Medical Plastics: The Innovative Beat Goes On

Society of Plastics Engineers
Plastics Engineering
April 2011

Strong and Flexible Implants

In the late 1990s, polyetheretherketone (PEEK) was first used for implants. PEEK-based spinal spacers were used to hold vertebrae upright after disk removal. Unlike titanium, PEEK parts didn’t eventually subside into bone, and they allowed visualization of the bone surrounding the implant in X-ray or CT images.

These advantages and others have led to many more PEEK-based implants. Marcus Jarman-Smith is a technology leader at Invibio Ltd., which has been making PEEK-Optima^R polymer for over a decade. He says that in addition to spinal implants, the material holds clear benefits for knee-replacement and hip-replacement parts. PEEK parts don’t produce the health concerns associated with the metal against metal wearing of traditional hip replacements. The strength of Invibio’s bearing grade, Motis^R, means it can be used alone to make hip-replacement cups instead of combining a metal cup with a polymer liner. The resulting thinner cup requires the removal of less bone. In addition, polymer parts flex and pass on stress to the bone rather than focusing the stress on the implant. This transfer of stress helps bone maintain strength and means that damage is less likely to occur.

“Because of the polymer’s high strength and bearing properties, it is starting to be looked at more for trauma applications,” says Jarman-Smith. He says that PEEK has a high strength-to-weight ratio and allows more flexing than metal plates and nails used to repair a broken arm or leg. If the patient is a child, or if the patient develops an infection, plates or nails may have to be need removed, which is difficult with metal because it tends to bind to the bone, whereas PEEK doesn’t. The Quantum^TM Humeral Composite Nailing System from N.M.B. Medical Applications Ltd. was the first PEEK intramedullary nail to gain FDA approval (March 2010). The nail is made of Invibio’s Endolign^R, a composite of continuous carbon fibers in a PEEK-Optima polymer matrix.

For future developments and applications, the company is considering options such as combining PEEK with additives that help it bind better with bone or encourage bone growth. “We also want to see if we can use it to make scaffolds or porous PEEK parts that can support tissue and allow tissue to grow inside and regenerate,” says Jarman-Smith.

Although PEEK has many advantages for medical implants, it can be difficult to mold in a clean-room environment because of its high melt temperature. PMC SmartSolutions^TM has been implementing new ways to handle this challenge. The company, founded in 1929, entered the medical molding market four years ago. It specifically focused on long-term surgical implants made of materials such as PEEK because of the potential growth in this market.

“Consistency is very important for implants,” says Lisa G. Jennings, president of PMC SmartSolutions^TM. Using heat-transfer oils for mold-temperature control can potentially cause product contamination. Electric cartridge heaters are subject to temperature problems that can affect product consistency because they cannot control the mold’s surface temperature in a tight window across a part and often have hot and cold spots along their length. Cartridge heaters are also unable to remove heat from the mold if it becomes too hot.

Thus, PMC examined using pressurized water for precise mold-temperature control. Pressurized water can be heated to temperatures as high as 400°F. The company partnered with Single Temperature Controls of Charlotte, North Carolina, USA, which sells temperature-equilibrium systems that pump water through the mold at a set temperature. Heat is transferred to the mold if the water is hotter than the mold and removed from the mold if the opposite is true.

In PMC’s experiments, it found a 44.2°F variance in mold temperature with electric heating and only 5.0°F variance with the water-heated system. The effects from this variance could be seen through a 0.003-inch increase in part shrinkage and average 18.5% reduction in relative crystallinity in the same parts produced using the electric-heated molds.

The pressurized water system has allowed the company to make complex parts of PEEK and other high-melt temperature plastics. For example, it has made insert-molded porous metal parts for orthopedic implants. “We were among the first to use high-pressure water to control mold temperature for making medical-device implants in a clean room,” Jennings says. Since PMC shared its data showing the benefits of using water to control mold temperatures, other companies have followed its lead in using this technology. The complete white paper is available on www.pmcsmartsolutions.com.