Mineral maturity and crystallinity index are distinct characteristics of bone mineral

Abstract

The purpose of this study was to test the hypothesis that mineral maturity and crystallinity index are two different characteristics of bone mineral. To this end, Fourier transform infrared microspectroscopy (FTIRM) was used. To test our hypothesis, synthetic apatites and human bone samples were used for the validation of the two parameters using FTIRM. Iliac crest samples from seven human controls and two with skeletal fluorosis were analyzed at the bone structural unit (BSU) level by FTIRM on sections 2-4 mum thick. Mineral maturity and crystallinity index were highly correlated in synthetic apatites but poorly correlated in normal human bone. In skeletal fluorosis, crystallinity index was increased and maturity decreased, supporting the fact of separate measurement of these two parameters. Moreover, results obtained in fluorosis suggested that mineral characteristics can be modified independently of bone remodeling. In conclusion, mineral maturity and crystallinity index are two different parameters measured separately by FTIRM and offering new perspectives to assess bone mineral traits in osteoporosis.

Figure 1

Evolution of the hydrated layer and crystal apatite. During the maturation and growth of the crystal, the hydrated layer, involved in a high surface reactivity, progressively decreased and led to a stable apatitic domain. The structure of the hydrated layer constitutes a pool of loosely bound ions which can be incorporated in the growing apatite domains and can be exchanged by foreign ions in the solution and charged groups of proteins (Pr). Courtesy of C. Rey (Rey et al. (2009); Osteoporos. Int :1013–1021)

Figure 2

Synthetic apatites, ν₄PO₄vibration (A) Crystallinity index [29] measured by Shemesh’s method [29], given by CI = {A₆₀₄ + A₅₆₄}/A₅₉₀. (B) Correlation between the CI measured by the Shemesh’s method and by FTIRM in which CI is inversely proportional to the full width at half maximum (FWHM) of the 604 cm⁻¹ peak.

Figure 3

Infrared spectra obtained from (1) a synthetic carbonated apatite and showing the ν₁ν₃PO₄, ν₂CO₃ and the ν₄PO₄ vibrations (bold curve), (2) new (full curve) and old (dotted curve) bone showing different vibrations, including those of amides (I, II and III).

Figure 4

Evolution of mineral maturity (ν₁ν₃PO₄) vibration in various synthetic apatites. (A) Non-apatitic phosphate (1110 cm⁻¹) decreases with the progression of mineral maturation, while the apatitic phosphates (1030 cm⁻¹) are constant. (B) Mineral maturity increases with the progression of maturation (d: days of maturation).

Figure 5

Evolution of mineral crystallinity and correlation between crystallinity index and mineral maturity in synthetic apatites. (A) Significant correlation between the crystallinity index measured by FTIRM (604 cm⁻¹ peak) and the crystallinity measured by X-ray diffraction [XRD, (002) reflection]. (B) Significant correlation between the crystallinity index measured by FTIRM (inversely proportional to the FWHM of 604 cm⁻¹ peak) and the crystallinity measured by the XRD (310) reflection. (C) Correlation between mineral maturity and mineral crystallinity index in synthetic apatites. These two parameters are closely linked (FWHM: full width at half maximum). (D) Correlation between mineral maturity and mineral crystallinity index in human bone.

Figure 6

In human bone and at the level of the ν₁-ν₃PO₄ vibration, the 1110 cm⁻¹ peak (non-apatitic domain) is less intense in old bone (A) than in new bone (B). This reflects that mineral maturation of old interstitial bone is greater than that of newly formed bone.

Figure 7

Example of one pair of samples analyzed by X-ray diffraction (XRD) (fluorotic bone in red and control bone in black) and by FTIRM. XRD diagram shows the decrease of the full width at half maximum (FWHM) (°2θ) of the (310) reflection in fluorosis, while (002) reflection is unchanged. Regarding the overall aspect of XRD diagram of the two samples, fluorotic bone is more crystallized than control bone. FTIRM analysis shows a decrease of the FWHM of the 604 cm⁻¹ (peak fitting inserted) of the fluorotic bone, indicating an increase in mineral crystallinity index.