Analysis of the effects of low-temperature environmental degradation on mechanical properties of Biolox® delta femoral heads through Fractographic study and Raman spectroscopic analysis.

Zirconia-toughened-alumina (ZTA) is one of the most used bioceramic material for artificial hip joints. Some of the reasons for its success are the mechanical properties of this material, in particular, the high flexural strength and fracture toughness. During the propagation of a microcrack, the polymorphic transformation from tetragonal zirconia to monoclinic zirconia occurs, triggering a volumetric expansion of the 3-4% zirconia grain size. The increase in volume allows the material to block the development of the crack and helps to preserve the integrity of the overall structure by recording the highest value of compressive stress at the tip of the crack. Over the years, numerous studies concerning the transformation of the ceramic material subjected to stress have been carried out, observing how, on the surface, the zirconia polymorphic transformation occurs spontaneously in a hydrothermal environment. In this work, the results of the fractographic study conducted on artificial femoral heads subjected to preliminary burst resistance tests are analyzed. Through the test, it is possible to study how the mechanical properties of the ZTA femoral heads have changed or how much the LTD could have influenced, for example, the flexural strength or fracture toughness. The ZTA "BIOLOX®delta" femoral heads (CeramTec, Plochingen, Germany) were all 28mm in diameter, made of a patented Alumina (Al2O3) matrix (80%) containing 17% Zirconia (ZrO2), partially stabilized with approximately 1.3mol% yttrium oxide (Y2O3). The burst strength test was performed according to ISO 7206-10 standards. The variation in fracture resistance evaluated before and after accelerated degradation at low temperatures, recording an average burst strength of 52 kN for pristine femoral heads, while for the aged ones had an average breaking value was ~13% lower. Furthermore, the aged samples showed an increase of the monoclinic zirconia volume fraction, ranging from about 6% (before hydrothermal degradation) to more than 50% (after hydrothermal degradation). All the analyzed ZTA femoral heads have exceeded the minimum fracture resistance value, according to the regulations stipulated by the US FDA despite the critical conditions applied. The samples were studied through a fractographic approach combining Raman spectroscopy analysis and optical analyses such as laser microscopy and SEM microscopy.