A Synopsis of AFP's "Acute Knee Effusions: A Systematic Approach to Diagnosis "

Acute Knee Effusions: A Systematic Approach to Diagnosis  
MICHAEL W. JOHNSON, MAJ, MC, USA, Madigan Army Medical Center, Tacoma, Washington  
Am Fam Physician. 2000 Apr 15;61(8):2391-2400     
http://www.aafp.org/afp/2000/0415/p2391.html

     When examining a swollen knee, the history is important. If the cause was from trauma, the diagnosis may be a fracture or tear. A non-injury cause would more likely be infectious, rheumatic, or due to gout.  If trauma is involved, the mechanism or injury is helpful.  Swelling will appear within four hours in a ligamentous injury, as well as for a hemarthrosis. An ACL tear will occur during a non-contact deceleration, a pivoting or cutting movement or during hyperextension. The patient may have heard a "pop". The patient will have a positive Lachman's test, anterior drawer test, and pivot shift test. A PCL tear will occur as a blow right below the knee during knee flexion. There will be a positive posterior drawer and sag test. Collateral ligament injuries will occur with force either left-to-right or right-to-left across the joint. There will be corresponding ligament laxity on the side opposite the blow. Meniscal injury will occur while the knee is twisted or while squatting. They will have positive McMurray's and Apley's test. In a fracture, there will be a history of a direct blow to the knee and an inability to bear weight in the joint.  There may be deformity, crepitus, or ecchymosis. 
     Radiography can be used to assess compartment space narrowing, knee effusions (with a 15-30 degree flexion), and fracture. According to the Ottawa knee rules, an x-ray is required if one or more of the following is present; 
     age of 55 years or older,
     tenderness at the head of the fibula, 
     inability to flex the knee 90 degrees, 
     can't bear weight on knee or take four steps.
An MRI is better for soft tissue damage.  Arthrocentesis can be diagnostic and therapeutic. The fluid should be sent for analysis. Bloody fluid may indicate a fracture or tear. An infection may have low glucose, elevated WBC count, and positive cultures. Gout will have aspirate with negative birefringent rods or needles. Pseudogout will show weakly positive birefringent rectangles or rhomboids. Rheumatoid arthritis will have increased protein, low glucose and a normal WBC count.