In vitro wear testing of Metal on Polymer artificial hip components under intermittent motion conditions.

Background
Total hip replacement (THR) was described as 'the operation of the century' in The Lancet in 2007, and is currently the most commonly implemented joint replacement procedure world-wide [1, 2]. Although this procedure has reputable success rates, complications can arise leading to a range of clinical issues which require revision surgery [1]. The most prevalent causes for failure are the adverse effects of wear debris released from articulating surfaces. This was particularly problematic in metal-on-metal implant designs, which wore more than expected and released damaging metal debris, causing detrimental side effects in patients [3, 4]. Subsequently, metal on polymer (MoP) joint designs are currently the most commonly implemented [1].

Pre-clinical wear testing of components is essential in avoiding such issues. It is also of high importance that these tests accurately simulate the conditions components are likely to encounter once they are implanted.

Currently, simulator testing of artificial hip components mostly apply a continuous walking cycle, at a speed of one cycle per second (frequency of 1Hz), as per the international standard ISO14242 [5]. This standard also implies that a person walks for 500,000 steps before stopping, which is inaccurate to how people live. People tend to walk for short periods and then stand, with patients tending to walk much slower than is suggested [6, 7]. Thus, simulator testing under an intermittent motion or "walk-stand-walk" loading and motion protocol makes more clinical sense.

Evidence suggests that, under these conditions, components will experience more wear, however research is minimal where MoP components are concerned [8-11].

Aim
To investigate wear characteristics of metal on polymer hip replacement components under clinically relevant intermittent motion and load conditions.

Objectives
* Develop clinically relevant testing protocol
* Update simulator programming to apply intermittent motion conditions
* Compare wear results from components obtained under ISO standard and intermittent motion conditions.

Methodology
A newly developed hip simulator (figure 2), at Newcastle University, will be utilised to conduct wear tests on the most common type of THR implant under ISO standard conditions and intermittent walking conditions at varied frequencies. A comparative analysis will then be conducted of results obtained.

Tests will take place as follows:
* ISO 14242 condition at 1Hz
* Walk-stand-walk condition at 1Hz
* ISO 14242 condition at 0.5Hz
* Walk-stand-walk condition at 0.5Hz

All tests will run for 5 million cycles, pausing every 500,000 cycles to take wear measurements.

Gravimetric measurements will be taken as well as analysis of surface roughness, to assess component wear.

Wear debris will also be analysed.

Test conditions which aren't being assessed will be maintained to ISO standard guidelines to ensure proper establishment of variable effects.

Impact
Almost 100,000 total hip replacements were conducted in 2018 in the UK alone [1]. Better testing of artificial joints prior to implantation will ensure that patients are able to live pain-free with as much mobility as possible, enabling them to live happier and more productive lives. It is envisaged that the results of this research could contribute to changes to the current standards for pre-clinical wear testing artificial hip joints to include more clinically relevant test conditions.


References
[1] W. National Joint Registry for England, Northern Ireland and the Isle of Man, "16th Annual Report," 2019, issue 2054-183X. [Online]. Available: www.njrcentre.org.uk

[2] I. D. Learmonth, C. Young, and C. Rorabeck, "The operation of the century: total hip replacement," The Lancet, vol. 370, no. 9597, pp. 1508-1519, 2007, doi: 10.1016/S0140-6736(07)60457-7.

[3] "REF Impact Case Study Supporting national and international policy development on the