Setting a Standard: Calculating the Specific Wear Index for the Atlas Cup

Mr D.J.Woodnutt, Mr M.M.Mullins
Morriston Hospital, Swansea, Wales.

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Previous work has identified that the flexible titanium and UHMWPE liner of the Atlas cup (FH, France) performs well at 10 years. However, wear is observed in certain cases (average linear <0.01mm/year)1. Similar work with other cups has allowed a wear rates to be compared after standardisation for load, activity and joint reaction force cup orientation. This paper describes a method for determining the Specific Wear Index (SWI) for a given bearing coupling.

For a single surgeon, from a sequential series of 802 Alas cups, 344 identical couplings were available with a 28mm head with a 30µm Titanium Nitride (TiN) micro-ceramic surface with an average follow-up of 47mths. Calculations were performed for a 1:10 sample. Parameters required comprised height, weight, activity liner wear vector and cup orientation (Imatri,SA) 2 as well as cup dimensions (Tiberius database) 3. A direct correlation of linear wear was found with patient weight and activity (r2=0.75 & 0.77 respectively). Non-statistical correlation was found with polyethylene thickness. Standardising for load, activity and cup orientation gave a provisional SWI for the Atlas cup of 1.12 (1-10: good-poor wear characteristics). Similar work with smaller numbers of other TiN-HMWPE couples indicates the Atlas cup to be the best in its class.

  1. Presentations at the 2011 GECO; 2011 combined AOA/NZOA meetings.

  2. Imatri Medical, software graphics analysis, South Africa.

  3. Tiberius Patient Management (Janus Medical, UK).