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Dissolution Of Discrete Calcium Phosphate Crystals From CandidateTi-based Implant SurfacesPadina Pezeshki, Stanley Lugowski, John E. Davies, Faculty of Dentistry, Institute of Biomaterials andBiomedical Engineering, Faculty of Engineering, University of TorontoCanadian Biomaterials Society25th Annual Meeting (May 26–28, 2006, Calgary, Alberta,Canada)IntroductionPlasma-spraying of dental and orthopedic implants withcalcium phosphate (CaP) was introduced in the mid-1980sas a means of accelerating early peri-implant bone healingand rendering the implant surface bonebonding [1]. While some success has been,and continues to be, achieved clinically;problems have also been identified due todisintegration and delamination of suchrelatively thick (approx. 50 micron) coatings.The latter, due to the high temperature plasmaspraymethod, results in a heterogeneous CaPcoating composition comprising severalpossible CaP phases of differing solubilities.Thus, the solubility of these and other coatingsachieved through biomimetic strategies or solgeltechnology has been a subject ofconsiderable study and different fabricationmethods have aimed to modify the propertiesthat influence the CaP dissolution rate such asphase, particle size, surface area, and porosity[2,3]. Recently, a new means of modifyingmetallic surfaces with CaP has been developedthat comprises the deposition of discretecrystals of CaP from solution onto the metaloxide surface. In suspension the CaP crystalsare of the order of 20-100nms, which can bedeposited on the surface as a function ofimmersion time. Our current study wasdesigned to quantify the amount anddissolution kinetics of these discrete CaPcrystals from Ti6Al4V alloy dental implantsurfaces and compare their behavior withcommercially-available plasma sprayed CaPcoated implants of a similar size. Sinceactivated macrophages and osteoclasts canlocally reduce the extracellular pH to four, wechose an investigational pH range of 4-7 forour experiments.Materials And MethodsTwo Ti6Al4V implant types were employed. TheDCD-treated implants were IOSS615 (6mmdiameter x 15mm length) and plasma sprayedCaP implants were HT 613 (6mm diameter x1.501.000.500.0035.0030.0025.0020.0015.0010.005.000.0013mm length): both supplied by BIOMET 3i in Palm BeachGardens, Florida. Experiments were carried out to (i) measurethe total amount of Ca on the surface of the implants and (ii)the amount of Ca released in pH-specific solutions. In orderto measure the traces of Ca, all experimental equipment wasinitially cleaned to clear Ca contamination. For determinationof total amount of Ca on the surfaces, each DCD-treatedimplant was immersed in 10ml of 1 M Ultrex Grade HCl (pHFigure 1: Comparison of total [Ca] on plasma sprayed and DCD-treated implants4.003.503.002.502.00Ca = -1.9205 pH 3 + 32.805 pH 2 - 188.26 pH + 383.04Ca = 0.0826 pH 3 - 0.9697pH 2 + 1.8537pH + 6.27783 3.5 4 4.5 5 5.5 6 6.5 7 7.5pH3 3.5 4 4.5 5 5.5 6 6.5 7 7.5pHFigure 2: Comparison of different implants’ dissolution response3
0, by JT Baker, Phillipsburg, NJ) for 15minutes and each plasma sprayed samplefor 16 hrs, (n=8). For the second part of theexperiment, samples were placed in 10ml ofsaline solution with fixed pH of 4, 5, 6 or 7(by adding clean HCl or NaOH in a drop-wisefashion) for 15 minutes (n=3). The sampleswere then removed and the total Ca in theremaining solution was measured usingatomic absorption spectrophotometry.Furthermore, the morphology of the surfaceswas qualitatively analyzed using fieldemission scanning electron microscopy(FESEM).4.504.003.503.002.502.001.501.000.500.001 2 3Sample #pH4pH5pH6pH735.00Results And Discussion30.00The results revealed a reproducible25.00difference of three orders of magnitude less20.00in the amount of calcium removed from15.00DCD-treated implants when compared to10.00plasma-sprayed implants (3.81 vs.5.0011551.29 µg respectively) (Figure 1).0.00Moreover, the dissolution of Ca in aspecified time as a function of pHdemonstrated a clear trend between themeasured Ca and the pH of the solution(Figure 2). This is relevant to initial steps ofbiological healing, where activated macrophages, which mayrandomly contact the implant surface, could reduce the localpH to four. Moreover, as the body pH proceeds to its naturalpH of 7.4, the dissolution of CaP from DCD implants isalmost zero, while dissolution does occur from multi-phaseplasma-sprayed implants. Finally, the pH-dependentdissolution of the two implant types were compared and avariation was observed (Figure 3), which demonstrated amore consistent CaP phase for the DCD samples and multiphaseCaP surface for the plasma sprayed samples.ConclusionDCD-treatment provides implants with a more consistent andstable phase of CaP as well as lower amounts of CaP thatwould not invoke the known problems associated withdissolution of CaP from plasma-sprayed coatings.1 2 3Sample #Figure 3: Comparison of distribution of data for the two implant typesReferences1. Sun et al., J Biomed Mater Res. 2001;58(5):570-922. Yang et al., J Biomaterials 2005;26;327-337.3. Zeng et al., J Biomaterials 1999;20(5):443-351pH4pH5pH6pH74
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