Modern orthopedic and dental implant devices often incorporate porous coatings or structures that are designed to promote bone infiltration for biological fixation. These coatings are regularly subjected to shear stresses in normal use and the coating must not shear off under those stresses. These test methods have been established primarily for plasma-sprayed titanium and plasma-sprayed hydroxyapatite coatings.
Test programs can be designed to determine the performance of these coatings under constant or cyclic application of shear stresses. Test samples are subjected to shear loads parallel to the surface plane in order to evaluate their strength and fatigue life properties. Fatigue testing methods can either be for developing S-N stress-life curves or for completing several run-out tests against predetermined acceptance criteria. ASTM specifies 2 standards for this type of testing:
ASTM F1044 - Standard Test for Shear Testing of Calcium Phosphate Coatings and Metallic Coatings
ASTM F1160 - Standard Test for Shear and Bending Fatigue Testing of Calcium Phosphate and Metallic Medical and Composite Calcium Phosphate/Metallic Coatings
These standards are specific to shear testing of continuous calcium phosphate coatings and metallic coatings adhering to dense metal (Titanium, Cobalt Chrome) and plastic (PEEK, PAEK, UHMWPE) substrates. Shear fatigue testing determines the adhesive or cohesive strength, or both, of the coating. Shear fatigue strength is typically assessed in ambient air at predetermined test frequency, load ratio and coating shear area. Specimens are bonded to metal or plastic substrate dummies, placed into shear fixtures and tested to 10M cycles or failure.
Data from these tests can be used to satisfy regulatory requirements, as well as to compare or evaluate coating types and suppliers.
IMR offers a full assay of fatigue testing services for the medical device industry. Click here to find out more about Shear Fatigue Testing services.