When medical implants are manufactured, one of the final steps is to mark each with a unique identifying serial numbers. These markings are vital to tracing the product throughout its lifecycle; and are especially critical in the event of a product recall or litigation stemming from failure.

Should the implant fail to meet performance or durability standards, others with similar serial numbers can be quickly and accurately traced. The manufacturer will know the exact place, time, and material batch, allowing them a chance to investigate materials and processes tied to the making of the product. On the other end, surgeons and hospitals will know exactly when and to whom the device was implanted.

Generally, these serial numbers are etched onto the surface of the metal with precision laser cutters. When properly administered, these shallow markings have no impact on product quality, performance, or longevity. However…

Spinal Rod Testing
Depending on the product and type of test, medical implant laboratory testing can take anywhere from a few days to months. In this instance, I was asked to oversee a six-week project that involved a number of stress tests on two dozen spinal stabilizing rods. As the test progressed, a concerning number of the rods began to show signs of excess stress and broke prematurely. Needless to say, the client was more than a little concerned.

The Culprit: Laser Etching
After careful inspection it was determined that micro cracks were present in the stress area. Because spines are not straight, spinal rods must be flexed when installed to match the curvature of the spine. The stress area is wherever these bends occur.

We were able to trace the root of the problem to the etching process. Some further investigation determined that micro cracks were formed where the laser heated the metal’s surface when applying the serial numbers. These micro cracks weakened the material causing the rods to fail under the stress of bending.

Etching
Serial numbers are absolutely vital for implants; and laser etching remains the most viable alternative. It is recommended that manufacturers etch serial numbers in a location outside areas where the rods must flex. This is a good rule of thumb for all implants including rods, hip, knee and spine. Premature failure is a costly issue for manufacturers. It can lead to recalls, lost contracts, and litigation.

Battle Fatigue
Stress-related failure is certainly nothing new and definitely not limited to any particular industry or material type or thickness. At the same time, failure traced to marking in the wrong location dates back to as long as metals were being engraved or stamped.

Case in point: Railway transportation played an important role in World War II for all sides. As such railroads were targeted to disrupt the movement of supplies and troops. One rail axle inspector unknowingly aided the enemies cause. The large iron wheels and axles were cast from the same material. The area where the two came together had a large 10 to 15 inch radius. The axle/wheel unit was routinely stamped to show it had passed inspection. After an unusual number of these units developed cracks, it was determined that cause could be traced to the placement of the inspector’s certification stamped into the metal. As it turned out, the process actually concentrated stresses at the point of the stamp that were otherwise dispersed across the entire radii.

John McCloy has plenty more disturbing details about implants gone wrong: 

• Just When I Thought I’d Seen It All…
• Metal Is Metal…Right?
• Spinal Rod Failure & the Cost of Higher Education

Look out for more horror stories in the future! Find more information on Engineered Assurance here.