrosis.

Opinions vary as to whether bone marrow edema is always associated with pain. Sequential MRI studies have shown persistence of bone marrow edema for 2 years in patients with degenerative arthritis whose symptoms have waned. Bone marrow edema may be associated with pain but may be absent or inconsequential in the presence of pain.

Because fluid-sensitive T2-weighted MRI is exquisitely sensitive for mobile water protons (ie, in bone marrow edema), it is important that a cause for the edema-like signal be sought on the MRI scan, since this finding is nonspecific and may be associated with articular disease, trauma, osteonecrosis, infection, or bone tumors. Additionally, clinicians need to be aware that the findings on MRI depend on the quality of the study, and are influenced by technical factors such as magnet strength, imaging planes, and use of surface coils.

MRI should be used in patients in whom surgical treatment, ie, arthroscopy, is being considered. As discussed above, several studies have shown that a significant number of unnecessary arthroscopies may be prevented when preceded by an MRI examination.

Figure 5. A 45-year-old man with left knee pain after a motorcycle accident. (A) Lateral radiograph shows an osseous fragment at the posterior aspect of the knee joint (long arrow). This was thought to represent an avulsion fracture of the posterior cruciate ligament. There is also a knee effusion (short arrow). (B) Sagittal proton-density-weighted MRI through the intercondylar notch shows an intact anterior cruciate ligament (white arrow). There is an avulsion fracture of the posterior proximal tibia (red arrow) at the attachment of the posterior cruciate ligament. The fragment, which is displaced proximally, is attached to the posterior cruciate ligament, which remains intact.
Other indications include cases in which clinical findings are equivocal in the setting of acute injury, in competitive athletes in whom an immediate diagnosis and treatment is required, and in patients who present a high surgical risk. MRI should not be routinely used to diagnose the painful or injured knee,13 and if the skilled physical examination does not indicate findings of ligamentous or meniscal injury, conservative therapy should be prescribed.1 MRI is also not useful and offers little for patients in whom changes of degenerative joint disease are evident on radiographs.

Figure 5 shows the use of MRI in the evaluation of a 45-year-old man with left knee pain after a motorcycle accident.

ULTRASONOGRAPHY HAS ONLY A LIMITED ROLE

Ultrasonography does not play a major role in the evaluation of acute knee pain in the United States, in part because the accuracy of the results depend much on the technical skills and experience of the operator.

Ultrasonography can be useful in evaluating for rupture of the quadriceps and patellar tendon, or to assess a repaired tendon after surgery,15 and it is a quick and reliable way to determine the presence of joint effusion and popliteal cyst. It is also used to guide needle placement for joint aspiration and injection.

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