Expanding the use of simulation could enable regulatory authorities to allow greater customization of medical implants. Simulation is a reliable alternative to testing, yet offers comparable safety assurances.
Technology has advanced so far in the last few years that medical-device manufacturers have the ability to customize implants to the specific weight, shape, and dynamics of each individual patient’s body. What they lack is the permission to customize implants for individual patients.
The permission they need comes, of course, from regulatory agencies--the U.S. FDA, the European Union’s “notified bodies”--who decide what implants surgeons can insert into the human body. Traditionally, these agencies have evaluated one definitive version of a medical implant. When they approve it for use in the human body, they approve it in that configuration.
In the abstract, that sounds like an illogical approach considering the almost infinite variations from one person’s body to the next. In reality, approving one version of a medical implant has been an essential part of protecting the public as well as medical-device manufacturers.
Regulatory agencies couldn’t possibly test every conceivable variant of an implant. The effects on price and patient care would be prohibitive. Producing a physical prototype of each variant would exponentially increase the price of medical implants. Then the agencies would need time to test the variants, which would delay patients’ treatment. Without sufficient testing, however, the agencies could not guarantee that every variation on a design was safe.
However, there is a trend afoot in medical-product design that could lead to regulatory agencies allowing greater customization without dialing back safety standards. Over the last few years, the FDA has been integrating simulation into its review and approval processes. The agency is issuing standards governing how simulations must be submitted and for what purposes they will be accepted.
Expanding the use of simulation could enable regulatory authorities to allow greater customization of medical implants. Simulation’s usefulness in medical design has already been proven by decades of experience, as seen in Figure 1 at left, which shows a simulation of an implanted replacement spinal disc.
Examining its current role in implant design reveals that simulation results correlate closely–in many cases precisely–to physical testing results. The Shiley Center for Orthopedic Research and Education at Scripps Health (SCORE) found that simulation results correlated to physical testing when it was working on knee joint replacements.
SCORE conducted an experiment to calculate wear performance--or coefficient--for two varieties of cross-linked polyethylene that might be used to reduce wear in knee joints. The objective was to determine if the wear values calculated through computer simulation could predict material wear in various designs under multiple conditions.
An analysis team used Finite Element Analysis (FEA) to virtually simulate 5 million cycles of typical knee-joint actions and their associated biomechanical forces. These simulations were used to determine wear patterns and approximate the life of a cobalt-chromium alloy knee-replacement-joint design. The team found that the computational analyses, when compared with physical tests, correctly predicted the location, shape, and depth of the wear pattern.
Further confirming the simulation was a predicted rate of wear that fell within one standard deviation of the experimental results. Figure 2 shows the similarity between physical testing results (left) of the padding in the knee joint with the simulated results.
Simulation that delivers results this accurate can also predict the behavior of implants that are modified during production to fit into a patient’s body seamlessly and perform trouble-free for longer duration. Simulation is a reliable alternative to testing, yet offers comparable safety assurances.
Currently, regulatory agencies have a tendency to view simulation data as supplementary information to a physically performed laboratory test. The FDA, for example, might use simulation to recreate a process to test a vendor’s conclusion. If, however, regulators have the numerically modeled representation of the product, they can rerun simulations themselves, not only to verify results and validate the underlying simulation methodology, but also to explore the design space by varying loading scenarios, material properties, localized geometries, or a host of other parameters inherent in the numerical model.
This level of flexibility provides regulators the tools they need to understand design sensitivities and set acceptable ranges of variation from the approved design. Within those ranges, physicians and technicians can customize implants to match each patient’s anatomy, stride, weight–anything that affects how long they last and contribute to the patient’s health.