Medical Materials
Implants: Biocompatibility and Wear Issues Implants, orthopedic and otherwise, are all FDA Class
Two and Class Tree devices, with stringent requirements, the foremost of which is biocompatibility, MacNeal said. “Of the materials that are favored from a biocompatibility standpoint in metals, titanium would be the number one choice for implants—it’s basically inert in the body. Tere are also some alloys of stainless steel—people talk about ‘surgical stainless steel’ and those two would be the two big ones.” But as manufacturers in other industries know, tita-
nium has its challenges: “Titanium is difficult to work with because it does catch fire. When you’re machining it, you really have to control your feeds and speeds. Its ratio of hardness to brittleness is not great, and it doesn’t have good wear properties—it abrades. In an articulating joint like a knee or hip, you can’t have metal-on-metal there, it’s much too soft.” Cobalt chrome, another popular medical metal, has been
used as a wear surface in orthopedic implants, but, as has been widely reported, it’s under fire right now: “People who have cobalt chrome metal-on-metal interfaces in their orthopedic joints get wear debris resulting in much higher than average levels of chromium ions in their body,” MacNeal noted. “Tose higher levels weren’t planned for and weren’t
in the original filing data, so even though they haven’t been linked to any health problems, they’re an unexpected outcome, and the FDA is asking questions. So orthopedics companies are trying to get away from cobalt chrome for such applications. “Instead of a metal-on-metal wear surface, companies
typically will have a HDPE—high-density polyethylene wear surface, which simulates cartilage. In an actual hip joint, the bone is covered with cartilage, which when lubricated with synovial fluid is essentially friction-free. Inside the orthopedic joint, a biocompatible metal is coated with HDPE to mimic
the cartilage role. But HDPE too can abrade, and in this case, the wear debris—inert polyethylene particles—builds up behind the metal, and as the body attempts to clean up these wear particles it can trigger an autoimmune reaction which causes resorption of bone tissue—a condition called osteoly- sis. Te bone pulls away from the metal joint that had been screwed into it, and the joint can start to become loose. Tat’s usually why some patients need revision surgery—and why people who are, say, 65 years old may elect to put off having replacement surgery, in order to not need to replace the joint at age 75. “So the hunt has been on for a better wear surface. Enter
ceramics. Ceramics are super-hard and are great wear sur- faces. Tey don’t abrade so you don’t have the wear debris is- sues. Tere have been two issues with ceramics, however. Te first is that if the clearance isn’t completely and totally per- fect, you end up with ceramic squeaking against ceramic: As people were taking steps, their joints were literally squeak- ing—loudly!—and these are permanent implants, so there’s no easy way to minimize the sound. Te other challenge with ceramics is that they’re comparatively brittle—if they receive the wrong impact, they break, creating a problem much worse than noisy joints.” While the squeaking is a quality of life problem, it is not
much of a wear problem. Ceramics are essentially self- lubricating. But there are also serious machining issues with ceramics. Tey are extremely hard, so shaping them is a prob- lem—especially when you need such a perfect fit.
Nitinol: Thanks for the Memory A material that is growing in popularity for certain applica-
tions is the titanium/nickel alloy nitinol, which has shape- memory capabilities that make it exceptional, MacNeal says, and there are players in the industry that specialize in making devices that take advantage of that ability.
Stainless steel surgical scissors component made via metal injection molding—MIM—at Parmatech. 44 Medical Manufacturing 2014
Photo courtesy ATW Companies
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