Abstract
Introduction
Laser marking of implants surfaces is necessary in order to provide traceability during revisions which will help identify product problems more quickly, better execute product recalls and improve patient safety. There are several methods of marking employed within the medical field such as chemical etching, electro pencil marking, mechanical imprinting, casting of markings, marking with vibratory type contact, ink jet, hot foil and screen printing. However, these methods have various drawbacks including marking durability or addition of potentially toxic chemical compounds. As a result laser marking has become the preferred identification process for orthopedic implants. Laser marking is known for its high visual quality, good reproducibility and precision. However there are concerns about the laser marking potential to affect fatigue life of a device. There is a limited number of research papers that studied the effect of laser marking on fatigue life of implants. The objective of the current study is to investigate the effects of laser marking on the fatigue life of titanium alloy material.
Material and Methods
Two groups of four point bend specimens were used to investigate the effect of laser marking on the fatigue life. The first group comprised of the specimens without laser marking while the second group comprised of specimens with laser marking currently utilized for the implant surfaces. Prior to conducting the fatigue testing, a non-destructive X-ray diffraction (XRD) residual stress analysis was conducted to determine if the laser marking had introduced any residual stresses. Imaging analysis was also conducted to examine any potential surface damage on the test sample's surface. A servo-hydraulic test machine was used for the fatigue four point bend testing regime where the inner and outer spans were 30 mm and 90 mm respectively. All testing was conducted at a frequency of 10 Hz, a stress ratio R=0.1, and sine-wave loading in air. Testing was stopped at 10 Million cycles or at failure of the specimen.
Results & Discussion
Figure 1 shows that laser marking process can create a fine network of surface cracks. Table 1 shows the results of residual stress measurements. Laser making introduced high tensile stresses on the components whereas “as machined” component without laser marking exhibited compressive stresses inherent due to machining. The result from the S-N curve testing is shown in Figure 2. The current laser marking components demonstrated 41% reduction in fatigue strength compared to non-laser marked specimens. The reduction in fatigue strength is due to the residual tensile stresses generated at the laser marking location which can lead to crack propagation from small micro fractures created during the surface melting process
Conclusion
This study has shown conclusively that laser marking of implants if located at high stress regions can lead to early fatigue failure. Based on the results from the study it is advisable to locate the laser markings at the region of lowest or compressive stress areas and when possible the laser marking process should be selected as to create the minimal damage to the surface.
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