Surface texture and shape play critical roles in the successful integration, lifetime, and effectiveness of bioimplantable devices. A variety of techniques is available to quantify the surface characteristics. Non-contact, 3D optical profiling using scanning white light interferometry (SWLI) is of growing interest for quantifying these critical characteristics. The NewView™, ZeGage™, and ZeScope™ systems from Zygo Corporation (Middlefield, CT) exemplify this class of surface analysis instrumentation.
Why Non-Contact SWLI?
Traditional 2D tactile techniques trace along the surface or sample a large number of discrete points with a probe to measure shape and texture. This type of contact can actually change the surface as it is being measured, compromising the integrity of the measurement and increasing the chances of developing bioimplantable devices that will ultimately fail because of problems in surface structure. SWLI is an attractive technology for medical implant metrology precisely because it requires no contact with the surface, and as a result, significantly improves the integrity and reliability of the measurement. Because it is an imaging technique based on light microscopy, SWLI also has the advantage that in many cases the data density and speed of the measurement are greater when compared with tactile results.
How Does it Work?
A SWLI system has specialized microscope objectives that perform two functions: one is magnification, and the other is measurement of the topography of the test surface. This process involves comparing the test surface to a reference surface — usually a very flat, smooth mirror integrated in the objective.
The microscope’s illuminator projects light through the objective, where a beam splitter sends some of the light to the reference mirror, and some to the part under test. When the optical path length from the beam splitter to both the reference and test surfaces are equal, the reflected light from both the test and reference surfaces recombine, resulting in light and dark bands called interference fringes. The shape and position of these fringes are directly proportional to the difference in height between the test surface and the reference mirror. They can be thought of as contours of the surface, where the contour intervals are proportional to the illumination wavelength, and processing algorithms can further refine this precision to small fractions of this illumination wavelength.
In order to profile the topography of the test surface, the microscope objective is scanned perpendicular to the test surface, thus varying the test length. As the objective is scanned, a camera and computer system monitor the changing fringe patterns. Sophisticated software interprets these patterns to construct a full 3D map of the surface under test. In contrast with other microscope-based 3D topography techniques, SWLI has the distinct advantage that the height resolution of the measurement is constant across all magnifications. Whether the field of view is 20 microns or 20 millimeters, the topography resolution is constant for a given tool.
The surface maps generated by SWLI characterize the surface shape as well as the texture, or roughness, of the surface. Both texture and form are critical in the manufacture and development of medical implants.
Applications for Implants
Surface Texture and Wear
Medical implant devices knit with the existing bone in order to function correctly and contribute to the lifetime of the device. Essential to this process is the device’s surface finish or texture. This is especially true for the roughness of the threads of bone screws, dental implants, prosthetic knees, and hip joints. Also of critical importance is when the implant has a strong weight bearing role, as poor bone knitting leads to a loose implant and a poorer quality of life for the patient.
Similarly, surface finish of an implant bearing surface, such as those in artificial knees, hips, and spinal implants help determine how well a system will move and how long it will last. Recent studies led by Ian Clarke at DARF (Donaldson Arthritis Research Found ation) utilized both NewView and ZeScope optical profilers to characterize the roughness of both new and worn metal-on-metal hip joints. Analysis of the data from the physically worn prosthetic joints’ surfaces has shown that the simulation of predicted wear modes has largely been accurate. Inspection by SWLI has helped uncover additional unpredicted wear patterns as well.1 This kind of revelation aids researchers and manufacturers in better understanding of their devices, and ultimately leads to better quality of life for the implant recipients.
Topography and Microstructure
With their microscopic field of view, SWLI profilers are ideal for measuring microstructure and topography that are critical to performance of a wide variety of implantable devices. Microstructure is a function of design, whereas texture is the result of a process. One example of microstructure that is regularly inspected on a production level is metal plating thickness in sensor devices. In addition to ensuring that there are no voids in the metal, metal plating thickness is important for maintaining consistent and reliable sensor performance.
Transparent Films Analysis
Implantable devices, which carry the risk of rejection or infection, and implantable sensor devices, need a more critical level of inspection. An advanced SWLI tool such as the NewView 7300 optical profiler provides this increased level of inspection with transparent film thickness analysis. SWLI accomplishes this by detecting not only the top surface of the transparent film, but also the interface between the device surface and the bottom of the film.
Typical examples of films that may be suitable for such inspection are anti-rejection drug coatings, sensor reagent coatings, and protective encapsulation coatings. Incomplete or inconsistent coverage of the device or sensor can lead to failure or rejection of the device once placed in the body, or in the case of a sensor, improper performance and faulty data.
As life expectancy increases, so will the need for more implanted devices. More exhaustive metrology will be necessary to ensure device integrity, performance, and lifetime use. Non-contact, 3D profiling with SWLI has a wide variety of applications that can be applied to the entire life cycle of implantable devices — beginning with product development all the way through to manufacturing. The combination of superb vertical resolution, fast measurement speed, and area-based measurement techniques make these tools attractive to researchers and manufacturers alike.
- Pathologic, Serologic, and Tribologic Findings in Failed Metal on Metal Total Hip Arthroplasty. C E. Pelt, J. Erickson, IC. Clarke, Donaldson TK, McPherson E, and Peters CL. In ’Scientific Exhibit# AAOS SE-12, Am. Acad. Orthop. Surg., San Francisco, 2012.