The past decade has seen a dramatic increase in the need for biomedical devices and materials offering life-long performance, particularly in the orthopaedics profession. It is only natural that this technological evolution and concurrent influx of innovative devices has created an increased demand for testing applications and solutions.

With demanding biomedical testing applications, ranging from low-force testing on native tissues to complex multiaxial simulation of spinal segments, testing systems must be tailored to meet specific needs. Biopuls, a range of application-centric solutions offered by Instron Corp. (Canton, MA) allows function and capability to be proven in vitro in a controlled and repeatable manner without putting patients at risk.

The hip wear test station generates loads and motions at the interface of the femoral head and the acetabular cup. This simulates physiological load and motion combinations to ensure accurate wear assessment of the hip implant.

Photo: Instron Corp.

Joint efforts

For decades, orthopaedic implants have been successfully used to restore normal function to arthritic joints and employed in post-

trauma stabilization and repair. As a result, a proliferation of new materials, designs and applications has emerged, and dozens of joint-replacement products are available for the hip, spine and knee. Articulating joints are among the most complex mechanical systems in the body, requiring that implant designers possess in-depth knowledge of joint kinematics and loads to accurately evaluate life-long functionality.

From basic static testing of raw materials, to impact loading of joint components, to evaluating fatigue and wear properties in vivo, the most effective testing solutions offer the ability to replicate joint loading conditions in a simulated physiological environment. This helps to provide consistent accuracy during the crucial phases of implant development.

The ASTM F 1714 is a standard guide for gravimetric wear assessment of prosthetic hip designs. This evaluates the bearing surfaces made of ceramic and polymeric materials used in hip joint replacement prostheses. Photo: Instron Corp.

Mechanically inclined

The interdisciplinary science of biomechanics uses the principles of mechanics to help understand issues relating to structure and function and to develop an understanding of the human body in motion.

Advanced biomechanics research has helped to enhance athletic activity by providing methods for preventing injury and improving safety. For example, shoe manufacturers use "energy return" materials in the soles of athletic footwear to reduce stress and decrease impact energies typically absorbed at the ankle, knee and hip joints. Additionally, mechanical testing assists with identifying impact associated with activities for which improved protective gear such as helmets and athletic braces help to minimize the risk of injury.

In a clinical setting, biomechanics testing also has helped to advance techniques in rehabilitation and physical therapy, as well as in the design and manufacture of prostheses and mobility aids. For example, prosthetic legs are tested to simulate impact loads at the heel-strike and toe-off phases of the gait cycle. Solutions such as the limb-testing fixture can be mounted into a fatigue testing system to determine prosthesis strength and fatigue durability from a single test force.

Material evidence

Over time, human tissue undergoes potentially painful and even immobilizing alterations affected by injury, the aging process and disease. Today, researchers and engineers are working to develop replacement tissues, advance surgical techniques and provide improved diagnoses of changes in tissue physiology and mechanics. The ability to accurately evaluate both natural and bioengineered materials in vitro is critical for the successful redevelopment of tissues.

For example, resorbable materials must combine strength with timely resorption, or breakdown of bone tissue through osteoclastic activity, and support tissue regeneration to be used effectively as replacement tissue. An extensive understanding of how these materials behave under a variety of mechanical and environmental stresses and throughout their lifecycle, including manufacture, sterilization, storage and in-vivo loading, is essential. Furthermore, testing is necessary to identify which materials, including metallic, ceramic and polymeric, are better able to withstand normal wear conditions and high-loading profiles.

Additionally, in preparing for an operation, surgeons employ advanced techniques such as surgical simulation and modeling. These computer-generated models require realistic boundary conditions and accurate stress and strain parameter values obtained through the mechanical evaluation of natural tissues.

Tissue samples, such as soft biomaterials, native tissues and tissue-engineered scaffolds are tested using a simple gripping method based on sutures and pulleys. This gripping technique distributes the load forces equally around the specimen for simultaneous testing along the X and Y axes. Photo: Instron Corp.

Meeting new standards

Regulatory agencies worldwide have set stringent performance standards for nearly every category of medical devices. To bring a new device to market requires in-depth knowledge of all of its characteristics, from raw material properties to long-term operation, under complex service conditions. The most comprehensive solutions seek to bring medical device manufacturers in compliance with regulations such as the Food and Drug Administration's (FDA) 21 CFR, Part 11 that deliver quality assurance to cutting-edge research and production.

For gloves, bandages and other standard items, as well as specialized products, including laparoscopy instruments and orthopaedic fixation devices, testing is required to meet the wide variety of standards requiring full-device characterization, including tension, compression, impact, fatigue and hardness properties. Sutures, bandages and other single-use products should feature materials that maximize the trade-off between performance and cost. For implantable devices such as stents, testing protocols help to ensure that the materials and designs meet FDA compliance and will stand up to prolonged use in vivo.

The prosthetic limb fixture tests a prosthesis. The diagram shows how the fixture applies axial-torsional motions to a prosthetic limb. Applying loads at the heel-strike and toe-off phases of the gait cycle ensure that a prosthetic leg mimics the functions of the natural leg. Source: Instron Corp

Getting no wear

Understanding the mechanisms of everyday processes such as chewing and trauma caused by dental hygiene can help dentists and oral surgeons find the best methods for maintaining healthy teeth. Simulation of the mastication cycle, including opening, crushing and gliding, tests the wear and durability of dental materials, including sealants and amalgams. For example, crowns and other dental restorations must withstand these everyday events, making the evaluation of these mechanical behaviors critical to performance. The examination of these behaviors helps to identify the functions these restorative materials are designed to perform, and ensures they provide years of pain-

free functionality.

Testing into the future

Instron's Biomedical Applications Team has developed solutions targeted to ensure that clinical performance meets or exceeds these and other challenging specifications. These turnkey solutions are central to a wide range of material testing protocols-everything from highly sensitive, low-force testing systems for analyzing soft skin tissues to systems that test the durability of complex materials used in artificial heart valves. The trends show that ideal partners for biomedical testing are those that remain firmly committed to working within the biomedical community to develop these and other advanced solutions for meeting testing requirements. Q

TECH tIPS

• The increased need for biomedical devices and materials has created an increased demand for testing applications and solutions.

• For demanding biomedical testing applications, testing systems must be tailored to meet specific needs.

• The interdisciplinary science of biomechanics uses the principles of mechanics to help understand issues relating to structure and function and to develop an understanding of the human body in motion.