Osteodistraction of a previously irradiated mandible with or without adjunctive hyperbaric oxygenation: an experimental study in rabbits

Mimicking oxygen delivery and waste removal functions of blood

In addition to immunological and wound healing cell and platelet delivery, ion stasis and nutrient supply, blood delivers oxygen to cells and tissues and removes metabolic wastes. For decades researchers have been trying to develop approaches that mimic these two immediately vital functions of blood. Oxygen is crucial for the long-term survival of tissues and cells in vertebrates. Hypoxia (oxygen deficiency) and even at times anoxia (absence of oxygen) can occur during organ preservation, organ and cell transplantation, wound healing, in tumors and engineering of tissues. Different approaches have been developed to deliver oxygen to tissues and cells, including hyperbaric oxygen therapy (HBOT), normobaric hyperoxia therapy (NBOT), using biochemical reactions and electrolysis, employing liquids with high oxygen solubility, administering hemoglobin, myoglobin and red blood cells (RBCs), introducing oxygen-generating agents, using oxygen-carrying microparticles, persufflation, and peritoneal oxygenation. Metabolic waste accumulation is another issue in biological systems when blood flow is insufficient. Metabolic wastes change the microenvironment of cells and tissues, influence the metabolic activities of cells, and ultimately cause cell death. This review examines advances in blood mimicking systems in the field of biomedical engineering in terms of oxygen delivery and metabolic waste removal.

Four-week histologic evaluation of grafted calvarial defects with adjunctive hyperbaric oxygen therapy in rats

Purpose

The aim of this study was to characterize the healing in the grafted calvarial defects of rats after adjunctive hyperbaric oxygen therapy.

Methods

Twenty-eight male Sprague-Dawley rats (body weight, 250–300 g) were randomly divided into two treatment groups: with hyperbaric oxygen therapy (HBO; n=14) and without HBO (NHBO; n=14). Each group was further subdivided according to the bone substitute applied: biphasic calcium phosphate (BCP; n=7) and surface-modified BCP (mBCP; n=7). The mBCP comprised BCP coated with Escherichia-coli-derived recombinant human bone morphogenetic protein-2 (ErhBMP-2) and epigallocatechin-3-gallate (EGCG). Two symmetrical circular defects (6-mm diameter) were created in the right and left parietal bones of each animal. One defect was assigned as a control defect and received no bone substitute, while the other defect was filled with either BCP or mBCP. The animals were allowed to heal for 4 weeks, during which those in the HBO group underwent 5 sessions of HBO. At 4 weeks, the animals were sacrificed, and the defects were harvested for histologic and histomorphometric analysis.

Results

Well-maintained space was found in the grafted groups. Woven bone connected to and away from the defect margin was formed. More angiogenesis was found with HBO and EGCG/BMP-2 (P<0.05). None of the defects achieved complete defect closure. Increased new bone formation with HBO or EGCG/BMP-2 was evident in histologic evaluation, but it did not reach statistical significance in histometric analysis. A synergic effect between HBO and EGCG/BMP-2 was not found.

Conclusions

Within the limitations of this study, the present findings indicate that adjunctive HBO and EGCG/BMP-2 could be beneficial for new bone formation in rat calvarial defects.

A rat model of radiation injury in the mandibular area

BACKGROUND

Radiation technology focuses on delivering the radiation as precisely as possible to the tumor, nonetheless both acute and long-term damage to surrounding normal tissue may develop. Injuries to the surrounding normal tissue after radiotherapy of head and neck cancer are difficult to manage. An animal model is needed to elucidate good treatment modalities. The aim of this study was to establish a rat model where a certain radiation dose gives reproducible tissue reactions in the mandibular area corresponding to injuries obtained in humans.

METHOD

The left mandible of male Sprague Dawley rats was irradiated by external radiotherapy (single fraction 15 Gy, total dose 75 Gy) every second week five times. Endpoint was six weeks after last radiation treatment, and the test group was compared to non-irradiated controls. Morphological alterations of the soft tissues, bone and tooth formation, as well as alterations of salivation, vascularity and collagen content were assessed. An unpaired, non-parametric Mann-Whitney test was used to compare the statistical differences between the groups.

RESULTS

Analysis of the soft tissues and mandible within the radiation field revealed severe unilateral alopecia and dermatitis of the skin, extensive inflammation of the submandibular gland with loss of serous secretory cells, hyperkeratinization and dense connective fiber bundles of the gingival tissue, and disturbed tooth development with necrosis of the pulp. Production of saliva and the vascularity of the soft tissues were significantly reduced. Furthermore, the collagen fibril diameter was larger and the collagen network denser compared to non-irradiated control rats.

CONCLUSION

We have established an animal model of radiation injury demonstrating physiological and histological changes corresponding to human radiation injuries, which can be used for future therapeutic evaluations.

Comparing calvarial transport distraction with and without radiation and fat grafting

The purpose of this study is to: a) assess transport distraction to reconstruct cranial defects in radiated and non-radiated fields b) examine adipose grafting's effect on the bony regenerate and overlying wound, and c) elucidate sources of bone formation during transport distraction osteogenesis. Twenty-three male New Zealand white rabbits (3 months; 3.5 kg) were used, 10 non-irradiated and 13 irradiated (17 treatment, 6 control) with a one-time fraction of 35 Gy. A 16 × 16 mm defect was abutted by a 10 × 16 mm transport disc 5 weeks after irradiation, and 11 animals were fat grafted at the distraction site. Latency (1 day), distraction (1.5 mm/day), and consolidation (4 weeks) followed. Fluorochromes were injected subcutaneously and microCT, fluorescence, and histology assessed. In distracted animals without fat grafting, bone density measured 701.87 mgHA/ccm and 2271.95 mgHA/ccm in irradiated and non-irradiated animals. In distracted animals with fat grafting, bone density measured 703.23 mgHA/ccm and 2254.27 mgHA/ccm in irradiated and non-irradiated animals. Fluorescence revealed ossification emanating from the dura, periosteum, and transport segment with decreased formation in irradiated animals. Transport distraction is possible for cranial reconstruction in irradiated fields but short-term osseous fill is significantly diminished. Adipose grafting enhances wound healing in previously irradiated fields but does not enhance ossification.

Treatment of irradiated mandibles with mesenchymal stem cells transfected with bone morphogenetic protein 2/7

PURPOSE

The study aimed to evaluate whether mesenchymal stem cells transfected with bone morphogenetic protein (BMP) 2/7 could increase bone regeneration after radiotherapy using a rabbit model of mandibular distraction osteogenesis.

MATERIALS AND METHODS

Twelve rabbits were randomly assigned to the sham control, radiotherapy control, nontransfected mesenchymal stem cells (MSCs), and MSCs transfected with BMP-2/7 groups. All rabbits, except those in the sham control group, received preoperative radiation of 9 Gy for 5 fractions. One month after radiotherapy, all rabbits underwent unilateral mandibular distraction at a rate of 0.9 mm/d for 11 days. At the end of active distraction, MSCs combined with bovine collagen were injected into the distraction zone. After 4 weeks of consolidation, the mandibular samples were collected and subjected to radiographic, microcomputed tomographic, and histologic examinations.

RESULTS

By radiographic examination, animals injected with nontransfected MSCs or MSCs encoding BMP-2/7 exhibited more bone formation than the control groups. Histologic examination showed that the group with MSCs encoding BMP-2/7 had a more mature medullary cavity than the nontransfected MSCs group.

CONCLUSIONS

MSCs encoding BMP-2/7 can increase bone healing in irradiated mandibular bone.

A hyperbaric oxygen chamber for animal experimental purposes

Facilities for hyperbaric oxygen therapy that are suitable for animal experimental research are scarce. In this paper, the authors introduce a hyperbaric oxygen chamber that was developed specifically for animal experimental purposes. The hyperbaric oxygen chamber was designed to meet a number of criteria regarding safety and ease of use. The hyperbaric oxygen chamber conforms to 97/23/EC (Pressure Equipment Directive), Conformity Assessment Module G Product Group 1. It provides easy access, and can be run in manual mode, semi-automatic mode and full-automatic mode. Sensors for pressure level, oxygen level, temperature, humidity and carbon dioxide level allow full control. This state-of-the-art hyperbaric oxygen chamber for animal experimental purposes permits the investigation of the biological mechanisms through which hyperbaric oxygen therapy acts at a fundamental level.

Quantitative histomorphometric assessment of regenerate cellularity and bone quality in mandibular distraction osteogenesis after radiation therapy

BACKGROUND

The use of mandibular distraction osteogenesis (MDO) for tissue replacement after oncologic resection in head and neck cancer could have immense therapeutic ramifications. We have previously demonstrated significantly decreased mechanical and microdensitomeric metrics of our MDO regenerate after 36-Gy radiation. Quantitative histomorphometry, a third metric, would permit objective investigation of the effects of radiation on tissue and cellular composition. Our hypothesis is that radiation-induced cellular depletion and diminution in function impair optimal bone regeneration.

METHODS

Five rats received radiation to the left mandible; 5 received none. All animals underwent surgical placement of external fixators, creation of mandibular osteotomies, distraction to a 5.1-mm gap width, and consolidation. Point counting and color thresholding were performed.

RESULTS

There was a significant increase in empty lacunae and a corresponding diminution in osteocytes after radiation. Whereas the volume fraction of mineralized, mature bone was not different, that of nonmineralized, immature osteoid was significantly increased in the radiated group compared with that in the nonradiated group.

CONCLUSIONS

Our findings confirm our prior 2 metrics. Actually, all 3 diverse metrics--microdensitometry, biomechanical analysis, and histomorphometry--corroborate our hypothesis of cellular depletion and diminution of function as the potential mechanism of radiation-induced attenuation in the distracted regenerate. Furthermore, our findings of tissue and cellular changes in the irradiated regenerate elucidate the pathophysiology of decreased bone quality when amalgamated with our previous results. Therapeutic agents may now be introduced, and their effects on the irradiated regenerate critically measured, so that MDO may be used as a viable reconstructive option in patients with head and neck cancer.

Amount of bone lengthening affects blood flow recovery and bone mineralization after distraction osteogenesis in a canine cleft palate model

OBJECTIVES

Distraction osteogenesis has been applied to the craniofacial region. To reduce the cleft width of patients with cleft lip and palate, alveolar bones are distracted toward the cleft. However, no reports have described limitations to the amount of lengthening that can be achieved by distraction osteogenesis in this area. Therefore, we investigated the healing process following different extents of distraction osteogenesis using a canine cleft palate model.

METHODS

A 10-mm bone defect was made in the palates. A bony segment including the canine was prepared and translocated into the defect area at a rate of 1 mm/d for 6 or 10 days, resulting in two groups (6- and 10-mm groups). Canine pulpal blood flow was monitored for 100 days with Doppler flowmetry. Then, the animals were sacrificed and the regenerated bone area was evaluated radiologically and histologically. Statistical significance was confirmed with the Mann-Whitney rank test.

RESULTS

Pulpal blood flow in the 6-mm group recovered to original levels earlier than in the 10-mm group. Cortical bone density in the regenerated bone, measured by peripheral quantitative computed tomography, was significantly greater in the 6-mm group than in the 10-mm group. The amount of regenerated bone in histologic sections was also significantly greater in the 6-mm group.

CONCLUSION

We clearly showed that healing progress depends on the extent of distraction osteogenesis, highlighting the importance of limited distraction osteogenesis in the alveolar area.

Bone healing with an in situ-formed bioresorbable polyethylene glycol hydrogel membrane in rabbit calvarial defects

OBJECTIVES

The aims of this study were to test whether or not the application of an in situ-formed synthetic polyethylene glycol hydrogel (PEG) used as a biodegradable membrane for guided bone regeneration with a variety of graft materials and ambient oxygen or hyperbaric oxygen (HBO) environments would result in enhanced bone regeneration, and to observe the histologic and histomorphometric aspects of bone healing of the calvarial defects with and without a PEG membrane.

STUDY DESIGN

Thirty adult, skeletally mature, male New Zealand white rabbits were randomly divided into 3 groups of 10 animals each. Bilateral 15-mm-diameter critical-size defects were created in the parietal bones of each animal. Group 1 served as a control with unfilled bilateral calvarial defects, group 2 had bilateral calvarial defects filled with morcelized autogenous calvarial bone, and group 3 had bilateral calvarial defects filled with a biphasic calcium phosphate ceramic. One of the calvarial defects was randomly protected with a PEG resorbable liquid membrane in each animal. Five animals from each group underwent a course of HBO treatment (2.4 ATA 100% oxygen for 90 minutes 5 days a week for 4 weeks) and the other 5 served as control and did not receive any supplemental oxygen (normobaric). The animals were killed 6 weeks after their surgery, and their parietal bones were harvested. The specimens were analyzed with microscopic computerized tomography (microCT) scans and histomorphometrics.

RESULTS

The unfilled normobaric control bony defects did not heal, proving the critical-size nature of these defects. The presence of autogenous bone or bone ceramic in the defects increased the bone volume fraction and bone mineral density of the defects (P < .001). The presence of a membrane in the ungrafted and autogenous bone grafted defects resulted in a decrease in the corrected bone volume fraction (P = .002) but not in the bone ceramic grafted defects (P = .580). Bony healing of defects where the membrane was unsupported was compromised; the membrane did not maintain the desired bone regeneration volume with the unfilled and autogenous bone grafted groups. The PEG resorbable liquid membrane worked best with the bone ceramic material. HBO did not ameliorate the healing of the autogenous bone graft or ceramic filled defects in the 6-week time period of this study.

CONCLUSIONS

Although the PEG resorbable liquid membrane is easy to use and forms an occlusive layer, caution is recommended when using the membrane over an unsupported defect. HBO did not ameliorate bony healing with the membrane at the early 6-week time point. The authors recommend future assessment with HBO at the 12-week time point.

Alteration in volumetric bone mineralization density gradation patterns in mandibular distraction osteogenesis following radiation therapy

BACKGROUND

The use of mandibular distraction osteogenesis for tissue replacement after oncologic resection or for deformations secondary to radiotherapy could have immense therapeutic ramifications. Radiotherapy, however, drastically impairs bone healing, potentially precluding the use of mandibular distraction osteogenesis as a durable reconstructive option. The authors have previously demonstrated significantly decreased mechanical and histologic metrics of the mandibular distraction osteogenesis regenerate after 36 Gy. The authors' goal is to now investigate the effect of these same radiation dosages on bone densitometrics using micro-computed tomographic scanning.

METHODS

Six Sprague-Dawley rats received 36-Gy fractionated radiotherapy sessions to the left mandible; six received none. All animals had external fixators placed, creation of osteotomies, distraction, and consolidation. Mandibles were scanned with micro-computed tomographic scanning. Volumetric density and microdensitometric measurements were analyzed.

RESULTS

There was a significant difference in volumetric bone mineralization patterns in irradiated animals. Bone volume fraction and bone mineral density, however, demonstrated no significant differences.

CONCLUSIONS

The authors discovered a significant increase of low mineralized, immature bone and a significant decrease of highly mineralized, mature bone in the irradiated regenerate. These findings corroborate the authors' hypothesis that radiation induces a diminution in cell function, impairing optimal bone regeneration. Overall densitometrics, however, were unchanged according to micro-computed tomographic measurements, despite documented significant changes in biomechanical and histologic metrics. An optimal radiation dose must now be sought that demonstrates a higher degree of reproducible degradation, but not irreversible destruction, in all three outcomes. Such an approach will allow formulation of therapeutic interventions designed to enhance mandibular distraction osteogenesis so that it may be used as a viable reconstructive option.

Experimental study of bone lengthening in dogs by means of backscattered scanning electron microscopy

OBJECTIVE

To describe the morphology of calcified tissues involved in distraction osteogenesis (DO) by means of backscattered scanning electron microscopy (BS-SEM).

STUDY DESIGN

Experimental study.

ANIMALS

Adult female Beagle dogs (n=12).

METHODS

Non-simultaneous and bilateral transverse mid-diaphyseal osteotomies performed in tibiae were stabilized and distracted by a Type Ia external skeletal fixation device. After a latency period of 5 days, distraction was applied at a rate of 0.5 mm every 12 hours for 10 days. Then, the external fixator was maintained in a static mode during the consolidation period until bone healing or euthanasia at 1, 2, 3, 4, 6, 8, 10, 12, 14, and 18 weeks after operations, whichever came first. Distracted regions were isolated and their structure was examined by BS-SEM.

RESULTS

Calcified chondroid tissue was prominent during distraction and calcified cartilaginous tissue during consolidation; both tissues were successively replaced by woven, lamellar, and osteonal bone.

CONCLUSIONS

In osteotomized tibia, chondroid tissue is the main component of the mineralization front during distraction, calcified cartilaginous tissue during consolidation, and then both tissues are replaced by woven, lamellar, and osteonal bone. The ossification mechanism of distraction callus is transchondroidal.

CLINICAL RELEVANCE

BS-SEM is an effective technique for studying progression of bone healing during DO. The presence of chondroid tissue during DO explains why callus mineralization occurs more rapidly during distraction than during static stabilization.

Biomechanical assessment of regenerate integrity in irradiated mandibular distraction osteogenesis

BACKGROUND

The role of mandibular distraction osteogenesis for reconstructing mandibular defects following radiation therapy depends on the quality of attenuated bone healing in the regenerate. This study investigated the regenerate properties after radiation therapy using yield and breaking load. The authors hypothesized that both would be significantly reduced in mandibular distraction osteogenesis following radiation therapy compared with mandibular distraction osteogenesis alone.

METHODS

Male Sprague-Dawley rats underwent left mandibular fractionated 36-Gy preoperative external beam radiation therapy and then 2 weeks of recovery (n = 7) or no radiation therapy (n = 10) before surgery. External fixators were secured and unilateral osteotomies were created behind the third molar, followed by 4 days of latency and then mandibular distraction osteogenesis: 0.3 mm every 12 hours for 8 days (5.1 mm) and 4 weeks of consolidation. Unoperated controls received no radiation therapy (n = 13). Mandibles were tension tested at 0.5 mm/second to failure, and yield and breaking load were determined.

RESULTS

There was a significantly lower breaking load for mandibular distraction osteogenesis following radiation therapy compared with mandibular distraction osteogenesis, alone, but there was no significant difference in yield between the groups. Both groups had significantly lower breaking load and yield when compared with unoperated controls.

CONCLUSIONS

The lowered breaking load in mandibular distraction osteogenesis following radiation therapy reflects the reduced biomechanical quality of the regenerate, despite evidence of radiographic union. These data show that radiographic union is not an adequate outcome measure for regenerate healing and support the need to define quantitative bone-healing metrics in mandibular distraction osteogenesis following radiation therapy before implementation in head and neck reconstruction.

Hyperbaric oxygen results in an increase in rabbit calvarial critical sized defects

OBJECTIVE

This study was undertaken to evaluate whether the effects of hyperbaric oxygen (HBO) therapy could alter the critical size for spontaneous healing of a bone defect in the rabbit calvarial model.

STUDY DESIGN

An animal trial of 12 weeks duration was conducted using 20 New Zealand white rabbits, which were randomly divided into 2 groups of 10 animals each. Calvarial defects were created in the parietal bones of each animal bilaterally. Defects were critical-sized, 15 mm on one side and supra-critical-sized, 18 mm on the contralateral side. Group 1 received a 90-min HBO treatment sessions at 2.4 absolute atmospheric pressure (ATA) per day for 20 consecutive days. Group 2 served as a control without any HBO treatment sessions. Five animals in each group were sacrificed at 6 and 12 weeks. Data analysis included qualitative assessment of the calvarial specimens, post-sacrifice radiographs, as well as histomorphometric analysis to compute the amount of regenerated bone within the defects. ANOVA and paired sample t test were used for statistical analysis.

RESULTS

Both radiographic analysis and histomorphometric analysis demonstrated that HBO-treated animals had significantly more new bone within their defects compared with the control group (P < .001). There was no statistically significant difference between the percentage of new bone forming in the 15-mm and 18-mm HBO-treated defects. There was no difference between the 6-week and the 12-week HBO-treated groups. HBO is effective in enhancing the bony healing of full thickness critical sized as well as supra-critical-sized defects in the rabbit calvarial model.

CONCLUSION

Bone regeneration was significantly greater in the HBO-treated animals regardless of the defect size. HBO may have increased the diameter of the rabbit critical-sized calvarial defect to more than 18 mm.

The effect of irradiation and hyperbaric oxygenation (HBO) on extracellular matrix of the condylar cartilage after mandibular distraction osteogenesis in the rabbit

The effects of irradiation and hyperbaric oxygenation (HBO) on the extracellular matrix of condylar cartilage after mandibular distraction were evaluated. Unilateral distraction was performed on 19 rabbits. Five study groups were included: control, low- and high-dose irradiation, and low- and high-dose irradiation groups with HBO. Additionally, four temporomandibular joints (TMJ) were used as control material. The high-dose irradiated animals were given in the TMJ 22.4 Gy/4 fractions irradiation (equivalent to 50 Gy/25 fractions). Low-dose irradiation group received a 2.2 Gy dosage. Two groups were also given preoperatively HBO 18 x 2.5ATA x 90 min. After a two-week distraction period (14 mm lengthening) and four-week consolidation period the TMJs were removed. Proteoglycan (PG) distribution of the extracellular matrix was evaluated using safranin O staining and collagen I and II using immunohistochemistry. The organization of fibrillar network was studied by polarized light microscopy. On the operated side of the control group and on the unoperated side in all, except for high-dose irradiated group, PG distribution and fibrillar network were normal appearing. In the irradiated groups, with or without HBO, the cartilaginous layer was partially or totally devoid of PG and the network structure was severely damaged. In conclusion, irradiation in conjunction with the pressure applied by distraction causes severe damage to extracellular matrix of condylar cartilage.

Distraction osteogenesis in the irradiated mandible. A case report

BACKGROUND

Distraction osteogenesis has been suggested as a relatively simple method of mandibular reconstruction following ablative head and neck surgery. Some authors report good results in irradiated patients while other authors report limitations with this group of patients.

PATIENT

In a 72-year-old male an attempt was made to reconstruct the irradiated mandible using distraction osteogenesis.

RESULTS

Distraction osteogenesis only resulted in an enlarged soft tissue envelope, while there was no evidence of bone formation in the distraction gap.

CONCLUSION

Based on this experience and a search of the literature, it is hypothesized that distraction osteogenesis is only a reliable reconstructive method in irradiated patients if the cumulative dose to the mandibular bone at the distraction site does not exceed a certain maximum still to be defined.

Bone Regeneration in Cranial Defects Previously Treated with Radiation

OBJECTIVES/HYPOTHESIS

Bone reconstruction in the head and neck region is frequently performed in the context of previous radiation treatment. Thus, the effectiveness of tissue engineering approaches for regenerating bone in radiated defects needs to be determined before considering application to patients. Incomplete healing is described when using osteoinductive protein therapy alone for bone defects previously treated with radiation. We hypothesized that a different approach using ex vivo gene therapy can heal these severely compromised defects.

STUDY DESIGN

Animal study using Fisher rats.

METHODS

Two weeks before surgery, rats received either no radiation or a 12 Gray radiation dose to the calvarium. Syngeneic dermal fibroblasts were transduced ex vivo using an adenoviral vector containing the cDNA for bone morphogenetic protein (BMP)-7. Critical-sized calvarial defects were created, and either a transduced cell-seeded scaffold or an autologous bone graft was placed into the defect. Nonradiated defects were harvested 4 weeks later for both groups. Radiated defects treated with bone grafts were harvested at 4 weeks, and those treated with gene therapy were harvested either at 4 or 8 weeks. Gross inspection and histology were used to evaluate wound healing.

RESULTS

None of the bone grafts had gross or histologic evidence of healing at the wound margins. The nonradiated gene therapy treated defects revealed gross and histologic near-100% bone regeneration by 4 weeks after surgery. By gross inspection, the radiated defects had soft tissue admixed with islands of bone at both 4 and 8 weeks. The histologic appearance revealed areas of dense bone in a nonconfluent pattern admixed with adjacent cells having the morphologic appearance of hypertrophic chondrocytes, suggesting continued endochondral ossification.

CONCLUSIONS

Preoperative radiation significantly impairs the ability of BMP-7 ex vivo gene therapy to heal rat critical-sized cranial defects. This finding has significant implications for translating this tissue engineering approach to patients with cancer-related segmental bone defects.

Angiogenesis is required for successful bone induction during distraction osteogenesis

The role of angiogenesis during mechanically induced bone formation is incompletely understood. The relationship between the mechanical environment, angiogenesis, and bone formation was determined in a rat distraction osteogenesis model. Disruption of either the mechanical environment or endothelial cell proliferation blocked angiogenesis and bone formation. This study further defines the role of the mechanical environment and angiogenesis during distraction osteogenesis.

INTRODUCTION

Whereas successful fracture repair requires a coordinated and complex transcriptional program that integrates mechanotransductive signaling, angiogenesis, and osteogenesis, the interdependence of these processes is not fully understood. In this study, we use a system of bony regeneration known as mandibular distraction osteogenesis (DO) in which a controlled mechanical stimulus promotes bone induction after an osteotomy and gradual separation of the osteotomy edges to examine the relationship between the mechanical environment, angiogenesis, and osteogenesis.

MATERIALS AND METHODS

Adult Sprague-Dawley rats were treated with gradual distraction, gradual distraction plus the angiogenic inhibitor TNP-470, or acute distraction (a model of failed bony regeneration). Animals were killed at the end of distraction (day 13) or at the end of consolidation (day 41) and examined with muCT, histology, and immunohistochemistry for angiogenesis and bone formation (n = 4 per time-point per group). An additional group of animals (n = 6 per time-point per group) was processed for microarray analysis at days 5, 9, 13, 21, and 41.

RESULTS AND CONCLUSIONS

Either TNP-470 administration or disruption of the mechanical environment prevented normal osteogenesis and resulted in a fibrous nonunion. Subsequent analysis of the regenerate showed an absence of angiogenesis by gross histology and immunohistochemical localization of platelet endothelial cell adhesion molecule in the groups that failed to heal. Microarray analysis revealed distinct patterns of expression of genes associated with osteogenesis, angiogenesis, and hypoxia in each of the three groups. Our findings confirm the interdependence of the mechanical environment, angiogenesis, and osteogenesis during DO, and suggest that induction of proangiogenic genes and the proper mechanical environment are both necessary to support new vasculature for bone induction in DO.

Osteoblastic activity and neoangiogenesis in distracted bone of irradiated rabbit mandible with or without hyperbaric oxygen treatment

The effects of irradiation and hyperbaric oxygenation (HBO) on osteoblastic activity and angiogenesis in rabbit mandibular distraction (DO) were evaluated. Three groups were studied. The mandible of two groups received a 22.4Gy dose of irradiation. One of the irradiated groups was also given HBO, 18 times at 2.5ATA for 90min per day preoperatively. The third group was given neither radiotherapy nor HBO. Mandibular lengthening was performed unilaterally. Osteoblastic activity was assessed ex vivo by [18F]fluoride digital autoradiography. Neovascularization of distracted bone was evaluated histomorphometrically. Osteoblastic activity was higher in non-irradiated than irradiated animals. In non-irradiated rabbits, the activity was evenly distributed over the distraction area. In the irradiated groups, the activity was greater in the central third of the lengthened bone than the peripheral thirds. HBO changed the osteogenic pattern towards that of non-irradiated bone. In the non-irradiated group the number of blood vessels was 1.7-fold as compared to irradiated rabbits without HBO (P=0.0012), and the fewest number of vessels was found in irradiated rabbits without HBO. Blood vessels were more numerous in the central region than in peripheral regions in non-irradiated animals and irradiated animals with HBO, but not in irradiated rabbits without HBO therapy. It is concluded that radiotherapy disturbs distraction bone formation and neovascularization related to DO. HBO increases osteoblastic activity, but not to the level of non-irradiated bone. Angiogenic response is markedly increased by HBO.

The effects of irradiation and hyperbaric oxygen on bone formation during rabbit mandibular distraction

[18F-]fluoride positron-emission tomography (PET) was used to assess bone formation during mandibular distraction osteogenesis. There were three study groups: irradiation, irradiation+hyperbaric oxygen and control. The two experimental groups received a tumoricidal dose of irradiation to the mandible, and one group was also given hyperbaric oxygen (2.5 ATA (atmospheres absolute) for 90 min) 18 times preoperatively. Control animals received neither irradiation nor oxygen. A unilateral osteotomy was made and, after a period of latency, bone distraction was started, continued for 2 weeks, and the distraction generated was then allowed to consolidate for 4 weeks. The first PET study was performed at the end of distraction and the second at the end of consolidation. At the end of distraction, the metabolic activity of bone in the distracted area was significantly higher in the controls than in either experimental group; differences between the experimental groups were not statistically significant. By the end of consolidation, activity in the control group had diminished to the same as in the two experimental groups, in which no significant change had occurred. Radioactivity was still significantly higher at second imaging on the distracted than non-distracted side in the control and irradiation+hyperbaric oxygen groups, but not in the group that was only irradiated. The results indicate that previous irradiation disturbs bone formation during mandibular distraction osteogenesis. Hyperbaric oxygen was not able to prevent the suppression of osteogenesis caused by radiotherapy but it might improve bone formation by prolonging high osteogenic activity.