DEVELOPMENT OF FATIGUE LIFETIME PREDICTIVE TEST METHODS FOR HIP IMPLANTS - PART I - TEST METHODOLOGY

The fatigue failure of hip joint prostheses will be expected to assume more importance in second generation implants aimed at younger, more active patients. Furthermore, new designs and material combinations in cluding coatings (e.g. hydroxyapatite) may introduce fatigue problems that as yet have not been considered. The current research makes an in itial attempt to develop accelerated fatigue testing procedures to enh ance the methodology of hip implant lifetime prediction. Tests conduct ed on a 'model' four point bendbar testpiece (mill-annealed Ti-6Al-4V) highlighted that the accelerated test must be conducted in a physiolo gical solution such as Ringer's at 37 degrees C. The introduction of s uperimposed block overloads (50 cycles) to signify stair ascent/descen t or fast walking and single overloads to signify sit/stand movements or stumbling were found to reduce fatigue life by > 50%. The findings of this fatigue study were combined with biomechanics studies to const ruct a variable amplitude 'in-service' load spectrum for testing hip i mplants. Using ambulatory trial data, a simple load sequence was desig ned containing 4 single (sit/stand movements) and 3 block (stair ascen t/descent) overloads that repeated ten times gave one days activity; s ingle overloads repeated every 110 base cycles (normal walking) and bl ock overloads 80, 110 (morning/evening) and 250 (daytime) base cycles. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.