Thanks in large part to data collected from explant testing, medical implant quality and reliability have come a long way. To truly appreciate where we are it’s important to understand where we’ve been. So let’s step back and look at just how far we’ve come.
Early Implants
Back pain is as old as mankind itself with evidence of spinal implants being found in Egyptian mummies. In the early 1900s spinal implants were a popular way of treating scoliosis. It wasn’t until the last part of the century however that spinal stabilization was a viable option for many less severe back pain sufferers. As discussed in previous posts, spinal rods have had a long history of issues (Broken screws, bending, corrosion, and sharp edges that lacerate surrounding tissue). As early as the 1940s, hip implants were being performed with ivory being the material of choice at that time. However, the industry soon learned that this material lacked durability and was prone to causing infections. The 50s saw the advent of knee replacements with wear, loosening, instability, stiffness, and infections were being common.
Early on the debate centered on a simple question: How long should an implant be expected to last? Five years? Ten Years? Today, with the help of testing, manufacturers are targeting a twenty year life for most implants.
Implant Failures – Explant Analysis
Implant failures are typically attributed to one of two areas: product flaws (including design, material selection, and manufacturing) or surgical errors (improperly installed or over torqueing of implant screws for example). It’s fair to say that most failures could be traced to the product itself.
Since little concrete data was available, design validation was not as thorough as it might have been. It was largely a trial and error process. As things progressed testing became more advanced. We began conducting failure analysis on failed parts to determine the exact cause(s) of breakage or excessive wear. The procedure known as “Explant Testing” provided analysis results used to improve designs, make better material decisions, and employ more effective manufacturing processes. The results were significantly more reliable implants. At the same time advancements in surgical procedures and instruments used to install the implants also had a positive impact on improving the overall success rate of implants.
Cycle Testing
Most early implants were designed to achieve a life expectancy of at least 5 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 10 million cycles. Five million cycles is roughly corresponding to 7-10 years of use in elderly patients. Younger folks with implants (tumor, trauma or congenital reasons) are expecting much more, hence the 10 million cycle 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. The procedure accelerates testing while providing reliable results.
Sharing the Results
Early on the industry held testing results close to the vest; guarding findings closely to provide a competitive advantage. Today, however, ASTM and ISO promote the sharing of research and test findings allowing best practices to be widely understood and followed. This is why these meetings are so important and we encourage folks to participate.
About John McCloy: With more than 30 years of experience, John McCloy is an expert in medical implant failure analysis. As a medical explant expert John provides the data to help manufacturers drive quality and reliability by determining causes of implant malfunction. Mr. McCloy is Founder & President of Engineered Assurance (www.engineeredassurance.com).
