Increased life expectancy and a more active lifestyle are causing more injuries and wear of critical body joints like knee, hip, shoulder, ankle, etc. Therefore, the demand for joint replacements is growing rapidly. Hip and knee replacements will see an especially fast growth. For North America alone, experts estimate the number of knee replacements to increase from 600,000 in 2020 to 3.5 million in 2030.
Tougher, more wear-resistant materials have dramatically improved the longevity of the implants, which can now last 20 to 30 years. However, the tougher materials are imposing considerable challenges for the surface refinement of the implants. In this article, we look at these challenges and how innovative mass finishing systems are helping to resolve them.
Why the surface finishing of
orthopedic implants is no easy task
Considering their function and the potential catastrophic consequences of a malfunction in the body, the surface finish of orthopedic implants must be no less than perfect!
Strict quality standards
With the possible exception of aerospace where similar strict standards apply, joint reconstruction implants must conform to the most rigorous quality demands.
For example, the implants must be biocompatible and resistant against bacterial infection. Considering that during stair walking a hip joint can be exposed to a load of 4,200 N (equivalent to > 400 kg!), implants must also have a high tensile and bending strength. At the same time, they can have no sharp edges and above all, must have tight dimensional tolerances. The fit, for example, between a knee femoral and the tibia plate cannot be too tight or too loose!
Tough, difficult-to-machine implant materials
Today, most implants are made from metals such as cobalt chrome, titanium or titanium alloys. Because of their numerous technical benefits (biocompatibility, resistance against compression and wear), ceramic implants are also becoming more popular. Probably the fastest growing materials in the area of joint reconstruction are coatings to either create a relatively rough, porous surface or a very fine surface to reduce friction and wear.
All these materials have one thing in common: They are extremely tough and, therefore, very difficult to machine and finish. This is especially true for ceramic materials.
3D printed implants pose entirely new finishing challenges
Additive manufacturing has evolved from a purely prototyping system into a technology for volume production. For example, the tibia plates for artificial knees today are, to a large extent, made by additive manufacturing. 3D printing allows to create novel, porous implant designs that are not only light weight but also promote bone growth. At the same time, 3D printing permits a design freedom that is impossible with traditional shaping methods like casting, forging or machining.
But the post-processing of 3D-printed components poses entirely new technical difficulties: These include the removal of support structures and loose powder from components made with powder-bed printing systems. By far the most difficult task is smoothing and/or polishing the implant surface. A 3D-printed component can have an initial surface roughness of Ra > 25 micron. Compared to this, the initial surface roughness of cast or forged components amounts “only” to Ra = 3.0 to 8.0 micron. This poses a considerable challenge for those surface areas that must be extremely smooth and polished with Ra values of < 0.05 micron!
How orthopedic implants are
The most common manufacturing methods for orthopedic implants are still investment casting (e.g. knee femorals, tibia plates) and forging (e.g. hip stems). Even though they are growing rapidly, ceramic shaping and 3D-printing are still relatively small.
The role of CNC contour / profile grinding and milling
After casting and forging, the raw implants do not yet have the required dimensional precision. Therefore, the initial shape (contour) must be further refined by CNC grinding, usually with grinding belts or grinding wheels. Contour grinding is followed by a profiling operation to create the final shape (profile). This can be a combination of CNC grinding and milling.
After the CNC contour and profiling operation, the surface readings on an implant such as knee femorals, usually amount to Ra = 0.2 – 0.4 micron.
Pros and cons of robotic surface grinding and buffing
Frequently, the contour and profile grinding function is handled by robots, whereby a robot is holding the implant against a grinding belt or wheel. Robots are also quite popular for placing the final polish on the implants with suitable buffing wheels.
While robots offer significant technical advantages, they also pose certain challenges.
The positives are:
Robots eliminate dirty, hazardous grinding and buffing operations and remove the inconsistencies of human work.
Robots, along with the respective grinding belts, wheels, etc., are programmable and run, therefore, fully automatically.
Some of the negatives are:
The grinding and buffing equipment, be it belt or wheel, is always running in the same direction. This produces a mono-directional, non-isotropic finishing pattern, which can cause stress in the implants and increase the probability of premature failure.
While the robotic movement is always perfect, over time the belts, grinding and buffing wheels wear. The programming of the robot must allow to compensate for this wear by changing the pressure the robot exercises against the belt or wheel. It may also require changing the belt and wheel speed.
Even the smallest design changes in an implant require extensive re-programming and teaching of the robot.
Compared to conventional grinding and buffing systems, robotic grinding and buffing requires the same careful consideration of dust issues and fire hazards.
Robotic grinding and buffing is not cheap! Apart from the high initial investment expenditures, it also causes relatively high operational costs in the form of replacement belts, wheels and general maintenance. Of course, a major cost factor is also the initial programming, re-programming and teaching of the robot by an expert.
Why mass finishing has become a leading technology for implant finishing
Mass finishing offers numerous technical benefits. Therefore, it is not surprising that today practically all renowned implant manufacturers are utilizing mass finishing systems for placing the finishing touch on their implants prior to insertion into the body.
Contrary to robotic grinding and buffing mass finishing produces non-directional (isotropic) finishes and has even a light peening effect. This improves the resistance of the implants against tensile and bending stress.
Mass finishing processes are very easy to manage. Once established, they produce absolutely consistent, repeatable finishing results. This represents a significant advantage over robotic grinding and buffing.
Mass finishing can handle the toughest materials such as cobalt chrome, titanium and ceramics.
Mass finishing is equally suitable for deburring/edge radiusing, surface smoothing and high-gloss polishing with Ra readings of as low as 0.01 micron.
Mass finishing operations can be easily automated. The degree of automation is entirely up to the customer.
Mass finishing offers an excellent cost-efficiency.
The latest mass finishing solutions for orthopedic implants
Special vibratory systems for handling small production lots
In so-called DL vibrators, two vibratory motors, mounted to the outside of the processing bowl, are generating the required vibratory energy. The work pieces, for example, acetabular cups, femorals, etc., are mounted to special fixtures in the bottom of the processing bowl. During the finishing process, the work pieces are completely immersed in the processing media.
DL vibrators are the ideal machines for generating extremely smooth, perfectly polished surface finishes on implants that are produced in low volumes, for example, a few dozen a day.
Drag finishing – automated surface smoothing and polishing for high-volume production
Drag finishers are built like merry-go-round rides in amusement parks: A rotary carousel contains several rotating work stations, onto which the individual work pieces are attached. The carousel “drags” the work stations with attached work pieces through a stationary processing bowl filled with grinding or polishing media. At a carousel speed of up to 80 RPM, the linear speed of the work stations can reach 2.0 m/sec. This creates a very high pressure between the stationary media and the moving work pieces.
Drag finishers are equally effective for intensive cut-down operations after CNC or robotic grinding for placing a high lustre finish on the implants prior to surgical insertion. Depending on the finishing job, drag finishers can finish between 100 and 200 orthopedic implants per day.
Surf finishing combines robotic work piece handling with the benefits of mass finishing
When it comes to the surface refinement of high-value orthopedic implants, surf finishing truly represents the next generation of automated grinding and polishing. Surf finishers combine all the technical possibilities of robotic work piece handling with the process stability and cost-efficiency of mass finishing.
At the center of the surf-finishing systems is a rotating 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 processing media. The bowl rotation of up to 170 RPM creates linear speeds of more than 4.0 m/sec.
The resulting high pressure exerted by the media on the work pieces produces not only excellent finishing results but also by far the shortest cycle times of any mass finishing system.
Contrary to robotic grinding and polishing, once a drag or surf finishing process has been established, it requires no further adjustments. The dressing of the abrasive belts and wheels, truing of the grinding wheels, force control of belts and wheels, or re-programming of the robots in case of even the smallest design changes, are no longer necessary!
Contributing Editor MFN and
Rösler Oberflächentechnik GmbH