|An additive-subtractive process produces random pores in the surface of a tibial tray in a single process.|
One side of orthopedic devices such as tibial trays features a complex geometry with variable pore sizes to promote osseointegration. However, the process of producing a random porous surface geometry on orthopedic implants generally involves a series of painstaking steps, such as spraying on a foam, infiltrating it with titanium, and then burning it away. In contrast, a new method developed by EOS of North America Inc. (Novi, MI), in conjunction with GF AgieCharmilles (Lincolnshire, IL), dispenses with these myriad steps, creating random pores in a single process.
The EOS and GF AgieCharmilles process chain is unique in that it blends both additive and subtractive manufacturing processes, explains Andrew Snow, sales director of EOS of North America. It pairs EOS's additive direct metal laser sintering (DMLS) technology with AgieCharmilles's subtractive electrical discharge machining (EDM) and high-speed milling technologies.
The manufacturing process chain starts with the generation of a 3-D CAD model. Next, a porous randomized lattice structure--known as a tribicular metal geometry--is developed and integrated into the model using software from Within Medical Software (London, UK). Once developed, the part program is loaded into an EOS laser sintering system containing a reservoir of a powdered metal such as low-oxygen-content, corrosion-resistant Ti64. The system then begins to build, or grow, the part layer by layer by repeatedly pushing the powdered material evenly across a flat work plate. Ranging in thickness from 20 to 60 µm, each powder recoat layer is solidified using the system's laser. After each layer is built, the system's table indexes downward so that the next layer can be solidified upon the previous one.
Once the parts are completed, they are cut from the metal work plate using a wire EDM system from GF AgieCharmilles. Using a 0.010-in.-diam brass wire to generate pulses of electricity, the system disintegrates thin layers of material as the wire advances through the part. This method, according to Eric Ostini, product manager at GF AgieCharmilles, helps to shorten the part build times required by the laser sintering equipment because it eliminates the need to add large amounts of cut-off stock. The EOS system typically builds parts with 0.005 to 0.010 in. of extra stock to accommodate subsequent surface finishing operations. "These small amounts of stock help reduce cutting-tool costs while increasing tooling life because parts require only light finishing cuts rather than rough milling," Ostini comments. "This capability is especially beneficial when parts are made from expensive materials, such as titanium, that require costly special tools for rough-machining the material efficiently."
In addition to producing medical implant parts in one process, the new manufacturing method reduces overall production times. "For example, the OES machine can build 12 tibia trays at a time, most likely in less than 30 hours," Snow says. "Manufacturing the implants one at a time using multiple conventional machining methods would take much longer and involve multiple operations, multiple machine setups, and human intervention." Moreover, the new process allows manufacturers to quickly produce diverse patient-specific implants within the same batch run.
Advancements in laser technology have driven interest in DMLS technology, Snow remarks. Thus, when industry transitioned from CO2 to diode-pump single-mode lasers five to seven years ago, EOS was able to process many more alloys than was previously possible. "As a result, manufacturers are changing how they view DMLS, and medical implants are a perfect example." In addition, the increased power and capabilities of today's PCs, as well as improved CAD/CAM and other software systems, has enabled EOS and GF AgieCharmilles to develop their additive-subtractive process, Ostini adds.
The companies are considering implementing their complete process chain in North America and Europe for FDA-approved implant manufacturing applications, Snow says. However, many medical device manufacturers are already using the EOS system to produce patient-matched knee implant tibia trays, acetabular hip cups, and other implants and instrumentation.