Introduction: The engineering of materials for bone regeneration that provide adequate mechanical support in the early phases after surgery and gradual replacement of the artificial scaffold by bone, is a current challenge for biomaterials development. The use of calcium phosphate ceramics, either alone or in combination with a polymeric phase, is now a common practice, since these materials provide good biological responses. The development of injectable materials for filling bone defects allows for the use of minimally invasive techniques. There is growing evidence that strontium (Sr), commonly used as an anti-osteoporotic agent, can be incorporated in biomaterials with beneficial effects in bone regeneration[1],[2]. The objective of the present work was to develop and characterize a hybrid polymer-ceramic injectable system that consists of an alginate matrix crosslinked in situ in the presence of Sr, incorporating a ceramic reinforcement in the form of Sr-rich microspheres.
Materials and Methods: Sr-rich porous hydroxyapatite (HAp) microspheres with a uniform size and a mean diameter of 555 mm were prepared, and their compression strength and friability tested. A 3.5% (w/v) ultrapure sodium alginate solution was used as the vehicle and its in situ gelation was promoted by the addition of calcium (Ca) or Sr carbonate and Glucone-d-lactone. Microspheres were added to the alginate solution to yield different weight percentages (10 to 35% w of the total solution). Injectability was evaluated using a device employed in vertebroplasty surgical procedures, coupled to a texture analyser. The compression strength of the extruded systems obtained after 24 h of incubation at 37ºC under controlled humidity was evaluated. Dynamic mechanical analysis (DMA) was used to study the viscoelastic properties of the vehicles and of the hybrid systems. The spatial distribution and size of the interstices between the microspheres was evaluated from high-resolution 3D micro-computed tomography (μCT) data sets.
Results: Significantly different rupture forces of 1.1±0.3 N and 0.5±0.2 N were observed for the Sr-HAp and HAp microspheres, respectively (p<0.001). Friability <0.1% was observed for the Sr-HAp and for the HAp microspheres.
Gelation times varied with temperature and crosslinking agent, being slower for Sr than for Ca, but adequate for injection in both cases. Compositions with 35% w of microspheres presented the best compromise between injectability and compression strength of the system, the force required to extrude it being lower than 100 N. DMA results showed that elastic behavior of the hybrid is dominant over the viscous one and that the higher storage modulus was obtained for the 3.5%Alg‑35%SrHAp‑Sr formulation. Micro CT analysis revealed a homogeneous distribution of the microspheres inside the vehicle, and a mean inter‑microspheres space of 220 mm.
Conclusions: The strontium rich viscoelastic hybrid system we have developed will offer structural support while providing a temporary scaffold onto which new bone can grow. It can be manually injected and sets in situ at body temperature, providing a scaffold for cell migration and tissue ingrowth. The incorporation of two Sr release kinetics, from the alginate and from the microspheres, may further improve effective bone regeneration, which can be especially useful in osteoporotic conditions.
This work was financed by FEDER funds, through the Programa Operacional Factores de Competitividade - COMPETE, and by Portuguese funds through FCT - Fundação para a Ciência e a Tecnologia in the framework of the project PTDC/CTM/103181/2008 and of project Pest-C/SAU/LA0002/2011.; The authors thankfully acknowledge the use of Micro-CT in the 3B’s Research Group facilities at University of Minho, the use of microscopy services at Centro de Materiais da Universidade do Porto, CEMUP and the technicians responsible for the experimental techniques mentioned, namely Vitor Correlo and Daniela Silva.
References:
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[2] Reginster JY, Seeman E, De Vernejoul MC, Adami S, Compston J, Phenekos C, et al. Strontium ranelate reduces the risk of nonvertebral fractures in postmenopausal women with osteoporosis: Treatment of Peripheral Osteoporosis (TROPOS) study. J Clin Endocrinol Metab 2005;90:2816-22.