Multimodal pore formation in calcium phosphate cements

Incorporation of synthetic water-soluble curcumin polymeric drug within calcium phosphate cements for bone defect repairing

Modified macroporous structures and active osteogenic substances are necessary to overcome the limited bone regeneration capacity and low degradability of self-curing calcium phosphate cement (CPC). Curcumin (CUR), which possesses strong osteogenic activity and poor aqueous solubility/bioavailability, esterifies the side chains in hyaluronic acid (HA) to form a water-soluble CUR-HA macromolecule. In this study, we incorporated the CUR-HA and glucose microparticles (GMPs) into the CPC powder to fabricate the CUR-HA/GMP/CPC composite, which not only retained the good injectability and mechanical strength of bone cements, but also significantly increased the cement porosity and sustained release property of CUR-HA in vitro. CUR-HA incorporation greatly improved the differentiation ability of bone marrow mesenchymal stem cells (BMSCs) to osteoblasts by activating the RUNX family transcription factor 2/fibroblast growth factor 18 (RUNX2/FGF18) signaling pathway, increasing the expression of osteocalcin and enhancing the alkaline phosphatase activity. In addition, in vivo implantation of CUR-HA/GMP/CPC into femoral condyle defects dramatically accelerated the degradation rate of cement and boosted local vascularization and osteopontin protein expression, and consequently promoted rapid bone regeneration. Therefore, macroporous CPC based composite cement with CUR-HA shows a remarkable ability to repair bone defects and is a promising translational application of modified CPC in clinical practice.

The osteogenesis effect of rhBMP2-loaded calcium phosphate cements in repairing dental extraction sockets

PURPOSE

After extracting impacted mandibular third molars (IMM3), the resulting bone loss at the distal surface of the distal root of mandibular second molars (MM2) is responsible for the poor stability of MM2. This study aimed to identify the clinical osteogenesis effect of recombinant human bone morphogenetic protein-2 (rhBMP-2)-loaded calcium phosphate cements (CPCs) and rhBMP-2 delivery systems (rhBMP-2/CPCs, named CPCII) on bone loss repair at the distal surface of the MM2 distal root after IMM3 extraction.

METHODS

Written informed consent was obtained from every participant whose IMM3 needed extraction. The impact of IMM3 on both sides was basically identical. From April 2014 to March 2016, extraction of IMM3 was performed in 9 patients (5 males/4 females, 26-42 years old). One side was randomly selected as the experimental group, and CPCII systems were implanted into the distal surface of the distal root in dental extraction sockets. The wounds on the other side were sutured and allowed to heal naturally (be treated as the control group). New bone formation in the alveolar fossa was detected 3 and 12 months after the operation by cone-beam computed tomography (CBCT) to measure the distance from the cementoenamel junction (CEJ) to the crest of the alveolar ridge (CAR).

RESULTS

The CAR-CEJ distance on the test side was less than that on the control side (P<0.5).

CONCLUSION

The quantity of new bone formation in the experimental group was greater than that in the control group. CPCII systems have osteogenic potential in the healing process of tooth extraction sockets.

A review of biomimetic scaffolds for bone regeneration: Toward a cell-free strategy

In clinical terms, bone grafting currently involves the application of autogenous, allogeneic, or xenogeneic bone grafts, as well as natural or artificially synthesized materials, such as polymers, bioceramics, and other composites. Many of these are associated with limitations. The ideal scaffold for bone tissue engineering should provide mechanical support while promoting osteogenesis, osteoconduction, and even osteoinduction. There are various structural complications and engineering difficulties to be considered. Here, we describe the biomimetic possibilities of the modification of natural or synthetic materials through physical and chemical design to facilitate bone tissue repair. This review summarizes recent progresses in the strategies for constructing biomimetic scaffolds, including ion-functionalized scaffolds, decellularized extracellular matrix scaffolds, and micro- and nano-scale biomimetic scaffold structures, as well as reactive scaffolds induced by physical factors, and other acellular scaffolds. The fabrication techniques for these scaffolds, along with current strategies in clinical bone repair, are described. The developments in each category are discussed in terms of the connection between the scaffold materials and tissue repair, as well as the interactions with endogenous cells. As the advances in bone tissue engineering move toward application in the clinical setting, the demonstration of the therapeutic efficacy of these novel scaffold designs is critical.

Calcium phosphate cements: Optimization toward biodegradability

Synthetic calcium phosphate (CaP) ceramics represent the most widely used biomaterials for bone regenerative treatments due to their biological performance that is characterized by bioactivity and osteoconductive properties. From a clinical perspective, injectable CaP cements (CPCs) are highly appealing, as CPCs can be applied using minimally invasive surgery and can be molded to optimally fill irregular bone defects. Such CPCs are prepared from a powder and a liquid component, which upon mixing form a paste that can be injected into a bone defect and hardens in situ within an appropriate clinical time window. However, a major drawback of CPCs is their poor degradability. Ideally, CPCs should degrade at a suitable pace to allow for concomitant new bone to form. To overcome this shortcoming, control over CPC degradation has been explored using multiple approaches that introduce macroporosity within CPCs. This strategy enables faster degradation of CPC by increasing the surface area available to interact with the biological surroundings, leading to accelerated new bone formation. For a comprehensive overview of the path to degradable CPCs, this review presents the experimental procedures followed for their development with specific emphasis on (bio)material properties and biological performance in pre-clinical bone defect models.

Fast dissolving glucose porogens for early calcium phosphate cement degradation and bone regeneration

Here, we demonstrate the in vivo efficacy of glucose microparticles (GMPs) to serve as porogens within calcium phosphate cements (CPCs) to obtain a fast-degrading bone substitute material. Composites were fabricated incorporating 20 wt% GMPs at two different GMP size ranges (100-150 μm (GMP-S) and 150-300 μm (GMP-L)), while CPC containing 20 wt% poly(lactic-co-glycolic acid) microparticles (PLGA) and plain CPC served as controls. After 2 and 8 weeks implantation in a rat femoral condyle defect model, specimens were retrieved and analyzed for material degradation and bone formation. Histologically, no adverse tissue response to any of the CPC-formulations was observed. All CPC-porogen formulations showed faster degradation compared to plain CPC control, but only GMP-containing formulations showed higher amounts of new bone formation compared to plain CPC controls. After 8 weeks, only CPC-porogen formulations with GMP-S or PLGA porogens showed higher degradation compared to plain CPC controls. Overall, the inclusion of GMPs into CPCs resulted in a macroporous structure that initially accelerated the generation of new bone. These findings highlight the efficacy of a novel approach that leverages simple porogen properties to generate porous CPCs with distinct degradation and bone regeneration profiles.

Reconstruction of Large Skeletal Defects: Current Clinical Therapeutic Strategies and Future Directions Using 3D Printing

The healing of bone fractures is a well-orchestrated physiological process involving multiple cell types and signaling molecules interacting at the fracture site to replace and repair bone tissue without scar formation. However, when the lesion is too large, normal healing is compromised. These so-called non-union bone fractures, mostly arising due to trauma, tumor resection or disease, represent a major therapeutic challenge for orthopedic and reconstructive surgeons. In this review, we firstly present the current commonly employed surgical strategies comprising auto-, allo-, and xenograft transplantations, as well as synthetic biomaterials. Further to this, we discuss the multiple factors influencing the effectiveness of the reconstructive therapy. One essential parameter is adequate vascularization that ensures the vitality of the bone grafts thereby supporting the regeneration process, however deficient vascularization presents a frequently encountered problem in current management strategies. To address this challenge, vascularized bone grafts, including free or pedicled fibula flaps, or in situ approaches using the Masquelet induced membrane, or the patient’s body as a bioreactor, comprise feasible alternatives. Finally, we highlight future directions and novel strategies such as 3D printing and bioprinting which could overcome some of the current challenges in the field of bone defect reconstruction, with the benefit of fabricating personalized and vascularized scaffolds.

Porous titanium fiber mesh with tailored elasticity and its effect on stromal cells

Porous titanium fiber mesh (TFM) is considered a suitable scaffold material for bone reconstruction. Also, TFM can be used to cover the surface of bone‐anchored devices, that is, orthopedic or dental implants. The titanium fiber size has an effect of the stiffness as well as porosity of the titanium mesh, which can influence the behavior of bone forming cells. Therefore, the aim of this study was to vary TFM composition, in order to achieve different stiffness, and to assess the effects of such variation on the behavior of bone marrow‐derived stromal cells (BMSCs). With that purpose, nine types of TFM (porosities 60–87%; fiber size 22–50 μm), were examined for their mechanical properties as well as their effect on the proliferation and differentiation of rat bone marrow‐derived stromal cells (rBMSCs) up to 21 days. Dynamic mechanical analysis revealed that the stiffness of TFM were lower than of solid titanium and decreased with larger fiber sizes. The stiffness could effectively be tailored by altering fiber properties, which altered the pore simultaneously. For the 22 and 35 μm size fiber meshes with the highest porosity, the stiffness closely matched the value found in literature for cortical bone. Finally, all tested TFM types supported the growth and differentiation of rBMSCs. We concluded that TFM material has been proven cytocompatible. Further preclinical studies are needed to assess which TFM type is most suitable as clinical use for bone ingrowth and bone regeneration.

Resorption of the calcium phosphate layer on S53P4 bioactive glass by osteoclasts

Clinically, S53P4 bioactive glass (BAG) has shown very promising results in bone infection treatment, but it is also known to degrade very slowly in vivo. To evaluate which mechanisms (cellular or dissolution) can play a role in the degradation of S53P4 BAG and S53P4 BAG putty, in vitro degradation experiments at different pH (7.4 and 4.6) were performed. Micro computed tomography showed a rapid dissolution of the synthetic binder in the putty formulation, within 12 h is simulated body fluid (pH = 7.4), leaving behind only loose granules. Therefore the degradation of the loose granules was investigated further. Significant weight loss was observed and ion chromatography showed that Ca2+, Na+ and PO43- ions were released from S54P4 BAG granules in the two fluids. It was observed that the weight loss and ion release were increased when the pH of the fluid was decreased to 4.6. Osteoclasts are known to create such a low pH when resorbing bone and therefore their capacity to degrade S53P4 surfaces were studied as well. Scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed that osteoclasts were able to create resorption pits in the calcium phosphate layer on S53P4 BAG surfaces. The silica of the BAG, located underneath the calcium phosphate, seemed to hinder further osteclastic resorption of the material. To our knowledge we were the first to observe actively resorbing osteoclasts on S53P4 bioactive glass surfaces, in vitro. Future research is needed to define the specific role osteoclasts play in the degradation of BAG in vivo.

Multimodal porogen platforms for calcium phosphate cement degradation

Calcium phosphate cements (CPCs) represent excellent bone substitute materials due to their biocompatibility and injectability. However, their poor degradability and lack of macroporosity limits bone regeneration. The addition of poly(d,l-lactic-co-glycolic acid) (PLGA) particles improves macroporosity and therefore late stage material degradation. CPC degradation and hence, bone formation at an early stage remains challenging, due to the delayed onset of PLGA degradation (i.e., after 2-3 weeks). Consequently, we here explored multimodal porogen platforms based on sucrose porogens (for early pore formation) and PLGA porogens (for late pore formation) to enhance CPC degradation and analyzed mechanical properties, dynamic in vitro degradation and in vivo performance in a rat femoral bone defect model. Porogen addition to CPC showed to decrease compressive strength of all CPC formulations; transition of the crystal phase upon in vitro incubation increased compressive strength. Although dynamic in vitro degradation showed rapid sucrose dissolution within 1 week, no additional effects on CPC degradation or bone formation were observed upon in vivo implantation. © 2019 The Authors. journal Of Biomedical Materials Research Part A Published By Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1713-1722, 2019.

Alendronate release from calcium phosphate cement for bone regeneration in osteoporotic conditions

Osteoporosis represents a major health problem in terms of compromising bone strength and increasing the risk of bone fractures. It can be medically treated with bisphosphonates, which act systemically upon oral or venous administration. Further, bone regenerative treatments in osteoporotic conditions present a challenge. Here, we focused on the development of a synthetic bone substitute material with local diminishing effects on osteoporosis. Composites were created using calcium phosphate cement (CPC; 60 wt%) and polylactic-co-glycolic acid (PLGA; 40 wt%), which were loaded with alendronate (ALN). In vitro results showed that ALN-loaded CPC/PLGA composites presented clinically suitable properties, including setting times, appropriate compressive strength, and controlled release of ALN, the latter being dependent on composite degradation. Using a rat femoral condyle bone defect model in osteoporotic animals, ALN-loaded CPC/PLGA composites demonstrated stimulatory effects on bone formation both within and outside the defect region.

Incorporation of fast dissolving glucose porogens and poly(lactic-co-glycolic acid) microparticles within calcium phosphate cements for bone tissue regeneration

This study investigated the effects of incorporating glucose microparticles (GMPs) and poly(lactic-co-glycolic acid) microparticles (PLGA MPs) within a calcium phosphate cement on the cement's handling, physicochemical properties, and the respective pore formation. Composites were fabricated with two different weight fractions of GMPs (10 and 20 wt%) and two different weight fractions of PLGA MPs (10 and 20 wt%). Samples were assayed for porosity, pore morphology, and compressive mechanical properties. An in vitro degradation study was also conducted. Samples were exposed to a physiological solution for 3 days, 4 wks, and 8 wks in order to understand how the inclusion of GMPs and PLGA MPs affects the composite's porosity and mass loss over time. GMPs and PLGA MPs were both successfully incorporated within the composites and all formulations showed an initial setting time that is appropriate for clinical applications. Through a main effects analysis, we observed that the incorporation of GMPs had a significant effect on the overall porosity, mean pore size, mode pore size, and in vitro degradation rate of PLGA MPs as early as after 3 days (p < 0.05). After 4 wks and 8 wks, these same properties were affected by the inclusion of both types of MPs (p < 0.05). Advanced polymer chromatography confirmed that the degradation of PLGA MPs coincided with an increase in composite porosity, mean pore size, and mode pore size. Finally, it was observed that the inclusion of GMPs slowed the degradation of PLGA MPs in vitro and reduced the solution acidity due to PLGA degradation products. Our results suggest that the dual inclusion of GMPs and PLGA MPs is a valuable approach for the generation of early macropores, while also mitigating the effect of acidic degradation products from PLGA MPs on their degradation kinetics.

STATEMENT OF SIGNIFICANCE

A multitude of strategies and techniques have been investigated for the introduction of macropores with calcium phosphate cements (CPC). However, many of these strategies take several weeks to months to generate a maximal porosity or the degradation products of the porogen can trigger a localized inflammatory response in vivo. As such, it was hypothesized that the fast dissolution of glucose microparticles (GMPs) in a CPC composite also incorporating poly(lactic-co-glycolic acid) (PLGA) microparticles (MPs) will create an initial macroporosity and increase the surface area within the CPC, thus enhancing the diffusion of PLGA degradation products and preventing a significant decrease in pH. Furthermore, as PLGA degradation occurs over several weeks to months, additional macroporosity will be generated at later time points within CPCs. The results offer a new method for generating macroporosity in a multimodal fashion that also mitigates the effects of acidic degradation products.

Bioinspired Mineral-Organic Bioresorbable Bone Adhesive

Bioresorbable bone adhesives have potential to revolutionize the clinical treatment of the human skeletal system, ranging from the fixation and osteointegration of permanent implants to the direct healing and fusion of bones without permanent fixation hardware. Despite an unmet need, there are currently no bone adhesives in clinical use that provide a strong enough bond to wet bone while possessing good osteointegration and bioresorbability. Inspired by the sandcastle worm that creates a protective tubular shell around its body using a proteinaceous adhesive, a novel bone adhesive is introduced, based on tetracalcium phosphate and phosphoserine, that cures in minutes in an aqueous environment and provides high bone-to-bone adhesive strength. The new material is measured to be 10 times more adhesive than bioresorbable calcium phosphate cement and 7.5 times more adhesive than non-resorbable poly(methyl methacrylate) bone cement, both of which are standard of care in the clinic today. The bone adhesive also demonstrates chemical adhesion to titanium approximately twice that of its adhesion to bone, unlocking the potential for adherence to metallic implants during surrounding bony incorporation. Finally, the bone adhesive is shown to demonstrate osteointegration and bioresorbability over a 52-week period in a critically sized distal femur defect in rabbits.