Long term functional simulation of large diameter metal on metal hip implants

The artificial hip replacement or total hip arthroplasty (THA), is a widely used solution to help restore hip function following arthritic disease. Pre-clinical in vitro evaluation of new designs is a vital aspect of the development of joint replacements to ensure devices are safe and to better predict their performance in vivo.

This thesis describes the development of a number of clinically relevant stop-dwell-start protocols for in vitro wear simulation of total hip replacement bearings. A new wear simulator was developed and commissioned to allow application of these complex cycles, and data from the literature examined to characterise activity patterns of real THA patients.

A number of different stop-dwell-start test scenarios were investigated, examining the effect on wear of altering both the duration of dwell periods, and the number of walking cycles between dwells. Wear did not appear to increase significantly with a longer dwell period, however it was observed that bearings did not bed in under these conditions therefore overall wear volumes may be higher.

A significant increase in wear was observed when the number of cycles between dwell periods was reduced down to one or two cycles; this is believed to be due to deterioration of the lubricant film during the dwell, leading to severe lubrication conditions at start-up, and subsequent high wear of the bearing surfaces. These findings may be particularly relevant to the use of MOM devices in very infirm patients, or those in the early stages of recovery, for whom walking in a single step pattern may be more common.

These results have clinical relevance in the loads and cycle combinations used, compared with earlier studies. Both the number of walking and dwell cycles, and the dwell period load are based on data from real hip patients in the literature, showing that activity bursts are generally short and change frequently, and the load during a pause is low compared with peak loads during walking.

It is believed this use of more clinically relevant activity simulations for pre-clinical in vitro testing of hip replacement bearings is vital for improved understanding of bearing tribology and more accurate prediction of implant performance in vivo.