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Diagnosing Periprosthetic Joint Infection After Unicompartmental Knee Arthroplasty

Key findings

  • 2018 International Consensus Meeting criteria don't suggest laboratory value thresholds specifically for diagnosing periprosthetic joint infection (PJI) after unicompartmental knee arthroplasty (UKA), but threshold values were proposed in a 2012 study
  • The current study reviewed 20 years of data at Massachusetts General Hospital (20 knees with PJI and 89 knees without) to determine which diagnostic tests and thresholds are most useful for the diagnosis of PJI after partial knee arthroplasty
  • Partial knee arthroplasty included medial UKA, lateral UKA, patello-femoral arthroplasty, and bicompartmental knee arthroplasty
  • Synovial nucleated count, synovial polymorphonuclear cell percentage, serum C-reactive protein, and serum erythrocyte sedimentation rate all demonstrated an excellent ability to diagnose PJI after partial knee arthroplasty and rule out infection
  • The sensitivity, specificity, and positive/negative predictive values of the optimal cut-offs for those four tests were generally higher than in the 2012 study, supporting the use of the 2018 ICM criteria unless criteria specific to UKA are developed

In 2018, the Second International Consensus Meeting (ICM) on Musculoskeletal Infection developed new criteria for diagnosing periprosthetic joint infection (PJI) that were later externally validated in the Journal of Arthroplasty. A limitation is the criteria don't address whether diagnostic thresholds should differ between total knee arthroplasty and unicompartmental knee arthroplasty (UKA).

The latter is often performed because of advances in implant design, expanded indications, and better outcomes. Even so, only a single study published in the Journal of Arthroplasty in 2012 has recommended diagnostic thresholds for PJI after UKA.

Researchers at Massachusetts General Hospital have concluded the laboratory values used in the 2018 ICM scoring system are more appropriate for the diagnosis of PJI after partial knee arthroplasty than the values proposed in the 2012 study. Christopher Melnic, MD, an orthopedic surgeon in the Hip and Knee Replacement Service of the Department of Orthopaedic SurgeryHany S. Bedair, MD, chief of the service, and colleagues explain their reasoning in The Journal of Arthroplasty.


The research team reviewed patient records from all relevant hospitals in the Mass General Brigham system for January 1, 2000, to January 31, 2021. They identified 100 patients (109 knees) who underwent partial knee arthroplasty and later required conversion to TKA, revision partial knee arthroplasty, or explant followed by insertion of an antibiotic-loaded bone cement spacer:

  • PJI group—20 patients (20 knees); 55% male; mean age at the time of revision, 55 (range, 43–69); mean time from index arthroplasty to revision, 292 days (range, 7–1,388 days)
  • Aseptic group—80 patients (89 knees); 60% female; mean age at revision, 58 (range, 41–91); mean time to revision, 1,101 days (range, 18–3,871 days)

There were no significant between-group differences in sex, age at revision, distribution of the type of partial knee arthroplasty performed (medial UKA, lateral UKA, patello-femoral arthroplasty, or bicompartmental knee arthroplasty), or Elixhauser Comorbidity Index. The mean time to revision was significantly longer in the aseptic group (P<0.001), an important finding since this cohort had a mean age under 60.

Diagnostic Performance

In analyses of the area under the receiver operating characteristic curve, the diagnostic performance of each test studied was:

  • Synovial nucleated cell (NC) count—0.97
  • Synovial polymorphonuclear cell (PMN) percentage—0.97
  • Serum C-reactive protein (CRP)—0.97
  • Serum erythrocyte sedimentation rate (ESR)—0.89
  • Serum white blood cell count—0.64

Diagnostic Thresholds

Optimal cutoff values were:

  • Synovial NC count—2,695 cells/microliter (94% sensitivity; 90% specificity; 68% positive predictive value; 99% negative predictive value)
  • Synovial PMN percentage—52.5% (94% sensitivity; 93% specificity; 75% PPV; 99% NPV)
  • Serum CRP—10.6 mg/L (100% sensitivity; 85% specificity; 60% PPV; 100% NPV)
  • Serum ESR—38.5 mm/h (84% sensitivity; 93% specificity; 73% PPV; 96% NPV)

A previous study in the Journal of Infection suggests, though, that serum CRP should not be used as a single diagnostic parameter.

Comparison to the 2012 Study

A table in the new article compares the thresholds identified in the current study against those proposed in the 2012 paper. In nearly every comparison of sensitivity, specificity, PPV, and NPV, the thresholds in this study were better.


This single-center study is not powered to make definitive recommendations about what laboratory tests or cutoffs to diagnose PJI after UKA or other types of partial knee arthroplasty. However, organizations such as the ICM and Musculoskeletal Infection Society should consider these results if they develop criteria specific to PJI after UKA.

accuracy of synovial nucleated cell count in ruling out infection after partial knee arthroplasty

accuracy of synovial polymorphonuclear cell percentage in ruling out infection after partial knee arthroplasty

accuracy of serum CRP in ruling out infection after partial knee arthroplasty

accuracy of serum ESR in ruling out infection after partial knee arthroplasty

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