Surface finesse

Courtesy of Acme Manufacturing.

Robotic gate-grinding operation for a hip stem. 

Pristine finishes are pivotal for safe and effective orthopedic implants.

It’s a bull market for orthopedic implants, which means that those who manufacture the devices are ever on the lookout to maintain their edge by staying abreast of evolving technology.

According to Cleveland-based market research company Freedonia Group, the $14.3 billion U.S. market for the devices is expected to grow 8.9 percent annually through 2012. The primary driver of this expansion is the on-the-go baby boomer generation, whose insistence on being as active as possible for as long as possible has helped put knees and hips in the No. 1 and 2 spots, respectively, of the most frequently implanted orthopedic devices.

Courtesy of Acme Manufacturing

Incoming hip stem with gate (top) and stem with gate removed via robotic grinding. 

While each step in the manufacturing of knee and hip implants is important to their longevity, safety and effectiveness, the surface finish is especially key, as it’s the implants’ frontline defense against fatigue and bacterial proliferation. 

For orthopedic implant OEMs and subcontractors, achieving optimal surface finishes has been the impetus behind the evolution from hand polishing to automated finishing. Yet the search to improve repeatability and productivity, while maintaining quality, is ongoing. Most recently, it has led to two noteworthy developments: the use of abrasive belts featuring microreplicated surfaces and a new surface finishing technology called the Micro Machining Process (MMP), which makes use of millions of microcutters to achieve “engineered surface finishes” for knee implants and other devices.

Automated Surface Finishing

Among the more prominent requirements artificial hips and knees share is to successfully bear loads at a point where they can also rotate. In the case of hips, the hip ball rotates in the acetabular shell, which is a “cup” made of cobalt chrome, lined with ultrahigh molecular-weight polyethylene. Knees move against a tibia tray, also composed of cobalt chrome with an ultrahigh molecular-weight polyethylene wear surface.

Implant knees and hips are usually composed of cobalt chrome, although titanium, zirconium and ceramic are sometimes used.

For both types of implants, medical device companies “are ultimately looking for a mirror finish,” according to John Barry, technical service engineer for 3M Abrasive Systems Div. Based in St. Paul, Minn., 3M develops surface-finishing methodologies for orthopedic implant OEMs. A pristine, precision finish is necessary to reduce premature wear of the implant’s mating surfaces. 

Courtesy of 3M Abrasive Systems

Microreplicated abrasive belts feature surfaces of micron-grade, precisely shaped, 3-D structures composed of aluminum oxide or silicon carbide. 

The implant surface-finishing methodology developed by 3M consists of several steps—from rough grinding to multistep polishing and buffing. 

Newly cast implants need to have the gates removed prior to machining. “For that, it’s common to use a grade 24 or 36 coated abrasive belt with ceramic mineral,” Barry said. “With the gates removed, a CNC machining process is used to remove the remaining cast skin and create the net shape of the implant.” 

Next, polishing begins with abrasive belts featuring microreplicated surfaces of micron-grade, precisely shaped, 3-D structures composed of aluminum oxide or silicon carbide. With their microreplicated structures, these abrasives work and wear with great consistency. While a conventional abrasive belt surface consists of peaks and valleys with individual abrasive grains bonded to the belt backing, its microreplicated counterpart consists of mineral-filled structures that uniformly finish and polish, minimizing the opportunity for unwanted scratches and imperfections.

Fritz Carlson, president of 100-year-old Acme Manufacturing Co., Auburn Hills, Mich., which provides medical OEMs with robotic belt-finishing systems for knees, hips and other devices, said the introduction of microreplicated abrasive belts has been among the biggest advances in surface finishing during the past decade. Compared to conventional ceramic abrasive belts that feature randomly sized and placed grains, the precisely cut, 3-D matrix design of the microstructured belts offer greater quality, according to Carlson, whose company also incorporates a surface-finishing process laboratory. 

“They last longer and provide a much more consistent surface finish,” he said. “With a product as expensive and critical as an implant, you want a process that you have total control over. But if you are using abrasives that are not made consistently, if there are changes from belt to belt, you’re introducing a change to your process that you don’t have control of.”

Courtesy of Acme Manufacturing

Incoming raw knee casting (left) and completed, robotically polished knee. 

To ensure that medical manufacturers make the best use of the microreplicated belts, 3M’s Barry advises them to implement a stepwise approach to abrasive finishing. To remove the CNC machining lines from an implant, a four-belt abrasive sequence is usually used. Depending upon the coarseness of the incoming CNC machining process, the first belt step typically starts with an A-80 (80µm-grade abrasive) belt, or an A-65 belt, followed by an A-45, or an A-30 belt, and on to an A-16 and A-6 belts, according to Barry. Applying sequentially finer abrasives helps to ensure the gradual removal and refinement of any previous belt scratches. In some instances, end users will also include a vegetable-based lubricant, sprayed onto the final abrasive belt, to help create an even finer finish. 

“Our goal is to develop optimal sequences in which we try to reduce processing steps,” Barry said. “But if you skip too many abrasive belts in the sequence, it is very possible that some of the deeper scratches from the previous, more aggressive abrasive belts may not be successfully removed. Going through a fairly systematic sequence of belts significantly increases the likelihood that the surface will be properly modified and refined throughout the process.” 

Buffing and Cleaning

One of the added advantages of using ever-finer abrasive belts is to reduce the amount of buffing needed. By going to the fine-grade A-6 belt, end users can usually skip the sisal buff step and go directly to the color buffing. The color buff step is used to further enhance the implants’ appearance by providing a bright, mirror-like finish. The final buffed finish for implants is 1µin. or 2µin. Ra. 

“From the time a knee comes off a CNC grinder to color buffing that knee, you’re probably looking at 6 to 8 minutes on a robot,” said Acme’s Carlson. “Then there’s still a small amount of hand touch-up, because there are certain areas of the knee the robot cannot [reach to] polish. But we are currently developing a technology to enhance our robotic polishing process that will eliminate the need for manual touch-up.”

Micromachining Process

Another recent technology that minimizes manual labor while achieving what the company offering it calls “engineered surface finishes,” is the Micro Machining Process (MMP). Introduced 8 years ago in Europe by Swiss-based Best in Class, the finishing method was unveiled in the U.S. last September by MicroTek Finishing LLC, Hamilton, Ohio, the exclusive North American provider of MMP.

“It’s a machining process that allows us to selectively remove different levels of surface roughness using millions of micromilling-type cutters that typically take off 5 microns to 20 microns of material,” said Tim Bell, director of operations for MicroTek. With the exception of aluminum and natural diamond, the technology can be used to machine materials such as cobalt chrome, ceramics, hardened metals, nickel-base alloys and titanium. With its ability to filter out roughness to eliminate friction, increase coating life and adhesion properties and ensure clean surfaces, the technology has found applications in the medical field as well as the aerospace, tool and die and gas turbine industries. 

Courtesy of Acme Manufacturing

Robotic gate-grinding operation for a knee femoral.

MMP, Bell explained, consists of three steps. “First, using a profilometer we measure the roughness profile and conduct a technical verification. We identify the material’s chemistry and hardness, and evaluate the roughness profile left behind by the final manufacturing process (milling, turning, grinding or EDMing, for example).”

With that data, the company divides the roughness profile into four components, beginning with the outermost primary roughness, followed by secondary roughness, waviness and form. The top three layers were imparted at some point during manufacturing. The underlying form is the configuration the designer intended for the material.

Exactly which layer(s) of roughness the MMP needs to remove is determined by the customer’s needs. The method allows companies to choose exactly how much roughness they want to remove. “Unlike other surface finishing processes, we can selectively remove, say, only the secondary roughness, which is on top of the peaks and valleys of the primary roughness,” Bell said. “Or we can remove the secondary and a part of the primary or go all the way down to the form. And we can do that by selecting, from our choice of more than 300 microcutters, the correct one to fit a certain range of roughness, which we call a frequency.” 

Courtesy of MicroTek Finishing

MicroTek engineers compile a profile of each part, which includes four layers: primary roughness, secondary roughness, waviness and form. 

The microcutters, which are made in Switzerland, consist of two parts: a microtool and a nanotool. (The materials they’re composed of are proprietary.) The nanotools attach to the microtools via chemical reaction when the devices are placed in the vat where machining occurs. “The chemical is nothing more than a catalyst that bonds the micro and nano parts,” Bell said. Though he wouldn’t reveal the exact size of the two-part tool, Bell noted that the tool can enter a hole as small as 0.02 " and machine an internal fillet radius down to 0.006 ". 

In the case of knee implants, which are often surface-finished down to the form, the production process begins with building fixtures to hold the as-cast or machined devices, which are then secured in the vat. “Depending on the size of the knee implant, we can do up to about 30 at a time,” Bell said.

The selected microtools and nanotools are uniformly applied to the implant. “Then there’s a high amount of mechanical energy supplied to the system to make the microtools move in a specific manner to cut,” Bell said. “There’s no electricity, no temperatures more than 150° F, no cold, no lasers; it’s mechanical energy. And we take the finish down to well below 1µin. Ra.”

Bell wouldn’t reveal how long machining typically takes, but did say that the entire operation—from receiving the work order to profiling the component’s roughness, completing paperwork, building fixtures, processing the part, disassembling, cleaning, inspecting and shipping—is competed in 3 days on a production order. CTE

About the Author: Daniel McCann is senior editor of Cutting Tool Engineering. He can be reached at dmccann@jwr.com or (847) 714-0177.

Contributors

Acme Manufacturing Co. 
(248) 393-7300
www.acmemfg.com

MicroTek Finishing LLC
(513) 766-5600
www.microtekfinishing.com

SAES Memry Corp.
(203) 739-1100
www.memry.com

3M Industrial Abrasives
(800) 362-3550
www.3m.com

Courtesy of SAES Memry

SAES Memry’s laser-form stents undergo a variety of surface finishing procedures, including the application of a passive oxide layer to help protect against corrosion. 

Stent surface finishing 

In addition to protecting against fatigue, the surface finishing of coronary, peripheral and aortic stents requires measures to protect against corrosion because of the devices’ continuous contact with blood.

A major OEM supplier of Nitinol medical products, SAES Memry Corp., Bethel, Conn., manufactures stents and other medical device components from foundry processing to packaging. “Because the stents are pushing up against an artery wall and they’re going to be in place for a long time, typically 10 years, they need to have the best fatigue and corrosion life available,” said Dennis Norwich, process engineering manager for SAES Memry.

Surface finishing to provide that protection begins as soon as the laser cutting operation concludes. When they leave the laser workstation, “there will be some slag on the stent, so it will get honed and tumbled,” Norwich said. SAES Memry relies on diamond tools to hone the inside of the stent and mechanical tumbling to clean slag from the interior. 

“It’s typically about a half-an-hour tumbling operation using various types of [proprietary] media,” Norwich said. “Because the stents are fragile, we don’t just drop them into a drum; we customize fixtures for every size and shape of stent, and build racks that fit inside the tumbling drum.” The fixtures and racks, he added, also maximize tumbling results while increasing throughput.

The stents are then cleaned in an ultrasonic tank and rinsed with alcohol. The next step is to acid etch the stent surface to remove the naturally occurring oxide layer. 

“If you have a bumpy surface, an acid etch will attack everything, everywhere at the same rate, so your geometry is basically going to be the same [as before etching],” Norwich said. “It doesn’t do much to smooth out the surface. For long life and fatigue resistance, a smooth surface is needed. You want to minimize surface defects where cracks can propagate and lead to failures. So that’s when you move on to electropolishing.”

The electropolishing process involves submersing the stents in a chemical bath with an electric current to create a galvanic action to remove more of the stent surface. “The high points, or peaks, in a material are densely concentrated with electrons,” Norwich said. “When that material is placed in an electrical bath, the electropolishing selectively removes the high spots to provide the part with a glass-like surface.” Norwich estimated the surface roughness at 2µin. Ra or better.

The final step, aimed at protecting against corrosion, provides the stent with a passive layer of oxide. “It’s a weak acid bath for about a half an hour, which deposits a very controlled oxide layer, which we know gives us the best qualities for corrosion resistance,” Norwich said. “And then the stent is ready.”

—D. McCann