A universal curve of apatite crystallinity for the assessment of bone integrity and preservation

Gregorio Dal Sasso,Ivana Angelini,Gilberto Artioli,Luca Nodari,Yotam Asscher

doi:10.1038/s41598-018-30642-z

Abstract

The reliable determination of bioapatite crystallinity is of great practical interest, as a proxy to the physico-chemical and microstructural properties, and ultimately, to the integrity of bone materials. Bioapatite crystallinity is used to diagnose pathologies in modern calcified tissues as well as to assess the preservation state of fossil bones. To date, infrared spectroscopy is one of the most applied techniques for bone characterisation and the derived infrared splitting factor (IRSF) has been widely used to practically assess bioapatite crystallinity. Here we thoroughly discuss and revise the use of the IRSF parameter and its meaning as a crystallinity indicator, based on extensive measurements of fresh and fossil bones, virtually covering the known range of crystallinity degree of bioapatite. A novel way to calculate and use the infrared peak width as a suitable measurement of true apatite crystallinity is proposed, and validated by combined measurement of the same samples through X-ray diffraction. The non-linear correlation between the infrared peak width and the derived ISRF is explained. As shown, the infrared peak width at 604 cm−1 can be effectively used to assess both the average crystallite size and structural carbonate content of bioapatite, thus establishing a universal calibration curve of practical use.

Highlights

Bone is a composite material constituted by the intimate association of an organic matrix and a mineral phase arranged in a complex structure described in terms of hierarchical levels of organization[1,2]
The effectiveness of infrared splitting factor (IRSF) as a parameter for describing crystallinity variations is here discussed by comparing results obtained from FTIR spectroscopy with an independent measurement of bioapatite crystallinity carried out by X-ray powder diffraction coupled with the Rietveld refinement of diffraction data, being this technique the most suitable and statistically robust to assess the average size of bioapatite crystallites
Archaeological bones from Sudan show significantly higher IRSF and lower FW85% than those of fresh bones; within this range, data referring to archaeological bones from Israel partially overlap to those of fresh bones and of archaeological bones from Sudan (Fig. 2)

Summary

Introduction

Bone is a composite material constituted by the intimate association of an organic matrix and a mineral phase arranged in a complex structure described in terms of hierarchical levels of organization[1,2]. Given the relevance gained by this diagenetic parameter to assess the preservation state of fossil bones, in this research a detailed study on the relationship between IR peak width, IRSF and crystallinity has been carried out by investigating fresh and archaeological bones This set of samples covers a wide range of crystallinity as fresh bones are inherently characterized by poorly crystalline bioapatite, whereas archaeological bones from different sites show a large variability in terms of recrystallization degree, depending on the types and extent of alteration processes encountered during burial. Bioapatite crystallinity can be indicative for bone quality or preservation, since nanocrystals tend to recrystallize into larger crystals with a more ordered structure[11,13] as soon as specific dysfunctions or the death of the individual hinder the metabolic processes normally controlling bioapatite chemical composition and crystal size[7,8,11]

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Conclusion