05.19.06
Implantable Plastics: Engineering for the Long Term
Manufacturers are taking note of the superior performance of implantable plastics
Marcus Jarman-Smith
Invibio Ltd.
To meet the demands of the growing orthopedic market and changing demographics of patients, implant design solutions are becoming increasingly sophisticated. Today, device manufacturers are leveraging implantable plastics that possess high performance and customized properties, as they allow for greater design freedom and ultimately lead to improved applications. The most widely used long-term implantable plastics include polyethylenes, polyetheretherketones (PEEK) and bioabsorbables—polylactic acid (PLA), polyglycolic acid (PGA) and their copolymers.
The increasing use of high performance plastics, composite materials and compounds can be seen in the development of a wide range of orthopedic applications, including spinal fusion cages and plates, artificial discs, acetabular cups, femoral stems and arthroscopic bone anchors and interference screws.
Any material considered for in vivo implantation must meet stringent requirements. According to the FDA, an implant is any device that is inserted into the body with the intention of remaining there for more than 30 days. The device must not elicit an adverse host response and should demonstrate stability in body fluids.
In addition to meeting these general implant requirements, orthopedic applications in particular must meet demanding structural and performance requirements. For this reason, only a limited number of suitable biomaterials are available for the development of implantable orthopedic devices, and the emergence of new biomaterials is rare.
Benefits of Implantable Plastics
Because of their biocompatibility and high performance, implantable-grade plastics have emerged as a leading biomaterial in the development of orthopedic applications. These biomaterials are attractive for both their mechanical properties and their associated processing technologies, which enable device manufacturers to tailor their characteristics to meet certain needs.
The ability to tailor the characteristics of certain implantable-grade plastics means that device designers can consider factors other than the structural substitution of the natural tissue. Physical characteristics such as the elastic modulus can be modified to recreate bone modulus. Mechanical properties such as the strength, wear resistance and impact performance of polymers and composites can be comparable to metals and offer additional benefits.
Since implantable-grade plastics are not metallic, they do not release metal ions into the body, which can trigger allergic reactions in certain patients. In addition to reducing or eliminating allergic reactions, these materials also eliminate artifacts during post operative examination by traditional techniques such as X-ray, CT and MRI technology. Non-metallic materials also resist corrosion, leading to a longer implant life span.
Polymers are also less dense than metals, have lower thermal conductivity and, in areas close to the translucent skin surface, provide better color aesthetics. In addition, polymers present the ability to be surface modified with such coatings as hydroxyapatite or titanium, to aid secondary fixation or with chemical species as with bone morphogenic proteins (BMPs).
Processing for some plastics can be easily scaled up to meet the increasing demand for product parts. Incorporating plastic technologies (for example, injection molding) means that the economics of production are viable on a larger scale, while complex shapes can be formed as required to aid device fabrication. Diverse processing technologies also present potential options for modifying the material’s form (for example, fibrous, porous or solid) and enhancing osteoconductive capabilities.
What’s Being Used Today
Biomaterials used in the development of orthopedic applications must be exceptionally strong and stiff and possess good wear characteristics. For example, while individuals participate in sporting activities, the loads imposed on their joints can be as much as 10 times their body weight, and it is estimated that a hip may be exposed to cyclic loading up to one million cycles a year.
For structural components, the stringent performance criterion rapidly excludes many implantable materials used at other body sites. Consequently, only the few plastics that are currently being used for the development of orthopedic applications, or are close to commercialization, are discussed as potential biomaterial solutions.
UHMWPE. Currently, the most common polymer used in the development of implantable orthopedic applications is ultra-high molecular weight polyethylene (UHMWPE). UHMWPE is biocompatible, has excellent impact strength, a low coefficient of friction and good fatigue resistance. For these reasons, it has been used for more than 30 years to manufacture orthopedic applications used in the hip, knee and other joints. It is also used to develop spinal disc implants. Though the material has proven successful in these areas, it is not used in the development of load-bearing applications, such as spinal cages, due to its relatively poor mechanical strength compared to other polymer alternatives.
Implantable-grade PEEK carbon fiber composite in combination with a metal insert has been used to create a unique elastically tailored prosthetic hip capable of withstanding high loads for many millions of cycles. Photo courtesy of Invibio. |
Furthermore, while UHMWPE parts can be machined from sheet or rod stock, the material cannot be injection molded.
Implantable-grade PEEK. A relatively new polymer, implantable-grade PEEK was introduced to the orthopedic device market in 1999. Since then orthopedic device manufacturers have become increasingly interested in the material because of its mechanical properties, which include good wear and impact resistance, long-term biocompatibility and biostability, inherent lubricity and high compression strength.
These mechanical properties can also be tailored to meet specific requirements. For example, the material’s strength and stiffness can be increased by adding carbon-fibers to the polymer matrix. This enables orthopedic device manufacturers to develop applications that satisfy high-strength requirements. The material’s imaging properties can also be tailored by adding variable concentrations of barium sulphate. This enables device manufacturers to develop applications than can be easily inspected, post operatively, via traditional imaging techniques without the generation of scatter or other imaging artifacts.
In addition to its mechanical and adaptable properties, implantable-grade PEEK can be processed via several methods, including injection molding, extrusion, compression molding, machining from plates/rods and powder coating.
Bioabsorbables. Also known as biodegradable polymers, specifically those belonging to the family of polylactic acid (PLA) and polyglycolic acid (PGA), bioabsorbables are playing an increasingly important role in orthopedic device development. Bio-absorbables are engineered to dissolve inside the body via hydrolysis and enzymatic activity and have a range of mechanical and physical properties, which can be engineered appropriately to suit particular applications. Their degradation characteristics depend on several parameters, including their molecular structure, crystallinity and copolymer ratio.
By dissolving in the body, bioabsorbables applications gradually transfer the load to the bone or soft tissue as it heals, eliminating a second surgery that may otherwise be needed to remove metallic implants. PLA, PGA and their compounds are the most common commercialized bioabsorbable polymers. The rate of degradation of these implants can be altered based on the material composition and offer the potential to match implant dissolution with healing rate. These polymers are used in orthopedics primarily for sutures, bone pins and screws. However, they do not have the mechanical properties required to be used in the development of load-bearing applications.
Drawbacks to Traditionally Used Materials
While titanium is still the most common material for the development of load bearing spine applications, the largest orthopedic growth area, spinal implant manufacturers are beginning to embrace implantable plastics as an alternative because of their superior properties.
For example, though metals and ceramics can satisfy high-strength requirements, their moduli are inherently high, which means they provide the structural strength but at the expense of shielding the adjacent bone from applied loads. Since bone naturally requires some exposure to stress in order to provide stimulation for natural bone synthesis, stress shielding is proposed to result in the weakening and fracture of the adjacent bone.
While some medical device manufacturers are substituting implantable plastics for traditionally used materials, others are using them in conjunction with metallic and ceramic materials.
The desire to preserve spinal motion and to eliminate transferred stress resulting from spinal fusion has fuelled extensive research in dynamic disc replacements. In 2005, one medical device manufacturer received FDA approval for the first artificial lumbar disc replacement. Another manufacturer developed the first cervical disc replacement.
Both of these applications are three-component implants consisting of two metallic endplates that affix to the vertebral body and a soft polymer core, which enables motion and reduces friction between the metal parts. The lumbar implant consists of two cobalt-chromium alloy endplates with an UHMWPE core. The cervical implant also uses an UHMWPE core, but it is used in combination with titanium endplates. UHMWPE is a high-quality material for these applications due to its comparative strength and toughness.
Implantable-grade PEEK polymer is also being evaluated for use in the development of dynamic disc replacements because of its good fatigue strength and wear resistance.
Joint Prostheses and Wear Debris
One of the main concerns in the development of joint prostheses is the creation of and biological reaction to wear debris. In metallic wear couples, metal ions released into the body can have localized effects such as osteolysis adjacent to the implant, which subsequently leads to loosening of the fixation. In addition, the potential effects of metal release may also be systemic if the ions travel by blood to the lymphatic tissue. An adverse reaction can result in failure of the device, complications and costly revision surgery.
The brittleness of ceramics has been improved through design and improved manufacturing techniques, but malpositioning of joints can still cause wear and fracture issues. Ceramics may also be prone to stress corrosion in humid environments. As a result, medical device manufactures have turned to implantable plastics to mitigate the potential for wear-related complications.
UHMWPE-on-metal or UHMWPE-on-ceramic couples have been frequently used for many hip joint replacement designs, as they demonstrate good wear and cost performance. On average, these implants typically wear about
0.1 mm per year (10 times faster than metal-on-metal) and have a replacement cycle of 10-20 years. The durability of these implants is decreased in younger, more active patients due to increased wear of the plastic component. However, the small particles of UHMWPE released into the body during wear can lead to osteolysis, and loosening of the implant and is a concern for medical device manufacturers.
The growing number of younger and more active hip joint replacement patients has commanded an increase in implant life span, which has led to the evaluation of new materials.
The latest generation of UHMWPE acetabular liners consist of polyetheylene crosslinked with gamma or electron beam radiation to improve wear performance. While results are encouraging, there is a lack of long-term implant history as well as concerns over residual free radicals, which may cause oxidative degradation and subsequent embrittlement of the material.
In the past, UHMWPE has been reinforced with carbon fibers and applied in acetabular and tibial components. However UHMWPE’s poor creep resistance resulted in time-induced extension when exposed to sustained stress and critically caused separation of the bonds between the UHMWPE matrix and the carbon fibers, leading to implant failure.
Implantable-grade PEEK polymer can be more successfully reinforced with carbon fibers, compared to UHMWPE, due to the significantly higher bond strength between carbon and the PEEK polymer. Implantable-grade PEEK polymer has a fiber/matrix bond at least 10-fold stronger than carbon fiber-reinforced UHMWPE, essentially eliminating fiber release over time. Additionally, the creep resistance of PEEK polymer ensures the integrity of the fiber and the polymer matrix interface.
Metals, usually cobalt-chromium alloys, are the materials of choice for resurfacing of the femoral and tibial articulating surfaces in knee joint replacement. A plastic insert is used to eliminate friction between the metal parts. UHMWPE is the most commonly used plastic for this insert due to its toughness and good wear properties. However, since concerns about tissue reaction to UHMWPE wear particles remain, other plastics, including implantable-grade PEEK polymer, are being considered for the development of knee applications.
Silicone polymers (eg, polydimethylsiloxane) are typically used to create hinged finger joints with 30-40 degrees of motion. Solid silicone is well tolerated, but other forms such as gels or injectables (now banned by the FDA) can unpredictably leak silicone oil and result in a chronic inflammatory response. Silicone finger implants can also be prone to implant fracture and implant dislocation. New versions of finger joints have capitalized on using implantable-grade PEEK polymer due to its radiolucency and ability to offer more natural flexing and rotation.
Arthroscopy Applications
Arthroscopic procedures, such as anterior cruciate ligament fixation and soft tissue attachment, require small, high-strength screws, sutures and anchors. Bioabsorbables and implantable-grade PEEK polymer are the primary materials used in the development of arthroscopy applications. One reason these polymers are attractive to arthroscopy device manufacturers is their ability to be injection molded. This enables the mass production of small, high-strength components with complex geometry. Bioabsorbables, such as PLA, PGA and their compounds are well-established in arthroscopy and offer the benefits of impermanence and imaging compatibility without creating artifacts or scatter to obscure the healing site.
They also eliminate the potential need for removal of a metal product. While the rate of polymer absorption can be altered, actual in vivo degradation rates cannot be strictly controlled and may vary depending on application and location in the body.
The Future of Implantable Plastics
The diversity of the human body and the natural heterogeneity of bone invariably mean that there will likely never be a single biomaterial to fit all the physiological and biological needs of different implant applications. However, because of these stringent criteria, only a few suitable materials currently exist, and the development of new materials is an evolutionary process.
Implantable plastics have been rapidly embraced by medical device manufacturers because of their superior performance and properties, which enable device manufacturers to develop improved applications that mitigate complications associated with wear and stress shielding.
Surgeons will have new polymers designed to meet specific biological needs at their disposal and will no longer need to rely on less suitable materials. Additionally, future implantable polymeric materials will have the ability to restore structure, function, biomechanical performance and biochemical behavior.
These advances, along with the ability to develop patient-specific, custom implants designed and manufactured specifically for individuals, will have a significant impact on the future of the implantable polymer market as well as on patient care and recovery.