Development of Phosphatized Calcium Carbonate Biominerals as Bioactive Bone Graft Substitute Materials, Part I: Incorporation of Magnesium and Strontium Ions.

Ingo Sethmann,Hans-Joachim Kleebe,Cornelia Luft

doi:10.3390/jfb9040069

Ingo Sethmann, Hans-Joachim Kleebe + Show 1 more

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https://doi.org/10.3390/jfb9040069

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Abstract

Synthetic materials based on calcium phosphate (CaP) are frequently used as bone graft substitutes when natural bone grafts are not available or not suitable. Chemical similarity to bone guarantees the biocompatibility of synthetic CaP materials, whereas macroporosity enables their integration into the natural bone tissue. To restore optimum mechanical performance after the grafting procedure, gradual resorption of CaP implants and simultaneous replacement by natural bone is desirable. Mg and Sr ions released from implants support osteointegration by stimulating bone formation. Furthermore, Sr ions counteract osteoporotic bone loss and reduce the probability of related fractures. The present study aimed at developing porous Ca carbonate biominerals into novel CaP-based, bioactive bone implant materials. Macroporous Ca carbonate biominerals, specifically skeletons of corals (aragonite) and sea urchins (Mg-substituted calcite), were hydrothermally converted into pseudomorphic CaP materials with their natural porosity preserved. Sr ions were introduced to the mineral replacement reactions by temporarily stabilizing them in the hydrothermal phosphate solutions as Sr-EDTA complexes. In this reaction system, Na, Mg, and Sr ions favored the formation of correspondingly substituted β-tricalcium phosphate over hydroxyapatite. Upon dissolution, the incorporated functional ions became released, endowing these CaP materials with bioactive and potentially osteoporotic properties.

Highlights

More than two million bone graft procedures are performed worldwide [1]
Chemical and structural characterization by means of energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) confirmed the coral material to consist of Ca carbonate (Figure 1c) with the crystal structure of polymorph aragonite (PDF 00-041-1475) (Figure 1d)
The use of Mg-bearing sea urchin calcite as starting materials instead of the more commonly used coralline aragonite skeletons offered an effective way of introducing Mg ions to the hydrothermal mineral replacement reaction in a phosphate solution

Summary

Introduction

More than two million bone graft procedures are performed worldwide [1]. The main objective is the repair of bone defects caused by trauma or tumor resection. Considered to be the ideal implant material, autografts of cancellous bone are sometimes not available in sufficient quantity, and harvesting has its own potential complications. To avoid the drawbacks of autografts, there is a critical need for biocompatible materials for bone graft substitutions with sufficient mechanical strength that become resorbed through the natural bone remodeling process after fulfilling their initial function of bridging and stabilizing the defect [7]. The mineral component of bone consists of hydroxyapatite that contains impurities, according to the generalized chemical formula Ca5(PO4,CO3)3(OH,F,Cl,CO3) with variable contents [10]. Bone contains 0.5 wt % Na, 0.25 wt % Mg, and 0.02 wt % Sr (among others) as impurities [11,12] that substitute for Ca

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