# Biomechanical analysis of additional vancomycin in articulating knee spacers: determining the threshold for structural failure

## TL;DR

This study examines how adding vancomycin to knee spacers affects their strength and finds that antibiotic concentration does not significantly impact structural integrity.

## Contribution

The study provides empirical evidence that vancomycin concentrations up to 20% in PMMA spacers do not compromise structural integrity.

## Key findings

- Vancomycin concentrations up to 20% in PMMA spacers do not significantly affect structural integrity.
- Dislocated spacer positioning significantly reduces load capacity compared to standard positioning.
- Fractures occurred exclusively in femoral components with dislocation.

## Abstract

Periprosthetic joint infections (PJI) pose considerable challenges in arthroplasty, with two-stage revisions involving the use of antibiotic-loaded spacers being the prevailing treatment modality for chronic low-grade PJI. Whilst the incorporation of antibiotics has been demonstrated to enhance infection management, the biomechanical impact of such agents on polymethylmethacrylate (PMMA) spacers remains to be elucidated. The present study evaluates the load-bearing capacity of spacers with varying antibiotic concentrations of vancomycin in order to determine structural failure thresholds.

A total of twenty PMMA knee spacers were subjected to testing, with the samples divided into two distinct groups based on the antibiotic concentration: a low concentration group (5% vancomycin) and a high concentration group (20% vancomycin). The spacers were subjected to uniaxial loading in two configurations: a standard weight-bearing position and a dislocated position with 10° femoral angulation. The breaking forces were measured using a servo-hydraulic testing machine. Statistical analyses were performed using Mann-Whitney-U tests, with significance at p < 0.05.

In the standard position, the mean breaking force for the low and high antibiotic groups was 38.7 ± 9.3 kN and 35.5 ± 5.8 kN, respectively (p = 0.421). In the dislocated position, breaking forces were significantly lower at 2.2 ± 0.7 kN and 2.0 ± 0.3 kN, respectively (p = 0.311). Spacer fractures occurred exclusively in femoral components, with a 17-fold reduction in load capacity in dislocated configurations. The antibiotic concentration of vancomycin exerted no significant effect on biomechanical integrity.

In the experimental ex vivo study, it was demonstrated that vancomycin concentrations of up to 20% by volume in PMMA knee spacers do not significantly affect the structural integrity. However, the positioning of the spacer has been shown to have a significant impact on biomechanical stability, with dislocation having a substantial effect on load capacity.