Isolation, Characterisation and Software Development for Polymeric Wear Particles from In Vitro Joint Simulators

Abstract

Wear of the Ultra-high molecular weight polyethylene (UHMWPE) articulating against metal in joint replacements is one of the primary reasons for long-term failure of prosthesis due to implant loosening. Amount, size and morphology of wear debris are important factors that affect the clinical performance of a joint implant. Major developments in UHMWPE as an orthopaedic bearing material are radiation crosslinking and recent introduction of vitamin E as an antioxidant. This thesis aims to investigate the size and shape characteristics of UHMWPE wear debris produced by current state-of-the-art noncrosslinked and vitamin E containing highly crosslinked UHMWPE implants in artificial hip, knee and ankle joint articulations.
Current polyethylene wear debris isolation methods were assessed for isolating wear particles from serum based simulator lubricants. The limitations of current isolation methods were incomplete digestion, presence of impurities, bacterial contamination and low reproducibility. Consequently, a novel UHMWPE wear debris isolation method was developed to overcome these limitations. This method successfully isolated UHMWPE wear debris by digesting simulator lubricants using 5M potassium hydroxide and by purifying particles using a two-stage density gradient ultracentrifugation. High-resolution scanning electron microscopy was used to capture digital images of wear debris particles deposited on 15 nm pore size membrane filters.
Two current commercial size analysers Nanosight and Mastersizer were compared to SEM image analysis for characterising UHMWPE wear debris. Large size range and complex shape of wear debris made both Nanosight and Mastersizer unsuitable for complete characterisation of UHMWPE particles. Moreover, no shape analysis was available in both commercial particle analysers. Therefore, SEM image analysis was chosen for particle characterisation. Custom software was developed to automatically analyse SEM images and characterise particles using a range of size and shape descriptors.
The isolation and characterisation methods developed in this study were used to investigate the influence of crosslinking, addition of vitamin E and change in molecular architecture on size and morphology of wear debris. All wear particles analysed in this study were found to be predominantly submicron in size. Crosslinking reduced the size of UHMWPE wear debris for particles generated in knee and multidirectional pin-on-plate. Noncrosslinked direct compression moulded (DCM) ArCom UHMWPE generated more elongated and fibrillar wear particles. Higher chain mobility and higher elongation to break was believed to be the reason for this particle morphology. Alternatively, highly crosslinked UHMWPE generated less elongated and more compact shaped particles.
Addition of vitamin E by blending, followed by crosslinking and annealing in ECiMA UHMWPE generated more elongated and fibrillar particles in comparison to vitamin E diffused highly crosslinked (E1) UHMWPE in hip. Multidirectional pin-on-plate wear testing of DCM UHMWPE moulded at 175°C and at 145°C generated similar particle size distribution and shapes.
Influence of type of joint articulation on size and morphology of wear particles was investigated by isolating and characterising E1 UHMWPE wear debris produced by ankle, knee, hip and multistation pin-on-plate articulation. E1 wear particles generated by ankle and knee had similar particle size and morphology. Moreover, both ankle and knee generated submicron as well as large micron sized particles. Submicron sized particles were more round, while large particles were elongated and fibrillar. Alternatively, hip generated mostly submicron-sized particles with rounder morphology. Wear particles generated in multidirectional pin-on-plate tester were also mainly submicron in size. However, morphology of these particles was more fibrillar and elongated.
In conclusion: Crosslinking, method of addition of vitamin E, change in processing conditions and type of joint articulation could influence the size and shape characteristics of UHMWPE wear debris.