Abstract
The objective of this study was to compare the low temperature degradation (LTD) behavior of femoral heads made of 3Y-TZP as observed on retrievals with that induced in vitro upon prolonged exposures to a hydrothermal environment. The time-dependent evolution of tetragonal-to-monoclinic transformation and the related residual stresses were nondestructively monitored by Raman microspectroscopy. An increasing intensification of tensile and compressive stresses was detected with increasing hydrothermal aging duration in tetragonal and monoclinic phases, respectively. The dependence of monoclinic fraction upon exposure time was rationalized through the Mehl-Avrami-Johnson (MAJ) formalism in order to interpret the LTD process according to a two-step mechanism of formation and growth of monoclinic nuclei. In vitro results were compared to in vivo monoclinic contents in the same type of 3Y-TZP head retrievals after implantation periods of 1.6–16.6 y, also including literature data previously reported by other authors. One-hour exposure under the selected aging condition is estimated to correspond to in vivo exposures of 4 and 2 years according to ISO and ASTM criteria, respectively. A critical review of these two criteria according to the present analyses revealed that the ASTM simulation predicts more closely the in vivo results as compared to the ISO one.
Highlights
Zirconia (ZrO2) bearings have been considered as a valid alternative to alumina bearings in total joint replacement (TJR)
The ZrO2 material used in orthopedics has predominantly consisted of a partially stabilized tetragonal phase with a content of 3 mol% of yttria (Y2O3), which is typically referred to as 3Y-TZP (i.e., 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystal) [1,2,3]
Unlike metallic joint implants made of cobalt-chrome or stainless steel [4], oxide ceramics are supposed to be more stable in biological environment since, in principle, they possess less driving force for structural degradation in air
Summary
Zirconia (ZrO2) bearings have been considered as a valid alternative to alumina bearings in total joint replacement (TJR). The most attractive characteristics of ZrO2 are its excellent flexural strength and fracture toughness, which are significantly higher than those of alumina (Al2O3) [1]. It has been well recognized that, in human body, the 3Y-TZP material is metastable and can transform from the tetragonal to the monoclinic (t → m) polymorph under the combined effects of biological (aqueous) and mechanical stress environments (e.g., frictional wear, body weight, and impingement). When acting against advancing cracks, this phenomenon plays a significant role in increasing fracture toughness by generating a strong crackshielding effect as a consequence of 3∼4% volume expansion (i.e., a mechanism referred to as transformation toughening) [1,2,3, 5]. If the t → m transformation becomes environmentally driven and uncontrollably occurs prior to crack propagation, the material undergoes a significant loss of BioMed Research International
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