Abstract
Introduction
The continued improvement of ceramic materials for total hip arthroplasty led to the development of Zirconia and Zirconia toughened Alumina materials such as BIOLOX® delta. Zirconia exists in a tetragonal phase in new ceramic heads and can transform to a monoclinic phase in response to loading giving the material improved fracture toughness. It is known that surface transformation occurs in this material under hydrothermal conditions (i.e. in vivo condition), and ISO standards recommend parts are artificially aged prior to testing to include any effect of surface transformation on new designs. Accelerated aging procedures have been used to predict the amount of phase transformation that will occur in vivo, but validation of these models requires the study of retrieved hip joints. Here 26 BIOLOX® delta retrievals are analysed to determine the degree of phase transformation that occurs in vivo. The levels of phase transformation measured are compared with those predicted based on accelerated aging tests.
Methods
Twenty-six retrieved BIOLOX® delta (CeramTec AG, Germany) femoral heads were investigated. Retrievals were obtained after implantation of between 1 month and 7 years with an average follow-up of 1.5 years. All retrievals were from ceramic-on-ceramic hip joints that were implanted between 2004 and 2012. Mean patient age was 69 years (range 48–87 years).
Raman spectra were collected using a confocal Raman spectrometer (WITec Gmbh, Germany) a laser wavelength of 532nm, a 50× objective and a 100μm pinhole. Twenty-five measurements were made on each retrieval at random locations outside any visible wear scars and inside visible wear scars. The average monoclinic content was calculated based on the method of Clarke and Adar [1]. For comparison 5 new BIOLOX® delta femoral head resurfacings were measured using the same procedure.
Results
The average monoclinic content of the new heads was 5%±2%. For unworn areas of the retrievals this increased to 23%±9% with the highest monoclinic content measured at 42%. There was no statistical correlation between implantation time and monoclinic content in the unworn areas. In worn areas of the retrievals the monoclinic content was higher and varied between 13% and 51%.
Discussion
This is the largest set of retrieved BIOLOX® delta heads to be examined for changes in monoclinic content building on the work of Affatato et al. [2] who analysed 5 retrievals. All retrievals have a higher level of phase transformation than new heads in the unworn areas showing that hydrothermal aging does take place in the body. The results also show that on average there is more monoclinic transformation in worn areas and hence the material toughening effect of the Zirconia phase is utilised. The mean phase transformation found in vivo matched the 23%±5% reported as occurring under the conditions described in the ISO standard for hydrothermal aging [3]. However, the ISO standard is thought to be equivalent to 40 years in vivo whilst the retrievals measured had a mean follow-up of 1.5 years.
