Bone Generation in the Reconstruction of a Critical Size Calvarial Defect in an Experimental Model

The Investigation of LRP5-Loaded Composite with Sustained Release Behavior and Its Application in Bone Repair

Low-density lipoprotein receptor-related protein 5 (LRP5) plays a vital role in bone formation and regeneration. In this study, we developed an injectable and sustained-release composite loading LRP5 which could gelatinize in situ. The sustained release of the composite and its efficacy in bone regeneration were evaluated. Sodium alginate, collagen, hydroxyapatite, and LRP5 formed the composite LRP5-Alg/Col/HA. It was found that the initial setting time and final setting time of LRP5-Alg/Col/HA containing 4% alginate were suitable for surgical operation. When the composite was loaded with 40 μg/mL LRP5, LRP5-Alg/Col/HA did not exhibit a burst-release behavior and could sustainably release LRP5 up to 21 days. Up to 18 days, LRP5 released from LRP5-Alg/Col/HA still present the binding activity with DKK1 (Wnt signaling pathway antagonist) and could increase the downstream β-catenin mRNA in bone marrow mesenchymal stem cells. Moreover, LRP5-Alg/Col/HA was found to significantly increase bone mineral density in the defect area after 6 weeks’ implantation of LRP5-Alg/Col/HA into the rats’ calvarial defect area. H&E staining detection demonstrated that LRP5-Alg/Col/HA could mediate the formation of a new bone tissue. Therefore, we concluded that Alg/Col/HA was a suitable sustained-release carrier for LRP5 and LRP5-Alg/Col/HA had a significant effect on repairing bone defects and could be a good bone regeneration material.

New and emerging strategies in platelet-rich plasma application in musculoskeletal regenerative procedures: general overview on still open questions and outlook

Despite its pervasive use, the clinical efficacy of platelet-rich plasma (PRP) therapy and the different mechanisms of action have yet to be established. This overview of the literature is focused on the role of PRP in bone, tendon, cartilage, and ligament tissue regeneration considering basic science literature deriving from in vitro and in vivo studies. Although this work provides evidence that numerous preclinical studies published within the last 10 years showed promising results concerning the application of PRP, many key questions remain unanswered and controversial results have arisen. Additional preclinical studies are needed to define the dosing, timing, and frequency of PRP injections, different techniques for delivery and location of delivery, optimal physiologic conditions for injections, and the concomitant use of recombinant proteins, cytokines, additional growth factors, biological scaffolds, and stems cells to develop optimal treatment protocols that can effectively treat various musculoskeletal conditions.

Sustained delivery of rhBMP-2 by means of poly(lactic-co-glycolic acid) microspheres: cranial bone regeneration without heterotopic ossification or craniosynostosis

BACKGROUND

Commercially available recombinant human bone morphogenetic protein 2 (rhBMP2) has demonstrated efficacy in bone regeneration, but not without significant side effects. The authors used rhBMP2 encapsulated in poly(lactic-co-glycolic acid) (PLGA) microspheres placed in a rabbit cranial defect model to test whether low-dose, sustained delivery can effectively induce bone regeneration.

METHODS

The rhBMP2 was encapsulated in 15% PLGA using a double-emulsion, solvent extraction/evaporation technique, and its release kinetics and bioactivity were tested. Two critical-size defects (10 mm) were created in the calvaria of New Zealand white rabbits (5 to 7 months of age, male and female) and filled with a collagen scaffold containing either (1) no implant, (2) collagen scaffold only, (3) PLGA-rhBMP2 (0.1 μg per implant), or (4) free rhBMP2 (0.1 μg per implant). After 6 weeks, the rabbits were killed and defects were analyzed by micro-computed tomography, histology, and finite element analysis.

RESULTS

The rhBMP2 delivered by means of bioactive PLGA microspheres resulted in higher volumes and surface area coverage of new bone than an equal dose of free rhBMP2 by micro-computed tomography (p=0.025 and p=0.025). Finite element analysis indicated that the mechanical competence using the regional elastic modulus did not differ with rhBMP2 exposure (p=0.70). PLGA-rhBMP2 did not demonstrate heterotopic ossification, craniosynostosis, or seroma formation.

CONCLUSIONS

Sustained delivery by means of PLGA microspheres can significantly reduce the rhBMP2 dose required for de novo bone formation. Optimization of the delivery system may be a key to reducing the risk for recently reported rhBMP2-related adverse effects.

Effect of recombinant human bone morphogenetic protein 2 associated with a variety of bone substitutes on vertical guided bone regeneration in rabbit calvarium

BACKGROUND

A major challenge for dental implantology is to consistently obtain appropriate bone augmentation before implant placement. The aim of this study is to evaluate the effect of recombinant human bone morphogenetic protein 2 (rhBMP-2) associated with bone substitute materials beta-tricalcium phosphate (β-TCP), biphasic calcium phosphate (BCP), and bovine bone mineral on vertical guided bone regeneration (GBR) in rabbit calvarium.

METHODS

Four titanium cylinders were fixed to the calvarium of 22 rabbits. In group 1 (n = 10), three cylinders were randomly filled with one of the test materials, and one cylinder was filled with a blood clot (CL). In group 2 (n = 12), the cylinders were randomly assigned to the same materials and CL but with the addition of rhBMP-2. Bone labels were injected over the course of 13 weeks, and euthanasia was performed 14 weeks after surgery in both groups.

RESULTS

The mean volume and area of tissue growth was greater in group 2 (with rhBMP-2) than in group 1 (without rhBMP-2), irrespective of the material used (P <0.001). The mean volume of tissue growth in the CL cylinder was smaller than that observed with all other materials (P <0.001) in both groups. The mean area of regenerated bone in the CL cylinder was smaller than that observed in the β-TCP cylinder (P = 0.028). The histologic study revealed more lamellar bone in the rhBMP-2 group, with a greater level of biodegradation of all the bone substitute materials tested.

CONCLUSION

The use of rhBMP-2/absorbable collagen sponge (ACS) combined with all of the bone substitute materials tested resulted in a greater amount of bone formation than that produced with the bone substitute materials alone or rhBMP-2/(ACS) and CL using the rabbit calvarium GBR model.

Repair of rat calvarial bone defects by controlled release of rhBMP-2 from an injectable bone regeneration composite

The objective of the present study was to enhance the regeneration ability of an injectable bone regeneration composite (IBRC) by the controlled release of recombinant human bone morphogenetic protein-2 (rhBMP-2). The IBRC comprised nano-hydroxyapatite/collagen (nHAC) particles in an alginate hydrogel carrier. First, bovine serum albumin (BSA) as a model protein was released from IBRC to evaluate its release rules. The results suggested that IBRC is a good controlled release carrier for BSA in the range 5-75 µg/ml. In the in vitro study the rhBMP-2 released from IBRC was determined by an enzyme-linked immunosorbent assay specific for rhBMP-2. The bioactivity of the released rhBMP-2 was evaluated through differentiated function of marrow mesenchymal stem cells (MSCs), as measured by alkaline phosphatase activity. The results of an in vitro study confirmed that rhBMP-2 released continuously for 21 days, and its bioactivity was well preserved during this period. The bone formation ability was assessed using a rat calvarial defect model of critical size. Micro-computed tomography (micro-CT) and histological analysis demonstrated that the IBRC had good bone formation ability, which was promoted through rhBMP-2 released from IBRC/rhBMP-2. In vitro and in vivo studies suggested that the present system is a potential bone critical defect repair material for clinical applications.

Biomaterials for craniofacial reconstruction

Biomaterials for reconstruction of bony defects of the skull comprise of osteosynthetic materials applied after osteotomies or traumatic fractures and materials to fill bony defects which result from malformation, trauma or tumor resections. Other applications concern functional augmentations for dental implants or aesthetic augmentations in the facial region. For ostheosynthesis, mini- and microplates made from titanium alloys provide major advantages concerning biocompatibility, stability and individual fitting to the implant bed. The necessity of removing asymptomatic plates and screws after fracture healing is still a controversial issue. Risks and costs of secondary surgery for removal face a low rate of complications (due to corrosion products) when the material remains in situ. Resorbable osteosynthesis systems have similar mechanical stability and are especially useful in the growing skull. The huge variety of biomaterials for the reconstruction of bony defects makes it difficult to decide which material is adequate for which indication and for which site. The optimal biomaterial that meets every requirement (e.g. biocompatibility, stability, intraoperative fitting, product safety, low costs etc.) does not exist. The different material types are (autogenic) bone and many alloplastics such as metals (mainly titanium), ceramics, plastics and composites. Future developments aim to improve physical and biological properties, especially regarding surface interactions. To date, tissue engineered bone is far from routine clinical application.

Effects of VEGF loading on scaffold-confined vascularization

Adequate vascularization of tissue-engineered constructs remains a major challenge in bone grafting. In view of this, we loaded ß-tricalcium-phosphate (ß-TCP) and porous poly(L-lactide-co-glycolide) (PLGA) scaffolds via collagen coating with vascular endothelial growth factor (VEGF) and studied whether the VEGF loading improves scaffold angiogenesis and vascularization. Dorsal skinfold chambers were implanted into 48 balb/c mice, which were assigned to 6 groups (n = 8 each). Uncoated (controls), collagen-coated, and additionally VEGF-loaded PLGA and ß-TCP scaffolds were inserted into the chambers. Angiogenesis, neovascularization, and leukocyte-endothelial cell interaction were analyzed repeatedly during a 14-day observation period using intravital fluorescence microscopy. Furthermore, VEGF release from PLGA und ß-TCP scaffolds was studied by ELISA. Micromorphology was studied from histological specimens. Unloaded ß-TCP scaffolds showed an accelerated and increased angiogenic response when compared with unloaded PLGA scaffolds. In vitro, PLGA released significantly higher amounts of VEGF compared with ß-TCP at the first two days resulting in a rapid drop of the released amount at the following days up to day 7 where the VEGF release was negligible. Nonetheless, in vivo VEGF loading increased neovascularization, especially in ß-TCP scaffolds. This increased vascularization was associated with a temporary leukocytic response with pronounced leukocyte-endothelial cell interaction at days 3 and 6. Histology revealed adequate host tissue response and engraftment of both ß-TCP and PLGA scaffolds. Our study demonstrates that ß-TCP scaffolds offer more suitable conditions for vascularization than PLGA scaffolds, in particular if they are loaded with VEGF.

The design and use of animal models for translational research in bone tissue engineering and regenerative medicine

This review provides an overview of animal models for the evaluation, comparison, and systematic optimization of tissue engineering and regenerative medicine strategies related to bone tissue. This review includes an overview of major factors that influence the rational design and selection of an animal model. A comparison is provided of the 10 mammalian species that are most commonly used in bone research, and existing guidelines and standards are discussed. This review also identifies gaps in the availability of animal models: (1) the need for assessment of the predictive value of preclinical models for relative clinical efficacy, (2) the need for models that more effectively mimic the wound healing environment and mass transport conditions in the most challenging clinical settings (e.g., bone repair involving large bone and soft tissue defects and sites of prior surgery), and (3) the need for models that allow more effective measurement and detection of cell trafficking events and ultimate cell fate during the processes of bone modeling, remodeling, and regeneration. The ongoing need for both continued innovation and refinement in animal model systems, and the need and value of more effective standardization are reinforced.

Effect of recombinant human bone morphogenetic protein-2 on bone regeneration in large defects of the growing canine skull after dura mater replacement with a dura mater substitute

OBJECT

This study was designed to evaluate the bone regeneration potential of the dura mater and dura mater substitute (Durepair) in the presence of recombinant human bone morphogenetic protein-2 (rhBMP-2) delivered in a collagen sponge-collagen-ceramic matrix (CCM; MasterGraft Matrix) in a large skull defect in growing canines.

METHODS

Forty immature male beagles were used to create two 2.5 x 4-cm cranial defects on each side of the sagittal suture. The dura mater on the left side was cut to make a 1 x 3-cm defect and replaced with bovine skin collagen (Durepair). The dura mater on the right side remained intact. Different doses of rhBMP-2 (none [8 animals], 0.11 mg/ml [4 animals], 0.21 mg/ml [4 animals], and 0.43 mg/ml [8 animals]) were infused on 2 Type I bovine absorbable collagen sponge (ACS) strips. The strips were layered with the CCM (15% hydroxyapatite [HA]/85% tricalcium phosphate [TCP]) to reconstruct both cranial defects. In a fifth group (8 animals), 0.43 mg/ml rhBMP-2 was directly infused into the CCM. Demineralized canine cancellous freeze-dried demineralized bone matrix (DBM; 8 animals) was used as a control in a sixth group. All materials were fixed under 2 resorbable protective sheets (MacroPore). Skulls were resected 16 weeks after operation. Histological and histomorphometric analyses on the percentage of the defect spanned by bone, and the percentage of residual HA-TCP granules and collagen were analyzed.

RESULTS

Calcified seroma was the only complication observed and only occurred in the 0.43-mg/ml rhBMP-2 groups (Groups 4 and 5). Dura mater repair appeared complete at 4 months in all animals. New bone was formed sporadically throughout the skull defect in the ACS+CCM and DBM groups without rhBMP-2. In all rhBMP-2 groups, mature new bone (compact and trabecular) was uniformly formed across the defect on both the repaired and intact dura mater sides. There was significant new compact bone formation on top of the repaired dura mater, which did not appear in the ACS+CCM and DBM groups lacking rhBMP-2. Greater HA-TCP and collagen scaffold resorption was noted in rhBMP-2 groups compared with non-rhBMP-2 groups. Statistical analysis showed there was a significantly lower percentage of bone spanning the defect in the ACS+CCM group compared with groups with rhBMP-2, with more residual HA-TCP and collagen on the repaired dura mater side than the intact dura mater side (p < 0.05). In all rhBMP-2 groups, there were no significant differences in new bone formation between the repaired and intact dura mater sides (p > 0.05).

CONCLUSIONS

The ACS+CCM combination had an effect similar to demineralized bone-on-bone regeneration in craniofacial reconstruction. The addition of rhBMP-2 to CCM directly or with ACS induces mature new bone formation in large cranial defects both in the presence of intact dura mater and repaired dura mater.

Effects of recombinant human bone morphogenetic protein-2 on midsagittal sutural bone formation during expansion

INTRODUCTION

The goal of this study was to evaluate whether human recombinant bone morphogenetic protein-2 (rhBMP-2) enhances sutural bone formation or causes premature sutural fusion.

METHODS

Thirty 6-week-old rabbits underwent midsagittal sutural expansion. The animals were randomly assigned to receive 0 (control), 0.1 mg per milliliter, or 0.4 mg per milliliter of rhBMP-2, delivered by an absorbable collagen sponge placed over the suture. A 100-g constant force was delivered for 33 days by using a nickel-titanium spring to expand the suture between 2 miniscrew implants anchored in the frontal bone. At days 10, 20, and 30, sutural separation was evaluated and modeled over time as polynomials by using multilevel statistical procedures. Bone formation and sutural gaps were analyzed histomorphometrically between days 10 and 20 and days 20 and 30.

RESULTS

The control group showed significantly greater overall sutural bone formation than did the 2 rhBMP-2 groups. Over time, bone formation decreased significantly in all groups. Between days 10 and 20, the 0.4 mg per milliliter group produced significantly more (58%) bone than did the 0.1 mg per milliliter group; there were no significant differences in bone formation between the 2 experimental groups between days 20 and 30. Both 0.1 and 0.4 mg per milliliter of rhBMP-2 in the absorbable collagen sponge caused premature fusion by forming a bony bridge connecting the ectocranial aspect of the sutural margins. Premature fusion significantly reduced sutural separation between 10 and 30 days (to 56% and 62% of control values for the 0.1 and 0.4 mg per milliliter groups, respectively). There were no significant differences in sutural separation between the 0.1 and 0.4 mg per milliliter groups.

CONCLUSIONS

Compared with the 0.1 mg per milliliter group, 0.4 mg per milliliter of rhBMP-2 accelerated sutural bone formation between days 10 and 20. After 10 to 20 days, rhBMP-2 in the absorbable collagen sponge caused premature sutural fusion, despite the constant expansion forces.

The challenge of establishing preclinical models for segmental bone defect research

A considerable number of international research groups as well as commercial entities work on the development of new bone grafting materials, carriers, growth factors and specifically tissue-engineered constructs for bone regeneration. They are strongly interested in evaluating their concepts in highly reproducible large segmental defects in preclinical and large animal models. To allow comparison between different studies and their outcomes, it is essential that animal models, fixation devices, surgical procedures and methods of taking measurements are well standardized to produce reliable data pools and act as a base for further directions to orthopaedic and tissue engineering developments, specifically translation into the clinic. In this leading opinion paper, we aim to review and critically discuss the different large animal bone defect models reported in the literature. We conclude that most publications provide only rudimentary information on how to establish relevant preclinical segmental bone defects in large animals. Hence, we express our opinion on methodologies to establish preclinical critically sized, segmental bone defect models used in past research with reference to surgical techniques, fixation methods and postoperative management focusing on tibial fracture and segmental defect models.