Medicine: Joint wear studies

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. The long test times usually do not allow sufficient resolution to test the impact of different activities, which are important for the patient and could discriminate between different types of prosthesis.

We received approval for a proposal at the Holifield Radioactive Ion Beam Facility to show the principal viability of a radiotracer method based on uniformly implanted ⁷Be for in-situ (at the motion simulator) wear analysis as well as for an off-line analysis with "daily-activity" resolution.

At the HRIBF we have in the past years developed (along the lines of the Bochum/Naples work) a ⁷Be ion beam with beam currents at the experimental station up to 2*10⁷ particles per second. The tandem accelerator delivers a mono-energetic beam, which for our purposes must be transformed into a tailored energy distribution from zero to maximum energy (for producing a flat implantation depth profile). We will achieve this by passing the incoming ion beam through an assortment of thin foils mounted on a rotating wheel in front of the plastic implantation sample. Transmission considerations led us in the following to start with a ⁷Be beam energy of 8 MeV with a maximum range of about 17 μm in Polyethylene.

The ⁷Be implantation dose is limited to 10¹¹ particles/cm² by our intent not to alter the mechanical properties of the investigated material. At a beam intensity of 10⁷ particles/second, one 10^{11 7}Be implantation will take about 3 hours per sample. 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 - 10.8 μm per 1 million motion cycles. Motion simulators work at a speed of about 1 Hz, giving a time of about 2 weeks for the million cycles, well matched to the ⁷Be half-life.

The wear studies will be performed with a specifically designed motion simulator at Rush University Medical Center in Chicago. The Pin-on-disk (POD) design replicates the motion trajectories of artificial joints. For future applications, more specific and complex testing machines allowing the application of hip-, knee, and spine implants are available. We will attempt to measure the change in activity in-situ, but expect that we can achieve higher sensitivity with an off-line analysis of the wear debris in the lubricating liquid. In the POD, the amount of this liquid can be reduced to a volume of the order of 10 ml, which will allow a relatively high efficiency measurement "on contact" with a Germanium detector.

Often mentioned in proposals for new facilities are the applications of nuclear physics to real life. In recent documents, one poster child has been the use of ⁷Be implanted as a radiotracer for wear studies. However, the usefulness and practical potential of the method has not been proven. This demonstration project aims at providing this development of a radiotracer method for plastics wear studies, specifically materials used in artificial joints. Uniform ⁷Be implantation and off-line low background activity measurements can lead to an improvement of about 2 orders of magnitude in wear resolution and should allow daily activity resolution for motion simulators.

Fig 1 - ⁷Be implantation setup on its beam line at HRIBF

We have built the ⁷Be implantation setup at the Colorado School of Mines. After installation at a new beam line at HRIBF (Fig. 1), we tested our beam energy attenuation approach with 8 MeV ⁷Li beam. Fig. 2 shows the energy distributions (measured with a silicon detector in the implantation position) achieved with a wheel of 20 foils (increasing thickness from zero to 10 μm in 0.5 μm increments) and the effect of additional energy "smearing" foils. It can be seen that relatively broad distributions can be achieved, which will lead to an activity plateau (with depth) in the polyethylene on which wear studies can then be performed.

Fig 2 - Energy deposited in a silicon detector of 8 MeV ⁷Li ions after passing through a rotating 20 foil wheel (pink), one additional "smearing" foil (blue) and two foils (black).

Due to the difference in stopping power, the ⁷Be distributions will be near the surface of the polyethylene pins. Additionally, we performed two ⁷Li implantations in polyethylene pins to evaluate possible radiation damage. One pin received 10^{11 7}Li (our projected implantation dose), the other was implanted with a factor 100 more. We are currently in the process of investigating those pins with AFM and spectrophotometer. Visual inspection showed a slight brownish tint on the pin with the high dose. The ⁷Be implantations will run at Oak Ridge in a parasitic mode during the next ⁷Be campaign probably in the summer of 2008.

References

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