||7Be Implantation in Plastics for Prosthesis Wear Studies|
||Oak Ridge National Laboratory, Holifield Radioactive Ion Beam Facility (HRIBF)|
||2004-present (multiple developments, some continuing|
|Result of NP research:
||Several developments in providing beams of long-lived isotopes for astrophysics and nuclear structure.|
|Application currently being supported by:
||Developments continue with support of base funding and DOE 09-13 ARRA funding.|
|Impact/benefit to spin-off field:
||Tools to provide radioactive isotopes implanted into materials to act as tracers for wear and similar studies.|
Currently in the United States about 200,000 hip joint replacement surgeries are performed each year and worldwide the numbers are 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 and knee 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. We tested the applicability of the prosthesis wear measurements technique by comparing the performance of two joint materials: one was untreated ultra-high density polyethylene (UHDE PE) and the other crosslinked UHDE PE (where the UHDE PE was irradiated with high gamma dose).
Realistic wear studies of medical implants are performed in simulators using liquid lubrication. Here the traditional wear determination via mass loss is imprecise due to the liquid soak. Also, 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.
To measure wear we implanted isotopes into the UHDE PE to act as traces. The Holifield Radioactive Ion Beam Facility (HRIBF) has a mono energetic 7Be ion beam line with beam currents at the experimental station of up to 2×107 particles per second. The mono energetic beam was energy tailored from zero to maximum energy to produce a uniform ion implantation profile of 8 micrometers deep in the UHDE PE. This was achieved by passing the incoming ion beam through an assortment of thin foils mounted on a rotating wheel in front of the plastic implantation sample (Figure 1).
The wear studies were performed with a specifically designed motion simulator operated by Rush University Medical Center at Argonne National Laboratory. The Pin-on-disk (POD) design replicates the motion trajectories of artificial joints. The wear of the pin was measured using a germanium detector (Figure 2) to measure traces of 7Be.
The wear resistance for the untreated UHDE PE and the crosslinked one is shown in Figure 3. The linearity of the wear indicates uniform 7Be distribution from surface to 80% of implantation depth. This result represents the first reliable measurement of liquid (bovine serum) lubricated crosslinked UHDE PE wear, revealing a factor of 13 lower mass loss than conventional UHDE PE.
This technique to estimate wear based on isotopes implanted into materials can also be applied to other materials (e.g. other plastics, bone, ceramics, etc.) and applications.
This effort was performed as a collaboration between the Colorado School of Mines, Rush University Hospital, Oak Ridge National Laboratory, and Argonne National Laboratory.
[DeG04] J. DeGaspari, Mech. Eng. 12 (2004).
[FEH00] P. Fehsenfeld et al., Nachrichten Forschungszentrum Karlsruhe 32 1-2 (2000) 91.