Advanced Materials for Orthopedic Implants

Written by William Fuller, Director of Business Development, DSM Biomedical | May 17, 2013 | Print  |
William FullerThis article is written by William Fuller, Director of Business Development, DSM Biomedical.

Innovation and constant advancement dominate the medical device industry due to the changing healthcare landscape and patient demand for high quality outcomes. Many of the advances in the medical device industry will come from revolutionary biomaterials that help enable the function of the devices. The impact of these novel biomaterials will be particularly significant to orthopedic implant devices. Orthopedic implants restore function by replacing or reinforcing damaged tissue, bone, and ligaments. Today's patients expect that orthopedic implants will improve the quality of life by restoring mobility and reducing pain — allowing them to return to active lifestyles. This is the challenge to the medical device manufacturers that want to deliver products that meet these demanding requirements.

Medical device manufacturers must balance between innovative solutions and proven materials when selecting resources for use in orthopedic implants. The device manufacturers seek trusted suppliers that demonstrate stability and commitment to the market, in addition to a portfolio of proven materials with clinical history and a pipeline of innovative next generation solutions. Success of any medical device is dependent upon a complex interplay of materials properties, device design and physiologic requirements. The unique characteristics of advanced biomaterials create solutions for orthopedic implants that help overcome the challenges of orthopedic ailments. Several categories of biomaterials play a prominent role in the advancement of orthopedic implant technologies including ultra-high-molecular-weight-polyethylene (UHMWPE) fibers, stabilization technologies and medical-grade polyurethanes.

UHMWPE Fibers

Ultra-high-molecular-weight polyethylene (UHMWPE) fibers are extremely strong, yet soft as silk, which makes them suitable for many surgical applications to treat conditions resulting from sports injuries, trauma or osteoarthritis. The fibers allow for design freedom and can be processed into many two- and three-dimensional constructions including extremely thin braids, weaves, flat cables, tubes and sheets of any shape and size. Smaller dimensions enable the fibers for use in the development and application of new, minimally invasive surgical techniques that can potentially lead to shorter hospitalization, faster recovery times and a lower total cost of care.

UHMWPE fiber is an ideal material for use with arthroscopic tools because of its softness, low elasticity and high modulus, which prevents unwanted stretching of fiber structures and gives good tactile feedback. For example, during ACL repair, ligaments are tied arthroscopically with UHMWPE fibers.

UHMWPE fiber is stronger than steel on a weight for weight basis. The fiber's strength relative to volume allows for a reduction in size of the device as compared to devices made with other materials. Some meniscus implants contain embedded UHMWPE fibers that help reinforce the device as it mimics natural knee movement. Steel does not conform to the body and might cut thru bone or soft tissue. UHMWPE fibers conform to the shape of the bone, increasing the contact surface and lowering contact pressure.

UHMWPE fibers are a leading material for use in orthopedic sutures used in soft tissue repair because the material provides uncompromising strength and minimizes the impact on surrounding tissue. Tissue inflammation and irritation are also minimized due to the fiber's biocompatibility and chemically inert properties. New grades of UHMWPE fibers have been developed to further increase the applications and potential usage for the material in medical devices. For example, DSM Biomedical offers three grades of its proprietary UHMWPE Dyneema Purity® fiber, including Dyneema Purity® UG and VG fiber grades, which were created specifically for use in orthopedic applications that demand the highest mechanical performance.

UHMWPE Powders & HALS Technology

UHMWPE powder that is compression molded into blocks and then machined into devices is the leading bearing material in total joint replacement devices. UHMWPE has a very low coefficient of friction, excellent mechanical properties, and biocompatibility making the material an obvious choice to withstand the movement and stress exerted by human joints.

Some hip replacement implants are made of metal-on-metal, but use of these types of devices are decreasing at a significant rate due to concerns of and suspected complications of metal-on-metal (MoM) devices. While there are concerns about the biologic consequences of metal ion release from MoM bearings, UHMWPE has proven biocompatibility and a significant clinical history.  

A drawback of UHMWPE material lies in its potential to oxidize from processing steps that crosslink the material. Stabilization technology can prevent or reduce the oxidation of UHMWPEs and thereby protect the polymer and extend its lifetime. The newest and potentially most effective stabilization technology takes advantage of a hindered amine light stabilizer (HALS). HALS, a known stabilizer in polymers for its long-term stabilization against UV radiation, can be easily blended directly into the UHMWPE powder. The UHMWPE is compression molded into bulk forms and then machined into devices. The HALS stabilizer remains in very low parts per million concentrations in the UHMWPE and demonstrates highly effective oxidative stabilization.

DSM's HALS technology offers a significant improvement over other stabilization methods because it does not interfere with the crosslinking of the UHMWPE. This is critical for orthopedics device manufacturers because it allows them to forgo changing their manufacturing processes.

In addition, HALS stabilizer regenerates itself; after scavenging a radical it can turn back into its original form. HALS is therefore not consumed like other stabilizers, such as Vitamin E. A lower concentration can be applied since the technology has a longer stabilization lifetime. HALS technology can be applied to total hip, knee, shoulder and ankle implants, or any other application where stabilization of UHMWPE is needed.

Stabilization presents the next key area for development in UHMWPEs for joint replacement devices. HALS is a leading solution because it stabilizes just as effectively as vitamin E, does not interfere with crosslink efficiency (as does vitamin E), processes into UHMWPE easily, eliminates the need for thermal treatment and saves costs while potentially increasing implant lifespan.  

Polyurethanes

Polyurethanes are an extremely versatile class of implantable materials due to the ability to tune the mechanical properties from flexible to rigid. In addition, certain polyurethane materials have significant clinical history making them one of the most biocompatible materials known today. These biomaterials have played a major role in the development of a wide variety of novel orthopedic implants. Properties including durability, elasticity, fatigue and wear resistance, compliance and acceptance or tolerance in the body during healing are crucial, especially for long-term implantation.

Biostable polyurethanes, such as polycarbonate-urethane (PCU) and silicone-polycarbonate-urethane (TSPCU) possess these essential traits, which are necessary for the development of devices like bearings for total joint replacement and artificial lumbar and cervical discs. These characteristics also apply to extremity applications and a wide variety of motion preservation devices for the spine. These specialized elastomeric materials display the best resistance to degradation, in vivo stress cracking, metal ion oxidation and calcification, and biostability. Medical device companies focused on motion preservation devices for the spine use polyurethanes to develop products with a better range of motion and shock absorption than traditional spinal fusion devices allow.

TSPCU materials are block copolymers systems that provide the best balance of biocompatibility and biostability, with the toughness of thermoplastic polycarbonate-urethanes. The silicone soft segment works synergistically with polycarbonate-based polyurethanes and is well suited to devices challenged by oxidative degradation. When AxioMed® Spine Corporation was developing their next generation total disc replacement devices used to treat patients with degenerative disc disease, they looked to TSPCU. The incorporation of DSM Biomedical’s Carbosil® TSPCU into AxioMed’s Freedom® Lumbar Disc and Freedom® Cervical Disc allowed the spinal implant maker to develop an artificial disc that would replicate the natural function of a human disc, providing a more biomechanically natural device with optimal flexibility and shock absorbing characteristics. The goal of this partnership was to deliver superior materials that would enable AxioMed’s devices to deliver the best possible patient outcomes for a condition that affects millions of people worldwide.

Future designs of artificial joints strive not only to replace, but also to imitate the natural joint and possibly allow for healing and regeneration. The natural joint is a complex system of cartilage, bone, and lubricating fluids that work perfectly when in good condition. The issue is that even without injuries or osteoarthritis natural wear and tear will occur during a person's lifetime. For a material to be selected for use in the joint space it must be matched as closely as possible to the properties of the natural tissue at the surfaces of articulating joints, which are both elastomeric and hydrodynamic. The modulus of elasticity of implantable polyurethanes is similar to that of articular cartilage. It is proposed that the low modulus of the PCU materials can simulate the function of articular cartilage in the natural joint and allow a fluid-film interface to form between the surfaces of the articulating prostheses.

Conclusion

Driving the development of new biomedical materials is a thorough understanding of how advanced materials can be used in the human body to strengthen or replace body parts that may succumb to the rigors of daily life. Partnerships between medical materials developers and medical device manufacturers are essential to the evolution of both orthopedic materials and orthopedic implants. Medical device manufacturers look to materials developers for their capability to deliver customized material solutions that can assist in getting their products to market faster. The broadening range of high-performance materials enables device manufacturers to broaden their horizons and develop increasingly innovative devices — from small yet strong devices, to low-profile devices delivered through less invasive procedures.

In turn, the benefits are not exclusive to patients, but also include clinicians and hospitals. Clinicians benefit from improved ease-of-use and maneuverability, while hospitals benefit from contained costs due to quicker recovery times, shorter hospital stays, and reduced ongoing treatment costs. Quicker recovery times for patients receiving novel orthopedic implants means a faster return to improved quality of life with restored mobility and relief from pain. Consequentially, next-generation orthopedic implants need to further improve upon current device standards in order to sustain continuous enhancement of clinician ease-of-use and patient outcomes. As all areas of society endeavor for advancement into new frontiers, the medical device industry will do the same in order to allow people to lead longer, brighter, healthier and more active lives.

More Articles on Orthopedic Devices:

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20 New Updates for Orthopedic & Spine Device Companies


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