The reaction of the dura to bone morphogenetic protein (BMP) in repair of skull defects

Analysis of rat calvaria defects implanted with a platelet-rich plasma preparation: histologic and histometric observations

OBJECTIVES

It has been suggested that degranulating platelet alpha-granules release growth factors having a potential to modulate bone formation. The objective of this study was to evaluate the osteoconductive potential of a platelet-rich plasma (PRP) preparation.

METHODS

Thirty adult male Sprague-Dawley rats were used. The PRP preparation was obtained from 10 ml of whole blood drawn from one age-matched donor rat. The preparation was processed by gradient density centrifugation and stored at -80 degrees C until use. Using aseptic techniques, the PRP preparation soak loaded onto an absorbable collagen sponge (ACS) or ACS alone was surgically implanted into contralateral critical size 6-mm calvaria osteotomies in 18 animals. Twelve animals received ACS versus sham surgery in contralateral defects. Animals were sacrificed at 4 and 8 weeks when biopsies were collected for histologic and histometric analysis.

RESULTS

The animals were maintained without adverse events. Bone formation was highly variable in sites receiving PRP and control treatments. Defect bone fill at 4 weeks averaged (+/-SD) 28.8+/-27.4% (PRP/ACS) versus 39.1+/-24.4% (ACS; p=0.2626) and 62.0+/-20.0% (ACS) versus 71.6+/-32.2% (sham surgery; p=0.1088), and at 8 weeks 81.0+/-12.9% (PRP/ACS) versus 64.5+/-28.1% (ACS; p=0.2626) and 75.6+/-34.1% (ACS) versus 74.1+/-24.2% (sham surgery; p=0.7353). Remnants of the ACS biomaterial were observed at both 4 and 8 weeks in sites implanted with PRP/ACS or ACS.

CONCLUSIONS

The results suggest that the PRP preparation has a limited potential to promote local bone formation.

Analysis of Rat Calvaria Defects Implanted With a Platelet-Rich Plasma Preparation: Radiographic Observations

BACKGROUND

Platelet-rich plasma (PRP) harbors growth factors identified in bone. It has been suggested that these factors enhance osteogenesis. The objective of this study was to conduct a radiographic evaluation on local bone formation following surgical implantation of a PRP preparation using a critical-size rat calvaria defect model.

METHODS

Thirty 22-week-old male Sprague-Dawley rats were used. The PRP preparation was obtained from 10 ml of whole blood drawn from one age-matched donor rat. The preparation was processed by gradient density centrifugation and stored at -80 degrees C until use. Using aseptic techniques, the PRP preparation soak-loaded onto an absorbable collagen sponge (ACS) carrier or ACS alone was surgically implanted into contralateral critical-size 6 mm rat calvaria osteotomies in 18 animals. Twelve animals received ACS alone versus sham surgery in contralateral defects. Animals were sacrificed at 4 and 8 weeks when biopsies were collected and radiographs were obtained using a standardized protocol. Three masked examiners independently evaluated the radiographic images of the defect sites. Examiner reproducibility was examined by repeat evaluation of all defect sites (r=0.6; P <0.0001).

RESULTS

The animals were maintained without adverse events. Defect sites in two animals receiving ACS versus sham surgery (4-week healing interval) were not evaluated due to specimen damage. Seventy-five percent of the sites (PRP/ACS or ACS) exhibited partial closure at 4 weeks; one site (ACS) exhibited full closure without significant differences between protocols (P=0.1797). Fifty percent of the sites receiving PRP/ACS exhibited full closure and 20% partial closure at 8 weeks versus 20% and 80%, respectively, for the ACS control (P=0.7532). There were no noteworthy differences between sites receiving ACS versus sham surgery at 4 or 8 weeks.

CONCLUSION

The results suggest that the PRP preparation does not have a significant effect on osteogenesis.

Mechanisms of action of demineralized bone matrix in the repair of cortical bone defects

Demineralized bone matrix commonly is used to enhance and to facilitate bone grafting after skeletal injury or disease; however, the biologic bases for its bone-inducing abilities remain obscure. We have taken advantage of a mouse model of cortical bone defect healing to elucidate its mechanisms of action in vivo. Demineralized bone matrix combined with hyaluronan improved skeletal healing by inducing early deposition of an osteoid matrix. Demineralized bone matrix combined with hyaluronan might accelerate bone formation because it serves as a scaffold on which osteoprogenitor cells attach. We tested this possibility by comparing demineralized bone matrix combined with hyaluronan with heat-inactivated demineralized bone matrix combined with hyaluronan and found that the intact material was superior in terms of its ability to stimulate new bone formation. We also compared the bone inducing capacity of demineralized bone matrix combined with hyaluronan with a synthetic collagen sponge and found that not only the synthetic collagen scaffold delayed bone healing but also impaired bony bridging at later stages of repair. Another important property of demineralized bone matrix combined with hyaluronan was its ability to become actively degraded by osteoclasts during healing. Therefore, demineralized bone matrix combined with hyaluronan may not only attract osteoblasts and stimulate their differentiation, but also induce bone matrix resorption, which is a critically important regulator of bone formation and mineralization.

Applications of a mouse model of calvarial healing: differences in regenerative abilities of juveniles and adults

Young children are capable of healing large calvarial defects, whereas adults lack this endogenous osseous tissue-engineering capacity. Despite the important clinical implications, little is known about the molecular and cell biology underlying this differential ability. Traditionally, guinea pig, rabbit, and rat models have been used to study the orchestration of calvarial healing. To harness the research potential of knockout and transgenic mice, the authors developed a mouse model for calvarial healing. Nonsuture-associated parietal defects 3, 4, and 5 mm in diameter were made in both juvenile (6-day-old, n = 15) and adult (60-day-old, n = 15) mice. Calvariae were harvested after 8 weeks and analyzed radiographically and histologically. Percentage of healing was quantified using Scion Image software analysis of calvarial radiographs. A significant difference in the ability to heal calvarial defects was seen between 6-day-old and 60-day-old mice when 3-, 4-, or 5-mm defects were created. The authors' analysis revealed that juvenile mice healed a significantly greater percentage of their calvarial defects than adult mice (juvenile mean percentage of healing: 3-mm defects, 59 percent; 4-mm defects, 65 percent; 5-mm defects, 44 percent; adult mean percentage of healing: <5 percent in all groups; p < 0.05). All three defect sizes were found to be critical in the adult, whereas significant healing was seen regardless of the size of the defect in juvenile mice. The establishment of this model will facilitate further, detailed evaluation of the molecular biology underlying the different regenerative abilities of juvenile versus adult mice and enhance research into membranous bone induction by making available powerful tools such as knockout and transgenic animals.

Effect of recombinant human bone morphogenetic protein-4 dose on bone formation in a rat calvarial defect model

BACKGROUND

Bone morphogenetic proteins (BMPs) are being evaluated for periodontal and bone regenerative therapy. The objective of this study was to evaluate the effect of recombinant human bone morphogenetic protein-4 (rhBMP-4) dose on local bone formation in a rat calvaria defect model.

METHODS

Calvarial, 8 mm diameter, critical-size osteotomy defects were created in 140 male Sprague-Dawley rats. Seven groups of 20 animals each received either 1) rhBMP-4 (2.5 microg) in an absorbable collagen sponge (ACS) carrier, 2) rhBMP-4 (5 microg)/ACS, 3) rhBMP-4 (2.5 microg) in a beta-tricalcium phosphate (beta-TCP) carrier, 4) rhBMP-4 (5 microg)/beta-TCP, 5) ACS or 6) beta-TCP carrier controls, or 7) a sham-surgery control, and were evaluated by histologic and histometric parameters following a 2- or 8-week healing interval (10 animals/group/healing interval).

RESULTS

Surgical implantation of rhBMP-4/ACS and rhBMP-4/beta-TCP resulted in enhanced local bone formation at both 2 and 8 weeks. Within the dose range examined, rhBMP-4 did not exhibit an appreciable dose-dependent response. Defect closure was not significantly different between the rhBMP-4/ACS and rhBMP-4/beta-TCP groups. New bone area of the rhBMP-4/ beta-TCP group was significantly greater than that of the rhBMP-4/ ACS group; however, bone density in the rhBMP-4/ACS group was significantly greater than that in the rhBMP-4/beta-TCP group at 8 weeks (P < 0.05).

CONCLUSIONS

rhBMP-4 combined with ACS or beta-TCP has a significant potential to induce bone formation in the rat calvaria defect model. Within the selected rhBMP-4 dose range and observation interval, there appeared to be no meaningful differences in bone formation.

Bone formation in calvarial defects of Sprague-Dawley rats by transplantation of calcium phosphate glass

The purpose of this study was to investigate the bone-regenerative effect of calcium phosphate glass in vivo. We prepared amorphous calcium phosphate glass powder having a mean particle size of 400 microm in the system CaO-CaF2-P2O5-MgO-ZnO. Calvarial critical-sized defects (8 mm) were created in 60 male Sprague-Dawley rats. The animals were divided into an experimental group and control group of 30 animals each. Each defect was filled with a constant weight of 0.5 g calcium phosphate glass powder mixed with saline. As a control, the defect was left empty. The rats were sacrificed 2, 4, or 8 weeks postsurgery, and the results evaluated using radiodensitometric and histological studies; they were also examined histomorphometrically. When the calcium phosphate glass powders with 400-microm particles were grafted, the defects were nearly completely filled with new-formed bone in a clean healing condition after 8 weeks. It was observed that the prepared calcium phosphate glass enhanced new bone formation in the calvarial defect of Sprague-Dawley rats and could be expected to have potential for use as a hard tissue regeneration material.

Influence of the in vitro culture period on the in vivo performance of cell/titanium bone tissue-engineered constructs using a rat cranial critical size defect model

The aim of this study was to investigate the in vivo performance in bone-regenerating capability of cell/scaffold constructs implanted into an orthotopic site. Bone marrow stromal osteoblasts were seeded on titanium fiber mesh scaffolds using a cell suspension (5 x 10(5) cells per scaffold) and cultured for 1, 4, and 8 days under either static or flow perfusion conditions forming six different treatment groups. A total of 16 constructs from each one of the six treatment groups were then implanted into an 8-mm critical size calvarial defect created in the cranium of adult syngeneic male Fisher rats. Half of the constructs from each group were retrieved 7 days postimplantation, and the other half of the constructs were retrieved 30 days postimplantation and examined for new bone formation and tissue response. Constructs retrieved 7 days postimplantation were filled with fibrous tissue and capillaries, but no bone formation was observed in any of the six treatment groups. Constructs retrieved 30 days postimplantation showed bone formation (at least 7 out of 8 constructs in all treatment groups). Titanium fiber meshes seeded with bone marrow stromal osteoblasts and cultured for 1 day under flow perfusion conditions before implantation appeared to give the highest percentage of bone formation per implant (64 +/- 17%). They also showed the highest ratio of critical size cranial defects that resulted in union of the defect 30 days postimplantation (7 out of 8) together with the constructs cultured for 1 day under static conditions before implantation. There were no significant differences between the different treatment groups; this finding is most likely due to the large variability of the results and the small number of animals per group. However, these results show that titanium fiber mesh scaffolds loaded with bone marrow stromal osteoblasts can have osteoinductive properties when implanted in an orthotopic site. They also indicate the importance of the stage of the osteoblastic differentiation and the quality of the in vitro generated extracellular matrix in the observed osteoinductive potential.

Effects on bone formation in ovariectomized rats after implantation of tooth ash and plaster of Paris mixture

PURPOSE

Our goal was to report on tooth ash and plaster of Paris mixture in bone defects in an ovariectomized rat osteoporosis model.

MATERIALS AND METHODS

Sixty rats were randomly assigned to 4 groups and each group was further divided into 3 subgroups: 4, 8, and 16 weeks after implantation. The defect was filled with different grafting conditions as follows: group 1, ovariectomy and nongraft group; group 2, ovariectomy and tooth ash-plaster graft group; group 3, nonovariectomy and nongraft group; and group 4, nonovariectomy and tooth ash-plaster graft group. Histologic sections and histomorphometric analysis of defects were obtained 4, 8, and 16 weeks after surgery.

RESULTS

For the 4-week ovariectomy group, there was significantly greater bone formation in tooth ash-plaster group compared with the nongraft group. In the nonovariectomy group, the tooth ash-plaster group also showed better bone formation than the nongraft group. However, there was no statistical significance. In both the ovariectomy and nonovariectomy groups, a significant increase in bone formation was observed according to the elapse of time. The nonovariectomy group showed increased new bone formation compared with the ovariectomy group, with the tooth ash-plaster group showing statistical significance in each subgroup (P =.048).

CONCLUSIONS

Ovariectomy acts as a negative factor in new bone formation. For a critical size bony defect, the tooth ash-plaster treatment of the osseous defect produces more stable, effective, and rapid new bone formation.

The Role of Growth Factors for Disease and Therapy in Diseases of the Head and Neck

Growth factors are a large family of polypeptide molecules that regulate cell division in many tissues by autocrine or paracrine mechanisms. Depending on what receptors are activated, growth factors can initiate mitogenic, antiproliferative, or trophic effects, that is, growth factors act as positive or negative modulators of cell proliferation. Therefore, growth factors do not only play an important role in embryonic development and adult tissue homeostasis, but also in pathological situations like infection, wound healing, and tumorigenesis. Consequently, the application of growth factors, or vice versa the application of substances which are directed against growth factors like antigrowth factor antibodies, may have therapeutic applications. This review provides a brief account of what we know regarding growth factors in otorhinolaryngology, particularly in the field of otology, wound healing, oncology, peripheral nerve regeneration, and rhinology.

Osteogenesis in calvarial defects: contribution of the dura, the pericranium, and the surrounding bone in adult versus infant animals

Guided bone regeneration is a promising means for reconstructing bone defects in the cranium. The present study was performed to better define those factors that affect osteogenesis in the cranium. The authors studied a single animal model, investigating the contribution of the dura, the pericranium, and the adjacent calvarial bone in the process of calvarial regeneration in both mature and immature animals. Bilateral, 100-mm2, parietal calvariectomies were performed in immature (n = 16) and mature (n = 16) rabbits. Parietal defects were randomized to one of four groups depending on the differential blockade of the dura and/or the pericranium by expanded polytetrafluoroethylene membranes. Animals were humanely killed after 12 weeks, and histometric analysis was performed to quantitate the area of the original bone defect, new bone formation, and new bone density. Bone formation was quantified separately both at the periphery and in the center of the defects. Extrasite bone formation was also quantified both on the dural and on the pericranial sides of the barriers. Bone regeneration was incomplete in all groups over the 12-week study period, indicating that complete bone healing was not observed in any group. The dura was more osteogenic than the pericranium in mature and immature animals, as there was significantly more extrasite bone formed on the dural side in the double expanded polytetrafluoroethylene barrier groups. In both the dural and the double expanded polytetrafluoroethylene barrier groups, dural bone production was significantly greater in immature compared with mature animals. The dura appeared to be the source of central new bone, because dural blockade in the dural and double expanded polytetrafluoroethylene groups resulted in a significant decrease in central bone density in both mature and immature animals. Paradoxically, isolation of the pericranium in mature animals resulted in a significant reduction in total new bone area, whereas pericranial contact appeared to enhance peripheral new bone formation, with the control group having the greatest total new bone area. The present study establishes a model to quantitatively study the process of bone regeneration in calvarial defects and highlights differences in the contribution of the dura and pericranium to calvarial bone regeneration between infant and adult animals. On the basis of these findings, the authors propose that subsequent studies in which permeability of the expanded polytetrafluoroethylene membranes is altered to permit migration of osteoinductive proteins into the defect while blocking prolapse of adjacent soft tissues may help to make guided bone regeneration a realistic alternative for the repair of cranial defects.

Effect of recombinant human bone morphogenetic protein-4 with carriers in rat calvarial defects

BACKGROUND

Bone morphogenetic proteins (BMPs) are being evaluated as candidates for periodontal and bone regenerative therapy. However, the research on recombinant human bone morphogenetic protein-4 (rhBMP-4) has been insufficient to evaluate its capacity to enhance bone formation and its carrier system. The purpose of this study was to evaluate the bone regenerative effect of rhBMP-4 delivered with an absorbable collagen sponge (ACS) or beta-tricalcium phosphate (beta-TCP). We also compared the potential of beta-TCP to that of ACS as a carrier system for rhBMP-4.

METHODS

Eight-mm calvarial critical-sized defects were created in 100 male Sprague-Dawley rats. The animals were divided into 5 groups of 20 animals each. The defects were treated with rhBMP-4/ACS (rhBMP-4 at 0.05 mg/ml), rhBMP-4/beta-TCP (rhBMP-4 at 0.05 mg/ml), ACS alone, beta-TCP alone, or left untreated for surgical control. The rats were sacrificed at 2 or 8 weeks postsurgery, and the results were evaluated radiodensitometrically, histologically, and histomorphometrically.

RESULTS

The results of radiodensitometric analysis were as follows: the rhBMP-4/ACS and the rhBMP-4/beta-TCP groups were more radiopaque than other groups at both 2 and 8 weeks (P < 0.01). The histologic observations were as follows: in the rhBMP-4/ACS and the rhBMP-4/beta-TCP groups, new bone was evident at the defect sites at 2 weeks and 8 weeks. The results of histomorphometric analysis were as follows: the rhBMP-4/ACS and the rhBMP-4/beta-TCP groups had more bone (%) than other groups at both 2 and 8 weeks (P < 0.01).

CONCLUSIONS

Surgical implantation of rhBMP-4/ACS may be used to support bone regeneration in the rat calvarial critical-sized defect, and rhBMP-4/beta-TCP may be able to regenerate bone in the rat calvarial critical-sized defect without complication. In addition, both ACS and beta-TCP may be considered as available carriers for rhBMP-4.

Roles of periosteum, dura, and adjacent bone on healing of cranial osteonecrosis

It has been reported that large cranial osteonecrotic areas can heal. It was hypothesized that optimal healing is possible by the synchronized contribution of the osteogenic structures (periosteum, dura, and adjacent bone) that envelop the necrotic cranium. This hypothesis was tested by preserving or isolating the contribution of these osteogenic tissues. A total of 37 4-old-month rats were included in the study. Twelve animals were killed immediately, and cranial bone samples were taken and processed for examination (from 6 animals as fresh samples [Group A] and from the rest as autoclaved samples [Group B]). Group B was created to test if the bone was completely nonviable. In Group C (n = 25), cranial bone disks 8 mm in diameter were taken from 4-month-old rats, autoclaved, and put back onto the defect area. This group was further divided into the four Subgroups C1 through C4 (n = 7 in C3; n = 6 in C1, C2, and C4). Dura mater was isolated from the overlying bone disk with a polytetrafluoroethylene sheet in Subgroups C1 and C2, whereas the bone contacted the dura in the rest. The bone samples were covered with healthy periosteum in Subgroups C1 and C3 and with skin in Subgroups C3 and C4. These animals were killed after a healing period of 12 weeks, and the relevant bone disks were obtained. Surrounding healthy bone was also harvested from the same animals after they were killed to create Group D. The data of Group A and D were compared with those of the experimental group to comment on the degree of bone healing in the latter group. Quantitative and qualitative assessment was performed by mammography, bone densitometry, computed tomography, and histological examinations to find out the density and cellular content (osteocytes and vessels) of the samples. Examination of Group B samples showed nonviable tissue with a preserved microstructure. Analysis of other samples showed that both the periosteum and, mainly, the dura play an important role in cranial bone healing. The periosteal reaction was observed to be more evident when the dura was not separated. Cellular repopulation was more evident when both structures contributed to the healing process. Newly formed bone progressed centripetally; however, adjacent bone without the support of the dura and periosteum was capable of producing limited neovascularization and bone formation.

The use of polylactic acid/polyglycolic acid copolymer and gelatin sponge complex containing human recombinant bone morphogenetic protein-2 following condylectomy in rabbits

PURPOSE

To examine the results of a polylactic acid/polyglycolic acid copolymer and gelatin sponge complex (PGS) with or without recombinant human bone morphogenetic protein-2 (rhBMP-2) used to treat condylar defects in rabbits.

MATERIAL AND METHODS

Adult male Japanese white rabbits (n=60; 3kg; 12-16 weeks old) were divided into three groups of 20 each. All rabbits underwent condylectomy. In the two implanted groups, PGS with or without 5 microg of rhBMP-2 was implanted to the condylar defect without fixation. No material was implanted into the control group. Animals were sacrificed at 2, 4, 8, 12 and 24 weeks postoperatively, and the temporomandibular joints (TMJs) were examined histologically.

RESULTS

Four weeks after implantation, growth of bone and cartilage-like tissue was observed in all rabbits that received PGS implants (with and without rhBMP-2). A cartilage-like layer was derived from the bone marrow at the operated surface. There was no growth of bone tissue in the control rabbits, but they also had a cartilage-like layer directly derived from the operated surface.

CONCLUSION

This study demonstrated that PGS with or without rhBMP-2 could induce regeneration of new bone and cartilage-like tissue in the TMJ.

Bone morphogenetic proteins: relevance in spine surgery

Bone morphogenetic proteins (BMPs) are low molecular weight glycoproteins that play a vital role in the development and maturation of skeletal tissue. Bone morphogenetic protein-induced mesenchymal cell recruitment and differentiation leads to the formation of chondroblasts and osteoblasts leading to the formation of de novo bone. Overwhelming pre-clinical and clinical evidence has suggested a promising role for BMPs for anterior and posterolateral spinal fusion. Strength of this approach lies in the potential ability of these growth factors to reverse inhibitory conditions common in the clinical setting and enabling predictable fusion. However, several issues related to carriers, costs, and dosages still need to be consecutively addressed. Gene therapy techniques producing in vivo osteoinductive factors and utilizing minimally invasive approaches are attractive options being developed for the future.

A trial to prepare biodegradable collagen-hydroxyapatite composites for bone repair

This paper is a trial to prepare collagen-hydroxyapatite composites in vitro by an alternate immersion method. Collagen sponges of different biodegradabilities were prepared through chemical cross-linking of Type I collagen with glutaraldehyde (GA) at concentrations of 0.2, 1.0, and 2.0 wt%. The sponges were immersed at 37 degrees C in Tris-HCl-buffered solution containing 200 mM CaCl2 (pH 7.4) for 2 h and then in an aqueous solution of 120 mM Na2HPO4 (pH 9.3) for a 2 h further (one immersion cycle). The alternate immersion cycle was repeated for different times to obtain collagen-hydroxyapatite composites. The characterization of the resulting composites was performed by Fourier transform infrared spectroscopy (FT-IR). X-ray diffraction (XRD), and scanning electron microscopy (SEM). The weight of composites increased with an increase in immersion cycles and the rate of increase became greater with higher GA cross-linking levels for collagen sponge preparation. The pH of the phosphate solution decreased with the immersion cycle, which suggests H+ generation accompanied hydroxyapatite formation. Irrespective of the GA concentration and immersion cycle, every composite showed IR absorption bands attributable to phosphate and hydroxyl groups at 950-1100 or 550-650 and 3000-3500 cm(-1) and broad peaks specific to hydroxyapatite on the XRD charts. SEM study revealed small white clusters of hydroxyapatite interspersed uniformly on/in the collagen framework without any preferential orientation. The composite prepared from 0.2 wt% GA cross-linked collagen sponge which showed favourable characteristics was applied to a rat skull defect to evaluate its osteoconductivity as well as biodegradability. The formation of new bone tissue was histologically observed at the defect 12 weeks after application in marked contrast to the collagen sponge alone. The composite degraded without any inflammation reaction. It is concluded that the collagen-hydroxyapatite composite prepared by the present method is a biodegradable biomaterial of osteoconductivity applicable to bone repair.

Enhancement of bone growth by sustained delivery of recombinant human bone morphogenetic protein-2 in a polymeric matrix

PURPOSE

The purpose of this study was to develop a polymeric sustained delivery system for recombinant human bone morphogenetic protein-2 (BMP-2) and to evaluate local bone growth induced by the sustained release of BMP-2 in an animal model.

METHODS

BMP-2 was incorporated in biodegradable poly(D,L-lactide-co-glycolide) (PLGA) microspheres to obtain different release rates. Two sustained and an immediate release implants were produced by suspending the BMP-2 loaded PLGA microspheres in aqueous sodium carboxymethylcellulose (CMC), lyophilizing, and cutting the dried materials to the size of the animal bone defects. The local in vivo release at the implantation site in rat calvarial defects was determined by gamma scintigraphy using radiolabeled BMP-2. The local bone induction in the critical size of rabbit calvarial defects was evaluated six weeks post implantation.

RESULTS

The immediate release implant showed about 65% initial drug release within 24 h and the remaining BMP-2 quickly exhausted from the implantation site within 7 days. The sustained release implants, showing 45-55% initial release followed by a prolonged release for 21 days, released a greater amount of BMP-2 at the implantation site and maintained higher serum BMP-2 for the longer period of time compared to the immediate release implant. Significant bone growth was observed in all BMP-2 treated defects while the defects without treatment or with BMP-2-free implant showed minimal bone healing. 75-79% of rabbit calvarial defect area was healed with newly induced bone matrix by the sustained release implants in 6 weeks as compared to 45% recovery from the immediate release implant.

CONCLUSION

The sustained delivery of BMP-2 based on the biodegradable PLGA microsphere system resulted in faster and more complete bone healing in the animal model.

Chemical and physicochemical characterization of porous hydroxyapatite ceramics made of natural bone

The properties of a porous hydroxyapatite ceramic produced by sintering of bovine bone were investigated by using a number of physicochemical methods such as scanning electron microscopy (SEM), SEM in combination with energy dispersive X-ray spectroscopy (SEM-EDX), mercury intrusion porosimetry, krypton-adsorption, contact angle measurements, wide angle X-ray diffraction. Fourier transform infrared spectroscopy, thermal analysis, inductively coupled plasma optical atom emissions spectroscopy and flame atomic absorption spectroscopy. The results indicate that there are considerable differences between the ceramic and native bone. However, the most important properties with respect to the use of such ceramics as a biomaterial for filling bone defects namely the high porosity (> or = 57 +/- 2%) and the interconnecting pore system are maintained. While macropores with an average diameter of approx. 300 microm contribute 97% to porosity, micropores with an average diameter of 1.3 microm account for only 3% of the total porosity. The surface area was found to be approx. 0.1 m2/g. The contact angles of water (44.6 +/- 15.4 degrees, n = 5) and tetrahydrofurane (10 degrees) allow the processing of the ceramic to a drug carrier by incubation with aqueous or organic drug solutions. The ceramic is highly crystalline with crystal sizes of 1-7 microm and contains crystal bridges. The investigation of its chemical composition revealed small amounts of other inorganic compounds such as Ca4O(PO4)2, NaCaPO4, Ca3(PO4)2, CaO, and MgO. Besides trace amounts of aluminum, iron, magnesium, potassium, silica, sodium, vanadium and zinc it contains probably carbonated apatite.

Effects of various implant materials on regeneration of calvarial defects in rats

The purpose of the present study was to determine the best implant material, the best conditions to substitute absorbable membrane for non-absorbable membrane, and the factors influencing guided regeneration of critical size defects using experimental rats. An 8-mm circular transosseous calvarial bony defect was made and implant materials, such as demineralized freeze-dried bone (DFDB), absorbable membrane (BioMesh; Samyang Co., Seoul Korea), non-absorbable membrane (Millipore filter; Micro Filtration System, MA, USA) or a combination of these materials, was placed on the defect. As for the results of sequential time-based guided bone regeneration, histological, histochemical, immunohistochemical and histomorphometric aspects were observed, and a statistical comparative analysis was performed, with control group of a soft tissue flap. Bone formation was significantly enhanced when DFDB was retained within the defect with a protective absorbable membrane. Inframembranous DFDB-filling was required to prevent membrane collapse and to preserve spaces for bone regeneration. The absorbable membrane which was recommended to overcome the disadvantages of the non-absorbable membrane should remain intact for more than 5 weeks in order for it to be effective. The macrophages recruited by grafts were involved partly in decreasing bone regeneration via the sequential events of releasing fibronectin, and in chemotactic effect of the fibronectin to fibroblasts and collagen lay-down. Thus, the activity of new bone formation was dependent upon the physical barrier effect of the membrane, such as the preserving ability to secure spaces and the suppression ability of early infiltration of collagen and epithelium, inducible ability of inflammation by the implant material, and potential in guiding bone regeneration of the grafts.

Osteogenesis by recombinant human bone morphogenetic protein-2 at skeletal sites

Osteogenesis was evaluated in the mandibular bone by combinations of various dosages of recombinant human bone morphogenetic protein-2, atelopeptide Type I collagen, and porous hydroxyapatite (four groups: Group I, 2 micrograms recombinant human bone morphogenetic protein-2, atelopeptide Type I collagen, and porous hydroxyapatite; Group II, 10 micrograms recombinant human bone morphogenetic protein-2, atelopeptide Type I collagen, and porous hydroxyapatite; Group III, 50 micrograms recombinant human bone morphogenetic protein-2, atelopeptide Type I collagen, and porous hydroxyapatite; Control Group, only atelopeptide Type I collagen and porous hydroxyapatite). The prepared materials were implanted in the mandibular bone hole (7 mm in diameter, 2 mm deep). Three weeks later, the alkaline phosphatase activity in the implanted region was determined, and the histologic features of the excised tissue were examined. There were significant differences in histologic and biochemical findings among the four groups. In the recombinant human bone morphogenetic protein-2 implanted groups, osteogenesis increased with the dosage of recombinant human bone morphogenetic protein-2, as assessed by alkaline phosphatase activity and histologic findings. The results suggest that atelopeptide Type I collagen is an effective carrier for recombinant human bone morphogenetic protein-2 and that porous hydroxyapatite would be advantageous for clinical application as a material to maintain its original form after implantation.

Bone morphogenetic protein but not transforming growth factor-beta enhances bone formation in canine diaphyseal nonunions implanted with a biodegradable composite polymer

BACKGROUND

The purpose of the present study was to create an effective bone-graft substitute for the treatment of a diaphyseal nonunion.

METHODS

A standardized nonunion was established in the midportion of the radial diaphysis in thirty mongrel dogs by creating a three-millimeter segmental bone defect (at least 2 percent of the total length of the bone). The nonunion was treated with implantation of a carrier comprised of poly(DL-lactic acid) and polyglycolic acid copolymer (50:50 polylactic acid-polyglycolic acid [PLG50]) containing canine purified bone morphogenetic protein (BMP) or recombinant human transforming growth factor-beta (TGF-beta1), or both, or the carrier without BMP or TGF-beta1. Five groups, consisting of six dogs each, were treated with implantation of the carrier alone, implantation of the carrier with fifteen milligrams of BMP, implantation of the carrier with 1.5 milligrams of BMP, implantation of the carrier with fifteen milligrams of BMP and ten nanograms of TGF-beta1, or implantation of the carrier with ten nanograms of TGF-beta1. At twelve weeks after implantation, the radii were examined radiographically and the sites of nonunion were examined histomorphometrically.

RESULTS

We found that implantation of the polylactic acid-polyglycolic acid carrier alone or in combination with ten nanograms of TGF-beta1 failed to induce significant radiographic or histomorphometric evidence of healing at the site of the nonunion. The radii treated with the carrier enriched with either 1.5 or fifteen milligrams of BMP showed significantly increased periosteal and endosteal bone formation on histomorphometric (p < 0.05) and radiographic (p < 0.02) analysis.

CONCLUSIONS

Bone formation in a persistent osseous defect that is similar to an ununited diaphyseal fracture is increased when species-specific BMP incorporated into a polylactic acid-polyglycolic acid carrier is implanted at the site of the nonunion. TGF-beta1 at a dose of ten nanograms per implant did not induce a similar degree of bone formation or potentiate the effect of BMP in this model.

CLINICAL RELEVANCE

The biodegradable implant containing BMP that was used in the present study to treat diaphyseal nonunion is an effective bone-graft substitute.

Implantation of octacalcium phosphate (OCP) in rat skull defects enhances bone repair

Synthetic octacalcium phosphate (OCP) enhances bone formation if implanted into the subperiosteal region of murine bone. Such implanted OCP may be resorbed and replaced by bone with time. We hypothesized that OCP could be used as an effective bone substitute. To test this hypothesis, we designed the present study to investigate if bone repair in a rat skull defect is enhanced by the implantation of OCP. Rats were divided into two groups: OCP-treated animals and untreated controls. Six rats from each group were fixed at 4, 12, and 24 weeks after implantation. A full-thickness standardized trephine defect was made in the parietal bone, and synthetic OCP was implanted into the defect. After being examined radiographically, the specimens were decalcified and processed for histology. OCP implantation significantly promoted bone repair compared with the controls. A statistical analysis showed an increase in the area of radiopacity within the skull defect between week 4 and week 12. Histologically, bone was formed on the implanted OCP and along the defect margin at week 4. At week 12, the implanted OCP was surrounded by newly formed bone. At week 24, the defect was almost completely filled with bone. In the control, bone formation was observed only along the defect margin. The present results demonstrate that OCP could be used as an effective bone substitute.

Incorporation of particulated bone implants in the facial skeleton

The purpose of this study was to compare the regenerative response of autogenous cortical and cancellous bone chips and a natural particulate resorbable bone mineral (RBM) (Bio-Oss, Geistlich-Pharma, Wolhusen, Switzerland) in standardized bony defects relating paranasal sinuses to one another and to bone blocks. On 13 skeletally mature female goats four standardized critical-sized full thickness bone defects were made in the frontal bone overlying the frontal sinus. These defects were filled at random with cortical bone chips, cancellous bone chips, spongiosa granules of a RBM or left empty. Fluorochrome bone markers were injected subcutaneously 1 and 5 weeks after transplantation, and one week before the animals were killed. The animals were killed at 3, 6, 12 and 24 weeks after surgery. Autogenous cancellous bone chips is the material of choice for bridging a bony defect in the maxillofacial area where there is no need for mechanical strength. They heal in the same way as cancellous bone blocks do. Cortical bone chips are not reliable enough to be used as a solitary bone-grafting material under these conditions. A cortical block as a solitary implant gives better results. RBM granules as solitary implant in a critical-sized defect do not stimulate osteoconduction but give rise to an extensive osteoclastic activity stimulated by the mutual loose relation. A solid block of RBM is in a similar case more reliable.

Osteogenesis with coral is increased by BMP and BMC in a rat cranioplasty

Autologous bone marrow cells (BMC), bone morphogenetic protein (BMP) and natural coral exoskeleton (CC) were used to enhance the repair of large skull bone defects in a craniotomy model. Nine millimeter calvarial defects were created in adult rats and were either left empty (control defects) or implanted with CC alone, CC-BMC, CC-BMP, or CC-BMC-BMP. After 1 or 2 months, osteogenesis was insufficient to allow union when defects were left empty or filled with CC. Addition of BMC alone to CC had no positive influence on osteogenesis at any time and increased CC resorption at 2 months (0.1 +/- 0.1 mm2 versus 0.5 +/- 0.3 mm2). In contrast addition of BM P or BM P/BMC to CC led to a significant increase in osteogenesis and allowed bone union after 1 month. At 2 months, the combination of CC-BM P-BMC was the most potent activator of osteogenesis. Filling a defect with CC-BMP-BMC resulted in significantly increased bone surface area (11 +/- 2.7 mm2) in comparison to filling a defect with CC-BMP (7.0 +/- 1.4 mm2), CC-BMC (3.5 +/- 1.1 mm2) or CC (4.5 +/- 0.4 mm2). CC resorption was significantly decreased in the presence of BMP with or without BMC at both times. These data are in accordance with the presence of progenitor cells in bone marrow that are inducible by BMP to the osteogenic pathway in a cranial site. The increase in material resorption in defects filled with CC-BMC could suggest that cells from the granulocyte-macrophage lineage survived the grafting procedure and were still active after 2 months.

In vivo endochondral bone formation using a bone morphogenetic protein 2 adenoviral vector

Bone morphogenetic proteins (BMPs) are polypeptides that induce ectopic bone formation in standard rat in vivo assay systems. Previous studies have demonstrated the clinical utility of these proteins in spinal fusion, fracture healing, and prosthetic joint stabilization. Gene therapy is also a theoretically attractive technique to express BMPs clinically, since long-term, regulatable gene expression and systemic delivery with tissue-specific expression may be possible in future. This study was performed to determine whether an adenoviral vector containing the BMP-2 gene can be used to express BMP-2 in vitro and promote endochondral bone formation in vivo. In vitro, U87 MG cells transduced per cell with 20 MOI of an adenoviral construct containing the BMP-2 gene under the control of the universal CMV promoter (Ad-BMP-2) showed positive antibody staining for the BMP-2 protein at posttransfection day 2. The synthesis and secretion of active BMP-2 into the conditioned medium of Ad-BMP-2-transduced 293 cells were confirmed by Western blot analysis and the induction of alkaline phosphatase activity in a W-20 stromal cell assay. In vivo, Sprague-Dawley rats and athymic nude rats were injected with Ad-BMP-2 in the thigh musculature and were sacrificed on day 3, 6, 9, 12, 16, 21, 60, and 110 for histological analysis. The Sprague-Dawley rats showed evidence of acute inflammation, without ectopic bone formation, at the injection sites. In the athymic nude rats, BMP-2 gene therapy induced mesenchymal stem cell chemotaxis and proliferation, with subsequent differentiation to chondrocytes. The chondrocytes secreted a cartilaginous matrix, which then mineralized and was replaced by mature bone. This study demonstrates that a BMP-2 adenoviral vector can be utilized to produce BMP-2 by striated muscle cells in athymic nude rats, leading to endochondral bone formation. However, in immunocompetent animals the endochondral response is attenuated, secondary to the massive immune response elicited by the first-generation adenoviral construct.

Potential of porous poly-D,L-lactide-co-glycolide particles as a carrier for recombinant human bone morphogenetic protein-2 during osteoinduction in vivo

Several different biodegradable bone graft materials are in clinical or preclinical use for the repair of bone defects in orthopedics, maxillofacial surgery, and periodontics. This study tested the hypothesis that poly-D,L-lactide-co-glycolide copolymer (PLG) can be used as an effective carrier of recombinant human bone morphogenetic protein-2 (rhBMP-2) and that the composite has osteoinductive ability. Porous PLG rods were shredded to a particle size ranging from 250 to 850 microm. Active and inactive demineralized freeze-dried bone allografts (DFDBA) with a comparable particle size were used as positive and negative controls, respectively. PLG particles were treated with vehicle or with 5 or 20 microg rhBMP-2. DFDBA and PLG particles were placed in gelatin capsules, mixed with vehicle or rhBMP-2, and implanted at intramuscular sites in male Nu/Nu (nude) mice. Each mouse underwent bilateral implantation with implants of the same formulation, resulting in five groups of four mice per group: active DFDBA, inactive DFDBA, PLG, PLG + 5 microg rhBMP-2, and PLG + 20 microg rhBMP-2. After 56 days, the implants were recovered and processed for histology. Bone induction was assessed by use of a semiquantitative scoring system based on the amount of new bone formed in representative histological sections. Histomorphometry was also used to measure the area of new bone formed and the area of residual implant material. The results showed that active DFDBA induced the formation of ossicles containing new bone with bone marrowlike tissue, whereas inactive DFDBA or PLG particles alone did not induce new bone. The addition of rhBMP-2 to PLG particles resulted in new bone formation that had a greater bone induction score than active DFDBA. Moreover, the histomorphometric analysis showed that the addition of rhBMP-2 to PLG particles induced the formation of a greater area of new bone and bone marrowlike tissue than active DFDBA. The resorption of the PLG particles was markedly increased with the addition of rhBMP-2, suggesting that rhBMP-2 may attract and regulate resorptive cells at the implantation site. The results of the present study indicate that PLG copolymers are good carriers for BMP and promote the induction of new bone formation. Further, the PLG copolymers with rhBMP-2 had a greater effect in inducing new bone formation and resorbing the implanted material than active DFDBA alone.

Tissue-engineered bone biomimetic to regenerate calvarial critical-sized defects in athymic rats

A tissue-engineered bone biomimetic device was developed to regenerate calvaria critical-sized defects (CSDs) in athymic rats. Well-documented evidence clearly confirms that left untreated, CSDs will not spontaneously regenerate bone. To accomplish regeneration, four candidate treatments were assessed: porous poly(D,L-lactide) and type I collagen (PLC), PLC and human osteoblast precursor cells (OPCs) at 2 x 10(5) (PLC/OPCs), PLC and 50 microg of recombinant human bone morphogenetic protein-2 (PLC/rhBMP-2), and PLC/OPCs/rhBMP-2 (the bone biomimetic device). The hypotheses for this study were PLC/OPCs/rhBMP-2 would promote more new bone formation in CSDs than the other treatments and the amount of bone formation would be time dependent. To test the hypotheses, outcomes from treatments were measured at 2 and 4 weeks postoperatively by radiomorphometry for percent radiopacity and by histomorphometry for square millimeters of new bone formation. Data were analyzed by analysis of variance and Fisher's protected least significant difference for multiple comparisons with p < or = 0.05. At 2 and 4 weeks, radiomorphometric data revealed PLC/rhBMP-2 and PLC/OPCs/rhBMP-2 promoted significantly more radiopacity than either PLC or PLC/OPCs. Histomorphometry data at 2 and 4 weeks indicated significantly more new bone formation for PLC/rhBMP-2, PLC/OPCs/rhBMP-2, and PLC/OPCs compared to PLC. By 4 weeks, PLC/OPCs/rhBMP-2 and PLC/rhBMP-2 had regenerated the CSDs with more new bone than the other treatments; the quantity of bone at 4 weeks for these treatments was greater than at 2 weeks.

Bone graft and bone graft substitutes: a review of current technology and applications

The morbidity associated with autogenous bone graft harvest and the recent concern regarding the transmission of live virus through use of allografts, have been the impetus for research into a variety of materials that could take the place of these standard materials for bone grafting. The positive results reported with various ceramics and/or bone derivatives suggest the possibility of a material with osteoconductive and/or osteoinductive properties for use with or in place of bone graft. This review discusses a variety of bone graft and bone graft substitute materials. Among the osteoconductive materials outlined are the hydroxyapatite and tricalcium phosphate ceramics as well as some reportedly osteoactive polymers. While osteoconduction is a favorable quality, much interest has focussed on the use of osteoinductive or osteogenic materials such as demineralized bone matrix or bone derivatives, that is, BMP, osteogenin, etc. It is increasingly apparent that these materials require a carrier vehicle for optimal expression of osteoactivity. Therefore, the review finishes with a comparison of the various materials suggested for use as carriers.

Bone morphogenetic proteins: an update on basic biology and clinical relevance

The regeneration of bone is a remarkable, complex physiological process, and BMPs are a formidable clinical tool to promote its regeneration. By defining roles played by BMPs in developmental biology and bone regeneration, significant progress has been made to identify cell-signaling molecules and their regulators. For example, the regulators of BMPs that include noggin, chordin, cerberus, dan, and gremlin may be harnessed as therapies to offset calcification encountered after total hip arthroplasties. Furthermore, exploiting BMPs and Smads may generate new therapeutic options for bone repair. Another compelling clinical consideration is the trans-acting factor osteoblast-specific factor-2, which can promote osteoblast differentiation. Moreover, the affiliation of osteoblast-specific factor-2 with heritable disorders merits exploration. A recognized daunting challenge includes a carrier/delivery system for the powerful morphogenetic therapeutic tools, as well as osteoprogenitor cells and intracellular transduction and transcriptional factors. In addition, the long-term effects of administering superphysiological doses of rhBMPs to patients must be assessed systematically. A new generation carrier/delivery system may be the answer to offset dosing liabilities as well as to provide residence for exogenous, BMP-receptive osteoprogenitor cells (111,112). The areas highlighted in this review offer fertile territory for thought and research to develop rational clinical treatments to promote bone regeneration and to understand some of the biological roles of BMPs.

Osteochondral progenitor cells in acute and chronic canine nonunions

This study examined the ability of cells isolated from early healing segmental defects and from tissue from chronic nonunions to support bone and cartilage formation in vivo and their response to transforming growth factor-beta1 in vitro. Ostectomies (3 mm) were created in the radial diaphysis of four dogs. The dogs were splinted 3-5 days postoperatively and then allowed to bear full weight. At 7 days, tissue in the defect was removed and any periosteum was discarded; cells in the defect tissue were released by enzymatic digestion. The dogs were splinted again and allowed to bear full weight for 12 weeks. Radiographs confirmed a persistent nonunion in each dog. Defect tissue was again removed, any periosteum was discarded, and cells were isolated. Cells were also obtained from the defect tissue by nonenzymatic means with use of explant cultures. One-half of the tissue and one-half of any preconfluent, first-passage cultures were shipped to Cleveland by overnight carrier. At second passage, cells were loaded into ceramic cubes and implanted into immunocompromised mice for 3 or 6 weeks. Harvested cubes were examined histologically for cartilage and bone with use of a semiquantitative scoring system. Confluent fourth-passage cultures of 7 and 84-day defect tissue cells were cultured with 0.03-0.88 ng/ml transforming growth factor-beta1 for 24 hours, and [3H]thymidine incorporation and alkaline phosphatase specific activity were determined. Donor-dependent differences were noted in the rate at which defect cells achieved confluence; in general, cells from 7-day tissue divided most rapidly. Seven-day defect cells formed less bone and at a slower rate than was seen in the ceramic cubes containing samples from day 84. Cells derived enzymatically behaved similarly to those from explant cultures. Ceramic cubes contained fibrous connective tissue, cartilage, bone, and fat, indicating that multipotent cells were present. Stimulation of [3H]thymidine incorporation in response to transforming growth factor-beta1 was donor dependent and variable; only two of six separate isolates of cells exposed to it had measurable alkaline phosphatase activity (which was relatively low), and none of the cultures exhibited an increase in response to transforming growth factor-beta1 for 24 hours. This indicates that mesenchymal progenitor cells are present in the healing defect tissue at 7 and 84 days and that the relative proportion of osteochondroprogenitor cells is greater at the later time. The response to transforming growth factor-beta1 is typical of multipotent mesenchymal cells but not of committed chondrocytes or osteoblasts, indicating that these committed and differentiated cells are not present in early stages of healing and suggesting that their differentiation is inhibited in chronic nonunion.

The effect of guided tissue regeneration on the healing of osseous defects in rat calvaria

The effect of guided tissue regeneration procedures on the healing of osseous defects was studied in Sprague-Dawley rats. A full thickness flap was reflected and two 5 mm defects were made in the calvaria of 17 rats. A resorbable membrane was placed over one defect in each animal, while the remaining defect served as a control. After 3, 5, or 7 wk, healing of the osseous defects was evaluated histologically. There was no statistical difference in bone regeneration between experimental and control sites at any time period.

Engineering bone regeneration with bioabsorbable scaffolds with novel microarchitecture

Critical-sized defects (CSDs) were introduced into rat calvaria to test the hypothesis that absorption of surrounding blood, marrow, and fluid from the osseous wound into a bioabsorbable polymer matrix with unique microarchitecture can induce bone formation via hematoma stabilization. Scaffolds with 90% porosity, specific surface areas of approximately 10 m2/g, and median pore sizes of 16 and 32 microm, respectively, were fabricated using an emulsion freeze-drying process. Contact radiography and radiomorphometry revealed the size of the initial defects (50 mm2) were reduced to 27 +/- 11 mm2 and 34 +/- 17 mm2 for CSDs treated with poly(D,L-lactide-co-glycolide). Histology and histomorphometry revealed scaffolds filled with significantly more de novo bone than negative controls (p < 0. 007), more osteoid than both the negative and autograft controls (p < 0.002), and small masses of mineralized tissue (< 15 mm in diameter) observed within the scaffolds. Based on these findings, we propose a change in the current paradigm regarding the microarchitecture of scaffolds for in vivo bone regeneration to include mechanisms based on hematoma stabilization.

Comparative study of intramuscular and intraskeletal osteogenesis by recombinant human bone morphogenetic protein-2

OBJECTIVE

The purpose of this study was to compare the osteoinducing activity of recombinant human bone morphogenetic protein-2 (rhBMP-2) at intramuscular and intraskeletal sites in rats.

STUDY DESIGN

Five tg of rhBMP-2 was implanted into the right calf muscle of each of 20 rats and into a hole (4 mm in diameter, 1.5 mm in depth) that was made in the mandibular body of each of 20 other rats, with atelopeptide type I collagen as a carrier. The alkaline phosphatase activity and calcium content were quantitatively analyzed 1, 3, 7, and 21 days after the implantation of rhBMP-2 into either mandibular bone (in the intraskeletal group) or calf muscle (in the intramuscular group). The new bone formation was evaluated histologically 21 days after implantation.

RESULTS

On days 1 and 3, the alkaline phosphatase activity and calcium content in the intraskeletal group showed no significant differences from those in the intramuscular group. On the 7th and 21st days after implantation, however, the alkaline phosphatase activity and calcium content in the intraskeletal group were significantly higher than those in the intramuscular group. Histometry of the microscopic views showed that the mean trabecular area was 0.87 mm2 in the intramuscular group and 2.66 mm2 in the intraskeletal group.

CONCLUSIONS

These results suggest that the new bone formation stimulated by rhBMP-2 in the intraskeletal group was greater than in the intramuscular group.

Healing response to various forms of human demineralized bone matrix in athymic rat cranial defects

PURPOSE

This study compared the ability of a bone autograft and four distinct forms of human demineralized bone (DBM) to elicit bone repair in a critical size cranial defect in athymic rats.

MATERIALS AND METHODS

Cranial defects were created in athymic rats and then grafted with either an autograft, rat DBM particles in glycerol (rGel), or one of four forms of human DBM: 1) hGel; 2) Putty (DBM fibers in glycerol); 3) Sheet (sheet of DBM fibers); or 4) Flex (DBM fiber sheet with glycerol). Histology, histomorphometry, and radiographic density of the graft sites were evaluated at 8 weeks.

RESULTS

Of the grafted defects, 29% to 58% were found to be filled with new bone. The rGel and human forms of DBM stimulated similar amounts of new bone growth in comparison with the autograft-filled defects. The fiber-based grafts produced the largest amounts of new bone.

CONCLUSIONS

Human DBM in gel, putty and sheet forms were found to perform as well as an autograft in a critical size cranial defect in the athymic rat.

Expression of bone microsomal casein kinase II, bone sialoprotein, and osteopontin during the repair of calvarial defects

The temporal expression of bone microsomal casein kinase II, osteopontin, bone sialoprotein, alkaline phosphatase, and the accumulation of a solid calcium-inorganic orthophosphate mineral phase, have been charted from day 2 to day 21 during the repair of calvarial defects in rats induced by the implantation of decalcified rat bone matrix. Unlike the sequence of events that occur when the same decalcified bone matrix is implanted subcutaneously or intramuscularly, in which cases the first tissue to form in response to the implant is cartilage that subsequently calcifies and is later resorbed and replaced by bone, the repair of cranial defects is quite different. In the latter case, the first cells induced are undifferentiated mesenchymal cells and early fibroblasts followed by osteoblastic direct bone formation. Somewhat later a few small islands of cartilage are formed, widely separated and spatially distinct from the newly formed bone matrix. All of the cartilage and most of the implanted decalcified bone matrix are later resorbed and replaced by new bone by day 21. This in vivo model of the repair of a bone defect by direct bone formation has provided an excellent system to follow specific biochemical and physicochemical events. The total accumulation and rate of accumulation of the mineral and the two noncollagenous phosphoproteins (bone sialoprotein and osteopontin), as well as the activities of alkaline phosphatase, and for the first time either in vivo or in cell culture, the activity of microsomal casein kinase II, the major enzyme that phosphorylates the bone phosphoproteins, have been determined as a function of healing time in vivo. The overall general pattern of accumulation of the phosphoproteins and calcium-phosphate mineral phase and their relationships are similar to those reported in osteoblast cell cultures also monitored as a function of time.

Developmental restriction of embryonic calvarial cell populations as characterized by their in vitro potential for chondrogenic differentiation

The mechanism(s) by which the cells within the calvaria tissue are restricted into the osteogenic versus the chondrogenic lineage during intramembranous bone formation were examined. Cells were obtained from 12-day chicken embryo calvariae after tissue condensation, but before extensive osteogenic differentiation, and from 17-day embryo calvariae when osteogenesis is well progressed. Only cell populations from the younger embryos showed chondrogenic differentiation as characterized by the expression of collagen type II. The chondrocytes underwent a temporal progression of maturation and endochondral development, demonstrated by the expression of collagen type II B transcript and expression of collagen type X mRNA. Cell populations from both ages of embryos showed progressive osteogenic differentiation, based on the expression of osteopontin, bone sialoprotein, and osteocalcin mRNAs. Analysis using lineage markers for either chondrocytes or osteoblasts demonstrated that when the younger embryonic cultures were grown in conditions that were permissive for chondrogenesis, the number of chondrogenic cells increased from approximately 15 to approximately 50% of the population, while the number of osteogenic cells remained almost constant at approximately 35-40%. Pulse labeling of the cultures with BrdU showed selective labeling of the chondrogenic cells in comparison with the osteogenic cells. These data indicate that the developmental restriction of skeletal cells of the calvaria is not a result of positive selection for osteogenic differentiation but a negative selection against the progressive growth of chondrogenic cells in the absence of a permissive or inductive environment. These results further demonstrate that while extrinsic environmental factors can modulate the lineage progression of skeletal cells within the calvariae, there is a progressive restriction during embryogenesis in the number of cells within the calvaria with a chondrogenic potential. Finally, these data suggest that the loss of cells with chondrogenic potential from the calvaria may be related to the progressive limitation of the reparative capacity of the cranial bones.

Low dosage of native allogeneic bone morphogenetic protein in repair of sheep calvarial defects

A sheep skull trephine defect model was used to test the efficacy of allogeneic partially purified sheep bone morphogenetic protein (sBMP), extracted using a low-cost alternative technique based on 60% ammonium sulphate saturation of the guanethidine-HCI extract of pulverized bone matrix. Eight mg of partially purified sBMP was implanted in six 22-mm right-side sheep calvarial critical-size defects trephined in the diploë area using a midline incision; left-side defects implanted with an equal amount of type IV collagen served as controls. After 16 weeks the sheep were killed and the defects removed. Formation of new bone was evaluated using radiomorphometry and histomorphometry. The healing percentage in sBMP-implanted defects was 60.8 +/- 8.1% and in controls 49.8 +/- 6.7% (P < 0.05) as assessed by radiomorphometry. In cross-sectional histomorphometry, newly formed bone regenerated 50.9 +/- 15.1% in the defects with sBMP and 16.1 +/- 10.6% in controls (P < 0.01). The good result, considering the low dosage of sBMP, can be explained by the strong osteoinductivity and low immunogenicity of native allogeneic sBMP.

Recombinant bone morphogenetic protein (BMP)-2 regulates costochondral growth plate chondrocytes and induces expression of BMP-2 and BMP-4 in a cell maturation-dependent manner

This study examined the effect of recombinant human bone morphogenetic protein-2 on several parameters of growth, differentiation, and matrix synthesis and on the endogenous production of mRNA of bone morphogenetic proteins 2 and 4 by growth plate chondrocytes in culture. Chondrocytes from resting and growth zones were obtained from rat costochondral cartilage and cultured for 24 or 48 hours in medium containing 0.05-100 ng/ml recombinant human bone morphogenetic protein-2 and 10% fetal bovine serum. Incorporation of [3H]thymidine, cell number, alkaline phosphatase specific activity, incorporation of [3H]proline into collagenase-digestible protein and noncollagenase-digestible protein, and incorporation of [35S]sulfate were assayed as indicators of cell proliferation, differentiation, and extracellular matrix synthesis. mRNA levels for bone morphogenetic proteins 2 and 4 were determined by Northern blot analysis. Recombinant human bone morphogenetic protein-2 increased the incorporation of [3H]thymidine by quiescent resting-zone and growth-zone cells in a similar manner, whereas it had a differential effect on nonquiescent cultures. At 24 and 48 hours, 12.5-100 ng/ml recombinant human bone morphogenetic protein-2 caused a dose-dependent increase in cell number and DNA synthesis in resting-zone chondrocytes. No effect was seen in growth-zone cells. Recombinant human bone morphogenetic protein-2 stimulated alkaline phosphatase specific activity in resting-zone chondrocytes in a bimodal manner, causing significant increases between 0.2 and 0.8 ng/ml and again between 25 and 100 ng/ml. In contrast, alkaline phosphatase specific activity in growth-zone chondrocytes was significantly increased only between 12.5 and 100 ng/ml. Recombinant human bone morphogenetic protein-2 increased the production of both collagenase-digestible protein and noncollagenase-digestible protein by resting-zone and growth-zone cells, but incorporation of [35S]sulfate was unaffected. Administration of recombinant human bone morphogenetic protein-2 also increased incorporation of [3H]uridine in both resting-zone and growth-zone chondrocytes; these cells produced mRNA for bone morphogenetic proteins 2 and 4. Bone morphogenetic protein-2 mRNA levels in both resting-zone and growth-zone chondrocytes increased in the presence of recombinant human bone morphogenetic protein-2; however, bone morphogenetic protein-4 mRNA levels in growth-zone cells decreased under its influence, and those in resting-zone cells were upregulated only with a dose of 10 ng/ml. This indicates that recombinant human bone morphogenetic protein-2 regulates chondrocyte proliferation, differentiation, and matrix production, and the effects are dependent on the stage of cell maturation. Resting-zone chondrocytes were more sensitive, suggesting that they are targeted by bone morphogenetic protein-2 and that this growth factor may have autocrine effects on these cells.

[Osteoinduction and -reparation]

Traumata, diseases, developmental deformities, and tumor resections frequently cause bone defects and atrophies. In general, three different mechanisms exist by which bone restoration can be achieved: (1) osteogenesis initiated by vital, osteoblastic cells of autografts; (2) osteoconduction (or creeping substitution); and (3) osteoinduction. The latter mechanism means the differentiation of pluripotent, mesenchymal-type cells (located in a recipient bed with strong regenerative capacity) into cartilage- and bone-forming progenitor cells under the influence of inductive bone morphogenetic proteins (BMPs). Some BMPs are physiologically included in low concentrations as organic components in bone tissue. They can diffuse from demineralized bone implants into the recipient bed and induce a differentiation into new bone tissue. Nine different BMPs have been isolated, characterized, and cloned. Some of these possess inductive properties and can initiate new bone formation in muscle tissue or in bone defects. In the future recombinant BMPs will be available in unlimited quantities. This will lead to completely new therapeutic concepts in reconstructive bone surgery.

Regeneration of rat calvarial defects using a bioabsorbable membrane technique: influence of collagen cross-linking

The aim of the present study was to examine the influence of cross-linking on collagen membranes used for guided bone regeneration of calvarial defects in rats. In 48 Wistar rats, divided equally into 4 groups, 1 control and 3 experimental, standardized transosseous circular calvarial defects were made midparietally. In the control group, the defect was only covered by the soft tissue flap while in the 3 experimental groups, 3 differently cross-linked collagen membranes were interposed between the osseous defect and the overlying flap before suturing. The healing was assessed at 10, 20, and 30 days after surgery. The results showed that augmenting the degree of collagen cross-linking diminished the membrane resorption rate. Compared to the sham-operated sites, the membrane protected defects showed significantly more bone regeneration (on average 4 times more) as attested by histology and measured by histomorphometric analysis. Although the bone gain seemed to augment with increasing degrees of cross-linking, the results within the 3 experimental groups were not statistically different. Since longer healing periods might have been necessary to substantiate results within experimental groups, a study is currently undertaken to evaluate this aspect. This study demonstrated the efficacy of collagen membranes in guiding bone regeneration, as well as the importance of the type and degree of cross-linking.

Influence of three membrane types on healing of bone defects

OBJECTIVES

To determine and compare osseous regeneration associated with three guided tissue regeneration membrane types (expanded polytetrafluoroethylene, dense polytetrafluoroethylene, and an absorbable polylactic acid/citric acid ester base) and removal forces required for expanded and dense polytetrafluoroethylene membranes.

STUDY DESIGN

Bilateral osseous defects were created in 30 adult rat calvaria; one defect was covered with a test membrane and the other received no membrane (control). After 2 or 4 weeks, forces required for membrane removal from the tissues were electronically determined, and the calvaria removed and decalcified. Sections through the defects were stained and evaluated electronically and microscopically. Data were analyzed statistically.

RESULTS

Microscopic evaluation with Mann-Whitney U test revealed that dense polytetrafluoroethylene was associated with significantly greater bone formation than expanded polytetrafluoroethylene (p = 0.02) at 2 weeks and absorbable polylactic acid/citric acid ester base (p = 0.004) at 4 weeks. Electronic evaluation of the linear degree of fill with one way ANOVA and Tukey's test found no significant difference (p > 0.05) among the experimental or the control groups. In addition, the Mann-Whitney U test indicated that removal forces required for dense polytetrafluoroethylene were significantly less than for expanded polytetrafluoroethylene (p = 0.003).

CONCLUSIONS

The use of dense polytetrafluoroethylene as a membrane barrier deserves further investigation as it allows osseous regeneration, it is easier to remove from healing soft tissues, and it is inexpensive. A study with larger sample sizes should be conducted.

Repair of calvarial defects with flap tissue: role of bone morphogenetic proteins and competent responding tissues

Bone morphogenetic proteins 2 through 8 have the ability to induce the in vivo transformation of extraskeletal mesenchymal tissue into bone. The aims of this investigation were to determine the optimal responding tissue and the specificity of the inductive effect of bone morphogenetic protein 3. The optimal responding tissue was found to be skeletal muscle. The specificity of this response to bone morphogenetic protein 3 was compared with that of recombinant human basic fibroblast growth factor, recombinant platelet-derived growth factor, and recombinant insulin-like growth factor. Bone morphogenetic protein 3 was the only factor that induced de novo bone formation. This ability to transform muscle into bone was tested in 7 x 7 mm irradiated skull defects in the rat. After 1500 rads of exposure, these defects showed no significant signs of healing by 8 months. When these defects were treated with the microvascular transfer of a nonirradiated muscle flap, they had 8 percent healing at 4 months and 37 percent healing by 8 months. Defects treated with 30 micrograms bone morphogenetic protein 3 (without the muscle flap) achieved 50 percent healing by 4 months and 64 percent healing by 8 months. When the defects were treated with both the muscle flap and bone morphogenetic protein 3, there was 96 percent healing by 4 months and 100 percent healing by 8 months (p < 0.015, compared with bone morphogenetic protein 3 alone at both time points). At 8 months, the transplanted muscle was entirely transformed into bone and healed the skull defect with newly generated bone indistinguishable from the surrounding calvarial tissue. These findings suggest a potential clinical utility of bone morphogenetic protein 3-induced bone formation in skeletal reconstructions. Furthermore, they also show that there is a collaborative requirement for both the osteoinductive factor bone morphogenetic protein 3 and the presence of competent responsive cells in the well-perfused muscle.

Xenogeneic moose (Alces alces) bone morphogenetic protein (mBMP)-induced repair of critical-size skull defects in sheep

A standardized skull defect in adult sheep was used to test the healing capacity of xenogeneic, partially purified, moose-derived bone morphogenetic protein (mBMP) extracted from the fresh long bones of moose (Alces alces) calves. An amount of 52 mg of mBMP mixed with 13 mg of purified type IV collagen (5:1) (mBMP/COL) in gelatin capsules was implanted into six 22-mm-diameter skull defects in adult sheep for comparison with six defects implanted with fresh autogenous bone marrow (BM) and six other controls implanted with a gelatin capsule containing 13 mg of type IV collagen (C). The amount of new bone formed was quantified from radiographs by computerized image analysis and histology. The healing percentage in the mBMP/COL group was significantly higher (93.18 +/- 4.51%) than in the BM (33.17 +/- 20.05%) or C group (31.32 +/- 17.41%) at 16 weeks after implantation. The difference between BM and C was not statistically significant. The level of anti-BMP antibody in the serum showed a significant increase in the group implanted with mBMP, but returned to normal after 6 weeks. The experiment demonstrated that xenogeneic mBMP possesses a strong osteoinductive capacity and weak immunogenicity.

The effect of insulin-like growth factor 1 on craniofacial bone healing

Human insulin-like growth factor 1, a known regulator of bone formation, was investigated for its possible effect on membranous bone formation in a rat model. Full-thickness bone defects (10 x 10 x 1 mm) were created in the rat calvarium, and insulin-like growth factor 1 was administered by an osmotic minipump directly into the defect enclosed by the periosteum and dura mater. The dose of insulin-like growth factor 1 was 100 micrograms every 2 weeks. The defects were studied radiographically, macroscopically, and microscopically at 3, 6, 9, and 12 weeks. The group treated with insulin-like growth factor 1 showed qualitative and quantitative differences when compared with the control group. The amount of new bone formation in the group treated with insulin-like growth factor 1 was significantly larger than that of the control group. In the insulin-like growth factor 1 group, the location of new bone formation occurred in the center and at the margin of the bone defect. In the control group, bone was formed only around the margin of the bone defect. This study suggests that insulin-like growth factor 1 improved membranous bone healing in vivo and that insulin-like growth factor 1 makes mesenchymal precursor cells of bone differentiate directly into bone-forming cells.

Growth factors in surgery

Surgical reconstruction of functional and aesthetic defects is often compromised by donor-tissue limitations and wound-healing constraints--problems that can potentially be overcome by peptide growth factor therapy. The effectiveness of growth factor therapy in animal models and in limited human clinical trials has been realized. Epidermal growth factor (EGF), fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), platelet-derived wound-healing formula (PDWHF), transforming growth factor (TGF), the bone morphogenetic proteins (BMPs), and general considerations of growth factor therapy are reviewed.

Heparan-like molecules induce the repair of skull defects

Heparin-binding growth factors (HBGFs) are known to stimulate bone repair when applied to bone lesions. Nevertheless, successful treatments are obtained with high protein doses since HBGFs are rapidly degraded in situ by multiple proteolytic activities associated with the inflammatory period of tissue healing. Like heparin or heparan sulfates, heparan-like molecules, named carboxymethyl-benzylamide-sulfonated dextrans (CMDBS), are known to potentiate fibroblast growth factor activities by stabilizing them against pH, thermal or proteolytic denaturations, and by enhancing their binding with cell surface receptors. We have postulated that CMDBS stimulate in vivo bone healing by interacting with endogenous HBGFs, spontaneously released in the wounded site. The effect of CMDBS on bone repair was studied in a skull defect model in rats by computer-assisted radio-morphometry and histomorphometry. Single application of CMDBS in a collagen vehicle to skull defects induced a dose-dependent increase in bone defect closure and new bone formation after 35 days. Complete bony bridging occurred in defects treated with 3 micrograms CMDBS, whereas bone formation was not observed in vehicle-treated defects which contained only dense fibrous connective tissue between the defect margins. These results indicate that heparan-like molecules, such as CMDBS, are able to induce bone regeneration of skull defects. This action is possibly mediated by potentiation of endogenous growth factor activities and/or by neutralization of proteolytic activities.

Repair of human skull defects using osteoinductive bone alloimplants

To estimate the efficacy of cranioplasty in clinical practice, autolyzed, antigen-extracted, allogenic (AAA) bone was prepared from cortical bones of human organ donors. AAA bone implants consisted of completely demineralized bone powder, completely demineralized pliable bone chips, surface-demineralized bone chips with pliable crevices, surface-demineralized rigid bone chips, or combinations thereof. 21 patients received AAA bone cranioplasties and were followed-up for between 12 and 58 months (average: 29 months). No infection or rejection of any of the AAA bone implants occurred. X-ray assessments as well as bone scintigraphies revealed osseous integration and remodelling of the AAA bone implants with minimal resorption, with the exception of completely demineralized AAA bone chips which showed partial resorption (2 cases). However, the partial resorption of completely demineralized AAA bone chips ceased after the implants had been remodelled. In 4 cases, the osteosynthesis material was removed between 10 and 18 months after the cranioplasty. In another case, a re-entry was necessary because of recurrence of an intracranial tumor. All of these five AAA bone reconstructions showed bleeding surfaces and osseous consolidations at the time of re-entry. A bone biopsy taken from one of these cranioplasties showed osteoinduction on the surface of the AAA bone implants. This first clinical review of cranial reconstructions using osteoinductive AAA bone implants emphasizes the therapeutical application of AAA bone for cranioplasty. Large AAA bone chips from human skull bones facilitate the reproduction of the skull's convexity especially when combined with preoperative stereolithography-based planning.

Repair of critical size rat calvarial defects using extracellular matrix protein gels

In this study the authors examined the capacity of gels of reconstituted basement membrane, laminin, and type I collagen to mediate repair of critical size defects in rat calvaria. Although autografts are widely used to repair bone defects caused by trauma or surgical treatment of congenital malformations, neoplasms, and infections, an adequate quantity of graft is not always available. Allogenic bone is readily available, but its use is associated with an increased incidence of nonunion, fatigue fracture, and rejection. Biologically active, purified components of basement membranes, which have been shown to promote osteogenic differentiation and angiogenesis in vitro and type I collagen (the major constituent of bone extracellular matrix) can be formed into native isotonic space-filling gels. In this study critical size calvarial defects were created in retired male Sprague-Dawley rats. Thirty-six animals were divided into seven groups. Group 1 (control) received no treatment for the defects. Group 2 animals were implanted with methylcellulose. Groups 3, 4, 5, and 6 were implanted with gels of type I collagen, reconstituted basement membrane, or laminin, respectively. The last group of three animals (Group 7) was implanted with 100 micrograms of type I collagen gels (identical to Group 3) and sacrificed at 20 weeks following a single CT scan to determine if complete healing could be obtained with this method given sufficient time. Except for rats in the type I collagen group that was evaluated by multiple computerized tomography (CT) scans biweekly from 2 to 12 weeks, bone repair was evaluated using CT at 12 weeks. Healing was quantified using three-dimensional reconstruction of CT. Following the final CT scan in each experimental group, animals were sacrificed, and a sample of tissues was evaluated by conventional histology. Animals treated with type I collagen gels showed 87.5% repair of the area of the defects at 12 weeks and 92.5% repair by 20 weeks. Increasing the gel volume 1.5 x accelerated complete repair to 3 months. Murine-reconstituted basement membrane and laminin gels induced 55.5% and 46.3% repair, respectively, at 3 months. In untreated control animals 7% repair of the area of the defects showed at 3 months. Histological analysis confirmed new bone formation in partial and completely healed defects. Bioengineered native collagen gels may have wide applicability for bone repair as an alternative bone graft material alone, in combination with autograft or marrow aspirate, or as a delivery system for osteogenic growth factors.