Cut-off values with highest sensitivity and specificity for the synovial fluid white cell and differential count will facilitate the accurate diagnosis of infection in total knee (TKR) and total hip replacement (THR). All patients undergoing revision TKR or THR for suspected prosthetic joint infection between 2009 and 2011 at two hospitals were identified. A total of 75 patients were included with a mean age of 70.3 years (38 to 89). Synovial fluid was aspirated pre-operatively and peri-prosthetic tissue samples were taken intra-operatively for histological and microbiological examination.

Receiver operating characteristic (ROC) plots were constructed for white cell and differential counts in aspirated fluid. The optimal cut-off for TKR and THR was 1590 white cells/µl and 65% neutrophilia. The white cell count cut-off value identified for THR was notably lower than previously quoted in the literature. A cut-off value for white cell count in synovial aspirate in suspected prosthetic joint infection of between 1100 and 1700 white cells/µl is likely to be applicable to both THR and TKR.

Cite this article: Bone Joint J 2013;95-B:554–7.

Although there is still no absolute diagnostic test for infection of the prosthetic joint, the synovial fluid leukocyte count and neutrophil percentage have been reported as having high sensitivity and specificity but to date the cut off values for these tests are not agreed upon. We investigated whether leukocyte count and differential of aspirated joint fluid are valuable tools in the diagnosis of suspected prosthetic joint infection.

In 39 patients who underwent revision arthroplasty the cell count and differential of joint aspirates were cross-referenced with culture results and histology to confirm that these were valid tests and to determine cut off values. Cases were identified via the microbiology service of two district general hospitals: 17 patients in whom infection was identified via culture and/or histology results and 19 who were uninfected.

Receiver operator characteristic curves were used to determine the correlation between the results of the two tests and to identify a suitable threshold level. For absolute leukocyte count the area under the curve was 0.997 indicating excellent agreement between the tests. A cut off value of 2000 gave a sensitivity of 100% and a specificity of 94%. Differential count had an area under the curve of 0.94. A threshold of 50% had a sensitivity of 100% and a specificity of 77%.

Despite the low number of patients this study shows that the leukocyte count and differential of aspirated joint fluid are valuable tools in the diagnosis of suspected prosthetic joint infection.

Aim

The aim of this FE study was to analyse the comparative behaviour of cement and metal based augments in TKR and quantify the stresses within these different augments and underlying cancellous bone.

Introduction

In complex primary and revision total knee replacement (TKR) the operating surgeon may encounter proximal tibial bone defects. The correct management of such defects is fundamental to both the initial stability and long-term survival of the prosthesis. Block or wedge-shaped metal augments are used to address some such type II unconstrained defects.