Imaging In Practice

The painful knee: Choosing the right imaging test

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ABSTRACTThe initial evaluation of acute knee pain should include plain radiography, which is a quick and cost-effective way to identify a wide range of problems, including fracture, degenerative changes, osteochondral defects, and effusions. Computed tomography (CT) is the test of choice to better delineate fractures in patients who have knee trauma. If the history and physical examination point to damage of the cartilage, the menisci, and the cruciate and collateral ligaments and arthroscopy is contemplated, then magnetic resonance imaging (MRI) is useful for evaluating these structures.

KEY POINTS

  • In the emergency department, most patients undergo plain radiography to assess for fracture, yet more than 90% of these studies do not show a fracture.
  • CT is useful in patients with knee trauma but normal radiographs.
  • MRI is the imaging modality for internal derangement of the knee.
  • Ultrasonography’s role in the evaluation of acute knee pain is generally limited to assessment of the extensor mechanism, joint effusion, and popliteal cyst.


 

References

Radiography plays a key role in the initial evaluation of acute knee pain in adults. Yet conflicting studies and the absence of clear guidelines may leave the primary care physician uncertain as to which imaging test to order—ie, whether radiography is sufficient, and when computed tomography (CT) or magnetic resonance imaging (MRI) is needed. This article reviews the indications for radiologic examination of the knee and discusses indications for cross-sectional imaging studies. Imaging in oncology patients is not discussed here.

ACUTE KNEE PAIN: A TYPICAL SCENARIO

A 47-year-old woman presents to the emergency department with left knee pain after a motor vehicle accident that occurred the day before. The car she was driving hit a tree, and she hit her knee on the dashboard. She was wearing a seatbelt at the time of the accident. She says she was unable to walk immediately after the accident because of knee pain.

The initial examination in the emergency room reveals swelling and pain throughout the range of motion. The anterior drawer test and the Lachman test are negative (see below).

Figure 1. This anteroposterior radiograph of a 47-year-old woman who was in a motor vehicle accident shows focal ossification adjacent to the medial femoral condyle (arrow) but no evidence of acute fracture.

Initial radiographs (Figure 1) reveal no acute fracture or effusion, but focal ossification adjacent to the proximal medial femoral condyle may indicate a past injury to the medial collateral ligament.

The patient is discharged home with a knee immobilizer, pain medication, and crutches, with instructions for a follow-up visit in the orthopedics clinic.

Five days later, she returns to the emergency department complaining of continuing knee pain. She says the knee gives way when she puts weight on it. The physical findings are unchanged, and she is discharged home with a follow-up appointment with orthopedics in 3 days.

At the follow-up visit, she complains of persistent knee pain in the medial aspect of the knee joint. Physical examination is difficult because of pain and swelling, and it reveals mild joint effusion with no gross instability. She has pain on the medial side with valgus stress, but there appears to be a hard end point. There is no posterior sag, and the Lachman test is negative.

Based on the physical examination and the patient’s complaints, she receives a diagnosis of medial collateral ligament strain and injury. She is given a hinged brace and is instructed to undergo a physical rehabilitation program.

Three weeks after the initial evaluation, she returns to the orthopedics clinic with continuing knee problems. Mild knee effusion persists, but she has less pain and swelling, allowing a more complete examination. The examination reveals less limitation of range of motion and a hint of positivity on the Lachman test. The knee is diffusely tender, and the pain seems out of proportion with the maneuvers used during the examination. She requests more pain medication. You suspect internal derangement of the knee. Which imaging test should you order to further evaluate this patient?

A SYSTEMATIC AND COST-EFFECTIVE APPROACH IS NEEDED

The case presented above represents a typical scenario for the presentation of acute knee pain and illustrates the diagnostic challenges.

Knee pain is a common reason for emergency room visits, and it accounts for approximately 1.9 million visits to primary care clinics annually.1 In the emergency department, most patients undergo plain radiography to assess for fracture, yet approximately 92% of radiographic studies do not show a fracture.2 Clearly, the evaluation of knee pain requires a systematic, accurate, and cost-effective approach.

Key elements of the physical examination

In acute knee pain, accurate diagnosis begins with a detailed history and physical examination.

The anterior drawer test is done to evaluate the anterior cruciate ligament. With the relaxed knee flexed to approximately 80° and the foot stabilized in a neutral position, the examiner grasps the proximal tibia in a firm yet gentle grip, and then applies anterior force, noting the degree of anterior displacement compared with the other knee.

The Lachman test, a variation of the anterior drawer test, is more definitive for the anterior cruciate ligament and is carried out with the knee in 15° of flexion and external rotation, in order to relax the iliotibial band. The upper hand grasps the distal thigh, and the lower hand, with the thumb on the tibial tubercle, pulls the tibia forward. The degree of anterior motion in millimeters is noted and compared with that on the other side, and the end point is graded as “soft” or “hard.” An end point is considered hard when a ligament abruptly halts the motion of the bone being tested against the stabilized bone. An end point is considered soft when the ligament is disrupted and the restraints are the more elastic secondary stabilizers.

Debate continues

Some authors contend that in skilled hands a thorough history, physical examination, and radiographic examination are sufficient to diagnose trauma-related intra-articular knee disorders.3 Others contend that MRI plays a key role in the initial evaluation. A number of studies4–8 have shown that using MRI in the initial evaluation not only identifies key lesions, but also may eliminate the need for an invasive diagnostic procedure (ie, arthroscopy).

For example, MRI can reveal fracture, stress fracture, insufficiency fracture, and transient patellar dislocation—conditions that may satisfactorily explain knee symptoms.

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