3D Optical Microscopy for Orthopedic Implants

This article discusses how 3D optical microscopy can be used effectively in the R&D and quality control phases of orthopedic implants manufacturing. Orthopedic implants can differ widely in terms of:

• Shape, ranging from basic round femoral heads to complicated saddle-shaped knee prostheses.
• Size, spanning from tens of centimeters through to millimeters.
• Material, covering from hydroxyapatite to stainless steel.
• Surface finish, spanning from intricately textured to promote stability to ultra smooth for reduced friction.

With these range of parameters, it becomes a challenging prospect to control the major specifications of a part, often requiring different types of metrology instrumentation designed for different tasks.

Usually, tolerances on measured quantities are very small with regard to the longevity and functionality of a component following successful surgical implantation. Bruker’s 3D optical microscopy technique offers several advantages, such as non-destructive and non-contact characterization; a large dynamic range to determine ultra smooth and ultra rough surfaces; insensitivity to the type of materials; fast, precise and repeatable areal measurements; and the ability for automation to determine a batch of parts and carry out pass-fail summaries based on the parameters specified by users.

Bruker’s 3D Optical Microscopy Solution

Orthopedic parts are precision produced to extremely high specifications. It is important to detect parts that have defects or have been damaged, or parts that have not been produced to the correct specifications. If such parts are detected, then they should be removed from the other parts passing the inspection process. Patient health is the major reason behind this rigorous compliance to part specification. This is because implantation of a device, with even a single flawed or defective component can lead to dire consequences.

Ambiguity about the interaction between a component and the patient’s body can ban the device from functioning as designed, or can lead to serious complications in the future, from patient discomfort, the need for additional surgeries and treatments, or even death. It is also important for device manufacturers to prevent the possibility of recall of a faulty product, as such a process incurs a significant financial burden and leads to loss of integrity among the medical fraternity.

Besides these significant downstream aspects for part inspection, a defective part can also have an effect on upstream manufacturing procedures. For instance, failure of a part due to greater than average surface roughness could point to an inaccurate polishing procedure. The resulting data can then be fed back to re-optimize the upstream tools, leading to less raw material wastage and fewer inferior parts being manufactured. This presents a direct example of return of investment to manufacturers.

Considering the aforementioned reasons, it is important that orthopedic parts are thoroughly checked in a rapid, repeatable, precise, and non-destructive manner. One of the most versatile, precise, and repeatable techniques for precision surface metrology is 3D optical microscopy based on white light interferometry. Systems built on this advanced technology can effectively characterize materials from the research stage to the production stage to sub-nanometer vertical resolution, in a variety of industries, such as medical, aerospace, automotive, electronics, data storage, MEMS, solar, precision machining, and general manufacturing. A 3D optical microscope provides a number of benefits as opposed to standard contact, stylus-based instruments that are still being widely utilized due to their simplicity and familiarity.

The fundamental operation of Bruker’s 3D optical microscopy instruments is shown in Figure 1. The incident light emitted from a high-brightness LED source is divided into two beam paths - one directed on the sample for inspection and the other reflected from a mirror.

Figure 1. Schematic of a 3D microscope with a self-calibrating He-Ne laser.

These beams are then reintegrated and directed onto a CCD. The ensuing display is a highly precise 3D contour map of the surface of the sample based on the varied optical paths traveled by the two original beams. A standard production inspection system will comprise a laser reference signal to enable continuous calibration, regardless of environmental differences.

3D Optical Microscopy Versus Stylus Based Techniques

For many years, stylus profilometers have been the systems of choice for surface texture measurements in industry. These rugged, yet simple instruments record the vertical deflection experienced by a stylus as it is traced over the surface of a sample. The resultant line trace can be utilized to verify surface properties such as trace widths, step heights, and trench depths. The form, roughness, and waviness of that line can also be reported.

However, stylus profilometers have certain drawbacks. As they are a contact-based method, the stylus can scratch or damage a sample. It also serves as a mechanical filter, because a large stylus can never view intricate details that are smaller than the size of the stylus on a sample surface. This can result in erroneously reported parameters, such as an undervalued roughness. Another issue with regard to the stylus methods is the validity of defining a surface based on a line trace. In order to prevent this issue, a large number of commercially available stylus profilers are integrated with a 3D option, which allows multiple line scans to be obtained side by side to create a 3D image. Nonetheless, this process can take a significant amount of time and is not viable for quality control of a bulk number of parts.

In contrast, 3D optical microscopy is a noncontact method, and sample damage is effectively prevented. Also, the system optics controls the lateral resolution, and at about 300 nm, it is relatively higher resolving than standard stylus measurements. In addition, vertical resolution is enhanced, with a usual 3D microscope resolution, almost an order better than that of a typical stylus instrument. The measurement method is intrinsically 3D, attaining a complete array of a 480x640-pixel CCD in a fraction of seconds. This would be comparable to obtaining 480 parallel line scans through the stylus profilometer, which can take several hours to complete.

The availability of this rich 3D data set enables more meaningful studies, allowing the characterization of the surface of the sample in a more robust way than that of a solo line trace. Volume measurements, large step measurements, and defect finding inside a region are some examples of measurements that can be made with a 3D microscope, but not with a stylus system. To study surface textures, Bruker’s 3D optical microscopes have been designed to create and database all the ISO 25178 established areal parameters for standardized assessment of sample surfaces.

Figure 2. A 3D optical microscopy image of a representative polished surface. Below the image is a software cross section that is exemplifying a single line trace that would have been seen by a stylus profiler across the center diagonal of the image. The parameters on the right are areal roughness parameters based on the entire image dataset and compliant with ISO 25178.

Quality Control – Automated Hip Cup Inspection Example

Hip replacement is the most frequent orthopedic surgery, and as a result parts related to this surgical procedure are produced in large quantities. The component tolerances with respect to their form, surface finish, and material composition have to be stringently met. So effective production of hip cups requires fast and precise assessment.

An R&D instrument utilized in a laboratory is entirely different from a production floor system being utilized for quality control. The former has dedicated technicians who are well aware of the technology and also of the detailed nuances of the software. However, on the shop floor there will be many different operators who will utilize the system as a black box. Considering this aspect, a different approach must be adopted with regard to the software and instrument design.

In the case of instrument design, Bruker offers new floor-standing NPFLEX™ and ContourGT^® 3D optical microscopes that come complete with integrated internal laser calibration, vibration isolation, rugged gantry designs to improve crash mitigation systems, sample fixturing possibilities, and extremely long working distance objectives to acquire data from areas that are difficult to access, such as those within small diameter hip cups.

A production interface, integrated into the systems, is separate from the standard interface (Figure 3). It has been designed to enable easy programming of measurement routines, and builds off of a generic manufacturing flow, in which the operator loads the component, the instrument identifies the component, and makes pre-determined measurements. This is followed by reporting pass/fail results and prompting for a subsequent batch to be displayed. Only basic knowledge is needed to operate the system, and easy-to-use features like barcode scanning can be implemented without any difficulty to facilitate keyboard-free measurements.

Figure 3. A screenshot of the production interface. The left side image shows the live video image from the camera. The right side shows the measured surface, as well as input fields for Operator ID, Part Number and Lot Number

Let us consider this process with respect to hip cups. A hip cup is usually mounted in a fixture or placed on the instrument to ensure that the part is properly and rigidly oriented during the measurement process. This is followed by scanning or entering the part number of that cup to access the related measurement routine. The instrument laterally shifts the hip cup to the measurement point, utilizing a motorized X-Y sample stage and subsequently lowers the objective toward the hip cup until the middle interior surface comes into focus. This is followed by collecting and processing the 3D dataset, taking the most optimum fit sphere from the data and applying filters according to a typical surface texture measurement. Finally, the roughness parameter is compared to the tolerance for that specific component and is eventually failed or passed. It takes less than 30 seconds to make one measurement from scanning the part barcode stage to returning the part and failing/passing stage. This process is highly efficient, making it possible to handle large quantities of parts without delaying the downstream processes.

Research and Development – Designing Future Orthopedic Components

As mentioned before, research laboratory and shop floor settings involve different usage, different users, and different specimens. In the research lab, throughput and speed is not important. While repeatability and precision are still important, flexibility is given more importance. In terms of an automated measurement, the specimen is a known quantity featuring a regular shape, known material composition, and definite measurement locations.

In the research lab, new materials are analyzed for certain properties that make them better performing than present-day implant materials. The nature of testing may also be more difficult when compared to basic roughness inspection. For instance, it could establish how the part wears over a period of time, which can involve testing a freshly manufactured part, applying accelerated aging process to it, and re-measuring and calculating the difference. Another instance could be qualifying a specific texturing or machining process that renders structure to the surface of the implant to give it beneficial anchoring and lasting rigidity in the body.

For these kinds of assessment, the standard software interface and bench top ContourGT 3D optical microscopes provide all of the functions required by researchers. In the advanced ContourGT 3D optical microscope, the integration of high numerical aperture interferometric objectives, and high-brightness LEDs for the light source objectives make it simple and easy to acquire data from a variety of materials, whether they be reflective and smooth, nonreflective, rough, or highly transmissive.

With regard to the example of the hip cup, but from an R&D perspective, hip implants often experience wearing mechanisms. Individual implant assembly exhibits a shell-like structure, keeping the femoral head in close contact with a line, and this liner, in turn, is kept in contact with an acetabular cup.

Different combinations of ceramics, metals, and plastics in contact can create debris and friction, which can lead to inflammation of the tissue covering the implant. The wear and the ensuing inflammation can cause pseudotumors, osteolysis (bone destruction), and loud frictional squeaking due to ceramic-on-ceramic “stripe wear” patterns. In such situations, it is important to define the wear and wear speed of the materials involved and thus, enhance the long-term stability and performance of these products. Here, 3D optical microscopy can be applied to readily obtain this kind of information.

Figure 4 depicts a sphere made of PolyEtherEtherKetone (PEEK), a thermoplastic that has excellent mechanical properties and chemical resistance, which led to its widespread adoption as a sophisticated biomaterial in medical implantations. Here, it has been demonstrated how the surface can be determined before and after wear testing to obtain a better understanding on how it would execute as part of a working device.

Figure 4. A triptic showing a PEEK (PolyEtherEtherKetone) sphere (left), the surface before wear testing (top right), and the surface after wear testing (bottom right). The images can be analyzed for volume of material lost, among other parameters.

Conclusion

The NPFLEX and ContourGT 3D optical microscopes from Bruker enable fast, versatile, and non-contact characterization of tribology and surface finish in a wide range of applications, for research laboratories as well as production floor. The repeatable and precise measurements afforded by this technology meet the rigorous requirements of the orthopedics sector. When compared to a one line trace from a contact stylus profilometer, the generated 3D datasets can be obtained much more quickly, and they also include more valid information about surface parameters. The automation case presented here underscores the ability of the 3D optical microscopy to provide simple, reproducible, and high-throughput analysis of hip cups.

The research-based analysis shows a highly precise wear metrology solution that is customized for the medical implant market. To sum up, 3D optical microscopy offers a superior metrology solution for the complete life cycle of orthopedic implants, from design and manufacture through to replicated aging and wearing of the product.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.