Knee pain is a common problem in adults, and acute knee pain often brings patients to the physician's office. Although plain knee radiographs are the most common test ordered, most radiographs are normal. Decisions about diagnostic testing in patients with acute knee pain and when to refer the patient to an orthopedist are often difficult. Jackson and associates reviewed the literature about the evaluation of acute knee pain to determine when diagnostic testing is necessary.

The authors attempted to answer the following questions about acute knee pain beginning less than one week before the clinical visit: (1) How useful are clinical decision rules in managing possible knee fractures? (2) How accurate is the physical examination in diagnosing cartilage and ligament injuries? (3) How sensitive and specific is magnetic resonance imaging (MRI) for cartilage and ligament injury? and (4) What is the utility of plain knee radiographs in patients with suspected osteoarthritis and pseudogout? The literature search showed that osteoarthritis was the most common diagnosis in patients with acute knee pain, followed by meniscal injuries, collateral and cruciate ligamentous injuries, gout, and fracture. Strains and sprains also were commonly diagnosed. Rheumatoid arthritis, infectious arthritis, and pseudogout were rare.

Five articles reported on four decision rules that may be useful. The Ottawa knee rules of when to order a radiograph in adults with acute knee pain have excellent sensitivity and a specificity of 49 to 54 percent, and are the only decision rules that have been validated in repeated studies. Other decision rules—the Pittsburgh knee rules, Weber and colleagues' rule, and Fagan and Davies' rule—have not been as well validated. Even with plain radiography, some fractures will be missed, and follow-up is recommended in patients with acute knee pain.

Meniscal injuries often accompany twisting leg injuries that occur while full weight is on the foot. Swelling occurs after several hours, and walking up and down stairs becomes painful. Ligamentous injury, which results in almost immediate swelling, usually is caused by direct stress or a blow to the knee while weight is on that leg. The ligament involved varies depending on the direction of the stress. The history often is not as helpful as the physical examination in distinguishing meniscal from ligamentous injury.

Physical examination by a knowledgeable examiner can detect meniscal, anterior cruciate ligament, and posterior collateral ligament injury with reasonable accuracy. The Lachman maneuver is the most sensitive test for a ligamentous tear, but the pivot test appears to be more specific. A negative physical examination by a competent examiner makes the likelihood of these injuries very low. Positive test results indicate that follow-up MRI has a moderate sensitivity and specificity for meniscal, ligamentous, and cartilaginous injuries.

The authors conclude that the history is not useful in evaluating an acute knee injury except as an adjunct to the physical examination. The latter is specific and moderately sensitive. Patients with a negative examination can be followed carefully without further evaluation. If the examination is positive, referral to an orthopedist may be a better strategy than MRI because of the latter's relatively low specificity. A plain radiograph may be helpful when there is a high suspicion of osteoarthritis or fracture, but because clinical criteria probably perform better in the diagnosis of osteoarthritis, radiography should be reserved for suspected fractures. Plain films can reveal markers for pseudogout, but their sensitivity and specificity in patients with acute pain is low. An algorithm, when used by clinicians with excellent knee examination skills, may lead to the correct diagnosis most efficiently (see the accompanying table).

editor's note: Guidelines for the use of radiologic investigation in evaluating acute knee injury in all persons except infants also have been compiled by the American College of Radiology.¹ These guidelines review indications for radiologic evaluation in a variety of circumstances. Radiographic studies are not indicated for patients who fall, have no focal tenderness or effusion, and are able to walk. If focal tenderness, effusion, or inability to bear weight is noted, the first study should be plain films. For patients who have had significant trauma or in whom posterior knee dislocation is suspected, plain films and angiography/ magnetic resonance arthrography/Doppler studies are considered most appropriate. Soft tissue injuries are best evaluated by magnetic resonance imaging or single-photon emission computed tomography.—R.S.