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Clinical lead, Dr Michael Cross
Diagnosis of a PJI after THA in the Setting of a MOM bearing or Corrosion Reaction: A Difficult Problem

The diagnosis of a periprosthetic joint infection (PJI) following total hip arthroplasty (THA) can be difficult. However, in 2014, the orthopaedic community is now faced with an especially difficult diagnostic problem.  Now, larger numbers of patients are presenting with pain after THA with metal-on-metal (MOM) bearings or corrosion reactions;1-5 in this particular setting, ruling out the diagnosis of a PJI becomes even harder.1

Standard diagnostic tests to diagnose a deep infection after THA include the C-reactive protein, Erythrocyte Sedimentation rate (ESR), and synovial fluid analysis including the white blood cell count (WBC), the differential (or % polymorphonuclear (%PMNs) cells in the fluid), and culture.  Previous studies have shown that the acceptable cutoffs (highest sensitivity and specificity) for diagnosing a chronic infection after THA, based on the synovial fluid analysis, include a synovial fluid WBC count >3,000 cells/μL and/or a differential with >80% PMNs;6 further, in the early post-operative period (< 6 weeks post-operative) after THA, optimal cutoffs for diagnosis of an acute infection include a WBC > 12,800 cells/μL and/or a %PMNs > 89%.7

While a PJI can occur concurrently with an adverse local tissue reaction (ALTR),8,9 during aseptic revision THA in the setting of metallic debris, the joint fluid often appears murky and purulent-like, mimicking a deep infection.10  Further, quite often the synovial fluid has “debris” that makes the synovial fluid difficult to interpret and/or read by automated machines.1,11-13  In these cases, manual counts or re-aspiration can be performed;1,11 however, the problem still remains of what to make of the data that is reported.  Studies have shown that the WBC count is elevated from baseline in aseptic revision THAs in the setting of metal debris from a MOM bearing1,2,8,10,14 or taper corrosion;2,13 thus, no optimal cutoffs exist for diagnosing an infection in this group. 

Future studies are needed to determine optimal cutoffs for the ESR, CRP, and especially the synovial fluid WBC count and differential, as this is often regarded by most to be the “gold standard” test to diagnose a PJI.  Until these studies exist, the literature suggests that each individual diagnostic test used to diagnose a PJI in this patient population should be used with caution. Instead, ALL diagnostic tests available to the clinician including the appearance of the wound, the general appearance of the patient, signs and symptoms of the patient, CRP, ESR, the synovial fluid WBC count and differential, AND the synovial fluid culture should be used together to assist in making the diagnosis of PJI after THA in the setting of metal-on-metal (MOM) bearings or corrosion reactions. 

Michael B. Cross, MD, Assistant Attending Orthopaedic Surgeon, Hospital for Special Surgery, New York, NY
March 2014


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