Porous calcium polyphosphate bone substitutes: Additive manufacturing versus conventional gravity sinter processing—Effect on structure and mechanical properties

Faculty of Dentistry, University of Toronto, Toronto, Ontario, M5G 1G6, CanadaReceived 15 March 2013; revised 2 June 2013; accepted 1 July 2013Published online 00 Month 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.33005Abstract: Porous calcium polyphosphate (CPP) structures pro-posed as bone-substitute implants and made by sinteringCPP powders to form bending test samples of approximately35 vol % porosity were machined from preformed blocksmade either by additive manufacturing (AM) or conventionalgravity sintering (CS) methods and the structure andmechanical characteristics of samples so made were com-pared. AM-made samples displayed higher bending strengths( 1.2–1.4 times greater than CS-made samples), whereaselastic constant (i.e., effective elastic modulus of the porousstructures) that is determined by material elastic modulusand structural geometry of the samples was 1.9–2.3 timesgreater for AM-made samples. X-ray diffraction analysisshowed that samples made by either method displayed thesame crystal structure forming b-CPP after sinter annealing.The material elastic modulus, E, determined using nanoin-dentation tests also showed the same value for both sampletypes (i.e., E 64 GPa). Examination of the porous structuresindicated that significantly larger sinter necks resulted in theAM-made samples which presumably resulted in the highermechanical properties. The development of mechanical prop-erties was attributed to the different sinter anneal proceduresrequired to make 35 vol % porous samples by the two meth-ods. A primary objective of the present study, in addition toreporting on bending strength and sample stiffness (elasticconstant) characteristics, was to determine why the two proc-esses resulted in the observed mechanical property differen-ces for samples of equivalent volume percentage of porosity.An understanding of the fundamental reason(s) for theobserved effect is considered important for developingimproved processes for preparation of porous CPP implantsas bone substitutes for use in high load-bearing skeletal sites.