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A comparison was conducted of measurement techniques used to quantify porous coatings applied to implant surfaces for biological fixation. Four types of coatings were measured, which included two bead size ranges, one wire pad, and one plasma flame-sprayed coating.
The purpose of the study was to determine the relationships between the measurement techniques and their resultant values when compared using two-dimensional sections. Basic stereological principles were used in the approach to this study. An automated image analysis system was utilized in making the determinations. Pore size measurement was conducted using two methods: (1) a line intercept method with serial measurements conducted at multiple levels parallel to and in plane with the substrate surface and (2) an interactive user-defined region measurement, in which the evaluator determined which features within the structure constituted individual pores. Comparison of data from both measurement methods was made contrasting the line intercept with the user-defined measurements and the planar-section with the cross-section measurements. Average values for the percentage of porosity in each coating were compared. The values obtained on cross sections were compared with values obtained on planar sections; the results indicated that the volume fraction of porosity observed in the cross sections was reproducibly 5% higher than that measured in the planar sections.
In the pore size measurements, two measurement methods and two section planes were compared. The line intercept method data showed a general tendency toward a smaller pore size when compared with the user-defined pore size data. This trend was observed in the pore size measurements in both cross sections and planar sections. The automated line intercept measurement did not exclude minimum values; therefore, its population of very small intercepts appears to skew the average pore size toward a lower average.
The choice of measurement method and the section plane selection appear to have significant influence on the average values obtained when measuring pore sizes on two-dimensional specimens.
porous implants, porous-coated implants, porosity, pore size, density
Manager, Metallurgical Laboratory, Zimmer, Inc., Warsaw, IN