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A. Pappas:
"Correlation of structural parameters, thermal and mechanical properties of UHMWPE with wear rate in hip implant applications";
Supervisor: V. Archodoulaki, P. Tarantili; Werkstoffwissenschaft und Werkstofftechnologie, 2011; final examination: 2011-06-27.

Hip replacement is a medical operation in which the natural human hip joint is replaced by a prosthetic implant. A joint replacement orthopaedic surgery of this kind is generally conducted in cases
that the patient suffers from arthritis in order to relieve pain, or in order to fix severe physical joint damage as part of hip fracture treatment. Hip replacement surgeries restore the original mobility of
the hip. The surgery counts as one of the greatest medical advances of our time.
Ultra-High Molecular Weight Polyethylene (UHMWPE) has been used for over four decades as a bearing surface in total hip joint replacements due to its special and unique material properties. The
extremely high average molecular weight of UHMWPE yields properties including high impact strength, high abrasion resistance and high frictional resistance.
Although these surgical procedures are common and generally successful, the lifespan of a hip Total Joint Replacement (TJR) with UHMWPE is often limited, typically only to 15-20 years. Upon
use, the joints can become unstable and fail from material wear [1]. The aim of this work was to correlate the wear of different material states with its structural parameters and also its thermal and
mechanical properties.
Over the last years, scientific community has concluded that gamma irradiation to the material,which induces crosslinking of the molecular chains, has tremendous positive effects on some properties
of the material, and consecutively to its wear rate. At the same time, it has been reported that irradiation creates oxidation to the material, which affects negatively its long-term effectiveness.
Thus, scientists have been trying to invent and apply new, improved irradiation methods, which preserve the beneficial effect of old methods without their drawbacks.
For the needs of the present thesis three different material states were investigated: GUR 1020 compression moulded, GUR 1050 compression moulded, both as virgin UHMWPE in the form of material plates, and an X3 implant, produced by Stryker Orthopaedics. This material is special because of its irradiation process, as it was irradiated and annealed in three consecutive cycles.
In order to evaluate the thermal and mechanical properties of the virgin UHMWPE blocks, as well as those of the X3 implant the following tests were used: Differential Scanning Calorimetry (DSC)
and Depth Sensing Indentation Testing (DSI). For evaluating the wear of each material state a pinon-disk experiments on a mechanical tribometer were used.
For all analyzed chemical-morphological and mechanical parameters no depth profile (variation of results with distance from the surface) and no local variations (variation of results with location in the implant, i.e. center and rim) were found. The results of the measurements revealed that molar mass has an influence on chemical-morphological structure. The degree of crystallinity influences
mechanical properties: a higher degree of crystallinity implies higher values for micro-hardness and indentation modulus.
Gamma irradiation and annealing of the X3 material proved to have a considerable influence on chemical-morphological structure, increasing the degree of crystallinity and the mechanical properties,
and also creating thicker lamellae and more perfect crystals. These modifications caused by the irradiation process induce a decrease of 34 % on the wear rate of X3 material in comparison with
GUR 1050 and GUR 1020.

PE-UHMW, Hip replacement surgery, mechanical properties, wear