Biological value and new trends in the use of bone graft substitutes

Introduction

Bone formation is a complex and closely regulated process. In this process the cellular events involve chemo-taxis of osteoblast precursors, proliferation of committed osteoblast precursors, and differentiation including expression of growth regulatory factors and of structural bone proteins (^{Mundy, 1996}). It is now generally accepted that the successful regeneration of bone requires cells that will synthesize bone matrix, an appropriate scaffold upon which the new bone can grow, and bioactive molecules to guide the process. Accordingly, reported techniques to regenerate bone include bone grafts or substitutes and more recently, bioactive molecules and transplanted cells (^{Yaszemski et al., 1996}).

Bone grafts or substitutes are used in a wide variety of orthopaedic and cranio-maxillofacial surgical procedures. In orthopaedics, they are needed to assist bony healing in the repair of osseous congenital deformities, or in the repair of deformity due to trauma or to surgical excision exceeding a certain size. In maxillofacial surgery they are also used to obliterate cysts, to correct jawbone contours and deficiencies and/or to promote sufficient bone support for the placement of dental implants. In periodontology, they are used in the treatment of infrabony and furcation defects with the objective of regenerating the structure of the lost periodontium (namely alveolar bone, periodontal ligament and cement) (^{Yukna, 1994a} ; ^{Yukna, 1994b} ; ^{Yukna, 1994c} ). The biological principles in maxillofacial applications are similar to orthopaedic ones but the load bearing requirements and the periodontal cell populations are somewhat different.

Historically, autogenous bone was the focus of initial attention as an accessible optimal material of choice for orthopaedic and oral applications. However, its use is problematic due to the need for a second surgical donor site which tends to increase morbidity, the frequent lack of intra-oral donor sites to obtain sufficient quantities of autogenous bone, and uncontrolled resorption (^{Aaboe et al., 1995}). Moreover, disadvantages of fresh frozen bone allografts, namely immunogenicity and risks of disease transmission, spurred the development of bone graft substitutes. Bone graft substitutes commonly used in periodontal and oral surgery are derived from human or bovine bone or consist of organic or non organic allo-plastic materials (^{Mellonig, 1996} ; ^{Ouhayoun, 1997} ; ^{Nasr et al., 1999}). These components have been used also combined with autogenous bone. Currently, they have gained wider applications as carriers of osteoactive molecules or osteoprogenitor cells, and as adjunct to guided tissue regeneration (GTR) barriers. In this paper, the biological potency of the bone graft substitutes commonly used in periodontics and their contributions to the bone forming mechanism and to periodontal repair are discussed. Finally, new trends of their use in the field of bone and periodontal engineering are introduced.

Functions and properties of bone graft substitutes

Rationale for use in bone formation

Generally, a bone graft substitute has to possess certain basic properties. It must be biocompatible, easy to sterilise and use, and inexpensive. Further, a bone graft substitute should be osteoactive, i.e. allowing new bone formation throughout the graft. Osteoactivity exhibits various forms and includes osteogenicity, osteoinductivity and osteoconductivity (^{Damien and Parsons, 1991}). When new bone is formed on a graft, it may be from surviving the tranfert cells or from cells of host origin. Osteogenesis represents all the steps and processes leading to bone formation. This term refers to bone formation from osteoblastic cells contained in the transplanted graft. It is believed that osteogenicity is achieved by autogenous bone grafts. A bone graft may also function as a source of osteogenesis by osteoinduction. Osteoinductive materials are designed to induce differentiation of host mesenchymal cells located near the graft into osteoprogenitor cells. A biomaterial is considered osteoinductive when bone formation occurs after implantation in a non osseous environment. Some factors known as bone morphogenetic proteins (BMPs) are involved in this process. Osteoconductive materials allow ingrowth of vessels as well as osteoprogenitor cells from the recipient bed into the graft. In other words, the graft acts as a scaffold. The differences observed between cortical and cancelous bone grafts suggest that osteoconduction requires porosity and that substances with greater porosity and surface area allow osteoconduction to proceed more quickly and completely (^{Cornell and Lane, 1998}). Nevertheless, resorbability and the chemical composition are also important for osteoconduction. Osteoconduction appears to be optimised in devices that mimic not only bone structure but also bone chemistry (^{Cornell and Lane, 1998}). In addition, a bone graft substitute should degrade in a controlled manner into non toxic molecules that the body can metabolise or excrete, allow the region to reconstitute itself with new bone, and allow the process of bone remodelling.

Rationale for use in periodontal surgery

In periodontics, the ideal bone graft should be osteoactive but moreover it should be able to trigger cementogenesis and formation of a functional periodontal ligament. Hence, two different aspects should be considered :

- its potential to promote new bone formation ;

- its potential to contribute to the formation of an attachment apparatus in periodontal bone defects.

New bone formation is a prerequisite for regeneration of the periodontium structure (^{Lynch, 1992}).

Osteogenesis during periodontal healing by transplanted surviving osteoblasts has only been achieved so far by autogenous bone grafts. Intra-or extra-oral (iliac bone and marrow) bone grafting, seems to result in comparable bone and periodontal regeneration (review in ^{Ouhayoun, 1997} ; ^{Nasr et al., 1999}). From an histological point of view, these grafts are able to induce osteogenesis, cementogenesis and regeneration of new attachment in intrabony defects in human (^{Hiatt et al., 1978} ; ^{Langer et al., 1981}). Therefore, in spite of the necessity of a second intraoral donor site, bone autograft remains a clinically useful and safe procedure. Despite their high regenerative potential, the use of allografts is limited by the risks of disease transmission (HIV, spongiform encephalopathy). Osteoconductive materials in periodontics have been shown to produce more bone and clinical attachment level gain and pocket depth reduction than flap debridement alone (^{Nasr et al., 1999}). However, in humans and animals they are not able to regenerate new connective tissue attachment or predictably induce cementogenesis. Osteoinduction seems to be the best scenario in periodontal regeneration. BMPs either extracted from demineralized bone matrix or obtained through cellular engineering have proven to possess this property. Current research in bone graft substitutes is focused on composites with osteoinductive or osteogenic and osteoconductive properties.

Bone derived graft substitutes

Rationale for use of bone-derived materials

Bone derived graft substitutes have been used in orthopaedic surgery since 1950 and were introduced to periodontics in the mid-seventies. These allografts include demineralized (DFDBA) or undemineralized (FDBA) freeze dried bone, obtained from autopsies of human donors within 24 hours after death and generally lyophilised according to the guidelines of the American Association of Tissue Banks. Freeze dried bone grafts undergo a series of processes, i.e. defatting, lyophilisation, sterilisation and/or demineralisation that result in cell death and alteration of the original composition. It there-fore enters the category of biomaterials used as bone graft substitutes rather than allografts (^{Ouhayoun, 1997}). Freezing or freeze drying the bone before use decreases the antigenicity of allografts, by partial distortion of the three dimensional presentation of human leukocyte anti-gens (^{Mellonig, 1996}). They are biocompatible and are resorbed (^{Mellonig, 1996} ; ^{Becker et al., 1996}) (^{fig. 1} and ²). They are available in sufficient quantities through bone banks. The ability of freeze dried bone, demineralized or undemineralized, to regenerate bone and periodontal tissues has been evaluated by several animal and clinical studies.

Freeze-dried human bone allografts (FDBA)

FDBA are regarded as osteoconductive, providing a scaffold on which new bone forms. The potential of freeze-dried bone as a periodontal graft material, alone or in combination with autogenous bone, was evaluated in a large non controlled clinical trial including all types of intraosseous defects. Bone filling (50 % or more) was observed in 60 % of FDBA treated defects (^{Mellonig, 1991}). However the only controlled blinded study comparing clinical results obtained with FDBA versus debridement alone showed the same amount of bone formation (^{Altiere et al., 1979}). Nevertheless, bone formation obtained with FDBA grafting is enhanced by the adjunction of autogenous bone (^{Sanders et al., 1983}).

Demineralized freeze dried human bone allografts (DFDBA)

In 1967, Urist et al. introduced the principle of a bone induction. They postulated the presence in bone matrix of a substance able to induce the differentiation of mesenchymal cells into osteoprogenitor cells. It was thought that the mineral blocked the effect of the « inductive agent » and so demineralization was necessary (^{Urist et al., 1970}). Clinical and histological studies have since evaluated DFDBA potential. Two controlled unpaired defect studies (^{Pearson et al., 1981} ; ^{Melloning, 1984}) and one controlled paired study (^{Meadows et al., 1993}) demonstrated a gain of attachment, a reduction of probing depth and bone filling in periodontal intraosseous defects grafted with DFDBA as compared to debridement alone. In another controlled study, DFDBA grafted sites showed greater bone filling as compared to debridement alone but no improvement of the clinical parameters (^{Masters et al., 1996}). Further, bone gain maintained over a period of 3 years has been described (^{Flemming et al., 1998}). The placement of DFDBA in the furcation defect resulted in a greater reduction of horizontal probing depth as compared to the GTR alone (^{De Leonardis et al., 1999}). Controlled human histological studies demonstrated that a new attachment can be obtained when intraosseous defects are grafted with DFDBA (^{Bowers et al., 1989a} ; ^{Bowers et al., 1989b}).

However, a controversy exists concerning the osteoinductive capacity of DFDBA. ^{Becker et al. (1996)} observed that grafted DFDBA failed to induce bone formation in human extraction sockets after 3 to 13 months. Histological analysis showed that bone formation was unpredictable in critical size supra alveolar periodontal defects in dogs after DFDBA cortical strip implant (^{Kim et al., 1998}), or in fenestration defects treated with DFDBA particles (^{Caplanis et al., 1998}). Further, investigation of the osteoinductive potential of some commercial preparations of DFDBA by implantation into ectopic sites of athymic mice showed a low percentage of bone formation or no formation at all (^{Garraway et al., 1998} ; ^{Becker et al., 1995}). A possible explanation for these discrepancies may be that some modifications introduced by bone bank preservation procedures resulted in insufficient amounts of bone inductive proteins, such as BMPs, to exert an appreciable effect on bone growth. Further, it seems that the ability of DFDBA to induce new bone formation varies considerably from one bone bank to another, and even inside the same bank. This variation appears to be donor age dependent (^{Schwartz et al., 1998a} ; ^{Schwartz et al., 1998b}).

In conclusion, a wide variation in the ability of human DFDBA to induce new bone formation is observed. So in spite of the human histological results showing bone and periodontal regeneration (^{Bowers et al., 1989a} ; ^{Bowers et al., 1989b}), the osteoinductive capacity of DFDBA can be questioned. The discrepancy between the work of Urist and the disappointing results obtained by recent studies may be explained by modifications introduced by bone bank preservation procedures mainly to reduce the risk of contamination. However, Creutzfeldt-Jacob disease transmitted via prions may remain a potential risk.

Bovine-derived bone material

Bovine bone material has regained recent interest as an alternative to FDBA or DFDBA due to decreased patient acceptance of allogenic bone derived biomaterials. However, some theoretical risks of prion disease transmission from bovine related products in humans can not be excluded . Bovine-derived bone materials are either sintered (Endobon^®, Osteograf/N^®) or unsintered (Bio-Oss^®). The unsintered materials are processed in order to remove organic components and keep the porosity (micro/macroporosity) of the original bone (^{Daculsi et al., 1997}).

Bovine bone mineral is reported to contain no organic components and to have 60-70 % porosities per volume unit. It has been used for the treatment of intraosseous defects and elevation of sinus floor as well as with GTR to support bone formation adjacent to dental implant or in extraction sockets. In a recent clinical trial this bone derived xenograft at 6 months gave similar attachment gain, probing depth reduction and bone filling (55,8 %) to that obtained with DFDBA in 30 infrabony defects filled with either one of the two materials (^{Richardson et al., 1999}). A 6 month follow-up study of 6 periodontal defects reconstructed with GTR barrier and a bone derived material exhibited improved clinical and radiological parameters (^{Lundgren et Slotte, 1999}). In another recent study this material used in conjunction with GTR barrier did not result in a better outcome than the use of membrane alone seule (^{Batista et al., 1999}). However, the propitious clinical results observed today have to be confirmed by other controlled studies.

Bone derived material has been histologically evaluated in dogs (^{Berglundh and Lindhe, 1997}), chimpanzee (^{McAllister et al., 1999}) and humans (^{Dies et al., 1996}).

New bone formation was observed in critical size defects in radii of rabbits grafted with porous bone mineral poreux (^{Schmitt et al., 1997}). In dogs, bovine bone mineral grafted in mandibular defects has been reported to be well incorporated by host bone in direct contact with the implanted grafted with anorganic bovine bone material exhibited a decrease of the graft and a replacement by newly formed bone (^{McAllister et al., 1999}). In humans, this material used with membrane resulted in mineralization, but bone regeneration seemed to be slower as compared to that obtained with membrane (^{Dies et al., 1996}). When it is implanted in ectopic sites no bone formation occurs (^{fig. 3} and ⁴). In summary, bovine-bone derived materials may be regarded as osteoconductive. The type of periodontal repair remains to be defined since no human histological study to our knowledge is available.

Alloplastic bone graft substitutes

Rationale for use of alloplastic bone grafts in periodontics

Prompted by severe problems in autogeneic and allogeneic bone grafts, intensive efforts were made to find adequate substitutes. The main challenge for these biomaterials is to obtain results comparable to those of autografts or bone derived materials. These results are related to the biological properties, i.e. the ability to form new bone, osseous integration and physiological remodelling.

The available alloplastic materials can be either resorbable or non resorbable. « Plaster of Paris », calcium carbonate, tricalcium phosphate and some polymers resorb totally or partially, whereas dense and porous hydroxyapatite and some other polymers, as well as bioglasses in bulk form, do not. Advantages of alloplastic materials include the absence of immunological reactions and risk of disease transmission, excellent biocompatibility and the ability to control graft composition. A lack of local or systemic toxicity, a lack of inflammation or of foreign body response and the ability to become directly bonded to bone are all positive attributes in favour of many of these materials (^{Yukna, 1994a} ; ^{Yukna, 1994b} ; ^{Yukna, 1994c}).

Ceramics

The use of bioceramics in bone healing applications is based upon their structural similarity to the mineral phase of bone. Calcium phosphates, calcium carbonates, calcium sulfates and bioglasses are the most commonly used. The main attractive feature of various calcium containing bioactive biomaterials such as hydroxyapatite, bioglass, glass-ceramics and coral is their ability to form a direct bond with the host bone, in contrast to bioinert or biotolerant materials that form a fibrous interface (^{Hench, 1991}). The mechanism of bonding to bone is variable and not completely elucidated. It is influenced by resorption mechanisms. Current research suggest that many biomaterials used may require surface modification before apposition of bone takes place. These modifications may be solution mediated (dissolution process) and/or cell mediated (^{Daculsi et al., 1997}). Dissolution results in removal of ions from the surface of the material in the biological fluid causing an increase in their concentration. This results in an increase of Ca and P ions around calcium phosphates (^{Daculsi et al., 1997}), calcium and carbonate around natural coral (^{Damien et al., 1994}), and calcium and silicate ions (among others) around Bioglass and glass ceramics (^{Hench, 1991}). These ions interact with others resulting in carbonate apatite precipitation and deposition. The formation of this layer seems important for direct bone apposition and is one of the caracteristics of bioactive materials.

Calcium phosphate materials

Several stoichiometries of calcium phosphate ceramics have been utilised for bone repair. The most commonly investigated have been : βtricalcium phosphate (βTCP), synthetic hydroxyapatite and biphasic calcium phosphate ceramics. The calcium phosphates have an enviable record of biocompatibility and can be modified through calcium to phosphate ratio, incorporation of ions, modification of cristallinity and porosity. Early studies using these materials focused on the principle that local release of calcium and phosphate ions would stimulate bone formation at the graft site. Despite desirable bioactive properties, their low mechanical properties make them unsuitable for use in load bearing areas. They are available in various forms (porous or dense, particles or blocks). More recently, dental implants with calcium phosphate coating have been also advocated.

β tricalcium phosphate

Tricalcium phosphate is a porous material produced by compacting tricalcium phosphate powder with naphtalene. Naphtalene is subsequently removed, giving rise to a porous structure. The porous volume thus created is approximately 36 % with a porosity varying from 100 to 300 μm. This porosity is said to be inadequate for complete bone ingrowth (^{Cornell and Lane, 1998}). Its resorbability is higher than that of hydroxyapatite. In bulk form, its rapid dissolution makes it a poor bone graft substitute. Clinical studies showed a reduction of probing bone depth (^{Louise et al., 1985}) and a bone gain was measured at 18 months re-entry in a non controlled study (^{Snyder et al., 1984}). Histologically, bone formation was observed around βTCP particles implanted in surgical periodontal-like defects in dogs (^{Barney et al., 1986}). In human periodontal defects, βTCP failed to promote osteogenesis and new connective attachment, as demonstrated by histological analysis after 3 and 8 months. However, after 36 and 40 months particles embedded in fully mineralised bone were observed (^{Saffar et al., 1990}) suggesting an unpredictable resorption profile. Today, it seems difficult to ascertain if β TCP alone is of any benefit in treating periodontal osseous defects.

Hydroxyapatites

Hydroxyapatites exist in dense or porous forms. The argument in favour of hydroxyapatite is that its structure resembles that of natural bone hydroxyapatite more closely than does βTCP (^{Yaszemski et al., 1996}).

Dense hydroxyapatite

Hydroxyapatite in its dense polycrystalline form is non resorbable, nevertheless some resorption may occur when preparation gives rise to amorphous or porous formation (^{Osborn and Newesely, 1980}). Controlled paired clinical studies of grafted sites compared to control sites showed improvement of clinical parameters (^{Galgut et al., 1992}) and of bone defect filling (^{Meffert et al., 1985}). Results of a clinical controlled study showed that these results were stable over long term periods (^{Yukna et al., 1989}). In animal studies bone formation was described around dense hydroxyapatite when implanted into surgical bone defects (^{Gumaer et al., 1986}). Fibrous encapsulation during the early healing stage was also observed (^{Takeshita et al., 1997}) and no osteoinduction in ectopic sites was elicited (^{Naaman Bou-Abboud et al., 1994}). New attachment or cementogenesis have not be demonstrated histologically in human biopsies (^{Ganeles et al., 1986}).

Porous hydroxyapatite

Porous hydroxyapatite (PHA) is obtained by an hydrothermal exchange process in which natural coral calcium carbonate is converted to calcium phosphate. It consists of interconnecting pores (100 to 230 μm diameter) and it is formulated in blocs and particles. It fulfils many criteria for a functional bone graft substitute, including osteoconductivity (^{fig. 5}) and new bone ingrowth due to its porosity. It is not an osteoinductive material (^{Naaman Bou-Abboud et al., 1994}). In clinical controlled pair defects studies, PHA-treated intraosseous defects demonstrate significantly more gain attachment level, pocket depth reduction and bone gain than corresponding defects treated by debridment alone (^{Kenney et al., 1986} ; ^{Kenney et al., 1988}). It gives similar results to FDBA, DFDBA and natural coral (^{Mora and Ouhayoun, 1995}). After surgical re-entry (^{Kenney et al., 1986} ; ^{Stahl et Froum, 1987} ; ^{Carranza et al., 1987}), new bone formation was observed but, in another study, PHA seems to delay healing (^{Ouhayoun et al., 1992}). In addition to these inconsistent results, PHA is not able to regenerate new connective attachment or cementum in humans and animals.

Injectable calcium phosphate biomaterials

Injectable calcium phosphate biomaterials have also been (^{Brown et al., 1998} ; ^{Gauthier et al., 1999}). In a controlled clinical study, calcium phosphate cement injected in periodontal defects was exfoliated in 11 of the 16 treated patients suggesting that there is no benefit for the use of hydroxyapatite cement as graft in its current formulation for the treatment of vertical intraosseous periodontal defects (^{Brown et al., 1998}). In an histological study of extraction sockets in dogs grafted with injectable calcium phosphate biphasic (BCP granules with hydrophilic cellulose derivative) new bone formation similar to that obtained in control sites was observed (^{Gauthier et al., 1999}).

In conclusion, reports of calcium phosphate ceramic use have demonstrated positive clinical results similar in magnitude and frequency to those obtained with other graft materials. New trends centre around the development of injectable resorbable forms of biomaterial in which osteoinductive molecules/growth factors may be incorporated without the need for high temperatures.

Natural coral skeleton (NCS)

The natural coral skeleton belongs to the Porites group and consists of calcium carbonate (99 %) in the form of aragonite. Its architecture is very similar to that of spongy bone with interconnecting pores (100 to 200 μm diameter). Although it originates from coral as does porous hydroxyapatite, it is not altered by heat treatment into a different non resorbable chemical structure. It has been reported to be biocompatible and resorbable (^{Guillemin et al., 1987}). It is well tolerated by osteoblasts and provides a surface for cell spreading, attachment and differentiation (^{Sautier et al., 1990}).

Controlled clinical studies of NCS treated infrabony defects demonstrate a significantly greater pocket depth reduction, clinical attachment gain and bone defect filling than debridement alone (^{Mora and Ouhayoun, 1995} ; ^{Jean et al., 1996} ; ^{Kim et al., 1996}), and similar to that obtained with porous hydroxyapatite (^{Mora and Ouhayoun, 1995}). A 5 year follow up of 16 patients treated with coral in infrabony defects demonstrated favourable long term results on clinical parameters, suggesting a beneficial effect in the long term clinical management of intrabony defects (^{Yukna and Yukna, 1998}).

NCS has been used experimentally and shown to be highly osteoconductive (^{fig. 6} and ⁷). Histological study in dogs showed that long bone resections grafted with coral were rapidly vascularized ; coral was gradually resorbed by osteoclast-like cells and replaced by newly formed bone (^{Guillemin et al., 1987}). Its gradual resorption depends on the particle size, the site of implantation and the experimental animal (^{Gross et al., 1990} ; ^{Ouhayoun et al., 1992} ; ^{Irigaray et al., 1993} ; ^{Roudier et al., 1995} ; ^{Moon et al., 1996}). In human periodontal defects, it seems more likely that total resorption-bone formation occurs within 8 to 10 months (^{fig. 8a}, ^8b and ^c). No fibrous encapsulation was observed during alveolar ridge enlargement by coral grafting in humans (^{Piattelli et al., 1997}). However, when implanted in ectopic sites no bone formation occurs, demonstrating the lack of bone induction potential (^{Naaman Bou-Abboud et al., 1994}). In addition, the type of periodontal repair obtained after NCS graft is unknown since no human study is available.

Bioglasses

Bioglasses are silicate-based materials that may bond ionically to compounds such as Na₂O, CaO, K₂O, P₂O5 NaO₂, ZnO, NiO. They include two brands marketed so far : Perioglass^® and Biogran^®. We have pooled the results of studies performed with these two types of bioglass. They are non resorbable in their bulk form and slowly resorbable as particles. Bioglasses can undergo ionic translocations ; consequently they can exchange ions or molecular groups with biological fluids (^{Hench, 1991}). They form an inorganic carbonated apatite layer on their surface (^{Rehman et al., 1998}). This property may enable bioglasses to bond chemically to bone. Their inherent weakness limits their use to non load bearing sites. These materials are suggested to be hemostatic and easy to manipulate (^{Shapoff et al., 1997}). Bioglass in a dense form may have clinical potential as a bone graft substitute in non load-bearing applications such as endosseous ridge implants (^{Stanley et al., 1997} ; ^{Yilmaz et al., 1998}) and as granules for periodontal defect treatment. More recently, dental implants with a bioglass coating have been also advocated. In vitro studies have shown that bioglass provides a favourable environment for human osteoblast proliferation and function.

In a controlled clinical trial with re-entry at 12 months, intraosseous defects treated with bioglass showed a significant improvement in clinical parameters as compared to those treated with surgical debridement alone (^{Froum et al., 1998}). Bioglass treated defects have been described by ^{Zamet et al. (1997)}. A significant increase in radiographic density and volume of bone was observed although there was no improvement in clinical parameters. In a controlled clinical study with a surgical re-entry at 9 to 13 months, ^{Ong et al. (1998)} failed to find statistically significant differences between groups treated with bioglass or surgical debridement alone for any parameter measured. In another recent study, bioglass produced comparable results to DFDBA with gain in clinical attachment, reduction of probing depth and bone filling (^{Lovelace et al., 1998}).

In animal experiments bone deposition around the bioglass particles within the surgically created defect has been histologically observed (^{Johnson et al., 1997}). In a recent study, more bone was quantified within defects filled with bioglass (90 to 710 µm) when compared to that obtained with a narrower range of particle sizes (300-355 µm). ^{Wheeler et al. (1998)} suggesting a role of particle size in the bone healing process. In what is to our knowledge the only study in surgically created defects ^{Fetner et al. (1994)} showed that bioglass produces new cementum. In humans the type of periodontal repair or regeneration is unknown since no human histology is available.

Available data being insufficient, further clinical and histological studies are required to clarify the effects of bioglass implants. New trends include the development of material with greater control of in vivo resorption when used as bone graft, and molecular control of the release rate of soluble chemical species. Further research oriented towards the optimisation of a favourable environment for osteoblast cell behaviour (^{Hench, 1998}).

Polymers

Polymers used as bone graft substitutes principally include PMMA (poly-methyl-methacrylate) polymer and various aliphatic polyesters. The advantages of such materials include the ability to control all aspects of the matrix, the lack of immunological reactivity and excellent biocompatibility. PMMA polymer, a non resorbable polymer, is formed by the combination of polymethylmetha-crylate and poly-hydroxyethyl and is coated by calcium hydroxide. Its biocompatibility has been demonstrated by in vitro studies which have shown that fibroblasts attach rapidly to PMMA polymer (^{Kamen, 1989}). In controlled clinical studies, when PMMA was implanted into periodontal intrabony defects or in Class II furcations, a significant bone fill increase was observed (^{Yukna, 1990} ; ^{Yukna, 1994a} ; ^{Yukna, 1994b} ; ^{Yukna, 1994c}). Similar results were observed in a 6-year clinical evaluation of a microporous composite of PMMA, PHEMA (poly-hydroxy-ethyl-methacrylate), and calcium hydroxide material implanted in Class II furcations (^{Yukna and Yukna, 1997}). However, these results need further confirmation. In addition, according to various histological studies, this material does not promote new attachment formation in periodontal (^{Plotzke et al., 1993} ; ^{Stahl et al., 1990}). Moreover, many researchers remain sceptical, believing that non resorbable PMMA could prevent normal healing.

Aliphatic polyesters comprise a large family of resorbable polymers. They possess two attractive properties : a great design flexibility and biodegradation (^{Yaszemski et al., 1996}). The most commonly used are polylactic and glycolic acid polymers. Their biodegradation characteristics vary according to their synthesis. Many reports indicate their good biocompatibility. Few studies have used them as bone graft substitutes (^{Meadows et al., 1993}). These materials do not seem to be adapted to bone grafting as osteoconductive material. Currently, they are widely used as resorbable membranes for GTR (^{Caffesse et al., 1994}) as linkers for other bone graft substitutes, or as delivery systems of BMPs and/or osteogenic cells (^{Boyan et al., 1999}).

Recent advances and future directions in bone graft substitutes

Bone graft substitute in bone and periodontal tissue engineering

The emerging discipline of tissue engineering uses principles of molecular developmental biology to design and construct spare parts that restore function to the human body. The three main ingredients for optimal engineering of tissues, including bone and periodontium, are regulatory/inductive factors, stem cells, and the extracellular matrix (^{Ripamonti and Reddi, 1997}). Regulatory/inductive molecules such as growth factors such as IGFs, PDGF, FGFs, cytokines, and BMPs such as BMP-2, -4, and -7 (osteogenic protein 1 or OP1) play a critical role in the development and healing of bone and periodontal tissues (^{Mundy, 1996} ; ^{MacNeil and Somerman, 1999}). Among them BMPs have received considerable attention for potential use in the regeneration of mineralized tissues, including periodontal tissues. They are the only growth factors known to provoke heterotopic bone formation by inducing differentiation of mesenchymal cells. Bone induction and differentiation is also governed by the context (microenvironment/extracellular matrix), which can be duplicated by biomimetic biomaterials such as collagens, hydroxyapatite, proteoglycans, and cell adhesion glycoproteins (^{Reddi, 1998}). Enamel matrix proteins have also been used in an attempt to regenerate periodontal tissues (^{Heijl et al., 1997}).

Current research in bone graft substitutes is focused on composites with osteoinductive or osteogenic and osteoconductive properties. Many combinations of biomaterials/biological factors such as collagen (^{Hollinger et al., 1996}), cell binding peptides (^{Yukna et al., 1998}), growth and differentiation factors are being tested. DFDBA (^{Schwartz et al., 1998a} ; ^{Schwartz et al., 1998b}), as well as alloplastic materials are used as delivery systems for growth factors and BMPs (^{Dard, 1997}) or plasmid DNA coding for these proteins (^{Goldstein and Bonadio, 1998}). Porous bone graft substitutes as carriers of cells with osteoblastic potential (^{Bruder et al., 1998a} ; ^{Bruder et al., 1998b} ; ^{Peter et al., 1998}) are also of interest in the field of tissue engineering.

Bone graft substitutes as carriers of bioactive molecules

Calcium phosphate ceramics, natural coral, collagen and polymers, demineralized human or bovine bone matrix (DBM), were tested for their ability to support BMP induced bone and/or periodontal regeneration (^{Sigurdsson et al., 1996} ; ^{Dard, 1997} ; ^{Schwartz et al., 1998a} ; ^{Schwartz et al., 1998b} ; ^{Arnaud et al., 1999} ; ^{Boyan et al., 1999}). The nature of the bone graft as carrier system may affect the ability of rhBMP-2 to regenerate both alveolar bone and periodontal attachment (^{Sigurdsson et al., 1996}). The surface characteristics and geometry of carrier were also found to be of critical importance in the induction of bone by BMPs (^{Ripamonti and Reddi, 1997}). Collagenous bone matrix was compared with hydroxyapatite, βTCP, PMMA and glass beads which were implanted alone or in combination with BMP-3. Results revealed that new bone formation occured only with a combination of collagen and BMP-3 (^{Mont et al., 1998}). However, the use of collagen is limited by its lack of mechanical strength and angiogenic potential. Nevertheless, the most important function of an osteogenic delivery system is the initiation of optimal osteoinductivity with relatively low doses of BMPs.

In oral cavity defects in animal experiments, collagen sponge, polymers and DFDBA were used as BMP carriers. BMPs with a collagenous matrix used as carrier, induced cementum and alveolar bone regeneration in surgically-created furcations in primates (^{Ripamonti and Reddi, 1997}). rhOP1 with collagen matrix as carrier also resulted in sinus augmentation in non human primates (^{Margolin et al., 1998}). The use of rhBMP-2 with a PLGA (prolyactic-acid-polyglyco acid) sponge material was able to induce periodontal tissue regeneration in ligature induced bone defects in dogs (^{Kinoshita et al., 1997}). DFDBA can also be used as an effective carrier of rhBMP-2 (^{Schwartz et al., 1998a} ; ^{Schwartz et al., 1998b}). In a recent human clinical study rhOP1 on a collagen type I carrier showed bone formation in a critically sized fibular defect (^{Geesink et al., 1999}).

In summary, the experimental results have shown that it is possible to combine osteoinductive proteins with carrier materials to obtain new composite « osteoinductive » biomaterials. However, further investigation is needed to develop carriers which are also able to allow sustained release of an appropriate quantity of inductive factor.

Biomaterials as gene based delivery systems

An alternative method of delivering osteoactive molecules in orthopaedic surgery to treat bone defects has been suggested. This method involves the delivery of DNA coding for osteogenic growth factors via a three dimensional matrix or the transplantation of transfected cells that express these molecules within the region of their placement. It offers the potential to deliver osteoregulatory factors in a more biological form than that achieved by the exogenous application of recombinant proteins (^{Niyibizi et al., 1998}). Polymers made from poly-glycolic-lactic acid were evaluated as a structural matrix for plasmid DNA encoding BMP-4 or a fragment of parathyroid hormone (^{Goldstein and Bonadio, 1998}) in the enhancement of fracture healing. Moreover, ceramic cubes were used to carry cells transfected ex vivo with BMP-2 cDNA (^{Lierberman et al., 1998}). Although encouraging, the data obtained are preliminary and more research on the basic biology of bone healing and the role of biomaterials used as gene delivery systems is required.

A delivery system for osteoprogenitor cells : hybrid biomaterials

Sufficient osteogenic progenitor cells to ensure osteoblastic differentiation and optimal secretory activity are necessary to obtain a successful regeneration of bone. Therefore, another current approach to bone regeneration envisions the delivery of the osteogenic cells themselves directly to the defect site (^{Caplan, 1997}). Biological data show that pluripotential stem cells from marrow (^{Caplan, 1997}) or cultured stromal vascular cells from extramedullary tissue may express osteoblastic phenotype markers under the influence of osteoinductive factors (^{Lecoeur et al., 1997}). Consequently, cells can be loaded onto the material surface prior to implantation or expanded in vitro for some period of time prior to implantation. New bone formation has been obtained using marrow stromal cells cultured in porous hydroxyapatite (^{Yoshikawa et al., 1996}) or hydroxyapatite/tricalcium ceramic powder (^{Krebsbach et al., 1997}). However, unsolved problems include the small population of osteogenic cells in marrow and the long delay before the appearance of an obvious osteoblastic phenotype of cells expanded in vitro.

In an attempt to refine a delivery system for osteoprogenitor cells, alloplastic materials which mimic the natural morphology of bone were investigated. The design and selection of an ideal cell carrier is based on several criteria. It should allow for uniform loading and retention of cells, support rapid vascular ingrowth, be resorbed and replaced by newly formed bone, enhance osteoconductive bridging of host bone by the new bone and be easy to handle in a clinical setting (^{Bruder et al., 1998a} ; ^{Bruder et al., 1998b}). For example, coralline hydroxyapatite which has a complete interconnected porous structure seems more appropriate than dense hydroxyapatite and βTCP (^{Bruder et al., 1998a} ; ^{Bruder et al., 1998b} ; ^{Caplan, 1997}). Natural coral, presenting the same porous structure and being resorbable would also be a good candidate as a delivery system for cells. Alternatively polymer-cell hybrids may be shaped easily to fit the precise dimensions of a bony defect (^{Peter et al., 1998}). Interestingly, poly (L-lactic acid), poly (glycolic acid) and poly (lactic co-glycolic acid) can be produced as three dimensional foams with a controlled porosity. Hence, they could facilitate cell attachment and therefore cell transplantation. Moreover, polymers have the potential to be assembled in various forms and integrated with growth factors or other compounds to create multiphase delivery systems for osteopromoting agents.

Conclusion

The multitude of bone graft substitutes, tested for their ability to promote bone and periodontal healing, evoked in this paper, demonstrates the continuing role which bone substitutes play in periodontics and oral surgery. From a clinical point of view, bone derived materials as well as alloplastic materials show improvement of clinical parameters as compared to debridement alone. From an histological point of view, periodontal regeneration in humans has been described only for DFDBA. However, a wide variation in its osteoinductive capacity exists and it appears to be « bank » and « donor » dependent. Osteoconductive properties have been described for bovine-derived bone materials as well as for various allo-plastic materials (porous hydroxyapatite, natural coral skeleton, bioglasses) but histological information from humans on periodontal regeneration are not available. Considering alloplastic materials, the absence of immunological reactions and risk of disease transmission and their biocompatibility are all positive attributes in their favour. Biological responses such bone bonding and resorption are very important in clinical application, but no convincing conclusions have been reached as to their mechanisms. Current research in bone graft substitutes is focused on composites with both osteoconductive and/or osteogenic and osteoinductive properties.

* We are grateful to Drs D. Etienne, G. Guillemin and M. H. Sawaf for critical review of the manuscript and for helpful discussions.

Demande de tirés à part :

Fani ANAGNOSTOU, Faculté de Chirurgie dentaire Paris-VII, Service d'odontologie Garancière-Hôtel-dieu, Unité de parodontologie, 5, rue Garancière, 75005 PARIS, FRANCE. E-mail:fanagnostou@hotmail.com

BIBLIOGRAPHIE

• AABOE M, PINHOLT EM, HJORTING-HANSEN E. Healing of experimentally created defects : a review. Br J Oral Maxillofac Surg 1995;33(5)312-318.
• ALTIERE ET, REEVE CM, SHERIDAN PJ. Lyophilized bone allografts in periodontal intraosseous defects. J Periodontol 1979;50(10):510-519.
• ARNAUD E, DE POLLAK C, MEUNIER A, SEDEL L, DAMIEN C, PETITE H. Osteogenesis with coral is increased by BMP and BMC in a rat cranioplasty [In Process Citation]. Biomaterials 1999;20:1909-1918.
• BARNEY VC, LEVIN MP, ADAMS DF. Bioceramic implants in surgical periodontal defects. A comparison study. J Periodontol 1986;57:764-770.
• BATISTA EL Jr, NOVAES AB Jr, SIMONPIETRI JJ, BATISTA FC. Use of bovine-derived anorganic bone associated with guided tissue regeneration in intrabony defects. Six-month evaluation at re-entry [In Process Citation]. J Periodontol 1999;70:1000-1007.
• BECKER W, URIST M, BECKER BE, JACKSON W, PARRY DA, BARTOLD M et al. Clinical histologic observations of sites implanted with intraoral autologous bone grafts or allografts. 15 human case reports. J Periodontol 1996;67:1025-1033.
• BECKER W, URIST MR, TUCKER LM, BECKER BE, OCHSENBEIN C. Human demineralized freeze-dried bone:inadequate induced bone formation in athymic mice. A preliminary report. J Periodontol 1995;66:822-828.
• BERGLUNDH T, LINDHE J. Healing around implants placed in bone defects treated with Bio-Oss. An experimental study in the dog. Clin Oral Implants Res 1997;8:117-124.
• BOWERS GM, CHADROFF B, CARNEVALE R, MELLONIG J, CORIO R, EMERSON J et al. Histologic evaluation of new attachment apparatus formation in humans. Part I. J Periodontol 1989(a);60:664-674.
• BOWERS GM, CHADROFF B, CARNEVALE R, MELLONIG J, CORIO R, EMERSON J et al. Histologic evaluation of new attachment apparatus formation in humans. Part II. J Periodontol 1989(b);60:675-682.
• BOYAN BD, LOHMANN CH, SOMERS A, NIEDERAUER GG, WOZNEY JM, DEAN DD et al. Potential of porous poly-D, L-lactide-co-glycolide particles as a carrier for recombinant human bone morphogenetic protein-2 during osteoinduction in vivo. J Biomed Mater Res 1999;46:51-59.
• BROWN GD, MEALEY BL, NUMMIKOSKI PV, BIFANO SL, WALDROP TC. Hydroxyapatite cement implant for regeneration of periodontal osseous defects in humans. J Periodontol 1998;69:146-157.
• BRUDER SP, JAISWAL N, RICALTON NS, MOSCA JD, KRAUS KH, KADIYALA S. Mesenchymal stem cells in osteobiology applied bone regeneration. Clin Orthop 1998(a);355 (suppl.):S247-256.
• BRUDER SP, KURTH AA, SHEA M, HAYES WC, JAISWAL N, KADIYALA S. Bone regeneration by implantation of purified, culture-exped human mesenchymal stem cells. J Orthop Res 1998(b);16:155-162.
• CAFFESSE RG, NASJLETI CE, MORRISON EC, SANCHEZ R. Guided tissue regeneration:comparison of bioabsorbable and non-bioabsorbable membranes. Histologic histometric study in dogs. J Periodontol 1994;65:583-591.
• CAPLAN AI. Cell molecular enginnering of bone regeneration. In : Lanza RL, Chik W, eds. Textbook of tissue engineering. Georgetown:Texas R.G. Les, 1997:603-618.
• CAPLANIS N, LEE MB, ZIMMERMAN GJ, SELVIG KA, WIKESJO UM. Effect of allogeneic freeze-dried demineralized bone matrix on regeneration of alveolar bone periodontal attachment in dogs. J Clin Periodontol 1998;25:801-806.
• CARRANZA FA, Jr, KENNEY EB, LEKOVIC V, TALAMANTE E, VALENCIA J, DIMITRIJEVIC B. Histologic study of healing of human periodontal defects after placement of porous hydroxylapatite implants. J Periodontol 1987;58:682-688.
• CORNELL CN, LANE JM. Current understanding of osteoconduction in bone regeneration. Clin Orthop 1998;355 (suppl.):S267-S273.
• DACULSI G, BOULER JM, LEGEROS RZ. Adaptive crystal formation in normal pathological calcifications in synthetic calcium phosphate related biomaterials. Int Rev Cytol 1997;172:129-191.
• DAMIEN CJ, PARSONS JR. Bone graft and bone graft substitutes:a review of current technology applications. J Appl Biomater 1991;2:187-208.
• DAMIEN CJ, RICCI JL, CHRISTEL P, ALEXER H, PATAT JL. Formation of a calcium phosphate-rich layer on absorbable calcium carbonate bone graft substitutes. Calcif Tissue Int 1994;55:151-158.
• DARD M. Delivery systems of growth factors for bone regeneration. Cellular Engineering 1997;2:84-92.
• DE LEONARDIS D, GARG AK, PEDRAZZOLI V, PECORA GE. Clinical evaluation of the treatment of class II furcation involvements with bioabsorbable barriers alone or associated with demineralized freeze-dried bone allografts [see comments]. J Periodontol 1999;70:8-12.
• DIES F, ETIENNE D, ABBOUD NB, OUHAYOUN JP. Bone regeneration in extraction sites after immediate placement of an e-PTFE membrane with or without a biomaterial. A report on 12 consecutive cases. Clin Oral Implants Res 1996;7:277-285.
• FETNER AE, HARTIGAN MS, LOW SB. Periodontal repair using PerioGlas in nonhuman primates:clinical histologic observations. Compendium 1994;15:932, 935-938, quiz:939.
• FLEMMIG TF, EHMKE B, BOLZ K, KUBLER NR, KARCH H, REUTHER JF et al. Long-term maintenance of alveolar bone gain after implantation of autolyzed, antigen-extracted, allogenic bone in periodontal intraosseous defects. J Periodontol 1998;69:47-53.
• FROUM SJ, TARNOW DP, WALLACE SS, ROHRER MD, CHO SC. Sinus floor elevation using anorganic bovine bone matrix (OsteoGraf/N) with and without autogenous bone:a clinical, histologic, radiographic, histomorphometric analysis. Part 2 of an ongoing prospective study. Int J Periodontics Restorative Dent 1998;18:528-543.
• GALGUT PN, WAITE IM, BROOKSHAW JD, KINGSTON CP. A 4-year controlled clinical study into the use of a ceramic hydroxyapatite implant material for the treatment of periodontal bone defects. J Clin Periodontol 1992;19:570-577.
• GANELES J, LISTGARTEN MA, EVIAN CI. Ultrastructure of durapatite-periodontal tissue interface in human intrabony defects. J Periodontol 1986;57:133-140.
• GARRAWAY R, YOUNG WG, DALEY T, HARBROW D, BARTOLD PM. An assessment of the osteoinductive potential of commercial demineralized freeze-dried bone in the murine thigh muscle implantation model. J Periodontol 1998;69:1325-1336.
• GAUTHIER O, BOIX D, GRIMI G, AGUADO E, BOULER JM, WEISS P et al. A new injectable calcium phosphate biomaterial for immediate bone filling of extraction sockets:a preliminary study in dogs. J Periodontol 1999;70:375-383.
• GEESINK RG, HOEFNAGELS NH, BULSTRA SK. Osteogenic activity of OP-1 bone morphogenetic protein (BMP-7) in a human fibular defect. J Bone Joint Surg Br 1999;81:710-718.
• GOLDSTEIN SA, BONADIO J . Potential role for direct gene transfer in the enhancement of fracture healing. Clin Orthop 1998;355 (suppl.):S154-S162.
• GROSS UM, MULLER-MAY C, VOIGT C. Comparative morphology of the bone interface with glass ceramics, hydroxyapatite, natural coral. In : Davies JE, ed. The bone biomaterial interface. Toronto : University of Toronto Press, 1990:308-320.
• GUILLEMIN G, PATAT JL, FOURNIE J, CHETAIL M. The use of coral as a bone graft substitute. J Biomed Mater Res 1987;21:557-567.
• GUMAER KI, SHERER AD, SLIGHTER RG, ROTHSTEIN SS, DROBECK HP. Tissue response in dogs to dense hydroxylapatite implantation in the femur. J Oral Maxillofac Surg 1986;44:618-627.
• HEIJL L, HEDEN G, SVARDSTROM G, OSTGREN A. Enamel matrix derivative (Emdogain) in the treatment of intrabony periodontal defects. J Clin Periodontol 1997;24:705-714.
• HENCH LL. Bioceramics:from concept to clinic. J Am Ceram Soc 1991;74:1487-1510.
• HENCH LL. Biomaterials:a forecast for the future. Biomaterials 1998;19:1419-1423.
• HIATT WH, SCHALLHORN RG, AARONIAN AJ. The induction of new bone and cementum formation. IV. Microscopic examination of the periodontium following human bone and marrow allograft, autograft and nongraft periodontal regenerative procedures. J Periodontol 1978;49:495-512.
• HOLLINGER JO, BREKKE J, GRUSKIN E, LEE D. Role of bone substitutes [published erratum appears in Clin Orthop 1997; 334: 387]. Clin Orthop 1996;324:55-65.
• IRIGARAY JL, OUDADESSE H, EL FADL H, SAUVAGE T, THOMAS G, VERNAY M. Efffet de la température sur la stucture cristalline d'un biocorail. J Thermal Analysis 1993;39:3-14.
• JEAN A, SOYER A, EPELBOIN Y, OUHAYOUN JP. Digital image ratio:a new radiographic method for quantifying changes in alveolar bone. Part II. Clinical application. J Periodont Res 1996;31:533-539.
• JOHNSON MW, SULLIVAN SM, ROHRER M, COLLIER M. Regeneration of peri-implant infrabony defects using PerioGlas:a pilot study in rabbits. Int J Oral Maxillofac Implants 1997;12:835-839.
• KAMEN PR. Attachment of human oral fibroblasts to a granular polymeric implant for hard tissue replacement. J Oral Implantol 1989;15:52-56.
• KENNEY EB, LEKOVIC V, ELBAZ JJ, KOVACVIC K, CARRANZA FA Jr, TAKEI HH. The use of a porous hydroxylapatite implant in periodontal defects. II. Treatment of class II furcation lesions in lower molars. J Periodontol 1988;59:67-72.
• KENNEY EB, LEKOVIC V, SA FERREIRA JC, HAN T, DIMITRIJEVIC B, CARRANZA FA Jr. Bone formation within porous hydroxylapatite implants in human periodontal defects. J Periodontol 1986;57:76-83.
• KIM CK, CHOI EJ, CHO KS, CHAI JK, WIKESJO UM. Periodontal repair in intrabony defects treated with a calcium carbonate implant and guided tissue regeneration. J Periodontol 1996;67:1301-1306.
• KIM CK, CHO KS, CHOI SH, PREWETT A, WIKESJO UM. Periodontal repair in dogs:effect of allogenic freeze-dried demineralized bone matrix implants on alveolar bone and cementum regeneration. J Periodontol 1998;69:26-33.
• KINOSHITA A, ODA S, TAKAHASHI K, YOKOTA S, ISHIKAWA I. Periodontal regeneration by application of recombinant human bone morphogenetic protein-2 to horizontal circumferential defects created by experimental periodontitis in beagle dogs. J Periodontol 1997;68:103-109.
• KREBSBACH PH, KUZNETSOV SA, SATOMURA K, EMMONS RV, ROWE DW, ROBEY PG. Bone formation in vivo: comparison of osteogenesis by transplanted mouse and human marrow stromal fibroblasts. Transplantation 1997;63:1059-1069.
• LANGER B, GELB DA, KRUTCHKOFF DJ. Early re-entry procedure. Part II. A five year histologic evaluation. J Periodontol 1981;52:135-139.
• LECOEUR L, OUHAYOUN JP, SEDEL L. In vitro osteogenic differenciation of cultured stromal cells from extramedullary adipose tissue induced by rhBMPH2 plus dexamethasone. J Cell Engineering 1997;2:100-106.
• LIEBERMAN JR, LE LQ, WU L, FINERMAN GA, BERK A, WITTE ON et al. Regional gene therapy with a BMP-2-producing murine stromal cell line induces heterotopic and orthotopic bone formation in rodents. J Orthop Res 1998;16:330-339.
• LOUISE F, FOUREL J, ROIG R. Bioceramics in periodontal bone surgery. Clinical trial of Synthograft. J Parodontol 1985;4:277-285.
• LOVELACE TB, MELLONIG JT, MEFFERT RM, JONES AA, NUMMIKOSKI PV, COCHRAN DL. Clinical evaluation of bioactive glass in the treatment of periodontal osseous defects in humans. J Periodontol 1998;69:1027-1035.
• LUNDGREN D, SLOTTE C. Reconstruction of anatomically complicated periodontal defects using a bioresorbable GTR barrier supported by bone mineral. A 6-month follow-up study of 6 cases. J Clin Periodontol 1999;26:56-62.
• LYNCH SE. Methods for evaluation of regenerative procedures. J Periodontol 1992;63(suppl.):1085-1092.
• MACNEIL RL, SOMERMAN MJ. Development and regeneration of the periodontium:parallels contrasts. Periodontol 2000 1999;19:8-20.
• MARGOLIN MD, COGAN AG, TAYLOR M, BUCK D, MCALLISTER TN, TOTH C et al. Maxillary sinus augmentation in the non-human primate:a comparative radiographic and histologic study between recombinant human osteogenic protein-1 and natural bone mineral. J Periodontol 1998;69:911-919.
• MASTERS LB, MELLONIG JT, BRUNSVOLD MA, NUMMIKOSKI PV. A clinical evaluation of demineralized freeze-dried bone allograft in combination with tetracycline in the treatment of periodontal osseous defects. J Periodontol 1996;67:770-781.
• MCALLISTER BS, MARGOLIN MD, COGAN AG, BUCK D, HOLLINGER JO, LYNCH SE. Eighteen-month radiographic and histologic evaluation of sinus grafting with anorganic bovine bone in the chimpanzee. Int J Oral Maxillofac Implants 1999;14:361-368.
• MEADOWS CL, GHER ME, QUINTERO G, LAFFERTY TA. A comparison of polylactic acid granules decalcified freeze-dried bone allograft in human periodontal osseous defects. J Periodontol 1993;64:103-109.
• MEFFERT RM, THOMAS JR, HAMILTON KM, BROWNSTEIN CN. Hydroxylapatite as an alloplastic graft in the treatment of human periodontal osseous defects. J Periodontol 1985;56:63-73.
• MELLONIG JT. Decalcified freeze-dried bone allograft as an implant material in human periodontal defects. Int J Periodont Restor Dent 1984;4:40-55.
• MELLONIG JT. Freeze-dried bone allografts in periodontal reconstructive surgery. Dent Clin North Am 1991;35:505-520.
• MELLONIG JT. Bone allografts in periodontal therapy. Clin Orthop 1996;324:116-125.
• MONT MA, JONES LC, EINHORN TA, HUNGERFORD DS, REDDI AH. Osteonecrosis of the femoral head. Potential treatment with growth and differentiation factors. Clin Orthop 1998;355 (suppl.):S314-S335.
• MOON IS, CHAI JK, CHO KS, WIKESJO UM, KIM CK. Effects of polyglactin mesh combined with resorbable calcium carbonate or replamineform hydroxyapatite on periodontal repair in dogs. J Clin Periodontol 1996;23:945-951.
• MORA F, OUHAYOUN JP. Clinical evaluation of natural coral and porous hydroxyapatite implants in periodontal bone lesions:results of a 1-year follow-up. J Clin Periodontol 1995;22:877-884.
• MUNDY GR. Regulation of bone formation by bone morphogenetic proteins and other growth factors. Clin Orthop 1996;324:24-28.
• NAAMAN BOU-ABBOUD N, PATAT JL, GUILLEMIN G, ISSAHAKIAN S, FOREST N, OUHAYOUN JP. Evaluation of the osteogenic potential of biomaterials implanted in the palatal connective tissue of miniature pigs using undecalcified sections. Biomaterials 1994;15:201-207.
• NASR HF, AICHELMANN-REIDY ME, YUKNA RA. Bone bone substitutes. Periodontol 2000 1999;19:74-86.
• NIYIBIZI C, BALTZER A, LATTERMANN C, OYAMA M, WHALEN JD, ROBBINS PD et al. Potential role for gene therapy in the enhancement of fracture healing. Clin Orthop 1998;355(suppl.):S148-S153.
• ONG MM, EBER RM, KORSNES MI, MACNEIL RL, GLICKMAN GN, SHYR Y et al. Evaluation of a bioactive glass alloplast in treating periodontal intrabony defects. J Periodontol 1998;69:1346-1354.
• OSBORN JF, NEWESELY H. The material science of calcium phosphate ceramics. Biomaterials 1980;1:108-111.
• OUHAYOUN JP. Bone grafts biomaterials used as bone graft substitutes. In : Lang NK, ed. Proceedings of the 2nd European Workshop on Periodontology. Londres:Quintessence Publishing, 1997:313-358.
• OUHAYOUN JP, SHABANA AHM, ISSAHAKIAN S, PATAT JL, GUILLEMIN G, SAWAF MH et al. Histological evaluation of natural coral skeleton as a grafting material in miniature swine mible. J Mater Sci Mater Med 1992;3:222-228.
• PEARSON GE, ROSEN S, DEPORTER DA. Preliminary observations on the usefulness of a decalcified, freeze-dried cancellous bone allograft material in periodontal surgery. J Periodontol 1981;52:55-59.
• PETER SJ, MILLER MJ, YASKO AW, YASZEMSKI MJ, MIKOS AG. Polymer concepts in tissue engineering. J Biomed Mater Res 1998;43:422-427.
• PIATTELLI A, PODDA G, SCARANO A. Clinical histological results in alveolar ridge enlargement using coralline calcium carbonate. Biomaterials 1997;18:623-627.
• PLOTZKE AE, BARBOSA S, NASJLETI CE, MORRISON EC, CAFFESSE RG. Histologic histometric responses to polymeric composite grafts. J Periodontol 1993;64:343-348.
• REDDI AH. Role of morphogenetic proteins in skeletal tissue engineering regeneration. Nat Biotechnol 1998;16:247-252.
• REHMAN I, KNOWLES JC, BONFIELD W. Analysis of in vitro reaction layers formed on bioglass using thin-film X-ray diffraction and ATR-FTIR microspectroscopy. J Biomed Mater Res 1998;41:162-166.
• RICHARDSON CR, MELLONIG JT, BRUNSVOLD MA, MCDONNELL HT, COCHRAN DL. Clinical evaluation of Bio-Oss:a bovine-derived xenograft for the treatment of periodontal osseous defects in humans. J Clin Periodontol 1999;26:421-428.
• RIPAMONTI U, REDDI AH. Tissue engineering, morphogenesis, regeneration of the periodontal tissues by bone morphogenetic proteins. Crit Rev Oral Biol Med 1997;8:154-163.
• ROUDIER M, BOUCHON C, ROUVILLAIN JL, AMEDEE J, BAREILLE R, ROUAIS F et al. The resorption of bone-implanted corals varies with porosity but also with the host reaction. J Biomed Mater Res 1995;29:909-915.
• SAFFAR JL, COLOMBIER ML, DETIENVILLE R. Bone formation in tricalcium phosphate-filled periodontal intrabony lesions. Histological observations in humans. J Periodontol 1990;61:209-216.
• SANDERS JL, SEPE WW, BOWERS GM, KOCH RW, WILLIAMS JE, LEKAS JS, MELLONIG JM, PELLEU GB, CAMBILL V. Clinical evaluation of freeze-dried bone allografts in periodontal osseous defects. Part III : Composite freeze dried bone allograts with and without autogenous bone frafts. J Periodontol 1983;54:1-8.
• SAUTIER JM, NEFUSSI JR, BOULEKBACHE H, FOREST N. In vitro bone formation on coral granules. In vitro Cell Dev Biol 1990;26:1079-1085.
• SERS JJ, SEPE WW, BOWERS GM, KOCH RW, WILLIAMS JE, LEKAS JS et al. Clinical evaluation of freeze-dried bone allografts in periodontal osseous defects. Part III. Composite freeze-dried bone allografts with and without autogenous bone grafts. J Periodontol 1983;54:1-8.
• SCHMITT JM, BUCK DC, JOH SP, LYNCH SE, HOLLINGER JO. Comparison of porous bone mineral and biologically active glass in criticalsized defects [see comments]. J Periodontol 1997;68:1043-1053.
• SCHWARTZ Z, SOMERS A, MELLONIG JT, CARNES DL Jr, DEAN DD, COCHRAN DL et al. Ability of commercial demineralized freeze-dried bone allograft to induce new bone formation is dependent on donor age but not gender. J Periodontol 1998(a);69:470-478.
• SCHWARTZ Z, SOMERS A, MELLONIG JT, CARNES DL Jr, WOZNEY JM, DEAN DD et al. Addition of human recombinant bone morphogenetic protein-2 to inactive commercial human demineralized freeze-dried bone allograft makes an effective composite bone inductive implant material. J Periodontol 1998(b);69:1337-1345.
• SHAPOFF CA, ALEXER DC, CLARK AE. Clinical use of a bioactive glass particulate in the treatment of human osseous defects. Compend Contin Educ Dent 1997;18:352-354.
• SIGURDSSON TJ, NYGAARD L, TATAKIS DN, FU E, TUREK TJ, JIN L et al. Periodontal repair in dogs:evaluation of rhBMP-2 carriers. Int J Periodont Rest Dent 1996;16:524-537.
• SNYDER AJ, LEVIN MP, CUTRIGHT DE. Alloplastic implants of tricalcium phosphate ceramic in human periodontal osseous defects. J Periodontol 1984;55:273-277.
• STAHL SS, FROUM SJ. Histologic and clinical responses to porous hydroxylapatite implants in human periodontal defects. Three to twelve months postimplantation. J Periodontol 1987;58:689-695.
• STAHL SS, FROUM SJ, TARNOW D. Human clinical histologic responses to the placement of HTR polymer particles in 11 intrabony lesions. J Periodontol 1990;61:269-274.
• STANLEY HR, HALL MB, CLARK AE, KING CJ, HENCH LL, BERTE JJ. Using 45S5 bioglass cones as endosseous ridge maintenance implants to prevent alveolar ridge resorption:a 5-year evaluation. Int J Oral Maxillofac Implants 1997;12:95-105.
• TAKESHITA F, AYUKAWA Y, IYAMA S, SUETSUGU T, OISHI M. Histological comparison of early wound healing following dense hydroxyapatite granule grafting barrier placement in surgically-created bone defects neighboring implants. J Periodontol 1997;68:924-932.
• URIST MR, URIST JM Jr, DUBUC FL, STRATES BS. Quantitation of new bone formation in intramuscular implants of bone matrix in rabbits. Clin Orthop 1970;68:279-293.
• WHEELER DL, STOKES KE, HOELLRICH RG, CHAMBERL DL, MCLOUGHLIN SW. Effect of bioactive glass particle size on osseous regeneration of cancellous defects. J Biomed Mater Res 1998;41:527-533.
• YASZEMSKI MJ, PAYNE RG, HAYES WC, LANGER R, MIKOS AG. Evolution of bone transplantation:molecular, cellular and tissue strategies to engineer human bone. Biomaterials 1996;17:175-185.
• YILMAZ S, EFEOGLU E, KILIC AR. Alveolar ridge reconstruction and/or preservation using root form bioglass cones. J Clin Periodontol 1998;25:832-839.
• YOSHIKAWA T, OHGUSHI H, TAMAI S. Immediate bone forming capability of prefabricated osteogenic hydroxyapatite. J Biomed Mater Res 1996;32:481-492.
• YUKNA RA. HTR polymer grafts in human periodontal osseous defects. I. 6-month clinical results. J Periodontol 1990;61:633-642.
• YUKNA RA. Clinical evaluation of coralline calcium carbonate as a bone replacement graft material in human periodontal osseous defects. J Periodontol 1994(a);65:177-185.
• YUKNA RA. Clinical evaluation of HTR polymer bone replacement grafts in human mandibular class II molar furcations. J Periodontol 1994(b);65:342-349.
• YUKNA RA. Synthetic grafts regeneration. In : Polson A, ed. Periodontal regeneration current status directions. Londres:Quintessence Publishing, 1994(c):103-112.
• YUKNA RA, CALLAN DP, KRAUSER JT, EVANS GH, AICHELMANN-REIDY ME, MOORE K et al. Multi-center clinical evaluation of combination anorganic bovine-derived hydroxyapatite matrix (ABM)/cell binding peptide (P-15) as a bone replacement graft material in human periodontal osseous defects. 6-month results. J Periodontol 1998;69:655-663.
• YUKNA RA, MAYER ET, AMOS SM. 5-year evaluation of durapatite ceramic alloplastic implants in periodontal osseous defects. J Periodontol 1989;60:544-551.
• YUKNA RA, YUKNA CN. Six-year clinical evaluation of HTR synthetic bone grafts in human grade II molar furcations. J Periodontal Res 1997;32:627-633.
• YUKNA RA, YUKNA CN. A 5-year follow-up of 16 patients treated with coralline calcium carbonate (Biocoral) bone replacement grafts in infrabony defects. J Clin Periodontol 1998;25:1036-1040.
• ZAMET JS, DARBAR UR, GRIFFITHS GS, BULMAN JS, BRAGGER U, BURGIN W et al. Particulate bioglass as a grafting material in the treatment of periodontal intrabony defects. J Clin Periodontol 1997;24:410-418.