Analysis of Synovial Fluid Aspirations in Aseptic Loosening and Instability After Total Knee Arthroplasty

Background: Aseptic total knee arthroplasty (TKA) complications can be challenging to diagnose. Many studies have defined periprosthetic joint infection (PJI) using synovial aspirations, but few studies have described aspiration characteristics in aseptic TKA problems. The aim of this study was to determine the synovial fluid characteristics of patients who had TKA failure caused by two common aseptic diagnoses: aseptic loosening and instability. We sought to compare the characteristics between these groups in addition to the failure caused by PJI.

Methods: A retrospective study was performed in which consecutive patients who had a preoperative knee aspiration and underwent TKA revision for one of three diagnoses (PJI, aseptic loosening, or instability) were evaluated. Clinical notes were used to obtain demographics, comorbidity data, aspiration cell count, and differential to compare among the groups. There were 399 patients included: 240 PJI, 103 aseptic loosening, and 56 instability.

Results: There were significant differences between mean white blood cell (WBC) count and polymorphonuclear, lymphocyte, and monocyte percentages between all groups. Findings consistent with a diagnosis of aseptic loosening included a mean WBC count of 1,021.9 cells/μL with 29.7% polymorphonuclear (PMNs), 32.7% lymphocytes, and 44.6% monocytes, and relatively elevated PMN/lymphocyte (2.1) and PMN/monocyte (3.5) ratios. Findings consistent with a diagnosis of instability included a mean WBC count of 1,261.2 cells/μL with 23.5% PMNs, 35.6% lymphocytes, and 50.0% monocytes, and a relatively lower PMN/lymphocyte (1.1) and PMN/monocyte (1.5) ratios compared to aseptic loosening. In both aseptic loosening and instability, there were significantly more lymphocytes and monocytes than in PJI. In addition, instability cases had a higher mean red blood cell count of 405,996.9 cells/μL (P = 0.012).

Conclusions: Differentiating between instability and aseptic loosening in TKA remains a diagnostic challenge. This study provides insight into the cellular pathophysiology of aseptic TKA complications and can be used to aid in clarifying the diagnosis of aseptic loosening versus instability.