Structure, biological and physical properties of strontium bioactive glasses
+1O’Donnell, M D; 2Gentleman, E; 2Candarlioglu, P; 3Miller, C, 2Stevens, M M
+ BioCeramic Therapeutics, London, UK, 2Imperial College London, UK, 3University of Sheffield, UK
[email protected]
1
Introduction: The strontium containing osteoporosis drug, Protelos [1], has been
shown in long-term clinical trials to reduce fracture risk and increase bone
mineral density due to the effect of the strontium ion on bone remodeling and cell
processes. Protelos, or strontium ranelate, is taken as a 2g daily oral dose.
Incorporation of strontium into a surface-active biomaterial could offer slow
release (e.g. 200mg over a 12 month period) and local delivery of strontium at the
defect site in addition to other therapeutic ions such as calcium, phosphorous and
silicon; for example in a synthetic bone graft or prosthesis coating. Here we
report on the structure and properties of a series of ten glasses based on a
modification of 45S5 Bioglass® [2] where we have progressively replaced
calcium with strontium on a molar basis as shown in Table (1).
the glass decreased as strontium expanded the glass network. The structure of the
glasses was probed using Raman spectroscopy displayed in Fig. (2). The
structure did not change across the series and the silicate network was
predominantly composed of linear chains (Q2) with isolated orthophosphate units
(Q0) [3]. This indicates all the glasses should show similar bioactive response
(dissolution, ion release kinetics, HA formation) with the added advantage of Srrelease.
Table (1): Glass compositions in mol. %.
ID
Sr0
Sr1
Sr2.5
Sr5
Sr7.5
Sr10
Sr25
Sr50
Sr75
Sr100
SiO2
46.13
46.13
46.13
46.13
46.13
46.13
46.13
46.13
46.13
46.13
Na2O
24.35
24.35
24.35
24.35
24.35
24.35
24.35
24.35
24.35
24.35
CaO
26.91
26.64
26.24
25.57
24.89
24.22
20.18
13.46
6.73
0.00
SrO
0.00
0.27
0.67
1.35
2.02
2.69
6.73
13.46
20.18
26.91
P2O5
2.60
2.60
2.60
2.60
2.60
2.60
2.60
2.60
2.60
2.60
Methods: Glasses were melted for 90 min. at 1350°C and frit-quenched into
water and milled and sieved to <38µm powder for characterization. Differential
thermal analysis (DTA) traces were recorded at 10°C.min.-1 from room
temperature to 1250°C. Raman spectra of glass powders were recorded between
200 and 1200cm-1 using a Renishaw RM 2000 spectrometer at 785nm. For the
cell studies 1.5g/L of glass powder (< 38 μm) was added to RPMI 1640 culture
medium and incubated on a roller at 37°C for 4 hours and then excess glass was
filtered. The dissolved ion concentrations were confirmed via inductively coupled
plasma-mass spectrometry (ICP-MS). Glass dissolution ions enriched culture
media was supplemented with 10% FBS, 2mM L-glutamine, 1% penicillinstreptomycin and bone mineralizing agents (5 mM β-glycerophosphate and 50 μ
g/mL ascorbic acid). SaOS-2 human osteosarcoma cells were plated at
30,000/cm2 for MTT and ALP analyses, whereas for tetracycline staining a
density of 34,400/cm2 was used. MTT activity based on reduction of a
tetrazolium salt was measured on 1,7,14, 21 and 28 days. Alkaline phosphatase
(ALP) activity was measured using p-nitrophenol phosphate as a substrate and
normalized to cell number against lactate dehydrogenase (LDH) enzyme activity
converting tetrazolium salt (INT) to red formazan product.
Results and discussion: The glass transition temperatures (Tg), onset (Tx) and
peak (Tp) crystallization, solidus (Ts), melting (Tm) and liquidus (Tl) temperatures
were obtained using DTA as shown in Fig. (1).
Fig. (2): Raman spectra of Sr-containing bioactive glasses.
Bioactive behaviour was assessed using the SBF test, an indicator of
hydroxyapatite forming ability; the speed of apatite formation, pH and glass
dissolution (ion release) increased with Sr content. Selected glasses (dissolution
products) were used in cell studies to assess cytotoxicity and cellular response.
The MTT assay plotted in Fig. (3) shows an increase in cell metabolic activity
with strontium content (and strontium release).
Fig. (3): MTT activity of Sr-containing bioactive glasses.
Fig. (4) shows the in vitro assay of the bone formation marker alkaline
phosphatase (ALP). This increases significantly with strontium glass content and
ion release.
Fig. (4): ALP activity of Sr-containing bioactive glasses.
Fig. (1): DTA traces of Sr-containing bioactive glasses.
All temperatures decreased with increasing Sr except Tx which decreased to a
minimum at 50% Sr-substitution then increased again. The glass transition
temperature decreased with Sr addition as the glass network expands to
accommodate this larger cation. Tx reached a minimum at 50% Sr as this is likely
to be close to a metasilicate eutectic point on the phase diagram. The addition of
strontium also suppressed crystallization. This can be seen by the shift to higher
temperature of the crystallization exotherm and the presence of only one melting
endotherm, compared to two in the Ca-rich glasses. Glass density increased as
strontium was added, a heavier ion than calcium, however the oxygen density of
Conclusions: Strontium has a clear positive effect on bone formation and
osteoblast activity also suppressing bone resorbing osteoclasts. Strontium can be
added to bioactive glasses in place of (and in combination with) calcium,
improving biological properties with minimal alteration of glass physical
properties.
References:
1. Meunier, Pierre J et al. ‘The effects of strontium ranelate on the risk of vertebral fracture in women with
postmenopausal osteoporosis’ N Engl J Med, 350 (2004) 459-68.
2. Hench, L L and Paschall, H A ‘Direct chemical bond of bioactive glass-ceramic materials to bone and
muscle’ J Biomed Mater Res 7 (1973) 25-42.
3. O'Donnell, M. D. et al. ‘Effect of P2O5 content in two series of soda lime phosphosilicate glasses on
structure and properties - Part I: NMR’ Journal of Non-Crystalline Solids 354 (2008) 3554-3560.
Poster No. 1241 • 56th Annual Meeting of the Orthopaedic Research Society