NanoSight

Orthopedic Implant Debris

Size Determination & Visualization of Wear Debris from Orthopedic Implants

Contents

1. Introduction
2. Background
3. The Technique
4. Sample Preparation
5. Case Study
6. Key Features
7. Relevant Third Party Citations

Introduction

The NanoSight technique can visualize sub-micron wear debris live and direct in liquid suspension. The technique uses these images to generate high resolution number versus size distributions for a sample. This information can be used in a research and development capacity allowing the manufacturer to test the suitability of materials/implant designs, with regards to the generation of sub-micron wear debris. The technique can analyze ceramic, metallic or polymeric wear debris from a sample of synovial fluid and is best suited once a protein digestion has been completed.

The technique can be used to analyze samples generated from an artificial joint simulator or clinical samples harvested from a patient.

Background

Characterizing the size and quantity of wear debris produced from an orthopaedic implant is of importance when determining the biocompatibility and longevity of the implant.

The production and size of wear debris can influence factors such as:

• Hypersensitivity
• Aseptic loosening of the implant and associated osteolysis
• Systemic distribution and accumulation of implant debris
• 3rd party degradation of articulating surfaces
• Bioavailability and bioreactivity of metallic species
• Design and choice of material for a specific implant

The Technique

The technique looks at the light scattered from individual nanoparticles as they move under Brownian motion in the path of a laser beam.

The speed at which the particles move under Brownian motion is related to particle size, temperature and solvent viscosity. With knowledge of the temperature and solvent viscosity, particle size can be directly calculated.

As the technique measures the particles on a particle by particle basis, the technique produces high resolution number distributions. This overcomes the inherent weakness in techniques such as DLS (Dynamic Light Scattering), which produce an average particle size intensity biased towards larger particles within the distribution. The technique complements Electron Microscopy as it looks at the sample in its natural, unprepared state. Analysis time takes only 1-5 minutes from sample preparation to result.

As the technique visualizes individual particles it can provide an estimate of the particle concentration within a sample. This allows the user to not only determine particle size but also relative concentrations within specific size classes.

Figure 1: Particle size distribution produced from a metal on metal CrN prosthesis.

Figure 2: Image of CrN wear debris produced by the NanoSight system. The polydispersity of the sample can be clearly seen.

Sample Preparation

Samples are prepared to remove the protein content from the synthetic/natural synovial fluid. Failure to remove the proteinaceous content would increase the signal to noise ratio but would not prevent analysis.

Polyethylene samples are treated for 1-2 days in KOH followed by solvent extraction using a chloroform/methanol mix. Hot enzymatic digestion is used to isolate metallic particles.

In general, the technique requires sample dilution to approximately 10⁹ particles/ml. From this, a sample of 250 µl is taken and injected into the viewing unit.

Figure 3. Particle size distribution produced from a polyethylene(PE) prosthesis.

Case Study

Kinbrum et al¹ tested the effect of heat treatments on the wear characteristics of cobalt chrome molybdenum prosthesis. The experiments used the NanoSight LM10 instrument and indicated that the ‘as-cast’ (i.e no heat treatment) produced less nanometer scale wear debris when compared with heat treated samples especially over long cycle periods. Figures 4 + 5 show the results of these findings, Figure 4 following 1.5 million cycles, Figure 5 following 2.5 million cycles. The experiments compared ‘as-cast’, single and double heat treated samples.

Figure 4. Particle size distribution after 1.5 million cycles.

Figure 5. Particle size distribution after 2.5 million cycles.

1.Amy Kinbrum, Anthony Unsworth and Amir Kamali (2008) ‘The Wear of High Carbon Metal-on-Metal Bearings After Different Heat Treatments’, 54th Annual Meeting of the Orthopaedic Research Society, San Francisco, 2nd-5th March, 2008, Poster #1905.

Key Features

• Particles can be measured in natural state (without drying/vacuum)
• Greater ability to size polydisperse samples due to particle-by-particle analysis
• Small sample volume
• Low cost of unit
• Visualization of individual particles without any pre-treatment such as labeling
• 1 to 5 minute analysis time
• Allows the study of time-based changes, such as agglomeration/ stability
• Accurate particle sizing from 10 nm— 2000 nm

Relevant Third Party Citations

1. Amy Kinbrum, Anthony Unsworth and Amir Kamali (2008) ‘The Wear of High Carbon Metal-on-Metal Bearings After Different Heat Treatments’, 54th Annual Meeting of the Orthopaedic Research Society, San Francisco, 2nd-5th March, 2008, Poster #1905.
2. Ward, P.A., Field, S.K., Sharif, K.Y.Z. and Shelton, J.C. (2008) The Influence of Coatings on the Wear of Metal-on-Metal Hip Prostheses, 8th World Biomaterials Congress, Amsterdam 28th May 1st June 2008, Poster # 1539.
3. Kinbrum A, Vasilliou K, Lee S.M, and Unsworth A. (2008) ‘A tribological and Particle debris Study of As-Castand Heat Treated CoCrMo Alloy, Journal of Bone and Joint surgery Vol 92-B Issue Supp_1,101