Magnetic Resonance Safety Evaluation of a Novel Alumina Matrix Composite Ceramic Knee and Image Artifact Comparison to a Metal Knee Implant of Analogous Design.

Background: Image artifacts caused by metal knee implants in 1.5T and 3T magnetic resonance imaging (MRI) systems complicate imaging-based diagnosis of the peri-implant region after total knee arthroplasty. Alternatively, metal-free knee prostheses could effectively minimize MRI safety hazards and offer the potential for higher quality diagnostic images. Methods: A novel knee arthroplasty device composed of BIOLOX delta, an alumina matrix composite (AMC) ceramic, was tested in a magnetic resonance (MR) environment. American Society for Testing and Materials test methods were used for evaluating magnetically induced displacement force, magnetically induced torque, radiofrequency-induced heating, and MR image artifact. Results: Magnetically induced displacement force and magnetically induced torque results of the AMC ceramic knee indicated that these effects do not pose a known risk in a clinical MR environment, as assessed in a 3T magnetic field. Moreover, minimal radiofrequency-induced heating of the device was observed. In addition, the AMC ceramic knee demonstrated minimal MR image artifacts (7 mm) in comparison to a cobalt-chromium knee (88 mm). The extremely low magnetic susceptibility of AMC (2 ppm) underlines that it is a nonmetallic and nonmagnetic material well suited for the manufacturing of MR Safe orthopaedic implants. Conclusions: The AMC ceramic knee is a novel metal-free total knee arthroplasty device that can be regarded as MR Safe, as suggested by the absence of hazards from the exposure of this implant to a MR environment. The AMC ceramic knee presents the advantage of being scanned with superior imaging results in 3T MRI systems compared to alternative metal implants on the market.

Zirconia Phase Transformation in Zirconia-Toughened Alumina Ceramic Femoral Heads: An Implant Retrieval Analysis.

BACKGROUND: Zirconia-toughened alumina ceramic was introduced as a femoral head material for total hip arthroplasty. The material combines the stability of alumina with the toughness of zirconia. Despite inherent benefits for bearing surfaces, concern exists in the medical field that phase transformation of the zirconia grains could worsen wear resistance and lower the strength of the head. We examined these concerns in retrieved and artificially aged ceramic heads. METHODS: Twenty-eight ceramic composite heads retrieved at revision surgery were combined with 5 pristine heads (as negative controls for phase transformation) and 5 artificially aged pristine heads (as positive controls). The extent of zirconia phase transformation at the bearing surfaces was examined through confocal Raman spectroscopy and X-ray diffraction. Burst testing was conducted on all pristine and aged heads and the 4 retrieved implants with the longest lengths of implantation. RESULTS: Retrieved heads had higher maximum average volume fractions of the monoclinic phase compared to pristine or aged heads. Length of implantation was not correlated to the volume fraction of the monoclinic phase. All the heads achieved a burst load far above the 46 kN Food and Drug Administration acceptance criterion; 3 of the 4 retrieved heads had burst strengths exceeding 100kN. CONCLUSION: Our results showed that phase transformation occurs in vivo in ceramic composite femoral heads, but the amount transformed did not increase with the length of time the head had been implanted. The negligible effect upon burst strength of the retrieved and artificially aged heads is reassuring. These results support continued clinical use of this alumina-zirconia composite material as a head material.

On the role of oxygen vacancies and lattice strain in the tetragonal to monoclinic transformation in alumina/zirconia composites and improved environmental stability.

The aim of this paper is to clarify at the nanometer scale the relevant factors influencing the hydrothermal resistance to polymorphic transformation of alumina/zirconia composites, primary candidates for artificial joint applications. The topographic distribution of oxygen vacancies and lattice strain on the composite surface were visualized by means of cathodoluminescence spectroscopy and mapped as a function of exposure time in a thermally activated water vapor environment (i.e., simulating the exposure in human body). Systematically monitoring the optical activity of oxygen vacancies in both alumina and zirconia phases also revealed the effect of surface lattice strain accumulation on the kinetics of polymorphic transformation. From the presented data, an explicit role is evinced for surface oxygen vacancy formation in the alumina matrix, an important step in the complex cascade of mechanochemical events determining the superior environmental resistance of the composite.

Raman spectroscopic analysis of phase-transformation and stress patterns in zirconia hip joints.

Confocal Raman piezo-spectroscopy has been used for the quantitative assessments of phase transformation and residual stresses in zirconia made artificial hip joints. This work can be considered to be a first step towards the development of a fully quantitative technique for the spectroscopic characterization of zirconia femoral heads and other zirconia parts for biomedical applications. After establishing reliable calibration procedures, Raman microprobe spectroscopy could be extended to provide quantitative assessments of zirconia metastability and microscopic stress fields along the z axis perpendicular to the joint surface. For the first time, we have directly visualized patterns of phase-transformation and related residual stresses on the very surface and along the subsurface of both in vitro tested and retrieved hip implants. These spectroscopic assessments may open a completely new perspective in understanding the micromechanical wear behavior of zirconia ceramics in biological environment and in developing new zirconia-based biomaterials with superior stability characteristics.