Surface structure integrity

Features - Cover Story

Optical surface metrology enhances quality control processes for production of modern orthopedic implants.

3D measurement devices offer new insights into surface structures and processing. This is an example of a confocal measurement instrument that can position samples with excellent accuracy for areal roughness measurements on a small target area in a very specific location. This multi-sensor configurable tool features automatable 5-axis motion control to measure required positions without user adjustment.
ALL PHOTOS COURTESY OF Mahr

The rising prevalence of orthopedic disease, increasing number of orthopedic procedures, and growing geriatric population are creating great demand for orthopedic implants. The global orthopedic implants market is projected to grow from $43 billion in 2022 to roughly $61 billion by 2029 . Additionally, today’s orthopedic implants are required to have a longer service life, bringing new materials considerations into the mix.

To successfully meet the growing market needs, orthopedic implants must be manufactured with extreme quality controls in place. Surface finish is increasingly important, affecting how a part will fit, wear, accept coatings, and more. With the introduction of new processes and materials, and a more advanced understanding of coating, bonding, lubrication, and friction, surfaces also have an increasingly technical function, and surface structure becomes an increasingly vital element to ensure performance and longevity.

Surface finish measurement

Proper analysis of these engineered surfaces relies on using the correct measurement procedure to ensure quality control in the production process. Surface finish measurement methods can generally be divided into contact and non-contact.

Contact methods include linear measurement (profile method), where the tip of a stylus traces across the sample's surface and rises and falls to determine roughness measurements. There are also 3D measurement devices offering insights into surface structures and processing. Suppose the application requires a more detailed understanding of the surface structure, and information from a single profile is insufficient. 3D measurements can cover a larger surface sampling area to provide in-depth information about the structure/characteristics of a surface.

Non-contact methods involve light, or optical measurements, to capture a 3D image of the surface. Light emitted from a tool such as a confocal microscope is reflected to obtain the measurement without contacting the sample, not harming the sample, and can measure very soft or viscous materials.

This optical technique creates a topography map including highly detailed height information across every point in the measurement area for an accurate representation of complex features. This type of 3D optical measurement is beneficial for more diverse surfaces, when there’s a requirement to focus on functional structures including protrusions or depressions, and for determining core parameters and the load-bearing ability of the surface. Today’s 3D measurement systems can provide micron and nanometer resolution and acquire more detailed information on aspects including surface roughness, depth, volume, flatness, and more.

A dynamic surface such as a femoral head incorporates a highly polished surface with a near-mirror finish. Confocal microscope technology offers high-resolution measurements without ever physically touching the part, preserving integrity of the surface finish measurements.

Today’s confocal microscope technologies enable the non-destructive, material-independent measurement of titanium, stainless steel, cobalt-based alloys, plastic, polymer, ceramics, and other materials, including inhomogeneous and porous surfaces.

Additionally, modern confocal measurement instruments position samples with accuracy for areal roughness measurements on a small target area in a precise location. There’s a generation of multi-sensor configurable tools featuring non-contact, 5-axis motion control positioning coupled with automation software to measure as many positions required without user adjustment, and for the system to adapt to different tasks.

Also, stylus-based measurements have been used for decades, and there’s a lot of valuable historical data associated with these measurement types. Leading confocal microscope technology will feature results correlating to those, making it easier for manufacturers to implement this technology.

Insights into surface structure

Dynamic surfaces. 3D optical measurements bring additional information to the surface finish measurement process. For example, a dynamic surface (part of the implant that slides or rotates while in service) such as a femoral head, must be smooth and defect-free. Joint implants have very tight tolerances to move freely with very low friction.

To accomplish this, femoral heads incorporate highly polished surfaces with near-mirror finish. Many manufacturers are reluctant to touch this type of surface with a profile measurement method, for fear of needing to re-polish.

Confocal microscope technology offers high-resolution measurements without touching the part. Furthermore, dynamic surfaces usually must be measured in multiple areas and an automated confocal measurement instrument will rotate and translate the part for access to all critical locations.

Rough surfaces. Surfaces where osseointegration is critical require extremely rough and complex features for bones to grow and adhere. How well that occurs depends on surface roughness and how convoluted it is or how much surface area exists for bone growth.

In this type of orthopedic implant, surface roughness of the root portion of the implant is essential because roughness increases overall surface area for implant stability. Newer confocal microscopes offer high accuracy and precision in measuring the roughness of this type of implant surface.

Industry standards

Surfaces where osseointegration is critical require extremely rough and complex for bone growth adherence. Surface roughness of the root portion of the implant is essential because roughness increases overall surface area and the implant’s stability. Confocal microscopes offer accuracy and precision in measuring this orthopedic implant surface roughness.

Parameters have been established for the measurement and assessment of surface roughness. The following standards outline the steps required to perform a roughness measurement. These engineered surfaces must meet specific surface finish parameter requirements for quality purposes, such as Ra, measured according to ISO 4287 or ASME B46.1.

ISO 4287 specifies the rules and procedures for assessing surface texture using stylus profilometers and typically records profiles with lengths of several millimeters. ASME B46.1 details the geometric irregularities of surfaces, defining surface texture and its components: roughness, waviness, and lay. It also defines parameters for specifying surface texture.

ISO 7206-2 and ISO 7207-2, standards specific to orthopedic implants, tell manufacturers about the surface finish and additional details about geometry and tolerances for making prostheses. For example, ISO 7206-2 covers partial and total hip joint prostheses implants for surgery, including surfaces made of metallic, ceramic, and plastic materials. It specifies the sphericity and surface finish requirements for articulating surfaces of prostheses providing a joint replacement of ball and socket configuration.

ISO 7207-2 details components for partial and total knee joint prostheses, specifying surface finish requirements for articulating surfaces made of metal, ceramic, and plastics.

The basis for worldwide 3D traceability is defined by ISO 25178, detailing 3D surface texture parameters and the methods for identification.

This is the first international standard to consider the measurement and specification of 3D surface textures and cover non-contact measurement techniques. ISO 25178 created new areal parameters to standardize the results from 3D measuring techniques. These include:

  • Hybrid parameters – Values involve height, spatial dimensions where vertical, horizontal parameters are combined to provide surface angle, slope information
  • Spatial parameters – Quantify how often surface features repeat themselves; whether a surface would have varying results when measured in different directions
  • Structure parameters – Describe functional structures such as grooves for lubricant transport, storage

Automation in surface finish measurements

As surface finish measurement increasingly moves to the point of manufacture, adding automation to the process creates more intelligent quality control operations in the production line, maximizing productivity and increasing quality and machining efficiency.

To use surface finish measurement tools in a fully automated measuring process, equipment must remotely transmit and receive data and controls, including instructions to start a measurement, change surface finish parameters for different parts, set up verification, and more. Fully automated surface finish systems providing a measuring sequence without operator intervention are ideal for orthopedic implants where numerous features must be measured with precision and maximum flexibility.

Automated surface measurement has numerous benefits including:

  • Making the right measurement – Processes ensure correct areas are measured in hard-to-reach locations; reliable data is generated from the measurements
  • Increased speed and reduced complexity – Tools enable quick processing of a high volume of parts regardless of shape, size, or type of material to be checked
  • Operator productivity – Allows operator to make efficient use of time without performing ongoing, tedious, repetitive movements by hand
  • Error reduction – Manual tasks can be monotonous, increasing chance of human error; automation removes this risk

Conclusion

As demand grows for increased precision, optical technologies such as confocal microscopes set new standards in 3D surface measurement. Widely used for quality control processes in medical applications, today’s tools enable faster, easier, automated measurement and higher-quality imaging. Benefits include non-contact, non-destructive material-independent measurement; accurate measurement of submicron distances; and realistic surface reproduction – all with results directly correlated to established contact surface measurement techniques for reliable, consistent results.

About the author: Christian M. Wichern Ph.D., is product manager, 3D surface metrology at Mahr Inc.

Mahr Inc. https://www.mahr.com