ARTÍCULO ESPECIAL

 

Base theories and the clinical application of bone morphogenetic proteins in maxillofacial surgery

Bases teóricas y aplicación clínica de las proteínas morfogenéticas óseas en cirugía maxilofacial

 

 

C.M. Ardila Medina

Profesor Asistente Universidad de Antioquia.
Presidente Sociedad Colombiana de Periodoncia y Oseointegración/Regional Antioquia. Antioquia, Colombia

Correspondence

 

 

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ABSTRACT

One of the fundamental advances in bone neoformation has been the identification of important growth factors like the bone morphogenetic proteins that regulate live cartilage and bone differentiation. The cleansing, genetic cloning and expression of recombinant human bone morphogenetic proteins (BMP) have laid the basis for cellular and molecular analysis of bone development and regeneration. The genetic study of the BMPs indicates that they are essential to the normal development and function of animals. BMP post-natal bone development is also very important in embryonic organic, skeletal, craniofacial and dental tissue development. The availability of BMPs provides several challenges and opportunities to improve insights into the mechanisms that regulate the regeneration of bone for optimal outcome in the patient.

Key words: Bone morphogenetic proteins; Bone regeneration.

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RESUMEN

Uno de los grandes avances en la neoformación ósea ha sido la identificación de factores de crecimiento importantes para ella como son las proteinas morfogenéticas óseas (PMO) que regulan la diferenciación ósea y cartilaginosa in vivo. La depuración, clonación genética y expresión de las PMO han establecido las bases para el análisis celular y molecular del desarrollo y la regeneración ósea. El estudio genético de las PMO señala que son esenciales para la función normal animal y en la osteogénesis postfetal es importante en el desarrollo embrionario orgánico, esquelético y de los tejidos dentales y craneofaciales. La disponibilidad de las PMO proporciona retos y oportunidades para mejorar los conocimientos que regulan la regeneración ósea con el fin de optimizar los resultados en el paciente.

Palabras clave: Proteína morfogenética ósea; Regeneración ósea.

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Introduction

The development of the bone morphogenetic proteins has allowed for an alternative to the traditional bone graft, which has been the golden standard for oral and maxillofacial reconstruction. The BMPs have been successfully used in primates to restore bisected mandibles and critical bone defects,^1,2 repair alveolar fissures,³ and induce bone formation in the maxillary sinus before implant placement.⁴ The clinical application of the BMP includes repairing facial, alveolar, and mandible bone defects.⁵

The purpose of this review is to present a clear look at bone morphogenetic proteins and their potential in the initiation and promotion of osteogenesis and how they can be applied in the regeneration process. Investigations regarding the molecular indicators of bone and cartilage differentiation have identified an entire group of bone morphogenetic proteins that have a regulating effect.

Morphogenetic proteins and their role in initiating bone formation

The post-natal bone development model induced by the extracellular matrix of demineralized bone has provided important expectations in material investigation. Muscular and subcutaneous implants of the demineralized bone matrix generate local differentiation in the endochondral bone.⁶ The tissue created by this implant is similar to the one seen in the embryological bone development. However, unlike the epispheo growth where there is continual bone and cartilage differentiation, in induced bone matrix model there is only one endochondral,⁶ formation cycle.

Due to the small quantity of osteogenic proteins that strongly attach to the organic and inorganic components of the extracellular bone matrix, it has been difficult to identify these bone matrix proteins. The existence of bone morphogenetic proteins was confirmed6 because the demineralized bone matrix can be dissociatively extracted and deactivated with agents such as 4M guanidine HCL. They can be osteogenetically restored with reconstituents that deactivate residues (insoluble collagen matrix) with parts of soluble proteins6. It should be mentioned that regardless of its solubility, the collagen matrix activates and a combination of the two forms restores osteogenic activity in an extra skeletal bio-assay in rodents.⁷ This bio-assay serves as a starting point for the depurification of bone morphogenetic proteins which were evaluated by molecular cloning and recombinant human protein expression.⁸

One of the bone morphogenetic proteins, the osteogenin, isolated itself from a bovine bone matrix through affinity to heparin chromatography. After electrophoresis it was purified by a electroendosmosys.⁸ The amino acid sequence of the peptides of the osteogenin revealed a primary structure identical to the sequence of amino acids found in the DNA clones of the bone morphogenetic protein BMP-3.⁸ Later, the osteogenin was isolated y purified from the bone matrix of a baboon. Other osteogenin proteins like BMP-2 and BMP- 4 have been isolated, recognized and cloned thanks to their identification via human molecular clones.⁹ Insoluble collagen from the bone matrix is obtained from the inactive residue after the dissociative extraction from the bone matrix using 4M guanidine- HCL.^8,9 The native and recombined human BMPs induce local endochondral bone formation when they are reconstituted with this insoluble collagen. The BMPs show sequences homologous to the members of the Beta transforming growth factors (TGF-Beta)⁷ which are amply distributed en vertebrates and invertebrates.^4,5 BMP-2,-3,-4,-5,-6,-7 and -8 are synthesized like polypeptide chain precursors with a terminal region of hundreds of amino acids and carboxyl terminals.^7,9 The mature proteins are processed from the precursors of the carboxyl terminals.

The surprising structural conservation of the BMPs changes the possibility that they will dominate the critical development and regulatory function not only of the cartilage and bone but also embryonic skeletal formation.⁶ Now that there are investigations that show that as well as postnatal osteogenesis the osteogenin (BMP-3) can influence the embryonic skeletal formation.^4,7 The distinct spatial patterns of BMP-2, BMP-3, BMP-4 and BMP-6 mRNAs suggest that they are also involved in the bone induction that controls the bone embryonic developmental formation pattern.¹ BMP-7 is present in high levels in the kidney, increasing the possibilities that it carries out its endocrine function. BMP-7 is also shown in the brain, suggesting a possible involvement in the development of the central nervous system.^1,2 BMP-3 is present mainly in the lungs, which suggests it has endocrinal involvement in the skeletal tissue homeostasis, as suggested in the case of the kidney,.^1,2 BMP-2 and BMP-2 mRNAS have not been detected in adult organs.¹⁰ Table 1 shows the BMP that leads to bone and cartilage induction.

Bone Morphogenetic Proteins and Regeneration

The interaction between the bone morphogenetic proteins and the macromolecules of the extracellular matrix has led to the development of new concepts concerning their regulatory potential in osteogenesis.^3,4 Inducted bone formation has been witnessed due to the recombined human and natural BMP's biological activity in extra skeletal sites of primates. ^3,6,8,9 Important studies have shown that osteogenesis can be used in reconstruction of the bone marrow for bone deformities in Bamboos.¹¹

The idea that optimal induction of bone formation depends on the combined action of the BMP and the complimentary substrate is an important one for future projects. When in a solid state, the substrate compound of the extracellular matrix, in association with soluble BMPs, starts the cascade of bone differentiation.³ After dissociative extraction and reconstruction of the BMP with an insoluble collagen matrix biological activity is restored. This indicates that the components of the extracellular bone matrix act as a vehicle for the functional expression of the bone morphogenetic proteins.^2,9 Some investigations have illustrated that the collagen bone matrix provides an optimal substrate for the recruitment and adhesion of regenerative cells, posterior proliferation and osteoblast differentiation.^1,2 The foundation of the collagen matrix in the development and differentiation was manifested early,⁸ showing that the collagen matrix can prevent the premature diffusion of the surgical implant,^2,9 as well as protect the BMP from unspecified proteolysis. The collagen transporter acts as a provisional substrate until it is replaced by new bone formation.

Clinical Application

Below the clinical use of BMP in mandible defects, preprosthethic maxillary and mandible regeneration, maxillofacial trauma and alveolar fissure reconstruction will be discussed.

Mandible continuity defects are secondary to the recession of tumours which have a high mortality when not surgically reconstructed, converting autogenetic implants into an important alternative for said reconstruction. Toriumi and collaborators, use a canine model to study BMP-2 to stimulate growth in defected mandibles.¹² They found that 68% of the defects were substituted by mineralized bone. Boyne found that the BMP-2 was an effective bone regenerative inductor in primates with critical bone defects.¹ In 2004, Warnke et al. used BMP-7 and blocks of mineralized bone to create vascularised human bone grafts they discovered bone restoration and mineralization.¹³ Development of effective reconstruction procedures using bone inductive factors without the need for bone grafts would have a strong impact on reconstructive head and neck surgeries.

Preprosthethic maxillary and mandibular lift augmentation: the use of bone implants and substitutes are frequent. The most commonly used procedures are alveolar ridge raise and maxillary sinus elevation. Jovanovic and cols. did a histological study where they carried out alveolar ridge raise on an animal model. When compared with resident bone, they did not find significant differences among the implants with bone induced BMP-2. With this they concluded that the BMP- 2 allows for the installation, bone integration and long term "functional load" in dental implants placed in dogs.¹⁴ Boyne and cols. recently published a study which evaluated recombinant human BMP in the maxillary sinus.⁴ They concluded that the BMP induced bone with certainty and predictability, which is suitable for the functional load of endosseous implants in those patients that need their maxillary sinus raised.

Maxillofacial Trauma: studies have been done that evaluate BMP-2 in trauma cases, showing their effectiveness in treating complete segment defects or partial long bones.^15,16 The severe pain in those that had soft and hard tissue loss led to challenges such as: procurement of soft tissue closure and contamination prevention. Another challenge was the loss of the BMP-2 in gunshot wounds, where there can be a significant necrotic pain zone which could later require debridement and secondary reconstruction.

Alveolar Fissure Reconstruction: the standard procedure for alveolar fissure treatment has been reported by Boyne and Sands. Using autogenous particles from the iliac crest¹⁷ they compared BMP-2 and iliac crest grafts in primates with alveolar defects.³ Statistically significant differences were not found. Currently many patients benefit from the use of BMP- 2 to induce bone formation in palatine fissures. It eliminates the need for a donor area intervention.⁵

Bone Morphogenetic Protein Perspectives

The ability of BMPs in mammals to start the bone formation process can be a useful procedure in embryonic development. It can also be synthesized in the post-natal bone formation. As well, it can be used to start therapeutic bone formation.¹⁸

Bone implants have been a golden standard in maxillofacial reconstruction. Their advantages include high success rates, numerous availability and biocompatibility. These advantages lead to autogenous implants being the second choice alternative. However, some not so obvious disadvantages also exist. They include, among others, the high cost of the procedure, the need for a donor site, the need for hospitalization and postoperative morbidity.¹⁸

The application of specific growth factors for bone induction without the need for a bone graft could have a tremendous effect on maxillofacial reconstruction. More that 20 BMPs have been isolated with different osteogenetic augmentation abilities. Type 2, 4 and 7 appear to have the greatest ability to stimulate bone formation.

From a therapeutic perspective it is necessary to focus investigations on creating distribution systems for the BMPs. An ideal distribution system substrate should be inorganic, non immunogenic and sensitive for optimal adhesion. It should also provide complete support to the mucoperiosteal flap.

A current advance will be the identification, isolation and cloning of bone protein receptors. The ability to clone BMP receptors will be an important aid in designing a therapeutic approach that generates bone formation.

 

 

Correspondence:
CM Ardila Medina
Carrera 47 No 20 sur 46 Envigado
Antioquia, Colombia 57(4) 3348122
Email: cmartin@odontologia.udea.edu.co

Recibido: 29.10.2008
Aceptado: 11.05.2009

 

References

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