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Optimal Synovial Marker Thresholds for Diagnosis of Infection Differ in Taper Corrosion of Metal-on-Polyethylene THA

Key findings

  • This prospective study evaluated the utility of serum and synovial fluid markers for diagnosing periprosthetic joint infection in patients whose metal-on-polyethylene total hip prostheses had failed due to head–neck taper corrosion
  • White blood cell count (optimal cutoff 2,144 cells/mL) and percentage of polymorphonuclear neutrophils (optimal cutoff 82%) in hip synovial fluid provided good to excellent accuracy in predicting infection
  • Diagnosis of infection in this setting requires manual testing of synovial fluid aspirate because tissue debris can falsely elevate automated cell counts

Over the past decade, most total hip arthroplasties (THA) in the U.S. have incorporated prostheses that have a metal-on-polyethylene (MoP) bearing surface with a modular femoral head. Corrosion at the femoral head–neck junction is emerging as an important mode of failure and can cause adverse local tissue reactions (ALTRs) previously seen in patients with metal-on-metal THA.

How to diagnose periprosthetic joint infection (PJI) in the setting of these reactions has been unclear. The Musculoskeletal Infection Society (MSIS) defines PJI according to elevations in certain laboratory markers, but ALTRs themselves can result in elevated values.

Christian Klemt, PhD, postdoctoral research fellow at the Bioengineering Laboratory in the Department of Orthopaedic Surgery at Massachusetts General Hospital, Young-Min Kwon, MD, PhD, program director of the Adult Reconstructive Surgery Fellowship Program and director of the Bioengineering Laboratory, caution that laboratory thresholds may need to be adjusted in these cases. They report the sensitivity and specificity of various cutoff values in The Journal of Arthroplasty.

Study Details

From a prospectively maintained database, the researchers reviewed consecutive patients with symptomatic ALTRs who underwent revision surgery for MoP head–neck taper corrosion. All revision surgeries were performed by the same surgeon.

Before revision, all patients had undergone synovial fluid aspiration with manual white blood cell (WBC) count and differential. Aerobic, anaerobic and fungal cultures were conducted for all aspirates and intraoperative biopsy samples (fluid and tissue).

Optimal Cutoffs

Two markers provided good to excellent accuracy in predicting PJI in the presence of head–neck taper corrosion:

  • Synovial WBC count (AUC, 96%; 95% CI, 88%–100%; optimal cutoff 2,144 cells/mL)
  • Synovial fluid percentage of polymorphonuclear neutrophils (AUC, 90%; 95% CI, 78%–98%; optimal cutoff 82%)

These cutoffs are lower than those reported for ALTRs associated with metal-on-metal hip implants and MoP implants without ALTRs.

Recommendations for Surgeons

Accurate diagnosis of PJI in patients who have MoP THA with head–neck taper corrosion requires testing of serum inflammatory markers and manual testing of synovial fluid aspirate. In these patients, normal erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) will be useful for excluding PJI, whereas optimal WBC count and neutrophil percentage in synovial fluid thresholds are lower than those reported for periprosthetic joint infection ALTRs associated with metal-on-metal hip implants and MoP implants without ALTRs.

Learn more about the Bioengineering Laboratory at Mass General

Refer a patient to the Department of Orthopaedic Surgery at Mass General

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