Tribology and testing of orthopaedic implants
Aia Malik and Chris Pickles discuss the bio-tribology of an implant, and review the three key areas to look at when assessing an implant – mechanical testing, debris analysis and surface analysis.
The orthopaedic implant industry is in a continual state of development, witnessing an explosion of novel materials, designs, and applications. This process is, however, often laced with challenges and articulating joints can present the greatest number of these. The biocompatibility of an orthopaedic implant is essential but, as an increased number of patients outlive the life expectancy of their implant; longevity is becoming a significant clinical problem. Thus, the bio-tribological performance of an implant becomes increasingly relevant.
Bio-tribology is the study of friction, lubrication and wear as they occur in the human body and, as such, are all important factors to consider in the design of implants. Assessment of an implant covers three areas – mechanical testing, debris analysis and surface analysis. In this paper we will review the key techniques available, focusing on the value of generating a complete picture and an understanding of an orthopaedic implant in terms of how the design, base material or coating behaves under friction and loading.
Where moving parts are at work, materials suffer tribological effects including applied stresses and frictional forces, particularly those associated with articulating surfaces. Issues associated with wear of an implant can be severe, from discomfort or heat due to friction or loosening of the stem to the production of loose particles, even nanoparticles, which could cause adverse effects on the surrounding tissue. Worse, delamination of a coating following cracking can be catastrophic and debilitating for the patient.
Properties like better wear and corrosion resistance will improve the longevity and behaviour of the implanted devices. The stresses associated with an implant depend on the patient’s body weight and physical activity, and thus both are factored into the process for designing an implant. Indeed the testing process must also be all-encompassing and physiologically relevant to generate valid findings.
Accurate characterisation, testing and analysis of these features can be used to assist with product development – helping manufacturers to determine wear parameters, how different materials will interact with each other and how coatings perform in the body. Chemical, physical and topographical changes can be determined and the information fed into the design process.
There are a number of techniques used during product development to help forecast changes to the implant’s base and coating material over its lifetime.
Wear testing involves mechanically mimicking human movements, load and, to some extent, the physiological environment, in order to establish the wear rate of an implant. Following mechanical testing, the affected surfaces can be quantitatively analysed for their chemistry and topography. The debris generated can also be characterised and quantified. Gathering the information together presents the designer with a more complete picture of how an implant would behave in the body, ensuring safety and efficacy for the patient. Mechanical testing, debris and surface analysis are all industry-recognised tests, with ISO standardised simulated wear testing viewed as a minimum for device approval by regulatory boards such as the FDA and MHRA. More robust protocols and investigations that include surface and debris analysis are favoured in order to better predict the performance of implants in the body. The tests together are relied on by regulatory authorities to inform on device safety for implantation and product marketing approval.
A variety of mechanical tests are used routinely in the study of wear phenomena on the surfaces of systems such as orthopaedic implants are fitted into specially engineered simulators which allow investigation of long-term wear of an implant over shorter time scales. Five years of in vivo activity can be simulated in three months. The method must accurately simulate the movements associated with the function of the device, and therefore simulators have been designed that better imitate the varying stresses on the device. A total hip or knee joint would undergo significant and irregular loads, rotations, and stresses, which will all affect the state of the materials. These complex loading patterns mean simple pin on disc wear tests are not really apt other than for early-stage screening. A full understanding and careful application of joint kinematics and loads is paramount.
The newer simulators attempt to replicate changes in compressive force as applied in the body to natural joints when walking, jumping or running. They also simulate the torque and different directions of movement applied to joints, together with investigating the potential impact of micro-separation on wear patterns. Furthermore, in the body, synovial fluid (SF) is normally present – a thick liquid that lubricates the joint and allows for ease of movement. It will help to reduce friction and wear in real joints and should be factored in during the mechanical testing of orthopaedic joints. It is important to use fluids with close rheological properties to human SF to generate a true response, and typically a 25 per cent bovine serum in saline solution is used.
During normal use, implants are subjected to cyclic loading and unloading, which can cause progressive, localised weakening of the material known as fatigue. Therefore, more specialised tests investigating material fatigue should be performed. Fatigue testing is carried out by applying repeated cyclic loading to the sample followed by surface analysis to determine the effects, as with all mechanical testing.
White Light Interferometry (WLI) is an established optical metrology tool which quantitatively measures and characterises the microtopography and surface roughness of materials. Topographical characterisation of wear scar features (depth and volume) can be determined from fully scaled 3D images generated using the technique of WLI. Spatial resolution is 0.5µm in x and y and 1nm in z. Output can be in the form of statistical roughness parameters, line scans, 2D colour height (thermal) maps, fully scaled 3D images and 3D videos. Where surfaces are coated, coating thickness can be mapped providing the coating is sufficiently transparent to white light.
3D SEM (Three Dimensional Scanning Electron Microscopy) and AFM (Atomic Force Microscopy) are also useful characterisation techniques for surface topography. 3D SEM combines the high resolution imaging of SEM with quantitative surface metrology information to generate a 3D model of the sample. A wide range of metrology information from the sample is available including high resolution height maps and 3D images to illustrate the surface topography, profilometry, surface roughness parameters and specific feature measurement.
Atomic Force Microscopy (AFM) can be used to produce topographical images of surface features too small to be seen in conventional optical microscopes – normally requiring SEM analysis. AFM testing however can be performed in air or under liquids with little sample preparation. Along with imaging of sub-micron features at nm resolution, the technique offers quantitative topographical measurements, relative hardness and friction distribution imaging.
The chemical analysis of material surfaces and interfaces can be achieved using X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) techniques. Both techniques are highly surface sensitive and sample only the outermost several nanometres of the test sample. XPS gives the quantitative elemental composition of the surface with a sensitivity of 0.1 atomic percent. SIMS generates mass spectra giving identification of the molecular species present in the surface region. The techniques are therefore complementary and are often used together. Both XPS and SIMS can be used for chemical profiling of the sub-surface region to a depth of up to 50µm, dependent on the material density. Where surfaces are coated, depth profiling can inform both coating thickness and chemical nature for either single layer or multilayer systems. The techniques can also generate chemical species maps which display the distribution of species across the surface. They are routinely used to explore on and off wear scars in tribological investigations.
In the body, the generation of debris from orthopaedic implants can have severe consequences for the patient. Therefore quantification and characterisation of debris generated following mechanical testing is paramount.
Gravimetric analysis on a sample implant after a series of cycles under various loadings can be performed. A sample is weighed before and after, and the weight loss represents the level of debris generated. However, the potential for contamination of the component, or moisture absorption, must not be neglected. Contamination may arise from the simulated lubricant on, or penetrated into, the surface. There may have been material transfer between surfaces. Either chemical exchange or physical deposition are possible and can affect the measurements. To account for this, the simulated SF can be tested for presence of debris generated during the mechanical testing of the component. Both laser diffraction techniques and SEM can be used to assess particulates for number, size, shape and composition.
Conclusion – Completing the picture
Wherever moving parts are operating, friction, wear and lubrication are inevitable phenomena and impact upon performance outcomes across a range of thermal, chemical and mechanical conditions. Designers must use better material couples with higher impact, wear and corrosion resistance in order to achieve better prognoses for implant patients.
Pre-clinical mechanical testing is essential for evaluation of new implant products, their validation against old designs, and in the support of regulatory approval. An understanding of how an implant’s material and design will perform is crucial. However, to generate valid results, mechanical simulators must be accurately representative of joint movements in the body – the current methods go some way to achieving this but more robust and representative methods must be developed to improve this. The advanced capabilities of surface analysis can provide invaluable information on what is really happening at the interface between implant materials, and should be used alongside the biomechanical data where possible. This would enable the designer and regulatory boards to make predictions for implant longevity and safety in body. This information, paired with analysis of debris, allow the designer to build a vital and complete picture of potential implant material behaviour.
Lucideon is a leading international provider of materials development, testing and assurance.
The company aims to improve the competitive advantage and profitability of its clients by providing them with the expertise, accurate results and objective, innovative thinking that they need to optimise their materials, products, processes, systems and businesses.
Through its offices and laboratories in the UK, US and the Far East, Lucideon provides materials and assurance expertise to clients in a wide range of sectors, including healthcare, construction, ceramics and power generation.
Aia has a BSc (Hons) in forensic science and has previously worked for Lucideon’s chemical analysis team. Working for Lucideon both in the UK and abroad, Aia pursues new products and services to benefit Lucideon’s clients.
Chris holds a degree in chemistry, a PhD in polymer science, and a postdoctoral fellowship. He is a consultant to Lucideon and has expertise in working in the areas of aerospace, automotive, polymers and surfaces and coatings.