Modular hip prostheses produce wear debris via various fretting wear processes. This has been well documented in many publications concerning implant retrieval and analysis. However, the results of this research are significant in that the debris is now confirmed to be composed of titanium alloy (primarily b phase); not oxides or hydrides of titanium. It is also demonstrable that hydrogen plays an important role in the generation process. the largest sizes of the in-vivo debris analyzed by this research (and particle geometries) differ substantially from those previously reported (400µm for this research as opposed to 75µm elsewhere). Two further important findings of this work are that surface defects such as hydrides and grit particles are present at the implant's surface and that the diffusion bonded mesh pad is far from completely bonded to the stem. These and other observations can be implicated in a 'cascade' of debris generation as follows:

The protective passive layer on the implant surface was broken mechanically (1) under the influence of localized micromotion between either the mesh surface and the implant surface below the mesh, (2) high cycle fretting between the CoCrMo alloy head and the titanium stem, or (3) high cycle fretting between the inner surface of the femur and the smooth implant surface.

Macroscopic presence of polyethylene debris in the specimens of this investigation implicates wear at the articulating surface.

Hydrogen from processing or in vivo absorption enhances the rate of debris generation. Debris particles commonly generated by the implant have been shown possess identical morphology to debris particles known to be hydrogen charged. Once shed, these hardened hydrogen charged debris particles can contribute to further damage.

After significant debris generation, local tissue reaction may have lowered the pH enough to elicit biological attack of implant and bone, initiating the loosening of the implant and pain.

This corrosive environment (and the resultant hydrogen uptake) coupled with continual micromotion (likely increased with bone loss) between surfaces released significant quantities of debris (including surface grit debris contaminants) into the region surrounding the implant. This debris is observed at revision surgery which is required to repair the joint and replace the failed implant.

Combining evidence from various particulate debris specimens with analysis of their source implants will add significant depth and focus to the above proposed mechanisms.

12Recommendations for Future Research

A primary concern for this research is the availability of specimens. Out of the thousands of implants that are revised each year, only a few are actually completely examined by a university or clinical research facility. If extensive patient condition and activity information is coupled with comprehensive implant retrieval and analysis, much more substantial evidence for the true mechanisms of wear debris generation can be developed. These mechanisms of debris generation must be well understood before design changes are applied to the surfaces of biomaterials. If they are not, they may come back to haunt us! Harder, more wear resistant surface treatments for titanium or the use of surface hardened zirconium alloys may appear to solve the wear debris problem in the short term, however, if the role of hydrogen in the substrate material is not fully understood and quantified in these new systems, they may fail even more catastrophically as the harder surfaces simply become harder debris particles under hydrogen's influence...

To improve the specimen preparation and analysis results, the following procedural changes are recommended:

• Use of a carbon paint on the SEM specimen mounts will reduce the background EDS detection of aluminum.
• Centrifugation will help concentrate debris in vial.
• Examination of debris obtained from in-vitro wear testing of implant material in simulated human environments will enhance the information obtained from wear debris obtained from human patients.
• Superposition of Ti Kb peak over V Ka peak combined with the small amount of vanadium in the alloy makes detection of the vanadium content in the particles difficult with EDS techniques. X-ray fluorescence, using a filter to eliminate the Kb contribution may be a simple and inexpensive method that can be applied to confirm or deny the presence of vanadium in the particle debris.

14 Acknowledgments

Stanford University Orthopedic Research Laboratory:

Dr. R. Lane Smith, Dr. William J. Maloney

Dr. David Su of Phillips-Signetics

Zimmer USA Warsaw, IN

Society for Biomaterials

I would especially like to thank Dr. K. S. Sree-Harsha for his helpful advice in class, and his assistance with X-ray diffraction, and lastly (but certainly far from least) Dr. Pat Pizzo, for his unfaltering encouragement when I was discouraged, vivacity when I was lethargic, and his sly wit when I needed a laugh.

Converted to HTML 04/13/97 by douglas j. wood

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