 |
 |
 |
|
| home | CME | help | feedback | subscriptions | archive |
|
The Journal of Bone and Joint Surgery (American). 2005;87:1367-1378. doi:10.2106/JBJS.D.02585 © 2005 The Journal of Bone and Joint Surgery, Inc.
Bone Morphogenetic ProteinsDevelopment and Clinical Efficacy in the Treatment of Fractures and Bone Defects
1 Department of Surgery and Traumatology, VU University Medical Center, P.O. Box
7057, 1007 MB Amsterdam, The Netherlands. E-mail address for M.F. Termaat:
mf.termaat{at}vumc.nl.
E-mail address for F.C. Bakker:
fc.bakker{at}vumc.nl.
E-mail address for P. Patka:
p.patka{at}vumc.nl.
E-mail address for H.J.Th.M. Haarman:
hjtm.haarman{at}vumc.nl
Investigation performed at the Department of Surgery and Traumatology,
VU University Medical Center, Amsterdam, The Netherlands
The discovery of bone morphogenetic proteins marks a major step forward in the understanding of bone physiology and in the development of advanced methods in skeletal surgery. The cornerstones for successful growth-factor therapy in skeletal surgery remain biomechanical stability and biological vitality of the bone providing an adequate environment for new bone formation. Knowledge of the biological characteristics, mechanisms of action, and methods of delivery of growth factors will become essential for skeletal surgeons. The current clinical application of bone morphogenetic proteins is safe and efficacious as a result of a well-regulated cascade of events leading to bone formation. Clinical trials have not yet determined whether different clinical indications each require a specific bone-tissue-engineering format or if a single pathway for stimulating bone-healing with growth factors is sufficient.
The treatment of fractures remains a challenge in skeletal surgery. Patients who have sustained trauma are often young and, consequently, the medical and socioeconomic costs of fractures are high1,2. As recently reported, the lifetime risk of fracture is one in two for males and one in three for females. These numbers are equivalent to the lifetime risk of coronary artery disease and demonstrate that fractures are a substantial public health problem2. Although the majority of fractures heal normally, between 5% and 10% of patients have impaired fracture-healing3. The cornerstones for successful bone-healing are biomechanical stability and biological vitality of the bone providing an environment in which new bone can form. Many conditions, such as insufficient vascularization, infection, mechanical instability, and systemic diseases, can impair this environment. Improving these conditions and preventing detrimental factors should guide surgical strategies. Increases in the understanding of bone physiology and advancements in biotechnology have led to a new surgical therapy for controlling and modulating bone-healing with growth factors. Skeletal regeneration, or bone-healing, is a recapitulation of embryonic bone development, as bone heals through the generation of new bone rather than by forming scar tissue. Growth factors in the transforming growth factor-ß (TGF-ß) superfamily, including bone morphogenetic proteins (BMPs), are the most intensively studied group of peptides involved in embryogenesis and in adult bone repair and are the most promising group of growth factors for use in the enhancement of bone repair. The effectiveness of applying BMPs has been extensively investigated preclinically and has led to the clinical introduction of the most potent BMPs (BMP-2 and BMP-7). Because the application of growth factors will play an important role in future surgical strategies, knowledge of the biological characteristics, mechanisms of action, and methods of delivery will be essential.
The phenomenon of bone induction, or osteoinduction, was first described by Urist in 1965, after he made the key discovery that new bone formation occurred locally in rodents after intramuscular implantation of bone cylinders decalcified with hydrochloric acid4. This phenomenon was ascribed to the presence of a protein in bone matrix, which he named bone morphogenetic protein5. In the late 1980s, it was found that not a single protein but a group of proteins in the bone matrix was responsible for osteoinduction6,7. Since then, the structure of more than sixteen different human BMPs has been identified with the aid of molecular biology techniques7-10. The BMPs are designated as BMP-1 to BMP-16 and are divided into several subtypes, according to structural similarity between the molecules within each subfamily (Table I). With the exception of BMP-1, the BMPs belong to the transforming growth factor-ß (TGF-ß) superfamily, a group of growth and differentiation factors that play an important role during embryogenesis and tissue repair in postnatal life7,8,10-14. The role of BMPs in bone formation during embryonic development and in adult bone-healing has been elucidated to a large extent15,16.
Osteoinductive Activity Only a subset of BMPs has the unique property of inducing de novo bone formation, or osteoinduction, by themselves16. BMP-2 through 7 and BMP-9 have been shown to have this property17-26, meaning that these osteoinductive BMPs have the capacity to provide the primordial signal for the differentiation of mesenchymal stem cells into osteoblasts. After ectopic implantation (e.g., subcutaneously in rats or intramuscularly in baboons), individual recombinant human (rh) BMPs induce endochondral bone formation. A typical sequence of events can be observed in endochondral bone formation induced by BMPs: recruitment and proliferation of monocytes and mesenchymal cells, differentiation into chondrocytes, hypertrophy of chondrocytes and calcification of the cartilage matrix, vascular invasion with associated osteoblast differentiation and bone formation, and finally remodeling of the newly formed bone and bone marrow formation (Fig. 1)27-29.
Cheng et al. recently demonstrated, in mesenchymal progenitor and osteoblastic cells infected by adenovirus-mediated gene transfer of BMPs, the relative osteoinductivity of different BMPs at various stages of the differentiation process26,30. BMP-2, 6, and 9 play an important role in the early phase of the differentiation of mesenchymal progenitor cells to pre-osteoblasts, while most BMPs promote the terminal differentiation of these pre-osteoblasts to osteoblasts. These findings could indicate that BMP-2, 6, and 9 may be more effective in a situation in which pluripotent cells are present (e.g., application of an autologous bone transplant or recruitment from surrounding muscular tissue). In the clinical situation of normal fracture-healing, in which pre-osteoblasts are predominantly present, the majority of the BMPs are adequate for promoting osteogenesis. However, the clinical relevance of this cascade, in which different BMPs are important at various steps in the differentiation process leading to bone formation, remains unknown. Interestingly, it has been demonstrated that the osteoinductive activity of BMP-3 depends on the experimental conditions and the stage of differentiation, as described above. BMP-3 has been shown to be osteoinductive in vivo, but it can also be inhibitory in the presence of other BMPs, such as BMP-2 and BMP-7, and it is a negative regulator of postnatal bone density in animal models26,30,31. Thus, BMP-3 appears to lack the stimulatory role of other BMPs, but these findings also illustrate that bone formation is the end-product of a complex intracellular and extracellular signaling cascade in which BMPs may have contradictory roles at different stages32. Although more BMP subtypes have been demonstrated to be osteoinductive, at present only rhBMP-2 and rhBMP-7 have been developed and used for clinical applications26.
Receptors and Intracellular Signaling Signaling by the TGF-ß family occurs through type-I and type-II serine/threonine kinase receptors, of which several submembers have been identified. Each member of the TGF-ß family, including the BMPs, binds to a characteristic combination of type-I and II receptors, which assembles a phosphorylated complex. Subsequently, this complex phosphorylates and activates receptor-regulated Smads (R-smads: Smad 1, 5, and 8 for BMPs). The Smad pathway is regulated by mediator Smads (Smad 4 for BMPs), inhibitory Smads (I-Smads: Smad 6 and 7), Smad binding proteins, and protein degradation34-37. The complex of Smad proteins are nuclear effectors, and they determine the outcome of the signal by activating or repressing target genes in the nucleus and they change the cellular activity, including growth, differentiation, and extracellular matrix synthesis (Fig. 2)38-40.
Growth and differentiation factors such as the BMPs play an essential role in cellular functioning but require a tempering of their signal for coordinated bone formation and remodeling. This is achieved by feedback mechanisms: extracellularly by antagonists (i.e., noggin) and intracellularly by inhibition and modulation of the Smad-signaling pathway41. Little is known about the downstream nuclear factors that inhibit or stimulate the intracellular BMP signal. Hence, BMP signaling is a complex cascade with many levels of mutual interactions of the signal at extracellular, membranous, cytoplasmic, and nuclear levels15.
Biological Activity of BMPs The production of individual recombinant human BMPs (rhBMPs) by biotechnology companies led to two important conclusions. First, single BMPs are osteoinductive by themselves. Second, the osteoinductivity of a single BMP has a dose-response ratio42,43. Patient characteristics do not determine the efficacy of the dose of BMPs, as these proteins act locally. Therefore, the concentration of BMPs at the site of implantation is more important than the total dose of the BMP. The dose of BMPs must overcome a threshold before they can effectively induce bone formation. If the dose of BMPs is too low there will be inadequate bone formation, and if it is too high there will be more bone formation and more rapid osteoinduction than desired43. This increased bone formation eventually results in direct (intramembranous) ossification, bypassing the intermediate phase of endochondral ossification that occurs when lower doses are used. However, with high doses of BMPs, initial localized resorption of bone can be seen as a result of increased osteoclastic activity, as BMPs also stimulate osteoclastogenesis and osteoclastic activity41,44,45. At this point, a higher dose of BMPs has no effectiveness, as the upper border, after which no further acceleration of bone formation occurs, has been reached43,46,47. Local overdoses of BMPs could be expected to lead to heterotopic ossification, but this phenomenon has not been shown to occur under physiological conditions. In a mouse model of BMP-4-induced heterotopic ossification, Glaser et al. demonstrated in vivo that heterotopic ossification in fibrodysplasia ossificans progressiva may not be due to the genetic overexpression of BMP-4 but rather to the underexpression of the extracellular antagonist of BMPs, noggin48. Excessive ossification in this animal model could be prevented by local delivery of noggin, illustrating the highly regulated negative feedback mechanisms for BMPs that prevent abnormal or heterotopic bone formation even with high doses. The dose of BMP needed for clinical efficacy must overcome a threshold, and the dose-response curve becomes steeper as one progresses from rodent to nonhuman primate models. The latter species, which is most closely related to humans, was used to derive the human therapeutic dosage of 3.5 mg/4 mL of sterile saline solution or 0.88 mg/mL of sterile saline solution for rhBMP-7 and 12 mg/8 mL of sterile water or 1.50 mg/mL of sterile water for rhBMP-2. RhBMPs are expensive and are used in current clinical applications at concentrations that are ten to 1000-fold higher than those of native BMPs. These high doses of BMPs are used in an attempt to produce a clinical effect comparable with that shown to be osteoinductive in animal studies. The strongly regulated signaling mechanisms and the rapid local and systemic clearance of BMPs in higher species also necessitate higher doses. It has also been assumed that higher species have fewer responding cells than do lower species. This raises important questions regarding combination therapies of BMPs and the development of advanced delivery systems that are more efficient and, not unimportantly, more cost-effective.
In the early 1990s, a major hurdle was cleared with the isolation, identification, and understanding of the physiological mechanisms of BMPs. However, it became evident that local application of BMPs is essential, as their systemic clearance is high. The natural delivery system of BMPs (i.e., bone matrix) is highly effective, as the proteins are incorporated into a matrix that serves as a reservoir49. As mentioned above, the biological activity of rhBMPs shows a dose-response relationship and, to increase the osteoinductive response, the activity must be maintained over a period of time. However, local tissue clearance of rhBMPs is rapid. Studies of animal fracture models have demonstrated that BMPs can be efficacious when administrated locally in a formulation buffer50-53. Despite this, it has become evident that bone repair is not efficiently stimulated in larger animal models, as the BMPs are not retained at the repair site for a sufficient period of time. Research has been concentrated on how to deliver these proteins efficiently at the site of implantation by varying the delivery systems. The need to develop an adequate delivery system for BMPs has hampered their clinical application54. In general, delivery of growth factors has been attempted through two approaches: (1) protein therapy, in which a recombinant protein is delivered directly to the regeneration site with or without a carrier matrix, and (2) gene therapy, in which a protein is delivered indirectly by its encoding gene.
Carriers Many different carrier materials have been used in animal experiments. These include demineralized collagenous bone matrix50,58-60, collagen products55,58,61, resorbable polymers such as polylactide62,63 and polylactide-co-glycolide64-68, gelatin hydrogels69,70, calcium phosphate ceramics such as tricalcium phosphate71,72 and hydroxyapatite73,74, resorbable calcium phosphate cement (alpha-BSM)75, and combinations of these materials76-79. Collagen-based carriers possess the appropriate characteristics, as type-I collagen, the main component of bone matrix, is biocompatible and bioresorbable. Collagen provides an osteoconductive matrix for the ingrowth of newly formed bone by promoting the deposition of minerals and by binding noncollagenous matrix proteins that initiate mineralization80. The rhBMPs available for clinical applications are delivered in a type-I bovine collagen carrier, either a collagen sponge (used with BMP-2) or collagen particles (used with BMP-7). The current clinical collagen carriers release BMPs in a bulk manner and require high doses of BMPs to produce an effect. Collagen-based carriers are effective for the current first-generation products, but they will need to be substantially improved in order to achieve clinical efficacy at lower doses and, therefore, at lower cost. It remains unknown if a sustained release of BMPs is required throughout the bone-healing process, and this question must be elucidated by future research.
The limitations associated with current carrier materials, such as immunological reactions and the inability to provide sustained release of BMPs, have led to the development of advanced approaches for the delivery of BMPs81. There are two ways to deliver the required BMP gene to the regeneration site: it can be delivered directly to the tissue so that host cells are transfected and express the protein (in vivo transduction)82-85, and it can be delivered through transfection of cultured cells, which are implanted at the regeneration site (in vitro transduction) and subsequently express the protein83,86-91. The extracellular signaling pathway of the BMPs advocates for the latter, in vitro, approach, as this technique provides a well-controlled transduction of the gene in a specific cell population that will express the protein at the required site. Despite the potential for using gene therapy to express growth factors, questions concerning the safety of employing viral vectors and the influence of immunological reactions to viral proteins need to be addressed before clinical application can be considered.
Local and Systemic Safety Excessive local bone formation has been reported only after the use of very high BMP doses, and it varies among species. Also, the excessive bone formation has been followed by eventual remodeling to the normal bone contour42. Moreover, bone formation requires the presence of mesenchymal cells expressing receptors for BMP, and these are unregulated only in specific clinical settings such as after injury92. When BMPs are used in the current therapeutic dosages, there seems to be a negligible risk of excessive bone formation at the implantation site93-96. Animal studies have demonstrated that, as a result of rapid clearance, systemic concentrations of rhBMPs are virtually undetectable after intravenous delivery. To our knowledge, studies of the use of rhBMPs in humans have not demonstrated any systemic toxicity thus far93,97-100. Currently used collagen carrier materials are immunogenic and induce the formation of antibodies. In recent clinical studies of small series, antibodies against BMPs developed in 6% to 10% of patients and antibodies against type-I bovine collagen developed in 5% to 20%93,96. In a study by Govender et al., the number of patients with an immunogenic reaction increased from one to nine when the dose of rhBMP-2 was increased from 0.75 to 1.50 mg/mL96. The antibody titers were low and transient, and this sensitization was not reported to have any adverse effects on the bone-healing process93,96. It is important to realize that, although sensitization was observed in a minority of the patients in these first clinical studies, the presence of antibodies was positively correlated with higher doses of both BMP and collagen. It remains unknown what immunogenic reactions will occur when even higher doses of BMPs or collagen are applied for the first time in a patient or for a second time in a previously sensitized patient. The long-term effects and possible genetic alterations due to the application of BMPs in humans remain unknown. In vitro and in vivo studies to date have not demonstrated any evidence of carcinogenicity, and they have even demonstrated an antiproliferative effect on human tumors101,102. We are not aware of any clear published data on the application of BMPs during human pregnancy or the effect on embryonic or adolescent development. Therefore, pregnancy is a contraindication to the use of BMPs, and applications in children must be considered carefully.
Clinical Efficacy of BMPs for Stimulating Fracture-Healing It is surprising that the effects of BMPs on fractures, potentially the most common clinical indication, have been studied only to a limited extent. BMPs and their receptors are unregulated during the fracture-healing process92,103. Animal models have demonstrated that fracture-healing can be accelerated by the local administration of rhBMPs50,52,58,103,104. The primary goal of the use of BMPs in fresh (open) fractures is to enhance the fracture-healing process in order to reduce the rate of secondary interventions and thus to shorten the healing time96.
Riedel and Valentin-Opran performed what we believe was the first feasibility study of the application of rhBMP-2 bound to an absorbable collagen sponge in patients with a fresh open tibial fracture105. This clinical trial demonstrated that implantation of rhBMP is surgically feasible and safe, and most patients had primary healing without secondary interventions. The BESTT study group evaluated the application of rhBMP-2 in an absorbable collagen sponge (ACS) in open tibial fractures with the primary end point for efficacy defined as the percentage of patients requiring secondary intervention within twelve months96. The patients were randomized to three treatment groups. Group I (147 patients) had fracture treatment with open reduction and internal fixation (the standard care) and was considered to be the control group, Group II (145 patients) had standard care with the addition of 0.75 mg/mL of rhBMP-2/ACS, and Group III (145 patients) had standard care with the addition of 1.50 mg/mL of rhBMP-2/ACS. The percentages of patients requiring secondary intervention were 46%, 37%, and 26%, respectively; thus, the percentages were lower in both groups treated with rhBMP-2. Compared with the control group, the group treated with 1.50 mg/mL of rhBMP-2 had a significant (44%) reduction in the rate of secondary interventions (p = 0.0005) and a significantly shorter healing time (145 compared with 184 days, p = 0.0022). A noteworthy limitation of the study was an imbalance in the use of unreamed and reamed nailing, with the latter favored in the rhBMP-2-treated groups. As multiple regression analyses indicated that both reaming and rhBMP-2 independently affected the primary outcome of the fracture treatment, the authors considered it appropriate to pool subgroups to assess the overall efficacy of rhBMP-2. Also, there was a risk of verification bias in this study because the treating surgeon, who was not blinded to the treatment assignment, performed the final clinical and radiographic assessments of fracture union. The implantation of rhBMP-2/ACS significantly reduced the rate of infections in patients with a Gustilo-Anderson type-IIIA or IIIB open fracture (p = 0.0219). The authors attributed this observation to increased fracture stability and increased vascular supply, both of which enhanced soft-tissue healing106. A randomized clinical trial of the use of rhBMP-7 to stimulate healing of fresh open fractures is ongoing, and the preliminary results were recently presented by McKee et al.107. Overall, 124 patients with an open tibial shaft fracture were randomized either to a group treated with the addition of rhBMP-7 in a type-I collagen carrier at the fracture site or to a control group treated with normal wound closure. In this preliminary report, the number of secondary interventions was lower in the rhBMP-7 group than it was in the control group (12% and 27%, respectively).
Clinical Efficacy of BMPs in the Treatment of Bone Defects and Nonunions In the late 1980s and early 1990s, Johnson et al. evaluated several small series of patients in whom resistant non-unions and segmental long-bone defects had been treated with human BMP (hBMP)112-114. As recombinant BMPs were not yet available, a purified mixture of BMP proteins (hBMP) was used, in combination with insoluble noncollagenous proteins; both were derived from human donor bone. At that point in time, the risk of immunogenicity associated with alloimplants was becoming apparent. Also, it remained unknown which specific proteins were responsible for the osteoinductive activity and how this activity should be managed to develop an applicable clinical product. However, these were the first studies to assess purified BMPs clinically, and they demonstrated that these alloimplants were tolerated and could be useful in the management of difficult nonunions. To our knowledge, Geesink et al. were the first to demonstrate, in a clinical model, that recombinant BMPs can induce healing of cortical bone defects in humans99. RhBMP-7 bound to collagen particles was shown to be effective in five of six patients with a critically sized fibular defect. Only three of six patients treated with the type-I collagen carrier alone had bone formation. We believe that this was the first evidence of the osteoinductive capacity and effectiveness of rhBMP-7 in humans. Friedlaender et al. assessed the efficacy of rhBMP-7 bound to type-I collagen (an OP-1 [osteogenic protein-1] implant) for the treatment of established tibial nonunions by comparing it with autologous bone-grafting, the so-called gold standard of treatment93. The union rate was similar in the two groups, which also did not differ with regard to clinical or radiographic healing characteristics. Although comparative efficacy was demonstrated, the goal of the BMP therapy, a higher healing rate, was not achieved. Unfortunately, the authors did not conduct a power analysis for their study, and the lack of differences between the groups may have been the result of a type-II statistical error. It is interesting to speculate that the rhBMP-7 might have been more effective, as the percentage of patients with atrophic nonunion was higher in the rhBMP-7 group (41%) than it was in the group treated with autologous bone-grafting (25%). However, no statistical analysis with adjustment for atrophic nonunion was performed. Also, the authors chose a difficult population of patients with nonunion to study, as the majority had had prior bone-grafting and intramedullary rod fixation before inclusion in the study and this could have resulted in heterogeneity of the results. Another factor causing heterogeneity may have been the fact that clinical and radiographic assessment of successful bone-healing is extremely difficult and subjective in such patients115. Friedlaender et al. did show that the risk of infection at the implantation site was significantly lower in the rhBMP-7 group than it was in the group treated with autologous bone-grafting (p = 0.002). The authors concluded that BMPs could be deemed an effective alternative to autografting if the morbidity associated with harvesting of the graft is taken into consideration93.
Clinical Efficacy of BMPs in Spinal Fusion Burkus et al. investigated the use of rhBMP-2/ACS to stimulate spinal fusion in a cohort of 279 patients with degenerative disc disease. In two prospective randomized trials, the effectiveness, safety, and rates of osteoinduction following anterior lumbar interbody fusion with use of rhBMP-2/ACS were compared with those following fusion with autologous bone graft, with both substances placed in the hollow central portion of an LT-Cage121,122. Plain radiographs and computed tomographic scans were used to evaluate the pattern of osteoinduction in the interbody space and the progression of fusion at six, twelve, and twenty-four months. All patients treated with the rhBMP-2/ACS demonstrated evidence of osteoinduction at six months. At twenty-four months, the fusion was successful in 94.5% of the patients in the BMP-2 group and 88.7% of those in the control group. This difference was not significant. In another study, Burkus et al. investigated the efficacy of rhBMP-2 for enhancing the incorporation of threaded cortical allografts in forty-two patients123. At twelve months, interbody fusion was demonstrated in 100% of the BMP-2 group compared with 89.5% of the control group. These studies demonstrated the effectiveness of BMPs in spinal fusion and the potential for eliminating the harvest of autograft121-123. In a randomized trial, Johnsson et al. compared the efficacy of rhBMP-7 with that of autologous bone-grafting for achieving posterolateral fusion in twenty patients94. The radiostereometric and radiographic results at twelve months showed no significant difference between the two methods of fusion. A major concern about the use of BMPs to enhance spinal fusion is the risk of bone overgrowth or heterotopic ossification leading to spinal or foraminal stenosis. In a laminectomy defect model with a dural defect in dogs, Meyer et al. found no evidence of abnormal mineralization within the the-cal sac or in the spinal cord after treatment with rhBMP-2124. However, caution with regard to the use of the BMPs should be exercised in cases of dural defects, even after repair. RhBMP-7 has been approved by the Food and Drug Administration for use with lumbar interbody spinal fusion cages in the United States. These cages have the advantage of providing proper containment of the BMP and offering resistance to axial loading. When BMPs are used in spinal surgery, the key considerations are accurate placement of the BMP at the fusion site, retention of the BMP by the carrier, securing hemostasis to prevent dilution of the BMP, and limiting the exposure of bone beyond the fusion area. It remains difficult to assess the clinical outcomes of the use of BMPs in spinal fusion, as objective standards for successful fusion are not available. Current evidence indicates that BMPs can be used safely and effectively when the above conditions have been met.
Infection
Angiogenesis
The discovery of BMPs marked a major step forward in the understanding of bone physiology and in the development of advanced methods of skeletal bone surgery. These growth and differentiation factors have a unique potential to induce new bone formation, even at extraskeletal sites. It has taken almost forty years since the initial discovery of these growth factors by Urist4 for them to become available for clinical application. The successful application of BMPs depends on elucidation of the optimal therapeutic dosage, delivery system, and local conditions for bone repair, and these factors are still under investigation. Basic surgical management to provide adequate environmental conditions of the implantation site, such as soft-tissue coverage, host-bed vitality, and biomechanical stability, remains essential. Prospective clinical trials of BMP-2 and BMP-7 for the treatment of fractures and nonunions, respectively, have shown solid outcomes. Clinical trials, although still limited in number compared with preclinical studies, have demonstrated that BMPs are effective and safe for human application and have an efficacy comparable with that of autologous bone-grafting. Growth-factor therapy with BMPs offers a new surgical approach that can augment or even replace bone-grafting procedures. However, it must be noted that treatment of fresh fractures with BMP in humans did not result in a significantly higher rate of bone-healing compared with that yielded by current treatment techniques such as autologous bone-grafting. This finding is in strong contrast to the preclinical results in animal studies. BMPs must be given in much higher doses to accomplish osteoinductive activity in humans. The first clinical studies of BMP-2 and 7 have demonstrated that bone formation is not always consistent. Possible explanations for this are the relative osteoinductivity of the applied BMPs in the presence of responding cells and the time at which the BMPs are presented locally by their carrier. An increased understanding of these mechanisms and additional research are still needed to develop better carrier systems and possible combination therapies with other BMPs or growth factors. Future preclinical and clinical research must clarify issues regarding the relative effectiveness of BMPs, the interaction between BMP subtypes, and the characteristics of responding cells in much greater detail. BMPs have great clinical potential, but in the next decades we will have to discern whether there is a single pathway to efficient bone-healing or whether different clinical situations require specific bone-tissue-engineering formats.
The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
|
Top
Abstract
Introduction
