Lateral x-ray of the left knee in a 57-year-old man who had a left total knee replacement 6 years earlier and now presents with pain in that knee.
Anteroposterior x-ray of both knees in the patient
A 57-year-old man comes in with left knee pain that has been present for 2 to 3 weeks. He had a left total knee replacement 6 years previously and was doing very well. He denies injury or known precipitating event. The pain is made worse with weight-bearing activities and occasionally throbs at night, but it seems to settle down at rest. On physical examination, no erythema is noted, but there is slight warmth to the knee on palpation. He has a +1 joint effusion. Range of motion measures full extension to 110 degrees of flexion, and collateral ligaments are intact. The patient is slightly febrile in the office, with a temperature of 99.5 degrees F. Anteroposterior and lateral radiographs are taken.
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The most damaging complication associated with primary total knee arthroplasty (TKA) is prosthetic joint infection. Fortunately, the infection risk is low, with an estimated 1% to 2% incidence of postoperative infection after primary TKA.
Occasionally, however, patients present with knee pain and joint effusion years after undergoing TKA. Often the pain is from a noninfectious source, such as synovitis of the knee, and the symptoms resolve after treatment with nonsteroidal anti-inflammatory drugs. However, prosthetic joint infection must be ruled out in anyone presenting with acute knee pain and effusion after TKA. Risks for acute hematogenous infection include any recent invasive procedure, including surgical and dental procedures, that might cause bacteremia.1
The first step in the workup of a potentially infected TKA is to draw serum inflammatory markers including erythrocyte sedimentation rate and C-reactive protein. If the patient has elevated inflammatory markers, including elevated C-reactive protein and/or erythrocyte sedimentation rate, joint aspiration should be performed for fluid analysis. Performing joint aspiration does involve a small chance of introducing infection with the needle stick; therefore, it should be done only if initial screening laboratory values are elevated or if the suspicion of infection is high. The aspirated fluid should then be sent for culture, synovial white blood cell count, and differentiation. Protein, glucose, and crystal levels are also commonly ordered after joint aspiration to help rule out other potential diagnoses.
It is also important to make sure patients abstain from antibiotic drugs for 2 weeks prior to knee aspiration. Starting patients on antibiotics for prophylaxis of suspected infection is not beneficial and inhibits subsequent efforts to diagnose the infection. A bone scan with or without labeled leukocyte imaging is often performed if an infection is likely (indicated by positive cultures and elevated inflammatory markers) and for preoperative planning.
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
Hatch D. Prosthetic joint infection. Orthobullets website. http://www.orthobullets.com/recon/5004/prosthetic-joint-infection. Updated July 3, 2016. Accessed September 19, 2016.
American Academy of Orthopaedic Surgeons. The Diagnosis of Periprosthetic Joint infections of the Hip and Knee. Guideline and Evidence Report. http://www.aaos.org/research/guidelines/PJIguideline.pdf. June 18, 2010. Accessed September 19, 2016.