Ectopic bone formation after implantation of a slow release system of polylactic acid and rhBMP-2

Animal models of tendon calcification: Past, present, and future

Tendon calcification is a common clinical condition that frequently occurs as a complication after tendon injury and surgery, or as an expression of fibrodysplasia ossificans progressiva. This condition can be referred to by various names in clinical practice and literature, including tendon ossification, tendon mineralization, heterotopic ossification, and calcific tendonitis. The exact pathogenesis of tendon calcification remains uncertain, but current mainstream research suggests that calcification is mostly cell mediated. To further elucidate the pathogenesis of tendon calcification and to better simulate the overall process, selecting appropriate experimental animal models is important. Numerous animal models have been utilized in various clinical studies, each with its own set of advantages and limitations. In this review, we have discussed the advancements made in research on animal models of tendon calcification, with a focus on the selection of experimental animals, the sites of injury in these models, and the methods employed for modeling.

In vivo studies on osteoinduction: A systematic review on animal models, implant site, and type and postimplantation investigation

Musculoskeletal diseases involving loss of tissue usually require management with bone grafts, among which autografts are still the gold standard. To overcome autograft disadvantages, the development of new scaffolds is constantly increasing, as well as the number of in vivo studies evaluating their osteoinductivity in ectopic sites. The aim of the present systematic review is to evaluate the last 10 years of osteoinduction in vivo studies. The review is focused on: (a) which type of animal model is most suitable for osteoinduction evaluation; (b) what are the most used types of scaffolds; (c) what kind of post-explant evaluation is most used. Through three websites (www.pubmed.com, www.webofknowledge.com and www.embase.com), 77 in vivo studies were included. Fifty-eight studies were conducted in small animal models (rodents) and 19 in animals of medium or large size (rabbits, dogs, goats, sheep, and minipigs). Despite the difficulty in establishing the most suitable animal model for osteoinductivity studies, small animals (in particular mice) are the most utilized. Intramuscular implantation is more frequent than subcutis, especially in large animals, and synthetic scaffolds (especially CaP ceramics) are preferred than natural ones, also in combination with cells and growth factors. Paraffin histology and histomorphometric evaluations are usually employed for postimplantation analyses.

Proteins, peptides and peptidomimetics as active agents in implant surface functionalization

The recent impact of implants on improving the human life quality has been enormous. During the past two decades we witnessed major advancements in both material and structural development of implants. They were driven mainly by the increasing patients' demand and the need to address the major issues that come along with the initially underestimated complexity of the bone-implant interface. While both, the materials and design of implants reached a certain, balanced state, recent years brought a shift in focus towards the bone-implant interface as the weakest link in the increasing implant long-term usability. As a result, several approaches were developed. They aimed at influencing and enhancing the implant osseointegration and its proper behavior when under load and stress. With this review, we would like to discuss the recent advancements in the field of implant surface modifications, emphasizing the importance of chemical methods, focusing on proteins, peptides and peptidomimetics as promising agents for titanium surface coatings.

Effect of Biomaterial Electrical Charge on Bone Morphogenetic Protein-2-Induced In Vivo Bone Formation

IMPACT STATEMENT

Biomaterials can play a dual role in bone regeneration: they enable local sustained delivery of growth factors, such as bone morphogenetic protein-2 (BMP-2), while they provide structural support as scaffold. By better imitating the properties of native bone tissue, scaffolds may be both osteoconductive and osteoinductive. The latter can be achieved by modifying the electrical charge of the surface. The present work uses tunable oligo[(polyethylene glycol) fumarate] hydrogel and demonstrates that negative charge enhances BMP-2-induced bone formation compared with neutral or positive charge. Altogether, this indicates that tissue-specific surface charge modifications of biomaterials hold great promise in the field of tissue regeneration.

Comparative analysis of bone regeneration behavior using recombinant human BMP-2 versus plasmid DNA of BMP-2

Bone regeneration and the osteoinductive capacity of implants are challenging issues in clinical medicine. Currently, recombinant growth factors and nonviral gene transfer are the most frequently investigated methods for bone growth enhancement, although the more favorable method remains unclear. There is a lack of knowledge in literature about the in vivo comparison of these methods for bone regeneration. BMP-2, which is the most commonly used growth factor for osteogenesis, was applied at its most efficient dose as a recombinant growth factor (rhBMP-2) and as a growth-factor-encoding copolymer protected gene vector (pBMP-2) in a critical size bone defect (CSD) model to determine the most suitable method for bone regeneration. CSDs were induced bilaterally in 32 Sprague-Dawley rats. RhBMP-2 (62.5 μg) or pBMP-2 (2.5 μg) was embedded in poly(d,l-)lactide-coated titanium discs. Survival times were set at 14, 28, 56, and 112 days. After euthanasia, samples were analyzed via micro-computed tomography, polychrome sequential fluorescent labeling, and immunohistochemistry. Whereas defects in both groups were bridged with new bone after 56 days, rhBMP-2 initially induced ectopic new bone formation that was later remodeled in an unorganized hypodense manner. In contrast, pBMP-2 led to slower but steady bone regeneration with physiological tissue morphology, as confirmed by high osteoblast activity shown by osteocalcin staining. CD68 and TRAP staining verified high osteoclast activity for the rhBMP-2 group. pBMP-2 successfully induced locally controlled physiological bone regeneration, whereas rhBMP-2 triggered rapid and ectopic but insufficient bone formation. Thus, nonviral gene transfer appears to be more favorable for clinical applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 163-173, 2019.

Bone morphogenetic protein-2 release profile modulates bone formation in phosphorylated hydrogel

The optimal release profile of locally delivered bone morphogenetic protein-2 (BMP-2) for safe and effective clinical application is unknown. In this work, the effect of differential BMP-2 release on bone formation was investigated using a novel biomaterial oligo[(polyethylene glycol) fumarate] bis[2-(methacryloyloxy) ethyl] phosphate hydrogel (OPF-BP) containing poly(lactic-co-glycolic acid) microspheres. Three composite implants with the same biomaterial chemistry and structure but different BMP-loading methods were created: BMP-2 encapsulated in microspheres (OPF-BP-Msp), BMP-2 encapsulated in microspheres and adsorbed on the phosphorylated hydrogel (OPF-BP-Cmb), and BMP-2 adsorbed on the phosphorylated hydrogel (OPF-BP-Ads). These composites were compared with the clinically used BMP-2 carrier, Infuse® absorbable collagen sponge (ACS). Differential release profiles of bioactive BMP-2 were achieved by these composites. In a rat subcutaneous implantation model, OPF-BP-Ads and ACS generated a large BMP-2 burst release (>75%), whereas a more sustained release was seen for OPF-BP-Msp and OPF-BP-Cmb (~25% and 50% burst, respectively). OPF-BP-Ads generated significantly more bone than did all other composites, and the bone formation was 12-fold higher than that of the clinically used ACS. Overall, this study clearly shows that BMP-2 burst release generates more subcutaneous bone than do sustained release in OPF-BP-microsphere composites. Furthermore, composites should not only function as a delivery vehicle but also provide a proper framework to achieve appropriate bone formation.

Histologic and Histomorphometric Comparison of Bone Regeneration Between Bone Morphogenetic Protein-2 and Platelet-Derived Growth Factor-BB in Experimental Groups

Efficacy of recombinant human bone morphogenetic protein-2 (rhBMP-2) and recombinant human platelet-derived growth factor-BB (rhPDGF-BB) delivered via absorbable collagen sponge (ACS) on bone formation was evaluated in guinea pig tibias. Three-millimeter-circular bone tibia defects were created in 24 guinea pigs assigned randomly to 4 groups according to the following defect filling materials: ACS only, rhBMP-2+ACS, rhPDGF-BB+ACS, or empty. New bone formation was evaluated histologically and histomorphometrically at 15 (early healing) and 45 days (late healing). Mean new bone per total defect area ratio was 0.73, 0.57, 0.43, and 0.42 in rhBMP-2+ACS, rhPDGF-BB+ACS, ACS only, and empty groups at early healing, respectively. During early healing, significantly more new bone formation was observed in rhBMP-2+ACS and rhPDGF-BB+ACS groups than in the control groups. New bone formation was significantly higher with rhBMP-2+ACS than with rhPDGF-BB+ACS. Mean new bone per total defect area ratio was 0.81, 0.86, 0.74, and 0.75 in the rhBMP-2+ACS, rhPDGF-BB+ACS, ACS only, and empty groups at late healing, respectively. During late healing, new bone formation was significantly higher in the rhPDGF-BB+ACS group relative to both control groups, but the results did not differ significantly from those in the rhBMP-2+ACS group. New bone formation in the rhBMP-2+ACS group did not change significantly between the healing periods. In the rhPDGF-BB+ACS group, however, new bone formation was significantly higher in the late healing period. Both growth factors accelerated new bone formation in the early healing period. Although rhBMP-2 was more effective in the early healing period, the effects of rhPDGF-BB were longer lasting.

Growth factor-eluting technologies for bone tissue engineering

Growth factors are essential orchestrators of the normal bone fracture healing response. For non-union defects, delivery of exogenous growth factors to the injured site significantly improves healing outcomes. However, current clinical methods for scaffold-based growth factor delivery are fairly rudimentary, and there is a need for greater spatial and temporal regulation to increase their in vivo efficacy. Various approaches used to provide spatiotemporal control of growth factor delivery from bone tissue engineering scaffolds include physical entrapment, chemical binding, surface modifications, biomineralization, micro- and nanoparticle encapsulation, and genetically engineered cells. Here, we provide a brief review of these technologies, describing the fundamental mechanisms used to regulate release kinetics. Examples of their use in pre-clinical studies are discussed, and their capacities to provide tunable, growth factor delivery are compared. These advanced scaffold systems have the potential to provide safer, more effective therapies for bone regeneration than the systems currently employed in the clinic.

Continuous delivery of rhBMP2 and rhVEGF165 at a certain ratio enhances bone formation in mandibular defects over the delivery of rhBMP2 alone--An experimental study in rats

The aim of the present study was to test the hypothesis that different amounts of vascular endothelial growth factor and bone morphogenic protein differentially affect bone formation when applied for repair of non-healing defects in the rat mandible. Porous composite PDLLA/CaCO3 carriers were fabricated as slow release carriers and loaded with rhBMP2 and rhVEGF165 in 10 different dosage combinations using gas foaming with supercritical carbon dioxide. They were implanted in non-healing defects of the mandibles of 132 adult Wistar rats with additional lateral augmentation. Bone formation was assessed both radiographically (bone volume) and by histomorphometry (bone density). The use of carriers with a ratio of delivery of VEGF/BMP between 0.7 and 1.2 was significantly related to the occurrence of significant increases in radiographic bone volume and/or histologic bone density compared to the use of carriers with a ratio of delivery of ≤ 0.5 when all intervals and all outcome parameters were considered. Moreover, simultaneous delivery at this ratio helped to "save" rhBMP2 as both bone volume and bone density after 13 weeks were reached/surpassed using half the dosage required for rhBMP2 alone. It is concluded, that the combined delivery of rhVEGF165 and rhBMP2 for repair of critical size mandibular defects can significantly enhance volume and density of bone formation over delivery of rhBMP2 alone. It appears from the present results that continuous simultaneous delivery of rhVEGF165 and rhBMP2 at a ratio of approximately 1 is favourable for the enhancement of bone formation.

Solvent free production of porous PDLLA/calcium carbonate composite scaffolds improves the release of bone growth factors

PURPOSE

Incorporation of alkaline nano-/microparticles for neutralization of acidic degradation products into degradable polymer foams requires the use of organic solvents, which may compromise biocompatibility and may be associated with biological hazards. The aim of the present study was to develop and validate a solvent-free method to produce porous poly (DL-lactic acid)/calcium carbonate composite scaffolds (PDLLA/CaCO3) for controlled release of incorporated osteogenic growth factors.

METHODS

Composite PDLLA/CaCO3 granules were produced using a milling process and compared to composite material fabricated through a solution precipitation process using organic solvents. Particle size and mineral content were comparable in both groups. Supercritical carbon dioxide pressure was used to incorporate rhBMP2 into both composites.

RESULTS

Gas foaming resulted in comparable pore structures in both groups exhibiting a homogenous distribution of CaCO3 microparticles in the polymer scaffolds. The elasticity modulus of both types of scaffolds was not significantly different whereas the bending strength of the solvent-free produced scaffolds was significantly lower. The pH values remained constant between 6.90 and 7.25 during degradation of both composites. Release of BMP2 was significantly higher and the induction of alkaline phosphatase was more reliable in the group of scaffolds produced without organic solvents.

CONCLUSION

Solvent-free fabrication of composite PDLLA/CaCO3 scaffolds for controlled release of bone growth factors through gas foaming significantly enhances the release of growth factors and improves the biological efficacy of the incorporated growth factors.

Application of bone growth factors--the potential of different carrier systems

Aim

The aim of the present review was to characterize the potential of different biomaterials as carriers for bone growth factors.

Introduction

Beyond mechanical and structural characteristics, one of the features that account for a potential carrier is the possibility to couple growth factor molecules to it. As simple adsorption of the growth factor to the carrier surface by soak loading produces a burst release of growth factors with rapid decrease of biological activity, the ability to accomplish controlled release of functional growth factor molecules is one of the crucial characteristics for an appropriate carrier material.

Conclusion

The variety of carrier materials requires different strategies to either couple growth factors to the material surface or to incorporate them into the carrier matrix. The present review outlines current technical approaches and discusses future trends in the use of carrier materials for bone growth factors.