Phase transformations during processing and in vitro degradation of porous calcium polyphosphates

Abstract

A 2-Step sinter/anneal treatment has been reported previously for forming porous CPP as biodegradable bone substitutes [9]. During the 2-Step annealing treatment, the heat treatment used strongly affected the rate of CPP degradation in vitro. In the present study, x-ray diffraction and (31)P solid state nuclear magnetic resonance were used to determine the phases that formed using different heat treating processes. The effect of in vitro degradation (in PBS at 37 °C, pH 7.1 or 4.5) was also studied. During CPP preparation, β-CPP and γ-CPP were identified in powders formed from a calcium monobasic monohydrate precursor after an initial calcining treatment (10 h at 500 °C). Melting of this CPP powder (at 1100 °C), quenching and grinding formed amorphous CPP powders. Annealing powders at 585 °C (Step-1) resulted in rapid sintering to form amorphous porous CPP. Continued annealing to 650 °C resulted in crystallization to form a multi-phase structure of β-CPP primarily plus lesser amounts of α-CPP, calcium ultra-phosphates and retained amorphous CPP. Annealing above 720 °C and up to 950 °C transformed this to β-CPP phase. In vitro degradation of the 585 °C (Step-1 only) and 650 °C Step-2 annealed multi-phase samples occurred significantly faster than the β-CPP samples formed by Step-2 annealing at or above 720 °C. This faster degradation was attributable to preferential degradation of thermodynamically less stable phases that formed in samples annealed at 650 °C (i.e. α-phase, ultra-phosphate and amorphous CPP). Degradation in lower pH solutions significantly increased degradation rates of the 585 and 650 °C annealed samples but had no significant effect on the β-CPP samples.

Calcium polyphosphate particulates for bone void filler applications

“…Unfortunately, due to overlapping of the stronger α‐CPP peaks (at 2 θ ≈ 25+ and 32°) unique identification of α ‐CPP by XRD is not possible. As reported elsewhere in a study in which more sensitive XRD methods and 31 P MAS NMR were used to analyze as‐made porous bulk FD‐ and SD‐CPP samples, the SD‐CPP was shown to be β‐CPP only while FD‐CPP indicated a multiphase structure consisting of β‐CPP and significant amounts of α‐CPP as well as traces of ultraphosphates and amorphous CPP …”

“…In an earlier study of bone formation in critical size defects created in rat calvaria and filled with 150–250 μm amorphous CPP powders for a 1‐month period, new bone formation was not observed. One possible reason for this lack of new bone forming was the too rapid degradation of the amorphous CPP, a characteristic noted in later in vitro experiments . These in vitro degradation studies, also demonstrated dramatic differences in degradation rates of two grades of CPP particulate formed using the different annealing schedules subsequently used in the in vivo investigation reported herein.…”

“…In the present study, two different Step‐2 temperature anneals were used namely 650 and 950°C. The Step‐2 annealing temperature used determines the final crystalline state of the CPP (i.e., the phase or phases that are formed) as well as the size of the CPP crystals (grains) within the particles forming the sintered structures …”

“…The 1–2 mm particulate size used in the present study was selected to approximate typically harvested autograft bone particle size and commercially marketed bone substitute products. Two different Step‐2 annealing temperatures were selected in order to create slower degrading (SD‐CPPp) and faster‐degrading (FD‐CPPp) calcium polyphosphate particulates as suggested by our prior in vitro degradation study . The SD‐CPPp samples were produced using the 950°C, 1 hour Step‐2 anneal, the condition previously used to form porous bulk CPP reported in earlier in vivo studies.…”

“…These in vitro degradation studies, also demonstrated dramatic differences in degradation rates of two grades of CPP particulate formed using the different annealing schedules subsequently used in the in vivo investigation reported herein. (A manuscript reporting on the in vitro studies has been submitted for publication and the results have been presented at meetings). It was this earlier in vivo rat calvarium study and the later in vitro studies that suggested the form of the two CPP particulate grades investigated in the in vivo study described herein.…”