Michael Barbella, Managing Editor12.17.21
His name may not carry the notoriety of Robert Jones, Hugh Owen Thomas, or even Sir John Charnley, but Duncan Dowson is nevertheless an orthopedic icon in his own right.
The former University of Leeds professor and decorated scientist who died last year at 90 is regarded as the father of biotribology and a pioneer in formulating elastohydrodyamic theory, a foundational concept used to describe the lubrication of gears, cams and bearings. That theory forms the basis for many of the analytical tools and methods Dowson created during his 70-year career, including a film thickness formula for hip joint prostheses that emanated from his 1960s-era research on total hip arthroplasty.
Industry pundits consider Dowson’s contributions to biotribology and elastohydrodynamic theory to be essential to the evolution of joint simulation and wear.
“Innovative numerial solutions for elastohydrodynamic lubrication problems pioneered by Dowson provided further insights into the mechanism of ankle, knee, and hip synovial joint lubrication,” a June 2020 editorial in lubricants stated. “Dowson investigated total joint replacement with an UHMWPE acetabular component, cushion bearing behaviour for knee and hip arthroplasty, and lubrication of total hip replacement joints created with materials of high elastic modulus...For more than 60 years, Duncan Dowson sustained invaluable contributions towards the advancement of total joint replacement prostheses.”
One of the most invaluable—and lasting—contributions arising from Dowson’s biotribology work was the development of knee simulators, which are used to perform wear testing of knee implants. Dowson detailed such a device for the first time in his 1977 book, “Evaluation of Artificial Joints.”
Simulators for knees, hips, and other joints reproduce both the active and natural motion of their respective parts to assess the kinematics and kinetics of total joint arthroplasty. The testing simulated by these devices allow researchers, product engineers, and manufacturers to evaluate the wear performance of their implants and bearing materials under physiological conditions. Such testing helps improve designs and leads to safer joint replacements.
ODT’s feature “Testing Complete” details the trends and market forces driving orthopedic testing and analysis. Mikaelle Giffin and Christopher Pohl, associate toxicologists at Nelson Labs, a Sotera Health company, were among the experts interviewed for the feature; their full input is provided in the following Q&A:
Barbella: Please discuss the latest trends in testing methodologies for orthopedic products.
Mikaelle Giffin: With the newly updated ISO 10993-18, we have seen a trend in notified bodies, where it is now the expectation to identify and assess any compounds that have been measured above the AET in the extractables/leachables testing. Furthermore, because lab and instrument variation produce known uncertainty factors that directly impact the derivation of an appropriate AET, notified bodies have been expecting lab-specific, verified uncertainty factors for each type of analytical chemistry process (GC/MS, LC/MS, etc.) in order to ensure the correct AET is set for the device and its expected patient exposure.
Christopher Pohl: The biggest questions from regulatory bodies is whether or not the chemistry study was conducted in such a way to adequately assess for compounds that have a variable response factor in the instruments. The Analytical Evaluation Threshold (AET) must be low enough to address this issue, and it is starting to hit the limit of detection at this point.
Barbella: What are the most pressing challenges facing orthopedic device testers, and what kinds of solutions are available to them?
Giffin: As previously discussed, notified bodies are expecting that an appropriate AET is established before testing takes place. This provides a challenge when testing these devices as these expected AET concentrations can often fall below the instrument’s sensitivity levels. In these cases more devices have to be pooled and extracted appropriately in order to meet each instrumentation's specific levels of detection/quantification. But on the flip side, if multiple devices can be used at a time, then that AET must be further adjusted to reflect the actual expected exposure per single device. This is an exciting, multi-level challenge in which solutions are still be investigated as more information becomes available. The current best solution for manufacturers and testers of these devices is to thoroughly identify the worst-case scenario of patient exposure when using these devices. Then, using that scenario, work together to ensure the correct AET is set before any testing is completed so results can accurately show the expected patient exposure when using the device.
Pohl: To go along with the concern listed above, the AET needs to be so low, that it is at the edge of what current science can do. This is a problem that is exacerbated when multiple devices can be used simultaneously. For most devices, the AET needs to be at or lower than the limit of detection of the instrument.
Barbella: How has patient-specific and customized implants impacted the testing methods for orthopedic devices?
Giffin: I personally have heard multiple presentations discussing the path to individualized medicine. While this is exciting, it does create some challenges when trying to design the appropriate extractables and leachables testing. When performing an extractables study we want to use the worst-case scenario, which often means that we want to extract the largest device and greatest number of those devices that could be exposed to a patient. With having multiple, customizable sizes, identifying a worst-case scenario may become more challenging and manufacturers may have to test more devices or prepare larger "monster" devices that would never be used in a patient in order to create that worst-case scenario. This creates some further challenges though; since it is understood this device would never be used in an actual patient, theoretical calculations are required to try and scale the amount of extractables measured from the device to a more appropriate patient exposure.
Pohl: This is an interesting one. Typically, when testing, a “worst-case” sample is needed when testing for biocompatibility, and this is usually the largest device that is available. However, if it is customizable, what is the “worst-case” or largest device? Testing can always be done on a device that is larger than would ever go into the human body, but then there is a higher chance of failing due to a compound or an unknown in the chemistry report.
Barbella: How do you expect orthopedic device testing to evolve over the next 5-10 years?
Giffin: Currently, the expected extractables testing that is performed on these type of devices is performed under exaggerated conditions (50 degrees celsius for 72 hours) and then after those 72 hours the extract is measured and assessed. Using a specific example of pure metal alloy implants, it is possible that potential residuals remain on these devices, which are expected to be eliminated by the patient within 30 days. However, when analyzed some of these residuals may be measured at or above the established limits of exposure. This then requires more testing and understanding as the limits for many elements and compounds are well-established and accepted by many notified bodies. A more appropriate way of evaluating some of the devices may be to perform simulated use extractions, where the extractions are done at lower temperatures and sampled over consecutive days. This type of testing may be able to provide a more relevant data and provide better kinetic information as to how these residuals could reduce over time. In the future I can see more instances of simulated use testing being performed in order to better understand the actual patient exposure.
Pohl: I would expect to see more additives that are embedded into the devices, especially if it is 3D printed. For example, we are seeing more cases of Vitamin E impregnated devices. I expect to see others as well.
The former University of Leeds professor and decorated scientist who died last year at 90 is regarded as the father of biotribology and a pioneer in formulating elastohydrodyamic theory, a foundational concept used to describe the lubrication of gears, cams and bearings. That theory forms the basis for many of the analytical tools and methods Dowson created during his 70-year career, including a film thickness formula for hip joint prostheses that emanated from his 1960s-era research on total hip arthroplasty.
Industry pundits consider Dowson’s contributions to biotribology and elastohydrodynamic theory to be essential to the evolution of joint simulation and wear.
“Innovative numerial solutions for elastohydrodynamic lubrication problems pioneered by Dowson provided further insights into the mechanism of ankle, knee, and hip synovial joint lubrication,” a June 2020 editorial in lubricants stated. “Dowson investigated total joint replacement with an UHMWPE acetabular component, cushion bearing behaviour for knee and hip arthroplasty, and lubrication of total hip replacement joints created with materials of high elastic modulus...For more than 60 years, Duncan Dowson sustained invaluable contributions towards the advancement of total joint replacement prostheses.”
One of the most invaluable—and lasting—contributions arising from Dowson’s biotribology work was the development of knee simulators, which are used to perform wear testing of knee implants. Dowson detailed such a device for the first time in his 1977 book, “Evaluation of Artificial Joints.”
Simulators for knees, hips, and other joints reproduce both the active and natural motion of their respective parts to assess the kinematics and kinetics of total joint arthroplasty. The testing simulated by these devices allow researchers, product engineers, and manufacturers to evaluate the wear performance of their implants and bearing materials under physiological conditions. Such testing helps improve designs and leads to safer joint replacements.
ODT’s feature “Testing Complete” details the trends and market forces driving orthopedic testing and analysis. Mikaelle Giffin and Christopher Pohl, associate toxicologists at Nelson Labs, a Sotera Health company, were among the experts interviewed for the feature; their full input is provided in the following Q&A:
Barbella: Please discuss the latest trends in testing methodologies for orthopedic products.
Mikaelle Giffin: With the newly updated ISO 10993-18, we have seen a trend in notified bodies, where it is now the expectation to identify and assess any compounds that have been measured above the AET in the extractables/leachables testing. Furthermore, because lab and instrument variation produce known uncertainty factors that directly impact the derivation of an appropriate AET, notified bodies have been expecting lab-specific, verified uncertainty factors for each type of analytical chemistry process (GC/MS, LC/MS, etc.) in order to ensure the correct AET is set for the device and its expected patient exposure.
Christopher Pohl: The biggest questions from regulatory bodies is whether or not the chemistry study was conducted in such a way to adequately assess for compounds that have a variable response factor in the instruments. The Analytical Evaluation Threshold (AET) must be low enough to address this issue, and it is starting to hit the limit of detection at this point.
Barbella: What are the most pressing challenges facing orthopedic device testers, and what kinds of solutions are available to them?
Giffin: As previously discussed, notified bodies are expecting that an appropriate AET is established before testing takes place. This provides a challenge when testing these devices as these expected AET concentrations can often fall below the instrument’s sensitivity levels. In these cases more devices have to be pooled and extracted appropriately in order to meet each instrumentation's specific levels of detection/quantification. But on the flip side, if multiple devices can be used at a time, then that AET must be further adjusted to reflect the actual expected exposure per single device. This is an exciting, multi-level challenge in which solutions are still be investigated as more information becomes available. The current best solution for manufacturers and testers of these devices is to thoroughly identify the worst-case scenario of patient exposure when using these devices. Then, using that scenario, work together to ensure the correct AET is set before any testing is completed so results can accurately show the expected patient exposure when using the device.
Pohl: To go along with the concern listed above, the AET needs to be so low, that it is at the edge of what current science can do. This is a problem that is exacerbated when multiple devices can be used simultaneously. For most devices, the AET needs to be at or lower than the limit of detection of the instrument.
Barbella: How has patient-specific and customized implants impacted the testing methods for orthopedic devices?
Giffin: I personally have heard multiple presentations discussing the path to individualized medicine. While this is exciting, it does create some challenges when trying to design the appropriate extractables and leachables testing. When performing an extractables study we want to use the worst-case scenario, which often means that we want to extract the largest device and greatest number of those devices that could be exposed to a patient. With having multiple, customizable sizes, identifying a worst-case scenario may become more challenging and manufacturers may have to test more devices or prepare larger "monster" devices that would never be used in a patient in order to create that worst-case scenario. This creates some further challenges though; since it is understood this device would never be used in an actual patient, theoretical calculations are required to try and scale the amount of extractables measured from the device to a more appropriate patient exposure.
Pohl: This is an interesting one. Typically, when testing, a “worst-case” sample is needed when testing for biocompatibility, and this is usually the largest device that is available. However, if it is customizable, what is the “worst-case” or largest device? Testing can always be done on a device that is larger than would ever go into the human body, but then there is a higher chance of failing due to a compound or an unknown in the chemistry report.
Barbella: How do you expect orthopedic device testing to evolve over the next 5-10 years?
Giffin: Currently, the expected extractables testing that is performed on these type of devices is performed under exaggerated conditions (50 degrees celsius for 72 hours) and then after those 72 hours the extract is measured and assessed. Using a specific example of pure metal alloy implants, it is possible that potential residuals remain on these devices, which are expected to be eliminated by the patient within 30 days. However, when analyzed some of these residuals may be measured at or above the established limits of exposure. This then requires more testing and understanding as the limits for many elements and compounds are well-established and accepted by many notified bodies. A more appropriate way of evaluating some of the devices may be to perform simulated use extractions, where the extractions are done at lower temperatures and sampled over consecutive days. This type of testing may be able to provide a more relevant data and provide better kinetic information as to how these residuals could reduce over time. In the future I can see more instances of simulated use testing being performed in order to better understand the actual patient exposure.
Pohl: I would expect to see more additives that are embedded into the devices, especially if it is 3D printed. For example, we are seeing more cases of Vitamin E impregnated devices. I expect to see others as well.