I recently saw an acquaintance who had undergone a hip replacement. All seemed to be going well. His only real concern was the limited range of motion he would have from now on. He explained that he couldn’t bend beyond a certain point, or even stoop to tie his shoes, in fear that the implant might “pop out of socket.” He cited the case of a relative whose hip dislocation had occurred simply from sitting in a chair that was a little shorter than the standard. My friend seemed to take comfort that his surgeon had reassured him that there was only about a 1% chance of this happening.
My jaw about hit the floor.
Can you name another industry that accepts a 1% failure rate? Would you book a flight, climb into a car or ride a rollercoaster if you knew that there was a 1 in 100 chance of failure? Even the world’s biggest risk-takers wouldn’t be comfortable with such odds. Why, then, does the medical industry seem to accept a 1% premature explantation rate?
It’s no secret that the medical industry represents the most litigated cases in today’s court rooms. Consequently, our industry gets hit with some costly verdicts, paying out hundreds of millions of dollars in settlements each year.
How do we fix this issue?
Let’s begin by defining “failure.” Medical implants are deemed to have failed if they fall short in producing satisfactory results in terms of quality and durability. Generally speaking, implant failure can be attributed to one or more of the following areas:
- Surgical error, including technique, judgment and infection
- Hypersensitivity, including rejection of foreign body, pain, sinus reactions
- Product, including design, manufacturing, materials, metallurgy and chemistry
Failure can be attributed to a number of factors, from human error on the part of the surgeon, to post-procedure complications such as infection, to implant device failure that may be traced to design or manufacturing deficiencies. Regardless of the source, when a medical device fails to meet lifecycle standards, the results are product recalls, lost market share, insurance claims and, of course, litigation. All of which are costly propositions.
These failures can be avoided by embracing a comprehensive approach to testing that combines design validation, failure analysis and explant testing. By proactively testing for potential and actual failures, you can gather data to make smarter design and manufacturing decisions moving forward.
What should each of these tests include?
When evaluating new product designs, we typically utilize two different, but related types of tests: static and dynamic. Both are required to validate a new implant design. Static testing is a single cycle process, wherein a steady force is exerted on a given part, assembly or system to determine in a short time when and where breakage will occur. Because this is a relatively quick test, it can be used to indicate pass/fail for a new design and draw good assumptions about designs and material strength.
By proactively testing for potential and actual failures, you can gather data to make smarter design and manufacturing decisions moving forward.
Dynamic testing is a multi-cycle testing process. In this type of test, a series of steady impulses are applied over a longer duration. This allows us to measure wear and durability to understand the expected life of an implant based on actual data rather than guestimates. All of this information—be it from explant, static or dynamic testing—is fed back to the manufacturer, helping to create better and longer-lasting implants.
Most early implants were designed to achieve a life expectancy of at least five million cycles; and laboratory tests were conducted to ensure that products met this requirement. Today, with the help of testing, implant life expectancy has been doubled to ten million cycles. Five million cycles is roughly corresponding to seven to ten years of use in elderly patients. Younger folks with implants (tumor, trauma or congenital reasons) are expecting much more, hence the ten million cycle/20-year target. In addition to extended product life, this also means longer testing cycle times and an opportunity for even more thorough testing and more accurate data to drive the development of next-generation products.
Personally, I’m an advocate of S/N curves: Stress vs. Cycles (N). This allows larger loads to be cyclically applied to the implant, forcing a break. S/N curves (run with doubles—two implants at the same load) give the designer a look into the natural variation of the design and material in elevated stress conditions. S/N curves also provide a glimpse into the potential extension of implant life. With a tweak here and there on the design, the S/N Curve will provide validation for more years of use; as we’re all aware, the first to extend implant life from what it is today will dominate the industry.
Widely used in various phases of the product lifecycle, Failure Modes and Effects Analysis (FMEA) helps identify potential failure based on experience with similar products or based upon common physics of failure logic. Effects analysis relates to the consequences of failures on different system levels. For example, to what degree might friction between the femoral head and acetabular component (resulting from failure in the plastic liner) shorten the life expectancy of an artificial hip?
Failure analysis answers questions that are important to the medical community as a whole:
- Did the implant break, or wear out?
- Why did it break or wear out?
- Where exactly did the device fail?
- When did failure occur?
- What factors caused or contributed to failure?
- How does component failure affect the system as a whole?
In failure analysis, it’s important to keep an open mind. We are, after all, detectives finding and following clues and employing proven methods, best practices and experience to determine exactly what happened and why. In addition to determining responsible parties, this information is fed back to the community of designers, manufacturers, surgeons, professional committees, allowing improvements to be made.
In determining why an implant failed, we must go all the way back to its origin and ask the following questions.
- Was the implant properly designed?
- Were the correct materials used?
- Were there defects in the implant occurring during fabrication?
- Did the surgeon implant the device properly?
- Did the patient correctly follow aftercare instructions?
- Was the patient compliant (weight, health, aftercare followed), or were these
conditions waived because a patient had no other option (obesity, etc.)?
As previously mentioned, implant failure can typically be attributed to one of two areas: product flaws (design, material selection and manufacturing) or surgical/installation errors. As an example, anchoring bone screws can break or become weakened and susceptible to breaking if they are over-torqued during installation. Once the anchoring bone screw is compromised, the device itself will become unstable or experience premature wear or breakage. However, while surgical errors certainly contribute to the problem, it’s fair to say that the majority of implant failures can be traced to the product itself. This is why explant testing is so important.