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Application of Particle Sizing to Joint Replacement Biomedical Research
Release Date:2009-Jun-18
Number:pr378

Hip, knee and other human joint replacements, constructed primarily from ultra high molecular weight polyethylene (UHMWPE), are widely and successfully used to alleviate the consequences of a variety of degenerative disorders. However, a small but significant failure rate persists so that developing an understanding of the processes that lead to the loosening of prostheses is important both for developing new materials and structures, and in devising strategies for patient management.

At the centre for Biomedical Engineering (CBME) at Durham University in the UK, research work focusing on the accurate and detailed characterisation of artificial joint wear debris is providing valuable information on the wear rate of prostheses and the role of debris in the development and progression of loosening. One aspect of this work involves particle size analysis of the debris, for which a Malvern Instruments Mastersizer 2000 is being used in a novel application. Results to date have shown that clinical samples (ex vivo) and those generated in the laboratory (in vitro) can be analysed reproducibly, avoiding some of the problems encountered with the more established microscopy techniques.

Role of particles


The role played by artificial joint wear debris in the promotion of osteolysis (bone breakdown) is currently the subject of extensive research. It is generally accepted that a foreign body reaction occurs due to the presence of wear particles, but the exact mechanism of osteolysis activation is uncertain. Many studies have examined the in vitro interaction of debris-like particles with macrophage cells. These have shown that a number of particle parameters, including size, material, morphology and surface charge, can elicit an adverse cellular response. It is therefore important that investigations to quantify the in vivo and in vitro response to wear debris should be underpinned by accurate, comprehensive particle characterisation of the debris.

Established techniques


Much characterisation of wear debris has involved two-dimensional information acquired from either optical or scanning electron microscopy (SEM). Typical processing protocols include alkali digestion of either debris-loaded ex vivo tissue, or bovine serum for in vitro origin debris, followed by filtering to isolate debris particles from the digest suspension. The debris-loaded filter membrane must then be dried and conduction sputter coated prior to SEM examination. This route enables imaging of individual particles and can thus provide useful morphological information. While adequate for sizing the debris in terms of absolute size range and modal size, this technique falls short of being able to provide the detailed information required to completely characterise the debris distribution.

The use of SEM observation to infer the size and mass distribution of the debris is unsatisfactory in a number of regards. Random sampling procedures can minimise observer bias, but the use of only a small number of sample micrographs taken from across the filter membrane introduces the possibility of misrepresentative sampling. The necessarily small number of particles characterised can introduce both systematic and observer bias that can be compounded by sampling errors.

Image analysis techniques may improve the sampling statistics by increasing the number of particles characterised, but the accuracy of the imaging software depends on resolution of discrete particles. Our research has found that aggregation of particles following filter drying is common. This causes clumping of particles and, because the phenomenon acts more strongly on the fine particles in the sample population, results in a systematic bias towards coarser particles in the population.

Non destructive measurement


Preliminary work at CBME has been the establishment of a method for the detailed, accurate and reproducible characterisation of wear debris. Our conclusions are that laser diffraction-based particle size analysis (LALLS – low angle laser light scattering) offers significant advantages over current techniques and will be of much use for the evaluation of joint prostheses of both in vitro and ex vivo origin. The entire debris sample is analysed non-destructively, whilst in suspension, so that artefacts associated with filtering, drying and agglomeration of debris are avoided.

Validated technique


The validated technique is now being put to practical use at Durham through the EPSRC funded project “The Role of Wear Debris in the Failure of Replacement Joints”. Information on debris number and volume distributions from ex vivo tissue harvested during total hip revision surgery for aseptic loosening, is being compared with debris generated in vitro in hip wear simulators. The aim is to improve understanding of the role that wear debris plays in the clinical failure of metal on UHMWPE total hip replacements and to improve the validity of hip wear simulators.

Continuing project


This project is now reaching an important stage and ethical committee approval has been obtained for the harvesting of revision arthroplasty ex vivo tissue. Investigation of the size and number distribution of wear debris particles extracted from ex vivo tissue has begun on tissue samples provided by several UK orthopaedic surgical teams. Comparisons will be made against characterised in vitro origin wear debris generated in the Durham hip wear simulator. Collaborative research with the University of Newcastle Bone Biology and Bioengineering research group is planned to investigate the cellular response to the UHMWPE wear debris.

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Kapler Communications Limited
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