- Tribology of articulating joints

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	Biocompatible Materials » Project survey » Main projects » Tribology of articulating joints

Background and description

One of the main problems with orthopaedic implants is the formation of wear particles, which cause osteolysis that leads to bone resorption and eventually implant loosening. Wear is influenced by a number of factors, e.g. the bearing combination, wear particle access and joint fluid pressure.

This project addressed the problem of wear in artificial joints (primarily knee and hip joints) from several different directions. It contained the following main ingredients: (i) tribological studies and development of wear testing protocols, (ii) exploration of interesting existing candidate materials, as well as development of new modified ma-terials, (iii) studies of retrieved clinical materials (donor programme), and (iv) clinical follow-up (national registers). The tribological studies were performed at the Swedish National Testing and Research Institute (SP) and included the following main parts: (i) basic model experiments where the material properties and experimental conditions were systematically varied in order to obtain general knowledge about the tribological system, (ii) development of wear testing protocols for screening of material combinations, and (iii) studies of materials developed within the project or by external collaborators.

Scientific results

A new screening method was developed for tribological investigation of joint implants. The testing period could be drastically reduced using a pin-on-disc method compared to a relatively long testing period (6-10 weeks) using a multiaxial joint wear simulator. For studying the amount of wear in a hip joint, radiostereophotogrammetry and X-ray single plane photogrammetric analyses were used. It was shown that a 32 mm femoral head had three times higher volume of wear particles compared to a 22 mm head. There was also a clear connection between the amount of fluid in the joint capsule measured with ultrasound with clear correlation to wear and clinical loosening. In order to reduce wear of joint couplings, both the metal and polymer parts have been studied in the project. The present sterilization treatment of UHMWPE using gamma irradiation gives a deterioration of the mechanical properties along with increased levels of oxidation. Different types of annealing treatment were tested and it was shown that high temperature annealing reduced wear. Addition of a radical scavenger to PE reduced the amount of free radicals and oxidation. Photos of different degrees of wear in explanted acetabular cups

Photos of different degrees of wear in explanted acetabular cups

A model for carbon nitride coating of orthopaedic bearing substrates (Si, Ti and ZrO₂) was developed in collaboration between Dept. of Biomaterials, GU, and Thin Film Physics Division, Dept. of Physics, LiU. The coating, which consists of a “fullerene-like” microstructure with curved, frequently intersecting and highly in-plane oriented basal lattice planes, has promising properties for further development in order to reduce friction and generation of wear particles in orthopaedic artificial joint bearings.

Four Ph.D. theses have been presented in the program.and one licentiate thesis.

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