An unfortunate consequence of a longer life expectancy and a more active lifestyle is the increased injury rate for joints like hips, knees, shoulders and ankles. Therefore, it is only natural that joint replacement has become the fastest growing surgical procedure in the world. The demand for artificial joints has literally exploded. Experts expect that knee replacements in the United States alone will grow from 600,000 in 2020 to over 3.5 million in 2030. Because of exceptionally wear-resistant, longer-lasting materials, new manufacturing methods, and dramatically improved surface finishing systems, patients can now expect the artificial joints in their body to perfectly function for 25 to 30 years. Over time, mass finishing has established itself as the dominant surface finishing method for these life-saving components, thanks to innovative new equipment and novel grinding and polishing media – specially tailored to the finishing requirements for orthopedic implants.

The most stringent quality standards in the industry

Orthopedic implants are subject to extremely strict quality standards. Understandably so: Bacterial infection, biological incompatibility of the implanted materials with the body tissues, material breakage and poor fit of the implant components can have catastrophic, even fatal, consequences for the patient. 

Generally, the implants must be biocompatible. In other words, they must not be toxic or harmful to the living tissue. In addition, they must be corrosion resistant to prevent reaction with body fluids. To prevent bacterial infection, implants must also be absolutely sterile. And implants must have a high tensile and bending strength. For example, a hip joint can be exposed to peak loads of 4,200 N when walking up stairs – equivalent to over 400 kg!

Furthermore, implants must have tight dimensional tolerances. To perfectly mesh with each other, implant parts like the femoral knee component and the tibia plate must be extremely precise!

Materials that work well and last for decades

Great progress has been made in the development of suitable implant materials. In this respect, metallic materials like titanium (Ti), various titanium alloys, and cobalt chrome (Co-Cr) offer excellent properties such as good biocompatibility, high mechanical strength and first-class osseointegration. Because they offer not only biocompatibility but also compressive strength and excellent wear resistance, in recent years, ceramic implants have become more popular. Currently, ceramics are probably the fastest growing material segment in the implant industry.

The most recent developments are coatings for metal implants. Some are intended to reduce friction and wear and to act as a barrier against metal ion release into the patient’s body. Other coatings create a porous surface that facilitates bone growth and anchoring of the implant to the surrounding bone tissue. 

Nothing less than a perfect surface finish

The needed surface finish on implants can vary from extremely smooth and polished to textured, somewhat rougher surfaces: 

Implant areas where two different components interact with each other must be exceptionally smooth. For example, the femoral knee component must move over the tibia plate with a minimum of friction. The same applies to the movement of the femoral head in the acetabular cup.

On the other hand, the implant sections anchored in the bone must have a textured, somewhat rougher surface to promote bone growth around the implant. This applies to sections such as the stem of the tibia plate, the backside of the femoral knee component, the hip stem and the outside of the acetabular cup. 

However, the surface grinding, smoothing and polishing of implant materials can be very demanding. Their toughness makes it difficult to machine and grind them. And it generally takes a long time to create a smooth, polished finish on these materials. Also, each material poses its own challenges: For example, titanium is softer than cobalt chrome or ceramics and, if finished too hard, a so-called “orange peel” effect can be generated on the titanium surface, rendering the component to scrap! On the other hand, cobalt chrome and especially ceramic implants are very hard, therefore requiring aggressive surface grinding and polishing.

Without question, there are also cosmetic reasons for smooth, shiny finishes. A polished component is generally associated with high quality. Therefore, it is not surprising that some implants like tibia plates are partially polished, even though this offers no functional or mechanical benefits. 

Surface finishing of orthopedic implants – a multi-stage process

Titanium and cobalt chrome components are usually made by forging or investment casting. Ceramic implants are shaped by isostatic pressing and then fired at temperatures of over 1000 C. Subsequently, they undergo different machining and finishing stages.

 For example, after casting, cobalt chrome and titanium femoral components are subject to a machining operation such as CNC grinding or milling. Depending on the material, the surface roughness of the implants after grinding amounts to Ra = 0.6 – 2.0 µm.

The subsequent surface grinding and polishing operations can be done by robotic grinding and buffing  or in suitable mass finishing equipment.

Mass finishing – the dominant finishing technology for orthopedic implants

In recent years, thanks to technical breakthroughs such as entirely new machine designs and automated systems, as well as innovative grinding and polishing media, mass finishing has become a major – if not the dominant - finishing method for implants. Today, mass finishing reduces initial surface roughness values of Ra = 2.0 µm on even the toughest materials to a perfectly smooth and polished surface of Ra < 0.01 µm. 

Compared to robotic grinding and buffing, mass finishing offers a significant advantage. Robotic grinding and buffing tools (belts and wheels) create a mono-directional (non-isotropic) finish, whereas mass finishing produces a non-directional (isotropic) surface. Another benefit of the mass finishing technology is that the high pressure of the media on the work pieces has a slight peening effect. This increases the resistance of the implants against tensile and bending stress.

The preferred mass finishing machines

After the preparatory CNC grinding operation, joint reconstruction implants are generally finished - be it surface grinding, smoothing or high gloss polishing - in drag and surf finishers. Even though vibratory systems require very long cycle times for the tough implant materials, they create excellent, high gloss finishes. And in case of low production volumes of work pieces like acetabular cups, vibratory finishing can be quite effective. But vibratory finishing equipment is not suitable for high-volume production.

Drag finishers produce perfect finishes at highly competitive costs

With dozens of equipment installations at leading implant manufacturers around the world, drag finishing has proven itself as a highly reliable, cost-efficient finishing method for implants like femoral knee components and hip stems. Drag finishers are equally effective for intensive surface grinding operations after CNC or robotic grinding, as well as for placing the final high gloss polishing on metal or ceramic implants.

Equipped with between 4 and 6 working spindles, drag finishers can handle up to 24 implants in one single batch.

After drag finishing, the implants are often ready for implantation. Occasionally, femoral knee components may need a final touch-up in the “box area”. 

Surf-Finishing – a new level of orthopedic implant finishing

Surf-finishers represent the ultimate finishing technology for orthopedic implants. Not only do they allow for the treatment of precisely targeted surface areas on the implants, but they are also the most intensive and, therefore, the most productive mass finishing machines currently available on the market. With surf-finishers, the entire mass finishing operation (surface smoothing and polishing) can take as little as 40 minutes!

At the center of the surf-finishing system is a rotating processing bowl filled with grinding or polishing media. The work pieces, attached to special rotary spindles or multi-axis robots, are partially or fully immersed in the moving media bed. The processing bowl rotation of up to 170 RPM creates linear speeds of more than 4.0 m (13 feet) per second. 

The resulting high pressure exerted by the media on the work pieces not only results in excellent finishing results, but also achieves the by far shortest cycle times of any mass finishing system: 

As illustrated in the chart below (Figure 9), compared to vibratory finishing equipment, surf-finishing generates an 80 times higher intensity, and it is about 4 times more intense than drag finishing. 

Surf-finishers are available in two different configurations: (1) the standard version RMSF 4/800 with 4 adjustable rotary spindles or (2) the version with robotic work piece movement in the processing bowl. 

Flexible process parameters in the surf-finishers, such as variable speed of the rotating processing bowl, variable movement of the work pieces through the media, different spindle speeds, use of different types of media, different immersion depths of the media in the bowl, etc., allow easy adaptation to the most challenging customer demands. For example, the surf-finishing technology allows for perfect finishing of the critical “box area” of femoral knee components, thus eliminating the need for manual touch-up prior to implantation.  

Automation and digitization

Drag- and surf-finishers can be completely automated, including robotic work piece loading and unloading. All that is needed is an “intelligent” work piece staging system and equipping the work spindles with quick-connect couplings that allow robotic handling. In addition, automatic media replenishment systems ensure absolute process stability. 

In particular, sophisticated controls with strategically placed digital sensors in the finishing equipment monitor the process parameters of the finishing operation as well as the physical condition of the machines. This permits quick adjustment of the parameters before they drift out of the preset acceptable range. The equipment is practically self-diagnostic, and the controls provide specific solutions. This practically eliminates the need for human supervision of the finishing process and greatly enhances process safety. 

Contributing Editor MFN and Rösler Oberflächentechnik GmbH 

E-mail: holzknecht.usa@gmail.com