Strategies towards injectable, load-bearing materials for the intervertebral disc: a review and outlook

Synthetic Calcium-Phosphate Materials for Bone Grafting

Synthetic bone grafting materials play a significant role in various medical applications involving bone regeneration and repair. Their ability to mimic the properties of natural bone and promote the healing process has contributed to their growing relevance. While calcium-phosphates and their composites with various polymers and biopolymers are widely used in clinical and experimental research, the diverse range of available polymer-based materials poses challenges in selecting the most suitable grafts for successful bone repair. This review aims to address the fundamental issues of bone biology and regeneration while providing a clear perspective on the principles guiding the development of synthetic materials. In this study, we delve into the basic principles underlying the creation of synthetic bone composites and explore the mechanisms of formation for biologically important complexes and structures associated with the various constituent parts of these materials. Additionally, we offer comprehensive information on the application of biologically active substances to enhance the properties and bioactivity of synthetic bone grafting materials. By presenting these insights, our review enables a deeper understanding of the regeneration processes facilitated by the application of synthetic bone composites.

Injectable bone cements: What benefits the combination of calcium phosphates and bioactive glasses could bring?

Out of the wide range of calcium phosphate (CaP) biomaterials, calcium phosphate bone cements (CPCs) have attracted increased attention since their discovery in the 1980s due to their valuable properties such as bioactivity, osteoconductivity, injectability, hardening ability through a low-temperature setting reaction and moldability. Thereafter numerous researches have been performed to enhance the properties of CPCs. Nonetheless, low mechanical performance of CPCs limits their clinical application in load bearing regions of bone. Also, the in vivo resorption and replacement of CPC with new bone tissue is still controversial, thus further improvements of high clinical importance are required. Bioactive glasses (BGs) are biocompatible and able to bond to bone, stimulating new bone growth while dissolving over time. In the last decades extensive research has been performed analyzing the role of BGs in combination with different CaPs. Thus, the focal point of this review paper is to summarize the available research data on how injectable CPC properties could be improved or affected by the addition of BG as a secondary powder phase. It was found that despite the variances of setting time and compressive strength results, desirable injectable properties of bone cements can be achieved by the inclusion of BGs into CPCs. The published data also revealed that the degradation rate of CPCs is significantly improved by BG addition. Moreover, the presence of BG in CPCs improves the in vitro osteogenic differentiation and cell response as well as the tissue-material interaction in vivo.

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Properties of injectable calcium phosphate bone cements and bioactive glasses are discussed.

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Benefits that BG addition to CPC could bring are highlighted.

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Desirable injectable properties of bone cements can be achieved by the inclusion of BGs into CPCs.

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The presence of BG in CPC advances in vitro and in vivo response of the composites.

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Future research direction of BG containing injectable CPC composites are provided.

Injectable microcryogels reinforced alginate encapsulation of mesenchymal stromal cells for leak-proof delivery and alleviation of canine disc degeneration

In situ crosslinked thermo-responsive hydrogel applied for minimally invasive treatment of intervertebral disc degeneration (IVDD) may not prevent extrusion of cell suspension from injection site due to high internal pressure of intervertebral disc (IVD), causing treatment failure or osteophyte formation. In this study, mesenchymal stromal cells (MSCs) were encapsulated in alginate precursor and loaded into previously developed macroporous PGEDA-derived microcryogels (PMs) to form three-dimensional (3D) microscale cellular niches, enabling non-thermo-responsive alginate hydrogel to be injectable. The PMs reinforced alginate hydrogel showed superior elasticity compared to alginate hydrogel alone and could well protect encapsulated cells through injection. Chondrogenic committed MSCs in the injectable microniches expressed higher level of nucleus pulposus (NP) cell markers compared to 2D cultured cells. In an ex vivo organ culture model, injection of MSCs-laden PMs into NP tissue prevented cell leakage, improved cell retention and survival compared to free cell injection. In canine IVDD models, alleviated degeneration was observed in MSCs-laden PMs treated group after six months which was superior to other treated groups. Our results provide in-depth demonstration of injectable alginate hydrogel reinforced by PMs as a leak-proof cell delivery system for augmented regenerative therapy of IVDD in canine models.