Bone healing process in critical-sized defects by rhBMP-2 using poloxamer gel and collagen sponge as carriers

Poloxamer-Based Scaffolds for Tissue Engineering Applications: A Review

Poloxamer is a triblock copolymer with amphiphilicity and reversible thermal responsiveness and has wide application prospects in biomedical applications owing to its multifunctional properties. Poloxamer hydrogels play a crucial role in the field of tissue engineering and have been regarded as injectable scaffolds for loading cells or growth factors (GFs) in the last few years. Hydrogel micelles can maintain the integrity and stability of cells and GFs and form an appropriate vascular network at the application site, thus creating an appropriate microenvironment for cell growth, nerve growth, or bone integration. The injectability and low toxicity of poloxamer hydrogels make them a noninvasive method. In addition, they can also be good candidates for bio-inks, the raw material for three-dimensional (3D) printing. However, the potential of poloxamer hydrogels has not been fully explored owing to the complex biological challenges. In this review, the latest progress and cutting-edge research of poloxamer-based scaffolds in different fields of application such as the bone, vascular, cartilage, skin, nervous system, and organs in tissue engineering and 3D printing are reviewed, and the important roles of poloxamers in tissue engineering scaffolds are discussed in depth.

Sustained Delivery of Lactoferrin Using Poloxamer Gels for Local Bone Regeneration in a Rat Calvarial Defect Model

Lactoferrin (LF) is a multifunctional milk glycoprotein that promotes bone regeneration. Local delivery of LF at the bone defect site is a promising approach for enhancement of bone regeneration, but efficient systems for sustained local delivery are still largely missing. The aim of this study was to investigate the potential of the poloxamers for sustained delivery of LF to enhance local bone regeneration. The developed LF/poloxamer formulations were liquid at room temperature (20 °C) transforming to a sustained releasing gel depot at body temperature (37 °C). In vitro release studies demonstrated an initial burst release (~50%), followed by slower release of LF for up to 72 h. Poloxamer, with and without LF, increased osteoblast viability at 72 h (p < 0.05) compared to control, and the immune response from THP-1 cells was mild when compared to the suture material. In rat calvarial defects, the LF/poloxamer group had lower bone volume than the controls (p = 0.0435). No difference was observed in tissue mineral density and lower bone defect coverage scores (p = 0.0267) at 12 weeks after surgery. In conclusion, LF/poloxamer formulations support cell viability and do not induce an unfavourable immune response; however, LF delivery via the current formulation of LF200/poloxamer gel did not demonstrate enhanced bone regeneration and was not compatible with the rat calvarial defect model.

The efficacy of sustained-release chitosan microspheres containing recombinant human parathyroid hormone on MRONJ

Treatment of patients with bisphosphonate usage is a significant concern for oral surgeons because it interferes with jaw bone turnover and regeneration. In case of adverse effects manifesting related to bisphosphonate use, oral surgeons are usually treating and keep the patient's symptoms under control. In this study, we aimed to investigate a new treatment protocol for medication-related osteonecrosis of the jaw (MRONJ). This treatment protocol consisted of administering human parathyroid hormone (hPTH) loaded chitosan microspheres which were prepared by ionotropic gelation method or/and the prepared microspheres were suspended in a poloxamer gel. After in-vitro optimization studies, the efficacy of the chosen formulations was evaluated in-vivo studies. Zoledronic acid was administered daily to forty-eight adult female Sprague-Dawley rats, divided into four experimental groups, at a daily concentration of 0.11 mg/kg over three weeks to induce the MRONJ model. At the end of this period, maxillary left molar teeth were extracted. In the first group, the subjects received no treatment. In the negative control group, poloxamer hydrogel containing empty microspheres were immediately applied to the soft tissues surrounding the extraction socket. The treatment group-1 was treated with local injections of poloxamer hydrogel containing hPTH. The treatment group-2 was treated with a single local injection of poloxamer hydrogel containing hPTH-loaded chitosan microspheres. Both treatment groups received a total of 7 µg of hPTH at the end of the treatment protocol. Our study demonstrates successful attenuation of MRONJ through a local drug delivery system combined with hPTH, as opposed to previously attempted treatment strategies.

Nonvascularized Bone Graft Reconstruction of the Irradiated Murine Mandible: An Analogue of Clinical Head and Neck Cancer Treatment

Nonvascularized bone grafts (NBGs) represent a practical method of mandibular reconstruction that is precluded in head and neck cancer patients by the destructive effects of radiotherapy. Advances in tissue-engineering may restore NBGs as a viable surgical technique, but expeditious translation demands a small-animal model that approximates clinical practice. This study establishes a murine model of irradiated mandibular reconstruction using a segmental iliac crest NBG for the investigation of imperative bone healing strategies. Twenty-seven male isogenic Lewis rats were divided into 2 groups; control bone graft and irradiated bone graft (XBG). Additional Lewis rats served as graft donors. The XBG group was administered a fractionated dose of 35Gy. All rats underwent reconstruction of a segmental, critical-sized defect of the left hemi-mandible with a 5 mm NBG from the iliac crest, secured by a custom radiolucent plate. Following a 60-day recovery period, hemi-mandibles were evaluated for bony union, bone mineralization, and biomechanical strength (P < 0.05). Bony union rates were significantly reduced in the XBG group (42%) compared with controls (80%). Mandibles in the XBG group further demonstrated substantial radiation injury through significant reductions in all metrics of bone mineralization and biomechanical strength. These observations are consistent with the clinical sequelae of radiotherapy that limit NBGs to nonirradiated patients. This investigation provides a clinically relevant, quantitative model in which innovations in tissue engineering may be evaluated in the setting of radiotherapy to ultimately provide the advantages of NBGs to head and neck cancer patients and reconstructive surgeons.

Injectable Shear-Thinning CaSO4/FGF-18-Incorporated Chitin-PLGA Hydrogel Enhances Bone Regeneration in Mice Cranial Bone Defect Model

For craniofacial bone regeneration, shear-thinning injectable hydrogels are favored over conventional scaffolds because of their improved defect margin adaptability, easier handling, and ability to be injected manually into deeper tissues. The most accepted method, after autografting, is the use of recombinant human bone morphogenetic protein-2 (BMP-2); however, complications such as interindividual variations, edema, and poor cost-efficiency in supraphysiological doses have been reported. The endogenous synthesis of BMP-2 is desirable, and a molecule which induces this is fibroblast growth factor-18 (FGF-18) because it can upregulate the BMP-2 expression  by supressing noggin. We developed a chitin-poly(lactide-co-glycolide) (PLGA) composite hydrogel by regeneration chemistry and then incorporated CaSO₄ and FGF-18 for this purpose. Rheologically, a 7-fold increase in the elastic modulus was observed in the CaSO₄-incorporated chitin-PLGA hydrogels as compared to the chitin-PLGA hydrogel. Shear-thinning Herschel-Bulkley fluid nature was observed for both hydrogels. Chitin-PLGA/CaSO₄ gel showed sustained release of FGF-18. In vitro osteogenic differentiation showed an enhanced alkaline phosphatase (ALP) expression in the FGF-18-containing chitin-PLGA/CaSO₄ gel when compared to cells alone. Further, it was confirmed by studying the expression of osteogenic genes [RUNX2, ALP, BMP-2, osteocalcin (OCN), and osteopontin (OPN)], immunofluorescence staining of BMP-2, OCN, and OPN, and alizarin red S staining. Incorporation of FGF-18 in the hydrogel increased the endothelial cell migration. Further, the regeneration potential of the prepared hydrogels was tested in vivo, and longitudinal live animal μ-CT was performed. FGF-18-loaded chitin-PLGA/CaSO₄ showed early and almost complete bone healing in comparison with chitin-PLGA/CaSO₄, chitin-PLGA/FGF-18, chitin-PLGA, and sham control systems, as confirmed by hematoxylin and eosin and osteoid tetrachrome stainings. This shows that the CaSO₄ and FGF-18-incorporated hydrogel is a potential candidate for craniofacial bone defect regeneration.

Biomimetic scaffolds facilitate healing of critical-sized segmental mandibular defects

OBJECTIVE

To investigate the efficacy of biomimetic PLGA scaffolds, alone and in combination with bone morphogenic protein (BMP-2) and adipose-derived stem cells (ASCs), to heal a critical-sized segmental mandibular defect in a rat model.

STUDY DESIGN

Prospective animal study.

METHODS

ASCs were isolated and cultured from the inguinal fat of Lewis rat pups. Using three-dimensional printing, PLGA scaffolds were fabricated and impregnated with BMP-2 and/or ASCs. Critical-sized 5-mm segmental mandibular defects were created in adult Lewis rats and implanted with (1) blank PLGA scaffolds, (2) PLGA scaffolds with ASCs, (3) PLGA scaffolds with BMP, or (4) PLGA scaffolds with BMP and ASCs. Animals were sacrificed at 12weeks. Bone regeneration was assessed using microCT, and graded on a semi-quantitative bone formation and bone union scale.

RESULTS

Twenty-eight rats underwent creation of segmental mandibular defects with implantation of scaffolds. Nine rats suffered complications and were excluded from analysis, leaving 19 animals for inclusion in the study. MicroCT analysis demonstrated no bridging of the segmental bony defect in rats implanted with blank scaffolds (median bone union score=0). Rats implanted with scaffolds containing BMP-2 (median bone union=2.0), ASCs (median bone union=1.5), and combination of BMP and ASCs (median bone union=1.0) demonstrated healing of critical-sized segmental mandibular defects. Bone regeneration was most robust in the BMP-2 treated scaffolds.

CONCLUSIONS

The current study utilizes a novel animal model to study the efficacy of biomimetic scaffolds carrying osteogenic factors to induce healing of a critical-sized segmental mandibular defect.

LEVEL OF EVIDENCE

N/A, Basic Science Animal Research.

Comparative study of chitosan/fibroin-hydroxyapatite and collagen membranes for guided bone regeneration in rat calvarial defects: micro-computed tomography analysis

This study aimed to utilize micro-computed tomography (micro-CT) analysis to compare new bone formation in rat calvarial defects using chitosan/fibroin-hydroxyapatite (CFB-HAP) or collagen (Bio-Gide) membranes. Fifty-four (54) rats were studied. A circular bony defect (8 mm diameter) was formed in the centre of the calvaria using a trephine bur. The CFB-HAP membrane was prepared by thermally induced phase separation. In the experimental group (n=18), the CFB-HAP membrane was used to cover the bony defect, and in the control group (n=18), a resorbable collagen membrane (Bio-Gide) was used. In the negative control group (n=18), no membrane was used. In each group, six animals were euthanized at 2, 4 and 8 weeks after surgery. The specimens were then analysed using micro-CT. There were significant differences in bone volume (BV) and bone mineral density (BMD) (P<0.05) between the negative control group and the membrane groups. However, there were no significant differences between the CFB-HAP group and the collagen group. We concluded that the CFB-HAP membrane has significant potential as a guided bone regeneration (GBR) membrane.

Defining the critical-sized defect in a rat segmental mandibulectomy model

IMPORTANCE

Advances in tissue engineering offer potential alternatives to current mandibular reconstructive techniques; however, before clinical translation of this technology, a relevant animal model must be used to validate possible interventions.

OBJECTIVE

To establish the critical-sized segmental mandibular defect that does not heal spontaneously in the rat mandible.

DESIGN AND SETTING

Prospective study of mandibular defect healing in 29 Sprague-Dawley rats in an animal laboratory.

INTERVENTIONS

The rats underwent creation of 1 of 4 segmental mandibular defects measuring 0, 1, 3, and 5 mm. All mandibular wounds were internally fixated with 1-mm microplates and screws and allowed to heal for 12 weeks, after which the animals were killed humanely.

MAIN OUTCOMES AND MEASURES

Analysis with micro-computed tomography of bony union and formation graded on semiquantitative scales.

RESULTS

Seven animals were included in each experimental group. No 5-mm segmental defects successfully developed bony union, whereas all 0- and 1-mm defects had continuous bony growth across the original defect on micro-computed tomography. Three of the 3-mm defects had bony continuity, and 3 had no healing of the bony wound. Bone union scores were significantly lower for the 5-mm defects compared with the 0-, 1-, and 3-mm defects (P < .01).

CONCLUSIONS AND RELEVANCE

The rat segmental mandible model cannot heal a 5-mm segmental mandibular defect. Successful healing of 0-, 1-, and 3-mm defects confirms adequate stabilization of bony wounds with internal fixation with 1-mm microplates. The rat segmental mandibular critical-sized defect provides a clinically relevant testing ground for translatable mandibular tissue engineering efforts.

Bone morphogenetic protein-2-impregnated biomimetic scaffolds successfully induce bone healing in a marginal mandibular defect

OBJECTIVES/HYPOTHESIS

To test the osteoregenerative potential and dosing of bone morphogenetic protein-2 (BMP-2)-impregnated biomimetic scaffolds in a rat model of a mandibular defect.

STUDY DESIGN

Prospective study using an animal model.

METHODS

Varied doses of BMP-2 (0.5, 1, 0.5, 0.5 in microspheres, 5, and 15 μg) were absorbed onto a biomimetic scaffold. Scaffolds were then implanted into marginal mandibular defects in rats. Blank scaffolds and unfilled defects were used as negative controls. Two months postoperatively, bone healing was analyzed with microcomputerized tomography (microCT).

RESULTS

MicroCT analysis demonstrated that all doses of BMP-2 induced successful healing of marginal mandibular defects in a rat mandible. Increasing doses of BMP-2 on the scaffolds produced increased tissue healing, with 15 μg demonstrating significantly more healing than all other dosing (P < .01).

CONCLUSIONS

BMP-2-impregnated biomimetic scaffolds successfully induce bone healing in a marginal mandibular defect in the rat. Percentage healing of defect, percentage of bone within healed tissue, and total bone volume are all a function of BMP-2 dosing. There appears to be an optimal dose of 5 μg beyond which there is no increase in bone volume.

LEVEL OF EVIDENCE

NA.