gms | German Medical Science

Objectives: Joint aspiration with analysis of synovial fluid white blood cell count (WBC) and microbiological culture is a widely established aspect in the diagnosis of joint infections. In case of a two stage revision for SJI with temporary implantation of a Polymethylmethacrylate (PMMA)-Spacer, joint aspiration before re-/implantation of an arthroplasty was used to rule out persistent infection for years. Recently, its value is under debate due to low sensitivity in hip and knee surgery. Shoulder specific data on these aspects is rare. The current study was designed to answer the following research questions: Joint aspiration has an insufficient predictive value in diagnosis of shoulder joint infections (SJI) in (1) initial workup and (2) before definite arthroplasty with PMMA-Spacer in place.

Methods: This retrospective evaluation (EbM-Level III) investigates 35 patients that were treated for SJI with a two staged implantation of a total shoulder arthroplasty (TSA) after debridement and implantation of an antibiotic loaded PMMA-Spacer. Joint aspirations were performed preoperative (PA) and before re/implantation of the prosthesis while spacer was in place (interstage aspiration, IA). Samples were taken for microbiological culture and analysis of WBC. Sensitivity, specificity and predictive values were calculated with reference to intraoperative microbiological samples from spacer implantation respectively spacer exchange/TSA implantation.Receiver Operating Characteristic (ROC) was plotted for WBC depending on culture results from spacer implantation. Area-Under-Curve (AUC) analysis and calculation of the Youden index were performed to quantify the quality of the test.

Results and Conclusion: The sensitivity of microbiological cultures from PA was 58.3% and the specificity was 88.9%. The positive predictive value (PPV) was 0.88 and the negative predictive value (NPV) was 0.64. The mean WBC was 27 800 leucocytes/mm3 (range 400-96 300). The Youden index (0.857) had a maximum for a cut-off of 2600 leucocytes/mm³ with a sensitivity of 85.7% and a specificity of 100.0%. IA was a dry tap in 9.0%. The sensitivity and specificity of IA were 0.0% and 88.5%, respectively. The PPV for IA was 0.00 and the NPV was 0.92.

PA aspiration is an important pillar in the diagnosis of SJI. Whilet sensitivity is moderate, specificity is high. WBC can be low even in cases with infection. Consequently, PA cannot rule out infection for sure. However, we strongly recommend PA for its advantages of targeted antibiotic therapy in case of germ identification. Surgeons should be aware that PA is likely to miss cutibacteria and CNS. Calculated antibiotic therapy should therefore cover these bacteria even if aspiration showed negative microbiological cultures. In contrast, diagnostic value of IA seems negotiable and does not qualify for routine use.