August 28, 2012, Rob Long
Porous coatings are an important component in many different medical implant types. These coatings serve many purposes, not the least of which is improving the useful life of implants, thus reducing the number of procedures performed on patients.
Many porous coatings are also osteoinductive, which means that the coating itself actually encourages the growth of new bone cells around the device. This translates to more reliable implants. Some non-metallic coatings even help reduce the risk of allergic reactions in patients who are metal-sensitive, and otherwise would not be able to accept the implant. In a recent ODT article, Mark Crawford talks about these, as well as other new developments in medical coating technology, including diamond-like carbon (DLC) films and layered, thin film coatings. 
In order to bring an implant with a porous coating to market, there are a number of tests that must be performed to verify the mechanical properties of the coating.
The FDA spells out their requirements for porous coatings in the guidance document entitled: “Guidance for Industry on the Testing of Metallic Plasma Sprayed Coatings on Orthopedic Implants to Support Reconsideration of Post-market Surveillance Requirements”. Any individual or company considering bringing a porous coating to market should consult with this document early in the process.
The FDA requires the following mechanical tests to be performed on the coating:
• Tensile Test: A standard tensile test is performed on a 1” diameter test specimen, with coating on one flat side. The coated specimen is bonded to a metal fixture and pulled until the coating fails. This will verify the tensile strength of the coating.
• Static Shear Test: This test is performed on a 0.75” diameter specimen, in a similar fashion as the tensile test. This test verifies the shear strength of the porous coating.
• Shear Fatigue Test: This fatigue test is typically run to 10,000,000 cycles, to verify the durability of the coating.
• Abrasion Test: Abrasion testing is very important for porous coatings, as coatings with high wear rates can have adverse effects if implanted. The two types of tests most commonly performed are Taber abrasion and blade on block abrasion.
• Rotating Beam Fatigue Test: Rotating beam is performed to test the fatigue endurance. This verifies that the coating does not compromise the fatigue strength of the substrate to which it is applied.
This battery of tests is a well-rounded verification of the material’s mechanical properties. As always, we suggest discussing your project with your regulatory consultant prior to testing, to ensure that you are meeting the proper requirements.
Eric Stang
August 28, 2012 at 8:56 am
Rob, it is important to remember that the shear tests run on 0.75” round coupons are called aligned interface shears. Lap shear testing is also covered by ASTM F1044. Some of our clients choose to run in the lap shear configuration for static shear. However, lap shear testing is for metallic coatings only. If the coating is ceramic or another material the aligned interface method is used exclusively. Also proper adhesion is critical to getting accurate test results.