*5.4. Significance of Vm*

Section 5.3 proposes a standard procedure for the evaluation of Vm from Raman spectrometry data. With this tool, one can now properly assess the amount of monoclinic phase on the surface of zirconia-toughened alumina hip prosthesis components. However, to determine whether the measured Vm has an influence on the performances of the components, one must also consider the origin of the transformation. Indeed, the origin can be twofold. First, the monoclinic phase can be formed by hydrothermal aging, after a spontaneous t–m transformation due to the presence of water. In this case, the t–m transformation is, in itself, a degradation mechanism. Second, the t–m transformation

can occur as a response to high mechanical stresses (phase transformation toughening), as can occur in or around a wear stripe [15], or during shocks. In the latter case, the t–m transformation is necessary to limit the damage. This is visible, for example, from the smaller width of wear stripes measured on ZTA than on alumina (that presents a comparable hardness but no phase transformation toughening) in vitro [34–36].

Stress-induced phase transformation is therefore required to obtain good crack and wear resistance, and the monoclinic content per se should not be considered as an indicator of degradation.

#### **6. Conclusions**

In our work, by measurements on both in vitro and in vivo aged BIOLOX®*delta* specimens, we determined that the Clarke/Adar equation is the most suitable equation to quantify the monoclinic content in the investigated material with the used experimental setup. Furthermore, we confirmed that metal transfer is not necessarily related to a high monoclinic content; previous studies showing the contrary might be affected by measurement artefacts leading to an exaggerated monoclinic content. This suggests that the discrepancy between in vitro and in vivo aged implants is rather ascribable to the effect of shocks than to the influence of metal transfer. Moreover, it must be considered that metastability of the tetragonal phase, to a certain extent, is necessary to guarantee good mechanical properties.

In addition, we demonstrated that Raman spectroscopy is a delicate procedure that is very much prone to errors. Critical aspects are associated with the used equation for the calculation of Vm, and with the definition of the related calibration coefficients. Other important issues are related to the spectral quality and data regression procedures. Our study demonstrates that there is a lack of standards concerning the quantification of the monoclinic content in zirconia by Raman spectroscopy. Such standards should be promptly put in place in order to avoid misinterpretations that could ultimately affect the well-being of THA patients.

**Author Contributions:** Conceptualization, A.A.P. and M.D.; methodology, A.A.P. and M.D.; software, M.D.; validation, A.A.P., M.D. and L.G.; formal analysis, M.D. and L.G.; investigation, A.A.P., M.D. and R.P.; resources, A.A.P., M.D. and R.P.; data curation, M.D. and L.G.; writing—original draft preparation, M.D. and A.A.P.; writing—review and editing, A.A.P., J.C., L.G. and R.P.; visualization, M.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** A.A.P. is an employee of CeramTec GmbH (Plochingen, Germany). M.D., L.G., J.C. and R.P. declare no conflict of interest.

#### **References**

