Wear testing of total hip replacements under severe conditions

Abstract

Controlled wear testing of total hip replacements in hip joint simulators is a well-established and powerful method, giving an extensive prediction of the long-term clinical performance. To understand the wear behavior of a bearing and its limits under in vivo conditions, testing scenarios should be designed as physiologically as possible. Currently, the ISO standard protocol 14242 is the most common preclinical testing procedure for total hip replacements, based on a simplified gait cycle for normal walking conditions. However, in recent years, wear patterns have increasingly been observed on retrievals that cannot be replicated by the current standard. The purpose of this study is to review the severe testing conditions that enable the generation of clinically relevant wear rates and phenomena. These conditions include changes in loading and activity, third-body wear, surface topography, edge wear and the role of aging of the bearing materials.

Keywords:

• hip simulator
• retrieval analysis
• total hip arthroplasty
• tribology
• wear simulation

Financial & competing interests disclosure

C Fabry is an employee of DOT (Rostock, Germany), a manufacturer of surface coatings. This work was partially funded through the Ministry of Art and Science of Baden-Württemberg, Germany. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Key issues

• Due to limited degrees of freedom with respect to motion and load in hip joint wear simulators as well as simulator-specific sets of parameters, the influence of different daily life activities on wear has rarely been investigated so far. Stop–dwell–start sequences influence the wear performance of THR with regard to a sufficient prediction of the clinical performance, in particular for MoM bearings.

• Third-body wear is deemed to be one of the severe types of implant wear. Third-body particles from different sources may be transferred into the lubrication gap and cause severe abrasive wear at the bearing surfaces, in particular with hard-on-soft bearings.

• Edge loading, including impingement, rim loading and microseparation, is a common phenomenon at THR, affecting the wear behavior of hard-on-soft and hard-on-hard bearings. However, hard-on-soft bearings seem to be less prone to edge loading compared with hard-on-hard bearings.

• The use of thin cross-linked UHMWPE liner should be critically examined concerning possible higher mechanical stress due to edge loading and the reduced mechanical properties by cross-linking.

• Clinically observed changes of surface properties, in terms of roughness of femoral heads, are frequently observed on retrievals. The influence on the wear propagation due to rougher surfaces and decreased friction parameters has been demonstrated in several simulator studies.

• The aging of bearing materials has an important influence on the wear performance. Mainly UHMWPE is affected by aging effects due to chemical and mechanical degeneration of material properties, leading to increased wear. Aging effects of hard bearing components are connected with surface roughening.

• The current test standards are well established and useful for comparing wear simulation studies. However, retrieval observations show an underestimation of wear patterns and amount of the wear during wear simulation. With severe test conditions, the wear mechanisms can approximate the in vivo situation.

Notes