FAQ

FAQ

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LifeLongJoints

(1) Are there employment and/or PhD opportunities within LifeLongJoints?

Yes the Consortium has a number of opportunities available including positions for Post-doctoral research fellows and PhD studentships. To find out more please contact lifelongjoints at llj_@_tutech.de, and keep checking the website under ‘News’.

(2) What training opportunities are available in LifeLongJoints?

As a person employed within LifeLongJoints, or as a student undertaking PhD studies, significant training opportunities are available both ‘on-the-job’ and by taking part in more formal courses. Of particular interest here are the excellent networking opportunities provided by LifeLongJoints with regular contact and exchanges between partners. These networks will comprise personnel from the University Research sector as well as Clinical Partners and the Commercial Environment. Scientifically, training will provided at the cutting edge through the development of the coating and new testing regimes. It is anticipated that researchers will present their research at leading conferences and publish in high ranking journals, subject to commercial interests and exploitation.

Total Hip Replacement

(3) What is a total hip replacement?

A hip replacement is a medical device used to replace a damaged hip joint. The procedure, which is usually undertaken under general anaesthetic, removes the diseased natural joint comprising the femoral head and the socket, which sits in the acetabulum of the pelvis. These two surfaces usual work well in the early part of life but can become painful and damaged as we age, making normal activities difficult. Pain may be such that it both disturbs sleep and also impacts on the wider quality of life. The artificial hip usually comprises a metallic stem and head and a metal or polymeric socket. Some designs of total hip replacement may utilise a ceramic head and/or socket. The surfaces of the metallic and ceramic components are usually very smooth to reduce the overall level of wear.

(4) How long do hip replacements last?

The lifetime of a hip replacements, or more accurately the procedure in which a replacement is implanted, are usually measured in terms of a survival curve or life-table. For contemporary total hip replacements the 10 years survivorship can be as good as 95 % to 98 %. However, whilst this appears relatively good, the sheer volume of hip replacements (50,000 per annum in the UK) means that the number of revision operations is large and, hence, affects a significant number of people. In the USA, Kurtz et al (2009) has predicted that the number of revision procedures could rise to 100,000 per annum which places significant demands on the healthcare system.

(5) Why do hip replacements fail?

Most hip replacements will last the rest of the patient’s life, i.e. usually in excess of 10-15 years. However, as life expectancy increases and the procedure is used in younger patients due to its relative success, revision surgery may be required as the artificial joint fails. For those failures which occur 5 or more years after the original surgery then the cause is often wear related with an inappropriate biological reaction to the wear debris being the prime cause. Once the joint has been deemed to be a failure, usually through increased pain and a reduction in the patient’s quality of life, a revision procedure takes places in which a new prosthesis replaces the damaged one. These revision procedures are more complex than the initial (primary) operation and been shown to be less successful, hence the drive to ensure that hip replacement survival is increased and that the replacement itself can deal with the variability in the demands placed on it due to different patient lifestyles, diverse surgical techniques and wide ranging pathologies. As well as this wear process, recent work in metal-on-metal implants has identified corrosion processes as a mechanism by which the hips may fail. These corrosive processes can occur at a variety of interfaces and is not limited to the bearing surface. Hence, if we are to combat failure then corrosion as well as wear will be included in our research to provide a fuller, more holistic approach to reducing the effects of wear.

(7) How are artificial hip replacements tested prior to being made available for clinical use?

New or modified hip replacements will undergo a number of tests that have been agreed within international and national bodies. These tests will test for the overall levels of wear using a pre-agreed protocol such that the wear from different prostheses can be compared. Other tests investigate the fatigue strength of the stem or the reaction to polymeric wear debris. Further, the range of materials are limited for reasons of toxicity and function. These materials are tightly specified such that their variability in mechanical properties is quite narrow. Once a body of evidence has been accrued that would suggest that the device is safe for use then a submission may be made to a regulatory authority to allow the implant to be used clinically and/or undergo a clinical trial. An issue highlighted around the current crop of poor-performing metal-on-metal total hip and resurfacing replacements has been the fact that none of the these standard tests appeared to highlight the potential for relatively high levels of wear and the possible high risk of failure. One reason for the lack of testing evidence was that the wear becomes problematic when adverse joints mechanics occur (e.g. when the femoral head is in contact with the socket edge) and that the testing regimes are largely focused on normal, and not adverse, activities.

(9) What is tribology?

Tribology is the science of lubrication, friction and wear. It is therefore of major importance in developing a fuller understanding of the artificial hip joint. The modelling work will analyse the bearing performance and predict the wear of the surface throughout the joint life. This will be used to inform the artificial joint design, and ultimately result in an optimised bearing. It will be further extended to consider the corrosion of the joint surface in the development of a new model for the tribo-corrosion of an artificial joint.

(13) What is the role of mechanical simulation in the project?

The purpose of mechanical wear simulation of orthopaedic implants is to replicate, as closely as possible, in-vivo wear. Destructively testing both materials and designs to get a more accurate picture of their performance envelopes and boundaries, which, in turn, helps to improve their longevity and performance.
Mechanical wear simulation is also a legal requirement for implants that are to be introduced into most markets, typically requiring a test of 5 million cycles following the respective ISO standard (hip – ISO14242-1 (2012); knee – ISO14243-1 (2009)) with a further 5 million cycles of ‘adverse’ wear testing now being advocated to identify potential areas of weakness or breakdown under extreme loading or ranges of motion.
Only through accurate mechanical wear simulation can weakness and performance deficiencies of materials and designs identified and ‘designed out’.

(15) How will wear be assessed in developing the coating technology?

In the development and optimization process of the SixNy coatings several screening tests will be performed to assess the quality of the coatings, including Rockwell indentation and scratch tests for adhesion, vertical scanning interferometry (VSI) and atomic force microscopy (AFM) tests for roughness, nanoindentation for hardness and profilometer tests for residual stresses in the coating. This will be performed at Linköping and Uppsala Universities. Coatings that are particularly promising will go on to ball-on-disk wear tests at Uppsala University (figure), and eventually hip simulator tests at the University of Leeds.
serum-solution.gif
Illustration of a ball-on-disk test setup for a preliminary assessment of the wear of the coatings. A ball (different materials relevant to the application are used) slides against metallic substrates coated with silicon nitride in a serum solution, simulating the in vivo situation.

(18) What types of corrosion are observed in hip replacements?

Corrosion in hip replacement
Advantages of modular connections between head, neck and stem can be offset by their failure risk. The interface between the stem and head, in particular, is known to be susceptible to corrosion, which can lead to a severe foreign body reaction and metal poisoning. Pitting and crevice corrosion have been documented, as well as mechanically assisted corrosion [Gilbert et al 2006]. Geometrical analysis of the surfaces of failed cobalt-chrome heads on titanium stems has suggested that joint loading, including high joint friction moments due to poorly lubricated bearings, may initiate corrosion by fretting, followed by material dissolution. In contrast, titanium heads on titanium stems demonstrate oxidation propagating radially from the interface, followed by crumbling. This can lead to neck fracture, but its potential biological consequences are yet to be clarified.

(20) Why did metal-on-metal prostheses demonstrate poor clinical performance?

Increasing data are available describing risk factors for the development of local and systemic adverse events based on the use of metal-on-metal hip implants. Especially the use of large head metal-on-metal implants with diameters greater than 36 mm show unsatisfactory results. The major drawback is the local and systemic risk of the explosion of the metal. The revision rate of metal-on-metal hip implants with a diameter of the head above 32 mm is 4 times higher than of comparable ceramic-ceramic hip implants. This has been reported after 10 years follow up.