Development and Validation of Test Methods for Hydroxyapatite Coatings for Commercial Use

This thesis develops and validates test methods to test critical to quality factors of the hydroxyapatite coated parts from the internal HA line. The aim is to develop robust and reliable test methods for process monitoring and internal testing. The properties in focus were crystallinity, coating adhesion, thickness, porosity, rate of corundum incrustation and calcium phosphate testing. This thesis was conducted in collaboration with DePuy Synthes. HA powder is removed from the Ti6Al4V coupons using a Diprofil reciprocating file which uses a diamond tool. The scraped powder is sieved using a test sieve with a < 40 µm stainless steel mesh. Uniformity within the crystals allows a thorough investigation of all crystal phases within a powder sample. HA powder was analysed using a 0.5g sample holder. Six coupons are used to create one sample. The powder was scanned through a range of 20 °2ϴ to 55 °2ϴ. The X-Ray diffraction pattern was produced in the range corresponding to the peaks selected (peak 210 or peak 211 for apatite, peak 0.210 for β-tricalcium phosphate and peak 200 for calcium oxide). The integrated intensities of the peaks selected for the apatite and the β-tricalcium phosphate were measured. DSC was performed on the 3 batches of FM1000 glue. Samples were placed in between crimped aluminium pans and subjected to a pre-determined temperature profile which replicates that of the current curing process – heated from ambient temperature to 180°C at 2.5°C per minute and then held at 180°C for 2.5hrs. All samples were run in triplicate. The exothermic peaks were then analysed on all DSC traces and results recorded. The ability to analyse porosity is an essential critical to quality component, as porosity influences both the physical and mechanical characteristics of hydroxyapatite, with the rate of bone ingrowth and the rate of dissolution directly proportionate to the degree of porosity of the HA coating. Images were taken across a minimum area of 20 micron and 10 equally spaced perpendicular lines were generated across each field with the distance material measured as the coating thickness. The average of the individual coating thickness measurements from all the fields measured is then calculated as the mean coating thickness. Samples were placed face down on the stage, rate of corundum incrustation was measured using Leica application suite V4.12. Five areas of the sample were imaged using brightfield settings on the Leica DMI inverted microscope. Images are captured at 100x Magnification (10x objective) to allow for distinction between features. Images taken across a minimum of area of 20 µm were loaded into the software for analysis. The average rate of corundum incrustation across the five fields that are measured then calculated as the mean rate of corundum incrustation. The zone of indifference for this test was identified as 6.5%. The mean porosity of external testing was determined to be 1.5%. The mean porosity of internal testing was determined to be 0.35%. Thus, the difference in means is 1.05% which lies within the zone of indifference is compliant to the acceptance criteria outlined above. The P-Value is statistically significant and determines that all the acceptance requirements as outlined above. The Ca/P ratio specification/tolerance as outlined above is 1.67 to 1.76, all the samples run have calculated Ca/P ratios within this range.