Synovial fluid differential cell count in wear debris synovitis after total knee replacement

Determining the cause of synovitis following total knee arthroplasty (TKA) can be challenging. The differential diagnoses include infection, hemarthrosis, instability, crystalline disease, wear debris or idiopathic causes. Wear particle synovitis can mimic periprosthetic infection with symptoms of pain and effusion. Radiographs and physical exam are often inconclusive in differentiating the two. Synovial fluid analysis is routinely used in evaluating periprosthetic infections.

We examined the association between synovial white blood cell count and differentials, and polyethylene wear and osteolysis, to see if fluid analysis can aid in establishing the diagnosis of wear particle synovitis.

A cell count and differential was obtained from synovial fluid samples from 54 TKAs undergoing revision for aseptic failure. Explanted polyethylene inserts were analyzed for linear and volumetric wear, oxidation (ketone peak height), and damage features. Analysis was performed to assess the relationship between cell counts and polyethylene wear indicators as well as severity of intra-operative and radiographic osteolysis.

Total and percent mononuclear (monocyte and lymphocyte) cell counts were found to be elevated in the presence of documented wear debris synovitis and an association was suggested between their levels and maximum ketone levels.

The present study implies that the differential cell count of knee fluid can help distinguish wear debris from infection as a source of synovitis following TKA and identifies the value of the mononuclear cell count as a possible tool to assess abnormal wear rates of the polyethylene insert. Further research into identifying the exact role of monocytes in the wear debris synovitis and osteolytic pathways is warranted.

Level II, diagnostic study.