Cutting Tool Engineering
June 2012 / Volume 64 / Issue 6

Replacement parts

By Alan Richter, Editor

Patients about to receive medical implants probably don’t anticipate participating in world-class athletic competitions, such as the London 2012 Summer Olympics, nor do they have the Olympics’ motto of citius, altius, fortius (Latin for higher, faster, stronger) on their minds. But they are still looking for harder, tougher, longer lasting implants that allow them to remain physically active. The last thing patients want is for implants to cause harm.

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Stryker’s ADM X3 Mobile Bearing Hip with BIOLOX delta ceramic technology features a large ceramic femoral head. The implant has two points of movement as opposed to a traditional design with one. Images courtesy Stryker.

Consider the case of Stephen Tower, M.D., 55, an orthopedic surgeon from Anchorage, Alaska, who competed in a double-century bike race within 6 weeks of having hip-replacement surgery in 2006, reported Consumer Reports magazine in the May 2012 issue. However, a year after the surgery, the DePuy ASR XL metal-on-metal hip implant, which was recalled in 2010, was causing Tower constant pain, and the levels of chromium and cobalt in his blood were high. Other physical ailments occurred, but after having the hip removed in 2009 and replaced with one made of ceramic and plastic, his symptoms markedly improved.

Material Issues

With the problems of metal-on-metal implants well documented, some implant manufacturers are turning to ceramic parts for a solution. Machining ceramic parts and using them in implants present their own challenges, but one of the major benefits is biocompatibility.

“From working with ceramics and surgeons, I know the body will react to anything foreign,” said Steve Cotton, president/owner of Micro Precision Parts Manufacturing Inc. (MPPM), a manufacturer of small medical prototypes and other components located in Qualicum Beach, British Columbia. While many implant recipients are sensitive to metals, such as cobalt, chromium and nickel, zirconia-toughened alumina (ZTA) ceramic has been shown to be biocompatible, he noted.

As they wear, metal surfaces generate particulates. “Debris is really not desirable at all,” said J.B. Lafon, president of Euro Industries Inc., Colorado Springs, Colo. “Nobody wants to talk about metal-on-metal implants anymore.” The company is the U.S. agent for SCT, a French manufacturer of ceramic implants, blanks and components.

Traditional metal-polyethylene hip replacement implants also wear over time, generating polyethylene particulate debris and causing osteolysis, or degeneration of bone tissue, according to ceramic implant manufacturer Morgan Technical Ceramics (MTC), Rugby, U.K. This, in turn, weakens the surrounding bone and results in an implant becoming loose—a main reason for expensive and complicated revision operations.

To reduce polyethylene wear, Kalamazoo, Mich.-based Stryker Corp. developed the Mobile Bearing Hip with its X3 advanced bearing technology. The company reports that acetabular inserts made of X3 ultrahigh-molecular-weight polyethylene (unsterilized) reduced volumetric wear 97 percent compared to the same insert made of N2/Vac gamma-sterilized UHMWPE in laboratory testing. The inserts tested were 7.5mm thick with a 32mm ID. Testing was conducted under multiple-axial joint simulation for 5 million cycles using a 32mm cobalt-chrome articulating counterface and calf serum lubricant. “This decrease in wear may extend the life of the hip implant, which is especially important for younger, more active patients,” a Stryker spokesperson stated.

The Mobile Bearing Hip system also has a large ceramic femoral head, or ball, and ceramic lines the acetabular shell, or socket, the spokesperson added. In addition, the hip has two points of movement as opposed to a traditional design that has one, which reportedly provides more natural movement, a greater range of motion and enhanced stability.

Although ceramics wear, the rate is minimal. According to an MTC study, its ceramic-on-ceramic hip implants made of Vitox AMC-brand ceramic, which contains 80 percent alumina and 20 percent zirconia, demonstrated a wear rate of 0.032mm3/million cycles. “Often these implants last more than 20 years,” said Yannick Galais, commercial manager for MTC. “Some surgeons still couple ceramic on polyethylene, but the best combination is ceramic on ceramic.”

Even so, the industry is fairly conservative and switching from one implant material to another requires extensive testing, validation and time. “There’s a great interest in the use of ceramic material but, possibly due to validation issues, it is taking time to be widely adopted by the industry,” Galais said.

Cracking Concern

One issue with ceramics is catastrophic failure, where an implant fractures within a body. “Reliability is probably the number one concern about using ceramics,” said Ricardo Heros, senior scientific and sales consultant for Laurens, S.C.-based CeramTec North America Corp., a manufacturer of high-performance ceramic and hermetic products, including hip and knee replacements and components for their articulation.

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SCT Ceramics uses its GH01 alumina grade, which is 99.8 percent pure, for the ceramic components in these hip implants. Image courtesy SCT Ceramics.

However, part of that concern derives from the general public’s perception that ceramics are fragile materials used to make products like dinnerware and ornamental items. “The ceramics we make are engineered specifically for the application,” Heros said.

He pointed out that there are three basic families of engineered, or technical, ceramics: silicon, alumina, or aluminum oxide, and zirconia. “You can tailor the ceramic as well to meet the demands of the application,” Heros said. For example, zirconia is often added to alumina as a toughening agent to enhance fracture and shock resistance.

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A profile superabrasive wheel from Abrasive Technology is effective for grinding ceramics. Image courtesy Abrasive Technology. Click here for a brief video report offering an overview of the company's superabrasive technology.

Ceramics, however, are desired for being a hard material, which also tends to be brittle. Ceramics are produced to be harder and have more compressive strength than metals, noted Costa Sideridis, president of Ferro-Ceramic Grinding Inc., Wilmington, Mass., which produces ceramic implants, surgical instruments and OEM parts, such as washer insulators. “Where ceramics fail is clearly in tensile strength,” he said. “They are not that strong when you bend them. They are not made to be dropped more than once.”

A ceramic implant has about a one-in-25,000 chance of catastrophic failure, according to published reports. “But if you’re the one out of that 25,000, you are not too happy,” Euro Industries’ Lafon quipped.

He noted that one option to reduce breakage is coating metal implant components, which won’t shatter, with a highly wear-resistant material, such as ceramic or diamond-like carbon, a ceramic-based coating.

Galais added that MTC can coat the heads and cups of ceramic hip implants with CVD diamond to reduce wear and friction. The technology is used to coat scanner windows at store checkouts to prevent scratching, and while MTC hasn’t gone into production with diamond coatings for implants, it has undertaken trials, he noted.

To reduce wear and the chance of dislocation while simultaneously increasing the range of motion, implant manufacturers often produce larger heads closer to anatomical geometry. As a result of these larger heads, the ceramic hip implants are less susceptible to breakage, according to Galais.

Ceramics’ inclination to break and not bend can be advantageous, however. That’s the case with housings for cochlear implants, an electronic prosthetic device for those with severe hearing problems. If a young child, for example, has a cochlear implant housed in titanium, a fall can deform the housing and reduce device performance but the child might not be aware that it’s defective, Galais explained. In contrast, a significant shock that breaks a ceramic housing causes the electronics to stop functioning, enabling a child to indicate a problem exists.

Cutting It

When a ceramic implant is in the green state (the ceramic powder is compressed and shaped but not yet heated and hardened), machining it is similar to cutting chalk. The vast majority of the material is removed at that stage with conventional cutting tools to create a near net-shape part.

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Users apply TSZtech diamond tools from Abrasive Technology for finishing zirconia-based applications. Image courtesy Abrasive Technology.

The part is then heated to achieve its desired properties. Heros noted CeramTec’s ceramic products go through three different thermal processes at temperatures higher than 1,400° C. The first one sinters and consolidates the material, the second one uses hot isostatic pressing (HIPing) to eliminate internal voids to increase ceramic density, and the third process tempers the material to remove residual stresses.

At that point, advanced ceramics don’t resemble chalk. When comparing hardness to diamond on a scale from 1 to 10, with diamond being 10, ZTA is about 9.0 and silicon nitride is 9.6, according to MPPM’s Cotton. While carbide tools can be applied in some ceramic machining operations, he noted carbide loses its edge after about 30 seconds in the cut.

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Dissection of a cadaveric head shows the divided sylvian fissure and a long-aperture, yttria-stabilized zirconia aneurysm clip. Image courtesy Micro Precision Parts Manufacturing, Indexable Cutting Tools.

The most suitable material for cutting such hard workpieces is the hardest one—diamond. Tom Namola, product development and applications engineering for single-layer products at Abrasive Technology Inc., Lewis Center, Ohio, indicated that the toolmaker’s brazed and plated single-layer diamond tools sometimes machine ceramics in the fired state but are more common for the green state, whereas resin- and metal-bond multilayer diamond wheels provide longer life when grinding fired-state ceramics. End users apply the company’s diamond products to produce some medical implants, he noted, but more frequently use them to make dental implants, such as crowns and bridges.

Namola added that many consider a cutting speed of 5,000 to 12,000 sfm appropriate for superabrasive wheels, but speeds as low as 500 sfm have been successfully employed when fine-finishing ceramics with small-diameter wheels. However, only about 0.002" to 0.007" of material is removed per pass at these lower speeds.

One of the main challenges when grinding ceramic is it doesn’t conduct heat, so the heat is directed into the wheel, according to Glen Rosier, product development and applications engineering for multilayer products at Abrasive Technology. A successful ceramic grinding operation, therefore, requires a wheel that’s able to readily dissipate heat and a setup for targeting the wheel/workpiece interface with a water-based coolant, he noted.

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Jaws from Micro Precision Parts Manufacturing are made of “very machinable” zirconia-toughened alumina and are for a new medical biopsy tool. The jaw top is 12.5mm long. Image courtesy Micro Precision Parts Manufacturing.

The same scenario holds for diamond routers, which can be 116" in diameter or smaller and rotate up to 200,000 rpm, Namola said. The coolant pressure doesn’t have to be high to be effective. He noted one customer uses a fish tank pump to deliver coolant from four nozzles on a small machine because the emphasis is on keeping the tool cool rather than flushing chips.

Without the coolant, tool life significantly diminishes. Namola estimated a 116" router running at 20 ipm and taking a 0.005” DOC will cut a ceramic part for more than an hour with proper coolant flow. “If I shut the coolant off, that tool might last 5 to 10 seconds,” he said.

In addition, coolant filtration is required to ensure ceramic chips are not recut. Rosier recommends a system to remove particles down to 1µm.

Polishing is often performed after machining ceramics, but Namola noted fine-grit wheels can achieve the finish requirements if the machine tool and fixturing are rigid enough and the workpiece is properly cut. “If you’re cutting in line with the direction of rotation, you’re going to get a rougher surface than if you’re cutting perpendicular or somewhat off-axis of the tool rotation,” he said.

Down to the Finish

With a fixed abrasive, whether it’s a wheel or mandrel, Sideridis said Ferro-Ceramic is able to impart finishes on ceramic parts from 20µin. Ra to 30µin. Ra—a typical commercially ground finish. He added that surface finishes from 4µin. Ra to 12µin. Ra are possible with fine-grit diamond wheels, but polishing is required to impart a finish finer than 4µin. Ra.

“The industry is trying to make it more predictable by mechanizing it as much as possible,” Sideridis said. “But how do you polish an ID counterbore on a 1"-dia. hole? Clearly, you can’t do it on a single- or double-sided lap, so you have to get creative about how you get in there.”

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A clip made from “very machinable” zirconia-toughened alumina from MPPM is 10mm long and has radii as small as 0.3mm. Image courtesy Micro Precision Parts Manufacturing.

For producing the parts, Sideridis pointed out that because he’s removing 0.001" or even 0.0005" of ceramic material per pass, the spindle is not being stressed. “Here, we’re seeing spindle loads of less than 10 percent, maybe 5 percent,” he said. Sideridis compared that to machining an aluminum part, taking 0.040" to 0.050" per pass and having a spindle load of 80 to 90 percent. “Our spindle replacements are few and far between.”

Another big difference between cutting those two materials is an aluminum part might take 30 seconds of machining while a ceramic one requires more than 30 minutes. Therefore, the shop performs as much lights-out machining to maximize throughput for the machines, Sideridis noted.

Although making parts from difficult-to-machine ceramics would seem to require high-end machine tools, according to Sideridis, that’s not the case. “A lot of people think you need the Cadillacs of the industry to generate these parts,” he said. “As long as a machine is robust enough to repeat within 0.0002", it works well for us. Ideally, what we’re trying to maximize is the surface feet per minute.”

That can be challenging when machining ceramics, but Cotton said MPPM and ceramic manufacturer Indexable Cutting Tools Inc., Welland, Ontario, developed a ZTA bisque material that is very machinable, does not flake or suffer edge breakout and can be “sintered to size” because the final sintering stage causes the bisque to shrink about 10 to 15 percent. “Indexable is also working on a silicon-nitride bisque with a machinability similar to the new ZTA,” he added. “Those two are probably the two most-favored ceramics used in the medical field.”

Although nondisclosure agreements prevent Cotton from revealing any details, he noted the new ZTA can’t be machined in its green state for numerous reasons, including catastrophic failure of the machined parts because of cutting stress. Development of the ZTA bisque took 2 years, including work with toolmakers to create custom PCD and PCBN cutting tools. “Hardness on hardness causes an issue,” he said. “If it’s too hard and too sharp, you blow the tool up. It’s pretty bad when you’re blowing up a $600 tool in 5 minutes, but it does happen. It’s still kind of a new frontier.”

Exploring unchartered territory can be exciting, but also quite expensive. “We’ve probably invested more than the value of our company into the research and development of this material,” Cotton said. “We’ve done a lot of prototyping and bisque analysis to come up with a final product, and we are ready to take that knowledge to production.”

As the company’s name implies, Micro Precision Parts Manufacturing specializes in small. In a test, MPPM was able to machine its’ ZTA as thin as 0.2mm before experiencing chipping and fracturing, Cotton noted.

Although producing ceramic medical implants has its rewards, the challenges can be daunting, requiring a high level of worker skill and development of on-the-job “tribal knowledge.” Ferro-Ceramic’s Sideridis said: “There’s a higher barrier to entry in what we do than with traditional metals. As an industry, we try to make it more systematic and symptomatic so we know what to expect and become more predictable.”

That’s important because a company’s survival can hinge on being consistent when machining ceramic implants. “The biggest caveat with the medical field is always liability,” Sideridis said. “When you make something that’s implantable or surgical, you want to ensure you have all the right controls and the right processes in place because there’s zero room for error.” CTE

About the Author: Alan Richter is editor of CTE. He joined the publication in 2000. Contact him at (847) 714-0175 or alanr@jwr.com.


Contributors

Abrasive Technology Inc.
(800) 964-8324
www.abrasive-tech.com

CeramTec North America Corp.
(800) 754-7325
www2.ceramtec.com

Euro Industries Inc.
(719) 264-6111
www.sct-ceramics.com

Ferro-Ceramic Grinding Inc.
(781) 245-1833
www.ferroceramic.com

Micro Precision Parts Manufacturing Ltd.
(250) 752-5401
www.precisionmicromachining.com

Morgan Technical Ceramics Inc.
(800) 433-0638
www.morgantechnicalceramics.com

Stryker Corp.
(269) 385-2600
www.stryker.com

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