Strontium(II) and mechanical loading additively augment bone formation in calcium phosphate scaffolds

From hard tissues to beyond: Progress and challenges of strontium-containing biomaterials in regenerative medicine applications

Tissue engineering and regenerative medicine have emerged as crucial disciplines focused on the development of new tissues and organs to overcome the limitations of traditional treatments for tissue damage caused by accidents, diseases, or aging. Strontium ion (Sr2+) has garnered significant attention for its multifaceted role in promoting regeneration medicine and therapy, especially in bone tissue regeneration. Recently, numerous studies further confirm that Sr2+ also plays a critical in soft tissue regeneration. This review firstly summarizes the influence of Sr2+ on critical biological processes such as osteogenesis, angiogenesis, immune modulation, matrix synthesis, mineralization, and antioxidative defence mechanisms. Then details the classification, properties, advantages, and limitations of Sr-containing biomaterials (SrBMs). Additionally, this review extends to the current applications of SrBMs in regenerative medicine for diverse tissues, including bone, cartilage, skeletal muscle, dental pulp, cardiac tissue, skin, hair follicles, etc. Moreover, the review addresses the challenges associated with current SrBMs and provides insights for their future designing and applications in regenerative medicine.

Concept and Evolution in 3D Printing for Excellence in Healthcare

Three-dimensional printing (3DP) has gained popularity among scientists and researchers in every field due to its potential to drastically reduce energy costs for the production of customized products by utilizing less energy-intensive machines as well as minimizing material waste. The 3D printing technology is an additive manufacturing approach that uses material layer-by-layer fabrication to produce the digitally specified 3D model. The use of 3D printing technology in the pharmaceutical sector has the potential to revolutionize research and development by providing a quick and easy means to manufacture personalized one-off batches, each with unique dosages, distinct substances, shapes, and sizes, as well as variable release rates. This overview addresses the concept of 3D printing, its evolution, and its operation, as well as the most popular types of 3D printing processes utilized in the health care industry. It also discusses the application of these cutting-edge technologies to the pharmaceutical industry, advancements in various medical fields and medical equipment, 3D bioprinting, the most recent initiatives to combat COVID-19, regulatory frameworks, and the major challenges that this technology currently faces. In addition, we attempt to provide some futuristic approaches to 3DP applications.

Mesoporous Bioactive Glass-Incorporated Injectable Strontium-Containing Calcium Phosphate Cement Enhanced Osteoconductivity in a Critical-Sized Metaphyseal Defect in Osteoporotic Rats

In this study, the in vitro and in vivo bone formation behavior of mesoporous bioactive glass (MBG) particles incorporated in a pasty strontium-containing calcium phosphate bone cement (pS100G10) was studied in a metaphyseal fracture-defect model in ovariectomized rats and compared to a plain pasty strontium-containing calcium phosphate bone cement (pS100) and control (empty defect) group, respectively. In vitro testing showed good cytocompatibility on human preosteoblasts and ongoing dissolution of the MBG component. Neither the released strontium nor the BMG particles from the pS100G10 had a negative influence on cell viability. Forty-five female Sprague-Dawley rats were randomly assigned to three different treatment groups: (1) pS100 (n = 15), (2) pS100G10 (n = 15), and (3) empty defect (n = 15). Twelve weeks after bilateral ovariectomy and multi-deficient diet, a 4 mm wedge-shaped fracture-defect was created at the metaphyseal area of the left femur in all animals. The originated fracture-defect was substituted with pS100 or pS100G10 or left empty. After six weeks, histomorphometrical analysis revealed a statistically significant higher bone volume/tissue volume ratio in the pS100G10 group compared to the pS100 (p = 0.03) and empty defect groups (p = 0.0001), indicating enhanced osteoconductivity with the incorporation of MBG. Immunohistochemistry revealed a significant decrease in the RANKL/OPG ratio for pS100 (p = 0.004) and pS100G10 (p = 0.003) compared to the empty defect group. pS100G10 showed a statistically higher expression of BMP-2. In addition, a statistically significant higher gene expression of alkaline phosphatase, osteoprotegerin, collagen1a1, collagen10a1 with a simultaneous decrease in RANKL, and carbonic anhydrase was seen in the pS100 and pS100G10 groups compared to the empty defect group. Mass spectrometric imaging by time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed the release of Sr2+ ions from both pS100 and pS100G10, with a gradient into the interface region. ToF-SIMS imaging also revealed that resorption of the MBG particles allowed for new bone formation in cement pores. In summary, the current work shows better bone formation of the injectable pasty strontium-containing calcium phosphate bone cement with incorporated mesoporous bioactive glass compared to the bioactive-free bone cement and empty defects and can be considered for clinical application for osteopenic fracture defects in the future.

Ready-To-Use and Rapidly Biodegradable Magnesium Phosphate Bone Cement: In Vivo Evaluation in Sheep

In clinical practice, hydroxyapatite (HA) cements for bone defect treatment are frequently prepared by mixing a powder component and a liquid component shortly before implantation in the operation theater, which is time-consuming and error-prone. In addition, HA cements are only slightly resorbed, that is, cement residues can still be found in the bone years after implantation. Here, these challenges are addressed by a prefabricated magnesium phosphate cement paste based on glycerol, which is ready-to-use and can be directly applied during surgery. By using a trimodal particle size distribution (PSD), the paste is readily injectable and exhibits a compressive strength of 9-14 MPa after setting. Struvite (MgNH4 PO4 ·6H2 O), dittmarite (MgNH4 PO4 ·H2 O), farringtonite (Mg3 (PO4 )2 ), and newberyite (MgHPO4 ·3H2 O) are the mineral phases present in the set cement. The paste developed here features a promising degradation of 37% after four months in an ovine implantation model, with 25% of the implant area being newly formed bone. It is concluded that the novel prefabricated paste improves application during surgery, has a suitable degradation rate, and supports bone regeneration.

3D Plotting of Calcium Phosphate Cement and Melt Electrowriting of Polycaprolactone Microfibers in One Scaffold: A Hybrid Additive Manufacturing Process

The fabrication of patient-specific scaffolds for bone substitutes is possible through extrusion-based 3D printing of calcium phosphate cements (CPC) which allows the generation of structures with a high degree of customization and interconnected porosity. Given the brittleness of this clinically approved material, the stability of open-porous scaffolds cannot always be secured. Herein, a multi-technological approach allowed the simultaneous combination of CPC printing with melt electrowriting (MEW) of polycaprolactone (PCL) microfibers in an alternating, tunable design in one automated fabrication process. The hybrid CPC+PCL scaffolds with varying CPC strand distance (800-2000 µm) and integrated PCL fibers featured a strong CPC to PCL interface. While no adverse effect on mechanical stiffness was detected by the PCL-supported scaffold design; the microfiber integration led to an improved integrity. The pore distance between CPC strands was gradually increased to identify at which critical CPC porosity the microfibers would have a significant impact on pore bridging behavior and growth of seeded cells. At a CPC strand distance of 1600 µm, after 2 weeks of cultivation, the incorporation of PCL fibers led to pore coverage by a human mesenchymal stem cell line and an elevated proliferation level of murine pre-osteoblasts. The integrated fabrication approach allows versatile design adjustments on different levels.

Does the incorporation of strontium into calcium phosphate improve bone repair? A meta-analysis

Background

The application of calcium phosphate (CaP)-based bone substitutes plays an important role in periodontal regeneration, implant dentistry and alveolar bone reconstruction. The incorporation of strontium (Sr) into CaP-based bone substitutes appears to improve their biological properties, but the reported in vivo bone repair performance is inconsistent among studies. Herein, we conducted a systematic review and meta-analysis to investigate the in vivo performance of Sr-doped materials.

Methods

We searched PubMed, EMBASE (via OVIDSP), and reference lists to identify relevant animal studies. The search, study selection, and data extraction were performed independently by two investigators. Meta-analyses and sub-group analyses were conducted using Revman version 5.4.1. The heterogeneity between studies were assessed by I². Publication bias was investigated through a funnel plot.

Results

Thirty-five studies were finally enrolled, of which 16 articles that reported on new bone formation (NBF) were included in the meta-analysis, covering 31 comparisons and 445 defects. The overall effect for NBF was 2.25 (95% CI 1.61–2.90, p < 0.00001, I² = 80%). Eight comparisons from 6 studies reported the outcomes of bone volume/tissue volume (BV/TV), with an overall effect of 1.42 (95% CI 0.65–2.18, p = 0.0003, I² = 75%). Fourteen comparisons reported on the material remaining (RM), with the overall effect being -2.26 (95% CI − 4.02 to − 0.50, p = 0.0009, I² = 86%).

Conclusions

Our study revealed that Sr-doped calcium phosphate bone substitutes improved in vivo performance of bone repair. However, more studies are also recommended to further verify this conclusion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-022-02092-7.

Strontium Functionalization of Biomaterials for Bone Tissue Engineering Purposes: A Biological Point of View

Strontium (Sr) is a trace element taken with nutrition and found in bone in close connection to native hydroxyapatite. Sr is involved in a dual mechanism of coupling the stimulation of bone formation with the inhibition of bone resorption, as reported in the literature. Interest in studying Sr has increased in the last decades due to the development of strontium ranelate (SrRan), an orally active agent acting as an anti-osteoporosis drug. However, the use of SrRan was subjected to some limitations starting from 2014 due to its negative side effects on the cardiac safety of patients. In this scenario, an interesting perspective for the administration of Sr is the introduction of Sr ions in biomaterials for bone tissue engineering (BTE) applications. This strategy has attracted attention thanks to its positive effects on bone formation, alongside the reduction of osteoclast activity, proven by in vitro and in vivo studies. The purpose of this review is to go through the classes of biomaterials most commonly used in BTE and functionalized with Sr, i.e., calcium phosphate ceramics, bioactive glasses, metal-based materials, and polymers. The works discussed in this review were selected as representative for each type of the above-mentioned categories, and the biological evaluation in vitro and/or in vivo was the main criterion for selection. The encouraging results collected from the in vitro and in vivo biological evaluations are outlined to highlight the potential applications of materials’ functionalization with Sr as an osteopromoting dopant in BTE.

3D printing of patient-specific implants for osteochondral defects: workflow for an MRI-guided zonal design

Abstract

Magnetic resonance imaging (MRI) is a common clinical practice to visualize defects and to distinguish different tissue types and pathologies in the human body. So far, MRI data have not been used to model and generate a patient-specific design of multilayered tissue substitutes in the case of interfacial defects. For orthopedic cases that require highly individual surgical treatment, implant fabrication by additive manufacturing holds great potential. Extrusion-based techniques like 3D plotting allow the spatially defined application of several materials, as well as implementation of bioprinting strategies. With the example of a typical multi-zonal osteochondral defect in an osteochondritis dissecans (OCD) patient, this study aimed to close the technological gap between MRI analysis and the additive manufacturing process of an implant based on different biomaterial inks. A workflow was developed which covers the processing steps of MRI-based defect identification, segmentation, modeling, implant design adjustment, and implant generation. A model implant was fabricated based on two biomaterial inks with clinically relevant properties that would allow for bioprinting, the direct embedding of a patient’s own cells in the printing process. As demonstrated by the geometric compatibility of the designed and fabricated model implant in a stereolithography (SLA) model of lesioned femoral condyles, a novel versatile CAD/CAM workflow was successfully established that opens up new perspectives for the treatment of multi-zonal (osteochondral) defects.

Graphic abstract

Adapting the Pore Size of Individual, 3D-Printed CPC Scaffolds in Maxillofacial Surgery

Three dimensional (3D) printing allows additive manufacturing of patient specific scaffolds with varying pore size and geometry. Such porous scaffolds, made of 3D-printable bone-like calcium phosphate cement (CPC), are suitable for bone augmentation due to their benefit for osteogenesis. Their pores allow blood-, bone- and stem cells to migrate, colonize and finally integrate into the adjacent tissue. Furthermore, the pore size affects the scaffold’s stability. Since scaffolds in maxillofacial surgery have to withstand high forces within the jaw, adequate mechanical properties are of high clinical importance. Although many studies have investigated CPC for bone augmentation, the ideal porosity for specific indications has not been defined yet. We investigated 3D printed CPC cubes with increasing pore sizes and different printing orientations regarding cell migration and mechanical properties in comparison to commercially available bone substitutes. Furthermore, by investigating clinical cases, the scaffolds’ designs were adapted to resemble the in vivo conditions as accurately as possible. Our findings suggest that the pore size of CPC scaffolds for bone augmentation in maxillofacial surgery necessarily needs to be adapted to the surgical site. Scaffolds for sites that are not exposed to high forces, such as the sinus floor, should be printed with a pore size of 750 µm to benefit from enhanced cell infiltration. In contrast, for areas exposed to high pressures, such as the lateral mandible, scaffolds should be manufactured with a pore size of 490 µm to guarantee adequate cell migration and in order to withstand the high forces during the chewing process.

Toward Biofabrication of Resorbable Implants Consisting of a Calcium Phosphate Cement and Fibrin-A Characterization In Vitro and In Vivo

Cleft alveolar bone defects can be treated potentially with tissue engineered bone grafts. Herein, we developed novel biphasic bone constructs consisting of two clinically certified materials, a calcium phosphate cement (CPC) and a fibrin gel that were biofabricated using 3D plotting. The fibrin gel was loaded with mesenchymal stromal cells (MSC) derived from bone marrow. Firstly, the degradation of fibrin as well as the behavior of cells in the biphasic system were evaluated in vitro. Fibrin degraded quickly in presence of MSC. Our results showed that the plotted CPC structure acted slightly stabilizing for the fibrin gel. However, with passing time and fibrin degradation, MSC migrated to the CPC surface. Thus, the fibrin gel could be identified as cell delivery system. A pilot study in vivo was conducted in artificial craniofacial defects in Lewis rats. Ongoing bone formation could be evidenced over 12 weeks but the biphasic constructs were not completely osseous integrated. Nevertheless, our results show that the combination of 3D plotted CPC constructs and fibrin as suitable cell delivery system enables the fabrication of novel regenerative implants for the treatment of alveolar bone defects.

3D Bioprinting of osteochondral tissue substitutes - in vitro-chondrogenesis in multi-layered mineralized constructs

For the generation of multi-layered full thickness osteochondral tissue substitutes with an individual geometry based on clinical imaging data, combined extrusion-based 3D printing (3D plotting) of a bioink laden with primary chondrocytes and a mineralized biomaterial phase was introduced. A pasty calcium phosphate cement (CPC) and a bioink based on alginate-methylcellulose (algMC) - both are biocompatible and allow 3D plotting with high shape fidelity - were applied in monophasic and combinatory design to recreate osteochondral tissue layers. The capability of cells reacting to chondrogenic biochemical stimuli inside the algMC-based 3D hydrogel matrix was assessed. Towards combined osteochondral constructs, the chondrogenic fate in the presence of CPC in co-fabricated and biphasic mineralized pattern was evaluated. Majority of expanded and algMC-encapsulated cells survived the plotting process and the cultivation period, and were able to undergo redifferentiation in the provided environment to produce their respective extracellular matrix (ECM) components (i.e. sulphated glycosaminoglycans, collagen type II), examined after 3 weeks. The presence of a mineralized zone as located in the physiological calcified cartilage region suspected to interfere with chondrogenesis, was found to support chondrogenic ECM production by altering the ionic concentrations of calcium and phosphorus in in vitro culture conditions.

Adjuvant Drug-Assisted Bone Healing: Advances and Challenges in Drug Delivery Approaches

Bone defects of critical size after compound fractures, infections, or tumor resections are a challenge in treatment. Particularly, this applies to bone defects in patients with impaired bone healing due to frequently occurring metabolic diseases (above all diabetes mellitus and osteoporosis), chronic inflammation, and cancer. Adjuvant therapeutic agents such as recombinant growth factors, lipid mediators, antibiotics, antiphlogistics, and proangiogenics as well as other promising anti-resorptive and anabolic molecules contribute to improving bone healing in these disorders, especially when they are released in a targeted and controlled manner during crucial bone healing phases. In this regard, the development of smart biocompatible and biostable polymers such as implant coatings, scaffolds, or particle-based materials for drug release is crucial. Innovative chemical, physico- and biochemical approaches for controlled tailor-made degradation or the stimulus-responsive release of substances from these materials, and more, are advantageous. In this review, we discuss current developments, progress, but also pitfalls and setbacks of such approaches in supporting or controlling bone healing. The focus is on the critical evaluation of recent preclinical studies investigating different carrier systems, dual- or co-delivery systems as well as triggered- or targeted delivery systems for release of a panoply of drugs.

3D Printing of Bone Grafts for Cleft Alveolar Osteoplasty - In vivo Evaluation in a Preclinical Model

One of the most common hereditary craniofacial anomalies in humans are cleft lip and cleft alveolar bone with or without cleft palate. Current clinical practice, the augmentation of the persisting alveolar bone defect by using autologous bone grafts, has considerable disadvantages motivating to an intensive search for alternatives. We developed a novel therapy concept based on 3D printing of biodegradable calcium phosphate-based materials and integration of osteogenic cells allowing fabrication of patient-specific, tissue-engineered bone grafts. Objective of the present study was the in vivo evaluation of implants in a rat alveolar cleft model. Scaffolds were designed according to the defect's geometry with two different pore designs (60° and 30° rotated layer orientation) and produced by extrusion-based 3D plotting of a pasty calcium phosphate cement. The scaffolds filled into the artificial bone defect in the palate of adult Lewis rats, showing a good support. Half of the scaffolds were colonized with rat mesenchymal stromal cells (rMSC) prior to implantation. After 6 and 12 weeks, remaining defect width and bone formation were quantified histologically and by microCT. The results revealed excellent osteoconductive properties of the scaffolds, a significant influence of the pore geometry (60° > 30°), but no enhanced defect healing by pre-colonization with rMSC.

Adjuvant drug-assisted bone healing: Part III - Further strategies for local and systemic modulation

In this third in a series of reviews on adjuvant drug-assisted bone healing, further approaches aiming at influencing the healing process are discussed. Local and systemic modulation of bone metabolism is pursued with use of a number of drugs with completely different indications, which are characterized by a pleiotropic spectrum of action. These include drugs used to treat lipid disorders (HMG-CoA reductase inhibitors), hypertension (ACE inhibitors), osteoporosis (bisphosphonates), cancer (proteasome inhibitors) and others. Potential applications to enhance bone healing are discussed.

A Novel Plasma-Based Bioink Stimulates Cell Proliferation and Differentiation in Bioprinted, Mineralized Constructs

Extrusion-based bioprinting, also known as 3D bioplotting, is a powerful tool for the fabrication of tissue equivalents with spatially defined cell distribution. Even though considerable progress has been made in recent years, there is still a lack of bioinks which enable a tissue-like cell response and are plottable at the same time with good shape fidelity. Herein, we report on the development of a bioink which includes fresh frozen plasma from full human blood and thus a donor/patient-specific protein mixture. By blending of the plasma with 3 w/v% alginate and 9 w/v% methylcellulose, a pasty bioink (plasma-alg-mc) was achieved, which could be plotted with high accuracy and furthermore allowed bioplotted mesenchymal stromal cells (MSC) and primary osteoprogenitor cells to spread within the bioink. In a second step, the novel plasma-based bioink was combined with a plottable self-setting calcium phosphate cement (CPC) to fabricate bone-like tissue constructs. The CPC/plasma-alg-mc biphasic constructs revealed open porosity over the entire time of cell culture (35 d), which is crucial for bone tissue engineered grafts. The biphasic structures could be plotted in volumetric and clinically relevant dimensions and complex shapes could be also generated, as demonstrated for a scaphoid bone model. The plasma bioink potentiated that bioplotted MSC were not harmed by the setting process of the CPC. Latest after 7 days, MSC migrated from the hydrogel to the CPC surface, where they proliferated to 20-fold of the initial cell number covering the entire plotted constructs with a dense cell layer. For bioplotted and osteogenically stimulated osteoprogenitor cells, a significantly increased alkaline phosphatase activity was observed in CPC/plasma-alg-mc constructs in comparison to plasma-free controls. In conclusion, the novel plasma-alg-mc bioink is a promising new ink for several forms of bioprinted tissue equivalents and especially gainful for the combination with CPC for enhanced, biofabricated bone-like constructs.

Recent developments in strontium-based biocomposites for bone regeneration

Recent advances in biomaterial designing techniques offer immense support to tailor biomimetic scaffolds and to engineer the microstructure of biomaterials for triggering bone regeneration in challenging bone defects. The current review presents the different categories of recently explored strontium-integrated biomaterials, including calcium silicate, calcium phosphate, bioglasses and polymer-based synthetic implants along with their in vivo bone formation efficacies and/or in vitro cell responses. The role and significance of controlled drug release scaffold/carrier design in strontium-triggered osteogenesis was also comprehensively described. Furthermore, the effects of stem cells and growth factors on bone remodeling are also elucidated.

3D Plotted Biphasic Bone Scaffolds for Growth Factor Delivery: Biological Characterization In Vitro and In Vivo

Bioprinting enables the integration of biological components into scaffolds during fabrication that has the advantage of high loading efficiency and better control of release and/or spatial positioning. In this study, a biphasic scaffold fabricated by extrusion-based 3D multichannel plotting of a calcium phosphate cement (CPC) paste and an alginate/gellan gum (AlgGG) hydrogel paste laden with the angiogenic factor VEGF (vascular endothelial growth factor) is investigated with regard to biological response in vitro and in vivo. Rat mesenchymal stromal cells are able to adhere and grow on both CPC and AlgGG strands, and differentiate toward osteoblasts. A sustained VEGF release is observed, which is able to stimulate endothelial cell proliferation as well as angiogenesis in vitro that indicates maintenance of its biological activity. After implantation into a segmental bone defect in the femur diaphysis of rats, a clear reduction of the defect size by newly formed bone tissue occurs from the distal and proximal ends of the host bone within 12 weeks. The CPC component shows excellent osteoconductivity whereas the local VEGF release from the AlgGG hydrogel gives rise to an enhanced vascularization of the defect region. This work contributes to the development of novel therapeutic concepts for improved bone regeneration which are based on 3D bioprinting.

Multiscale analyses reveal native-like lamellar bone repair and near perfect bone-contact with porous strontium-loaded bioactive glass

The efficient healing of critical-sized bone defects using synthetic biomaterial-based strategies is promising but remains challenging as it requires the development of biomaterials that combine a 3D porous architecture and a robust biological activity. Bioactive glasses (BGs) are attractive candidates as they stimulate a biological response that favors osteogenesis and vascularization, but amorphous 3D porous BGs are difficult to produce because conventional compositions crystallize during processing. Here, we rationally designed a porous, strontium-releasing, bioactive glass-based scaffold (pSrBG) whose composition was tailored to deliver strontium and whose properties were optimized to retain an amorphous phase, induce tissue infiltration and encourage bone formation. The hypothesis was that it would allow the repair of a critical-sized defect in an ovine model with newly-formed bone exhibiting physiological matrix composition and structural architecture. Histological and histomorphometric analyses combined with indentation testing showed pSrBG encouraged near perfect bone-to-material contact and the formation of well-organized lamellar bone. Analysis of bone quality by a combination of Raman spectral imaging, small-angle X-ray scattering, X-ray fluorescence and focused ion beam-scanning electron microscopy demonstrated that the repaired tissue was akin to that of normal, healthy bone, and incorporated small amounts of strontium in the newly formed bone mineral. These data show the potential of pSrBG to induce an efficient repair of critical-sized bone defects and establish the importance of thorough multi-scale characterization in assessing biomaterial outcomes in large animal models.

Design of Complexly Graded Structures inside Three-Dimensional Surface Models by Assigning Volumetric Structures

An innovative approach for designing complex structures from STL-datasets based on novel software for assigning volumetric data to surface models is reported. The software allows realizing unique complex structures using additive manufacturing technologies. Geometric data as obtained from imaging methods, computer-aided design, or reverse engineering that exist only in the form of surface data are converted into volumetric elements (voxels). Arbitrary machine data can be assigned to each voxel and thereby enable implementing different materials, material morphologies, colors, porosities, etc. within given geometries. The software features an easy-to-use graphical user interface and allows simple implementation of machine data libraries. To highlight the potential of the modular designed software, an extrusion-based process as well as a two-tier additive manufacturing approach for short fibers and binder process are combined to generate three-dimensional components with complex grading on the material and structural level from STL files.

Bone regeneration capacity of magnesium phosphate cements in a large animal model

Magnesium phosphate minerals have captured increasing attention during the past years as suitable alternatives for calcium phosphate bone replacement materials. Here, we investigated the degradation and bone regeneration capacity of experimental struvite (MgNH₄PO₄·6H₂O) forming magnesium phosphate cements in two different orthotopic ovine implantation models. Cements formed at powder to liquid ratios (PLR) of 2.0 and 3.0 g ml^-1 were implanted into trabecular bone using a non-load-bearing femoral drill-hole model and a load-bearing tibial defect model. After 4, 7 and 10 months the implants were retrieved and cement degradation and new bone formation was analyzed by micro-computed tomography (µCT) and histomorphometry. The results showed cement degradation in concert with new bone formation at both defect locations. Both cements were almost completely degraded after 10 months. The struvite cement formed with a PLR of 2.0 g ml^-1 exhibited a slightly accelerated degradation kinetics compared to the cement with a PLR of 3.0 g ml^-1. Tartrat-resistant acid phosphatase (TRAP) staining indicated osteoclastic resorption at the cement surface. Energy dispersive X-ray analysis (EDX) revealed that small residual cement particles were mostly accumulated in the bone marrow in between newly formed bone trabeculae. Mechanical loading did not significantly increase bone formation associated with cement degradation. Concluding, struvite-forming cements might be promising bone replacement materials due to their good degradation which is coupled with new bone formation.

STATEMENT OF SIGNIFICANCE

Recently, the interest in magnesium phosphate cements (MPC) for bone substitution increased, as they exhibit high initial strength, comparably elevated degradation potential and the release of valuable magnesium ions. However, only few in vivo studies, mostly including non-load-bearing defects in small animals, have been performed to analyze the degradation and regeneration capability of MPC derived compounds. The present study examined the in vivo behavior of magnesiumammoniumphosphate hexahydrate (struvite) implants with different porosity in both mechanically loaded and non-loaded defects of merino sheep. For the first time, the effect of mechanical stimuli on the biological outcome of this clinically relevant replacement material is shown and directly compared to the conventional unloaded defect situation in a large animal model.

Strontium-modified premixed calcium phosphate cements for the therapy of osteoporotic bone defects

In this study a premixed strontium-containing calcium phosphate bone cement for the application in osteoporotic bone defects has been developed and characterised regarding its material and in vitro properties as well as minimally invasive applicability in balloon kyphoplasty. Strontium was introduced into the cement by substitution of one precursor component, CaCO₃, with its strontium analogue, SrCO₃. Using a biocompatible oil phase as carrier liquid, a cement paste that only set upon contact with aqueous environment was obtained. Strontium modification resulted in an increased strength of set cements and radiographic contrast; and the cements released biologically relevant doses of Sr²⁺-ions that were shown to enhance osteoprogenitor cell proliferation and osteogenic differentiation. Finally, applicability of strontium-containing cement pastes in balloon kyphoplasty was demonstrated in a human cadaver spine procedure. The cement developed in this study may therefore be well suited for minimally invasive, osteoporosis-related bone defect treatment.

STATEMENT OF SIGNIFICANCE

Strontium-releasing calcium phosphate bone cements are promising materials for the clinical regeneration of osteoporosis-related bone defects since they have been shown to stimulate bone formation and at the same time limit osteoclastic bone resorption. Today clinical practice favours minimally invasive surgical techniques, e.g. for vertebral fracture treatment, posing special demands on such cements. We have therefore developed a premixed, strontium-releasing bone cement with enhanced mechanical properties and high radiographic visibility that releases biologically relevant strontium concentrations and thus stimulates cells of the osteogenic lineage. In a pilot experiment we also exemplify its excellent suitability for minimally invasive balloon kyphoplasty procedures.