Aim

We report on the performance of a simple algorithm using a combination of synovial fluid White blood cell count(WBC), C-reactive protein(CRP) and α-Defensin(AD) tests to aid in the diagnosis of prosthetic joint infections.

Infection is a leading indication for revision arthroplasty. Established criteria used to diagnose prosthetic joint infection (PJI) include a range of laboratory tests. Leucocyte esterase (LE) is widely used on a colorimetric reagent strip for the diagnosis of urinary tract infections. This inexpensive test may be used for the diagnosis or exclusion of PJI. Aspirates from 30 total hip arthroplasties (THAs) and 79 knee arthroplasties (KA) were analysed for LE activity. Semi-quantitative reagent strip readings of 15, 70, 125 and 500 white blood cells (WBC) were validated against a manual synovial white cell count (WCC). A receiver operating characteristic (ROC) curve was constructed to determine the optimal cut-off point for the semi-quantitative results. Based on established criteria, six THAs and 15 KAs were classified as infected. The optimal cut-off point for the diagnosis of PJI was 97 WBC. The closest semi-quantitative reading for a positive result was 125 WBC, achieving a sensitivity of 81% and a specificity of 93%. The positive and negative predictive values of the LE test strip were 74% and 95% respectively.

The LE reagent strip had a high specificity and negative predictive value. A negative result may exclude PJI and negate the need for further diagnostic tests.

Cite this article: Bone Joint J 2015;97-B:1232–6.

Introduction

A new conservative hip stem has been designed to address the complex problem of total hip arthroplasty in the younger population.

Introduction & Aims: A new femoral component for hip arthroplasty has been designed for a younger patient population. The design makes use of a higher femoral cut, which conserves bone stock, increasing options for future revision surgery. It uses the existing load bearing properties of the proximal femur, and therefore distributes load more evenly. The stem is longer than that of a resurfacing, so will be easier to insert at the correct orientation, minimising failure rates in inexperienced hands. The cross-sectional dimensions have been designed to produce torsional stability. The collar maximises the loading of the calcar, reducing stress resorption. The surface is hydroxyapatite coated and porous, which will produce a long-term biological fixation.

This project assessed the long-term stability of this design at different orientations, by measuring the change in surface strain distribution following its insertion.

Methods: Ten composite bones were coated in a Photoelastic material, positioned at a simplified single leg stance, and loaded at 2.3 KN. The surface strain was measured at one-centimetre intervals down the medial cortex. Then the prostheses were inserted into the bone at 135°, 145° and 125° to the femoral shaft, and the surface strains reread.

Results: The results were compared with an FEA model, and analysed statistically using the Wilcox signed rank test. The prosthesis inserted at 135° produced no significant difference in surface strain distribution compared with the intact bone.

Conclusions: This study suggests this stem design will be stable in the long term following insertion, and there were no areas of excessively high or low strain.