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6. Recent HRIBF Research - Use of 7Be Beam in Wear Studies (Proof-of-Principle Experiment)
[U. Greife (Colorado School of Mines), spokesperson]

Currently in the United States, about 200,000 hip-joint replacement surgeries are performed each year. Worldwide, the number is nearly 1 million [DeG04, Feh00]. Unfortunately, these implants do not last forever, but seem to have useful lifetimes (limited by wear) between 10 and 20 years. The aim of wear studies on artificial hip joints is to extend the lifetime of the implants through development of more durable materials, as well as to make recommendations for the patients on lifestyle (activity) choices.

Substantive work has been going on in the past decade to improve the lifetime of metal-plastics joints. The wear of plastics is nearly exclusively measured by gravimetric methods. Due to the low wear, long test times are necessary to achieve reasonable accuracy. Also, fluid soak from the lubrication fluids can lead to high systematic errors in this method. We have performed a proof-of-principle experiment at the Holifield Radioactive Ion Beam Facility (HRIBF) to show the principal viability of a radiotracer method based on uniformly implanted 7Be for wear analysis.

The radioactive 7Be for the experiment was obtained from the ATOMKI cyclotron institute in Debrecen, Hungary, where it had been produced via the 7Li(p,n)7Be reaction. After chemical extraction from the 7Li matrix at HRIBF, the 7Be matrial was transferred to a sputter cathode for injection in the HRIBF tandem accelerator. A new 7Be implantation setup had been developed at the Colorado School of Mines and was installed at a free beam line at HRIBF (Fig. 6-1). The activity available resulted in 7Be-beam currents of 105-106 ions per second at the sample location.

Figure 6-1: 7Be implantation setup during beam time at HRIBF.

The 8-MeV energy of the beam was transformed into a broad energy distribution (measured with a silicon detector in the implantation position) using a wheel of 20 foils (increasing thickness from zero to 10 μm in 0.5 μm increments) and additional energy "smearing" foils. Based on the foil thicknesses an activity plateau (with depth) in the polyethylene from 0 to approximately 9μm was achieved on which wear studies were subsequently performed. Total 7Be implantation doses varied from 109 to 1010 nuclei on the 7 pins used.

For artificial hip joints with metal-plastic couplings, a typical wear range lies between 0.1-1 mg/(cm2*1 million motion cycles). This corresponds to a depth wear of about 1.08-10.8μm per 1 million motion cycles. Motion simulators work at a speed of about 1 Hz, giving a time of about 2 weeks for one million cycles, well matched to the 7Be half-life. The wear studies were performed with a specifically designed motion simulator supplied and operated by Rush University Medical Center in Chicago. The Pin-on-disk (POD) design replicates the motion trajectories of artificial joints. For this experiment 2 types of plastic materials were available: One conventional high-density polyethylene, the other cross-linked high-density polyethylene. The latter material had been advertised as significantly superior to the previously used conventional materials. However, due to the lower wear and the problems of the gravimetric method with fluid soak a direct quantitative comparison had not been possible.

Figure 6-2: Pin-on-disk orthopedic wear testing system.

The wear studies were performed at Argonne National Laboratory and used a 20% Germanium detector setup for 7Be gamma detection. The plastic pins underwent a known number of wear cycles in the POD system (lubricated with bovine serum) before being cleaned and transferred for activity measurement. The results of the complete wear experiment (extending over 4 months) are depicted in Fig. 6-3. Shown in the figure is the fraction of activity worn off (natural decay corrected) as a function of wear cycles. Clearly visible is the different behaviors of the conventional material (rising group of 4 samples) and the cross-linked material (relatively flat group of 3 samples) with a preliminary result of a wear ratio of app. 13 (40.6%/106 cycles conventional; 3.1%/106 cycles cross-linked). Error analysis and further simulations of implantation depth are still ongoing.

Figure 6-3: Cumulative 7Be activity wear loss as a function of wear cycles.

This proof-of-principle experiment shows the usefulness and practical potential of the use of 7Be implantion as a radiotracer for wear studies. Further analysis and experiments have to look at improvements in the activity measurements (to reduce scatter and systematical error), the influence of radiation dose on mechanical properties (will provide upper limits on allowable 7Be implantation dose) and the possibilities of extending the method to natural materials.

The experiment was performed as a collaboration between the Colorado School of Mines (U. Greife, L. Erikson, N. Patel), Rush University Hospital (M. Wimmer, Y. Dwiwedi, M. Laurent), Oak Ridge National Laboratory [K. Chipps (Rutgers), D. Bardayan, J. Blackmon (LSU), C. Gross, D. Stracener, M. Smith, C. Nesaraja, R. Kozub (TTU)] and Argonne National Laboratory (E. Rehm, I. Ahmad)).

References:

[DeG04] J. DeGaspari, Mech. Eng. 12 (2004)
[FEH00] P. Fehsenfeld et al., Nachrichten Forschungszentrum Karlsruhe 32 1-2 (2000) 91



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