Clinicians should use different criteria to diagnose periprosthetic joint infection (PJI) in the setting of a partial knee replacement versus a total knee replacement, according to a new study presented at the 2018 annual meeting of the American Academy of Orthopedic Surgeons. PJI is one of the most devastating and frequent complications after lower extremity joint arthroplasty, often requiring revision surgery.

"The standards that we have used for diagnosing PJI in total knee replacement don’t necessarily apply to partial knee replacement. If you use the standard for a total knee replacement, then potentially you could be revising knee replacements for knees that are not actually infected," said principal study investigator Michael Cross, MD, an orthopedic surgeon at Hospital for Special Surgery, in New York City. "I think the guidelines need to be changed, so that partial knee replacement is a separate entity."

The International Consensus on Periprosthetic Joint Infection has established diagnostic criteria for diagnosing a PJI following total knee replacement, but cutoffs for diagnosis after a partial knee replacement remain unclear. "We have never really understood whether or not PJI should be diagnosed using the cutoffs for a total knee replacement," said Dr. Cross.

In the new study, Dr. Cross and colleagues reviewed records for all patients at HSS who had a partial knee replacement between August 2000 and December 2016 (n=234). They focused only on patients who had the complete data they were interested in analyzing, such as information on erythrocyte sedimentation rate (ESR), serum CO reactive protein (CRP), and synovial fluid white blood cell count (WBC). The researchers compared lab values for patients who were being revised for infection (n=18) to those who were being revised for reasons other than infection (n=76), to determine where the appropriate cutoffs were for diagnosing a PJI infection in the setting of a partial knee replacement.

The mean ESR (52.1 vs 17.2; P<0.01), CRP (106.4 vs 11.9; P<0.01), WBC (49,448 vs 967; P<0.01), and synovial PMNs (85.3 vs 47.3; P<0.01) were significantly higher in the infected group. The researchers identified optimal cutoff values for PJI in a multivariate model (ESR= 25.2 mm/h, CRP=17.2 mg/L, synovial WBC=6592.7, and %PMNs=71.7%). The synovial fluid WBC were higher than diagnostic cutoffs for total knee replacement.

"We looked at what is called the aspiration results, which is the amount of white blood cells and neutrophils in the joint fluid prior to revision. It is often one of the best tests we have," said Dr. Cross. "To make the diagnosis of infection in a partial knee replacement, the white blood cell cutoff was higher than in a total knee replacement. The cutoffs for diagnosis of infection in a partial knee replacement are somewhere in between a normal knee and a total knee replacement."

Dr. Cross said the findings are important for people trying to accurately treat patients with a partial knee replacement. "Just because the levels are slightly above the normal cutoffs for total knee replacement doesn’t necessarily mean that the knee is infected," said Dr. Cross.

PJI accounts for between 1.9 percent and 5 percent of revision procedures following partial knee arthroplasty. Roughly 10 percent of all knee replacement patients are a candidate for a partial knee replacement.