Does artificial aging correctly predict the long-term in-vivo degradation behavior in zirconia hip prostheses?

BACKGROUND

Accelerated hydrothermal aging has long been one of the most widely accepted quality control tests for simulating low-temperature degradation (LTD) in zirconia-containing implants used in total hip arthroplasty (THA). However, it is still unclear how much consistency there is between the experimental prediction from the internationally-standardized tests and the actual measurements from surgically-removed implants after a long period of implantation. This question is fundamentally related to a lack of understanding of mechanical/tribological contribution to the in-vivo LTD kinetics.

OBJECTIVE

The main purpose of this study is to validate the clinical relevance of standardized accelerated aging by comparing artificially-aged and in-vivo used prostheses, and to clarify the long-term effects of in-vivo mechanics/tribology on the LTD progression upon service in the body environment.

METHODS

Surface magnitudes of phase transformation and residual stress in zirconia femoral head retrievals (13.1-18.4 yrs) were evaluated by using confocal Raman microspectroscopy.

RESULTS

The long-term aging behavior in unworn head surface was in agreement with the experimental prediction estimated as 1 h aging at 134 °C = 4 years in-vivo. However, the current aging protocols based on ASTM and ISO criteria were not accurately predictive for the worn surfaces, and the tribologically-induced phase transformation and tensile stress were up to 6.5-times and 3.3-times higher than the environmentally-induced ones.

CONCLUSION

Our study suggests that wear/scratching, frictional heating, tribochemical reactions, and metal transfer may become far more intense triggers to phase transformation than the mere exposure to body fluid.