Bone Graft Substitutes for the Promotion of Spinal Arthrodesis

Neurosurg Focus 10 (4):Article 4, 2001, Click here to return to Table of Contents

Bone graft substitutes for the promotion of spinal arthrodesis

GREGORY A. HELM, M.D., PH.D., HAYAN DAYOUB, M.D., AND JOHN A. JANE, JR., M.D. Departments of Neurosurgery and Biomedical Engineering, University of Virginia, Charlottesville, Virginia

In the prototypical method for inducing spinal fusion, autologous bone graft is harvested from the iliac crest or local bone removed during the spinal decompression. Although autologous bone remains the “gold standard” for stimulat- ing bone repair and regeneration, modern molecular biology and bioengineering techniques have produced unique materials that have potent osteogenic activities. Recombinant human osteogenic growth factors, such as bone morphogenetic proteins, transforming growth factorÐ, and platelet-derived growth factor are now produced in highly concentrated and pure forms and have been shown to be extremely potent bone-inducing agents when delivered in vivo in rats, dogs, primates, and humans. The delivery of pluripotent mesenchymal stem cells (MSCs) to regions requiring bone formation is also compelling, and it has been shown to be successful in inducing osteogenesis in numerous preclinical studies in rats and dogs. Finally, the identification of biological and nonbiological scaffolding materials is a crucial component of future bone graft substitutes, not only as a delivery vehicle for bone growth factors and MSCs but also as an osteoconductive matrix to stimulate bone deposition directly. In this paper, the currently available bone graft substitutes will be reviewed and the authors will discuss the novel therapeutic approaches that are currently being developed for use in the clinical setting.

KEY WORDS ¥ stem cell ¥ spinal fusion ¥ graft

A variety of internal fixation techniques can be used in Typical biosynthetic bone grafts rely on one, or a combi- most patients to achieve immediate spinal stability; how- nation, of these components of the osteogenic cascade for ever, long-term stability typically requires bone fusion their activity. of the involved region. A solid bone fusion is usually The three processes by which bone can be repaired or achieved after placement of autologous or allogenic bone regenerated are osteoinduction, osteoconduction, and grafts, each of which has specific advantages and dis- osteogenesis. Osteoinduction is defined as the ability to advantages. Although autologous bone is the “gold stan- stimulate the proliferation and differentiation of pluripo- dard” graft material, donor-site morbidity (pain, infec- tent MSCs. In endochondral bone formation, stem cells tion), limited supply, and inconsistent osteogenic activity differentiate into chondroblasts and chondrocytes, laying continue to be associated problems.34 Allogenic bone down a cartilaginous ECM, which subsequently calcifies grafts are significantly less osteogenic than autologous and is remodeled into lamellar bone. In intramembranous bone, mainly because allogenic bone typically acts only as bone formation, the stem cells differentiate directly into a passive scaffold for vascular ingrowth and bone deposi- osteoblasts, which form bone through direct mechanisms. tion. Because of these limitations, basic science and clini- Osteoinduction can be stimulated by osteogenic growth cal researchers are aggressively developing biosynthetic factors, although some ECM proteins can also drive bone grafts as an alternative to autologous and allogenic progenitor cells toward the osteogenic phenotype. Osteo- bone grafts. Research has also focused on defining the conduction is defined as the ability to stimulate the at- physiological mechanisms involved in bone repair and tachment, migration, and distribution of vascular and regeneration. These mechanisms have only been partially osteogenic cells within the graft material. The physical elucidated but appear to involve complex interactions characteristics that affect the graft's osteoconductive activ- between a variety of different angiogenic and osteogenic ity include porosity, pore size, and three-dimensional ar- growth factors, ECM proteins, and pluripotent MSCs. chitecture. In addition, direct biochemical interactions between matrix proteins and cell surface receptors play a major role in the host's response to the graft material. The Abbreviations used in this paper: BMP = bone morphogenetic protein; DBM = demineralized bone matrix; ECM = extracellular ability of a graft material to produce bone independently matrix; HA = hydroxyapatite; MSC = mesenchymal stem cell; is termed its direct osteogenic potential. To have direct rhBMP = recombinant human BMP; TCP = tricalcium phosphate; osteogenic activity, the graft must contain cellular compo- TGF = transforming growth factor. nents that directly induce bone formation. For example, a

Neurosurg. Focus / Volume 10 / April, 2001 1

Unauthenticated | Downloaded 10/01/21 11:33 PM UTC G. A. Helm, H. Dayoub, and J. A. Jane, Jr. collagen matrix seeded with activated MSCs would have Ceramics. Hydroxyapatite and -TCP have been the the potential to induce bone formation directly, without two most intensely studied ceramics for bone repair and recruitment and activation of host MSC populations. Be- regeneration.12 Their most unique property is chemical sim- cause many osteoconductive scaffolds also have the abil- ilarity to the mineralization phase of bone; this similarity ity to bind and deliver bioactive molecules, their osteo- accounts for their osteoconductive potential and excellent inductive potential will be greatly enhanced. biocompatibility.24 An important difference between these compounds is their resorption rates. In long bone models BONE GRAFT SUBSTITUTES at 3 months, 85% of -TCP is resorbed compared with only 5.4% of HA. Since implanted HA requires many Biodegradable Scaffolds years for complete resorption, it may actually prevent Biocompatible matrices are currently being developed newly formed bone within the porous HA from experi- encing the mechanical stresses required for bone remodel- to stimulate osteogenesis via osteoconduction and to promote osteoinduction by using osteogenic growth factors, ing. Unlike -TCP, however, HA possesses a structure that is very similar to natural bone and, therefore, has im- mesenchymal cell implants, and genetically engineered cells.5,9,15 Many of the scaffolds currently being designed proved osteoconductive properties. Both HA and -TCP have outstanding osteoconductive properties, rapidly stim- have been shown to be excellent carriers of osteoinduction ulating the migration of fibrovascular and osteoprogenitor growth factors and osteogenic cell populations, which will greatly add to their utility as bioactive delivery vehicles cells into a porous structure, which is subsequently incor- 18 porated into and replaced by bone tissues. The specific in the future. Interestingly, the authors of recent studies properties of the scaffold, which will optimize bone have demonstrated that HA can directly induce osteogenic growth, are dependent on the site requiring bone repair, differentiation of MSCs in tissue culture. although the most effective scaffolds have specific char- Extracellular Matrix Scaffolds. Various ECM proteins, acteristics. The material should: 1) readily incorporate and including collagen, laminin, fibronectin, and glycosami- retain mesenchymal cells in tissue culture; 2) rapidly in- noglycans (that is, hyaluronic acid, heparin sulfate, chon- duce fibrovascular invasion from the surrounding tissues; droitin sulfate A, and dermatan sulfate), may be fabricat- 3) have significant osteoconductive properties to improve ed into excellent biological scaffolds.3 These proteins incorporation with the host bone; 4) not induce significant have the advantages of supporting the migration and dif- acute immune responses or a chronic foreign body re- ferentiation of osteoblastic progenitor cells, facilitate the sponse; 5) have biomechanical properties similar to those binding of growth factors responsible for osteogenesis, of normal bone, such as a compressive modulus of 50 and resorbing within a reasonably short period of time via MPa and a compressive strength of 5 MPa for human tra- nontoxic mechanisms. In addition, the matrices can be becular bone, which will limit stress shielding resulting in made porous and biomechanically robust, leading to a va- bone loss adjacent to the implant; 6) be biodegradable, riety of clinical applications. In the majority of previous with a controllable absorption rate that parallels the rate of clinical studies concerning biological scaffolds the authors new bone deposition; 7) have biodegradation products have focused on collagen matrices. Although more than that are nontoxic and easily secreted by normal physio- 15 different kinds of collagen have been identified, Type I logical pathways; and 8) contain sites that can noncova- collagen is the most ubiquitous and has been the most lently bind osteogenic biomolecules to enhance osteoin- heavily studied as a possible osteogenic scaffold. duction. Numerous materials have been evaluated for their At the cellular level, ECM molecules exhibit a variety osteoconductive potential, but to date, the optimal scaffold of activities, including acting as a substrate for cell migra- has yet to be developed. The advantages and disadvan- tion; an adhesive for cell anchorage; a ligand for ions, tages of some of the most interesting osteoconductive sub- growth factors, and other bioactive agents; and a signal for strates will be briefly reviewed. contacting cells. Matrix components can be reassembled Polymers. Numerous polymeric systems have been into a theoretically limitless number of combinations, with studied, including poly--hydroxy esters, polydioxanone, varying sizes and shapes, making them ideal to form im- propylene fumarate, poly-ethylene glycol, poly-erthoes- plantable devices and substrates for cell delivery. The high ters, polyanhydrides and polyurethanes, poly-L-lactic number of polar and nonpolar residues in collagen matri- acid, poly-glycolic acid, and poly-lactic-co-glycolic ces provides excellent regions for noncovalent interac- acid.2,13 Poly-glycolic acid, poly-L-lactic acid, and poly- tions with osteogenic growth factors and can be designed lactic-co-glycolic acid have the advantages of being al- to optimize the amount of “free” and “bound” protein, to ready approved for use in humans, can be constructed control the rate of protein release, and to provide high lo- with varying porosities and in any three-dimensional cal concentrations of osteoinductive biomolecules. The shape, and have been shown to be an excellent substrate engineering of osteoconductive delivery vehicles will be for cellular or bioactive molecule delivery. Perhaps the an important component of any successful bone graft sub- major drawbacks to these unique polymers is that they stitute. may induce mild lactic acid releaseÐrelated toxicity during degradation and do not contain bioactive ligands. In Demineralized Bone Matrix addition, some of the materials may induce a significant Prior to the identification and cloning of the human foreign body inflammatory response, which may inhibit bone growth factor genes, BMPs were isolated from dem- bone deposition. With further development, however, ineralized bone extracts. The DBM production process is these materials may prove to be excellent osteoconductive fairly straightforward, involving the initial pulverizing of scaffolds and delivery vehicles.20 bone specimens into particles 70 to 450 m in diameter,

2 Neurosurg. Focus / Volume 10 / April, 2001

Unauthenticated | Downloaded 10/01/21 11:33 PM UTC Bone graft substitutes and spinal arthrodesis followed by their demineralization with 0.5 N hydrochlo- terestingly, these growth factors can initiate the complex ric acid and subsequent rinsing with sterile water.32 The cascade of endochondral bone formation. The BMPs ini- application of DBM to promote spinal arthrodesis has tially recruit MSCs to the region via chemotaxis and been studied in detail in both preclinical and clinical tri- stimulate their rapid proliferation and differentiation in- als.25 Oikarinen, et al.,27 initially demonstrated that DBM to chondroblasts and chondrocytes.31 These cells sub- could be successfully used to fuse the rabbit spine, a find- sequently lay down a cartilaginous matrix, and this calci- ing that was later supported by studies by Ragni and Lind- fies into woven bone. This tissue subsequently remodels, holm.29,30 Despite initial enthusiasm for this approach, forming mature lamellar bone, which contains active bone clinical studies in which the investigators evaluated the marrow elements. In a variety of femoral defect models in use of DBM for inducing spinal fusion have yielded dis- rats, rabbits, dogs, sheep, and primates, BMP-2 and BMP- appointing results. For example, An, et al.,1 performed a 7 delivered on an osteoconductive carrier have been prospective, nonrandomized study comparing autologous shown to accelerate bone healing significantly. In other bone alone with DBM and freeze-dried allogenic bone in studies, numerous groups have also demonstrated the util- anterior cervical fusions. The authors demonstrated that ity of rhBMPs for inducing spinal arthrodesis in a variety the pseudarthrosis rate was significantly lower in the auto- of different spine models in rats, rabbits, dogs, and pri- graft group (26%) compared with the DBM group (46%). mates. In several studies, the BMPs have been shown to In addition, Jorgenson, et al.,21 demonstrated that when be more effective at fusing the spine—in terms of fusion DBM was compared with autologous bone for posterolat- rate and biomechanical strength—than autologous bone eral lumbar fusions, the DBM graft was inferior to the grafts. autologous bone, as reflected by lower solid fusion rates Recombinant human bone growth factors have recently and radiographically demonstrated bone density. On the been used in clinical trials and have been shown to have other hand another research group32 demonstrated radio- osteogenic potential in several locations; however, there graphically in a retrospective study that posterolateral do appear to be significant differences in the physiologi- lumbar fusion sites treated with autologous bone alone (54 cal activity of the various BMPs depending on the clinical cases) or with a composite of DBM and autologous bone situation and the particular patient. Boyne et al.,6 demon- (36 cases) had similar characteristics at 12 months, al- strated that rhBMP-2 delivered on collagen sheets was though the fusion rates were not determined. The principal capable of increasing the height of atrophied maxillary limitations of DBM preparations include batch-to-batch bones, although the grafts were less effective at treating variability, suboptimal processing methods, and potential alveolar ridge resorption. Geesink, et al.,14 subsequently infectious agents contaminating the material. It is proba- demonstrated significant osteogenic activity of BMP-7 ble that delivery of highly concentrated, rhBMPs (alone delivered on a collagen carrier in five of six patients in or in combination) will replace DBM formulations in the whom fibular osteotomies were performed. In the spinal near future. region, Boden, et al.,4 also showed that rhBMP-2 delivered on a collagen sponge placed inside an interbody fu- Bone Growth Factors sion cage can induce spinal arthrodesis at a rate higher than cages filled with autologous bone In patients with In the last 10 years, remarkable advances have been thoracolumbar burst fractures, Laursen, et al.,23 implanted made in the field of osteoinductive growth factors.16,17,33 BMP-7 on a collagen carrier into the lesions and demon- The results of numerous studies indicate that bone regen- strated that the BMP actually increased early bone resorp- eration involves the complex interaction of a variety of tion at the treatment site, most likely resulting in de- different regulatory factors. The TGF superfamily con- creased biomechanical strength at the treatment site. The tains some of the most important growth factors involved results of these initial studies clearly indicate that addi- in bone healing, including TGF1 through 5, BMPs, tional work needs to be performed to determine which and growth and differentiation factors. In addition, other specific BMPs have the most potent osteoinductive activ- growth factors, including fibroblast growth factors (acidic ity and whether combinations of different growth factors and basic), platelet-derived growth factor and insulin-like will also improve the rate and quality of bone deposition. growth factor, are clearly present at sites undergoing bone In addition, determination of the ideal BMP dose, the most healing. Using current molecular biology techniques, re- effective BMP carrier, and the optimum rate of release of combinant human bone growth factors can now be pro- the BMP from the carrier are clearly required before con- duced with purity and in high concentrations, and their sistent clinical results can be achieved. availability has led to a large number of preclinical and clinical studies to determine the efficacy of each protein. Cell-Based Approaches for Bone Formation Bone growth factors are typically delivered on an osteoconductive scaffold, such as Type I collagen, HA, or bi- The osteogenic potential of osteoconductive scaffolds odegradable polymers. The peptides that have shown the can be increased by seeding the material with cells that greatest osteogenic activity to date include two members have osteoblastic potential. Various cellular approaches of the BMP family, BMP-2 and BMP-7, which stimulate have been attempted, including the use of: 1) unfraction- specific serine/threonine transmembrane receptors on ated fresh bone marrow; 2) marrow derivedÐculture ex- MSCs and osteoblasts.26 Activation of the receptors leads panded MSCs; and 3) MSCs predifferentiated into osteo- to the transphosphorylation of second messengers within blasts.7 As with any tissue-engineering approach, each of the cytosol, which subsequently translocate to the nucleus these cell-based therapies has its own advantages and dis- where they induce the transcription of a variety of genes advantages. involved in cellular proliferation and differentiation.26 In- Fresh Bone Marrow. The authors of several preclinical

Neurosurg. Focus / Volume 10 / April, 2001 3

Unauthenticated | Downloaded 10/01/21 11:33 PM UTC G. A. Helm, H. Dayoub, and J. A. Jane, Jr. and clinical studies have demonstrated that fresh autolo- marrow-derived MSCs at a concentration of 7.5 106 gous bone marrow has the potential to induce osteogene- cells/ml. The MSC-loaded carriers demonstrated signifi- sis in a variety of locations, including the spine.7,10,11 The cantly improved bone formation compared with the cell- principal advantages of using this approach are that the free control implants. In a similar study in canines, Bruder harvesting/implantation of autologous bone marrow cells and Fox7 demonstrated that HA/-TCP implants seeded from the iliac crest is relatively inexpensive, simple, and with culture-expanded autologous MSCs also induced sig- not subject to regulation by the Food and Drug Admin- nificantly greater bone formation within the ceramic car- istration. The mechanism by which fresh bone marrow rier and appeared to induce a more rapid union with the induces bone formation is the proliferation and differenti- surrounding host bone compared with control grafts. Hu- ation of osteoprogenitor cells contained in the marrow man MSCs loaded onto a ceramic carrier in a similar fash- cavities. Because only 0.001% of the nucleated cells in ion also promoted bone formation in a femoral defect normal bone marrow are MSCs, it may be difficult to model in athymic nude rodents. Immunocompromised an- implant a large enough number of these cells to achieve imals were used to attenuate the host immune response satisfactory clinical results. In addition, in the debilitated against the human xenograft. It remains unclear whether or aged patient, because the number of MSCs in the bone human-derived allogeneic MSCs will also induce signi- marrow significantly decreases, the use of this approach ficant immune responses when they are applied in clini- may be ineffective in this population. In appropriate pa- cal trials. tients and in selected clinical situations, however, the use There has been only one reported clinical trial of MSCs of bone marrow aspirates to augment osteoconductive for the treatment of human disease. Horwitz, et al.,19 dem- scaffolds or in combination with osteogenic growth fac- onstrated that allogeneic bone marrowÐderived MSCs tors may be an attractive treatment option. transplanted into three children with osteogenesis imper- Mesenchymal Stem Cells. Numerous research groups fecta significantly improved bone deposition in trabecular are currently focusing their efforts on the purification and bone. In addition, a mean increase of 28 g in bone miner- expansion of pluripotent stem cells for the treatment of a al content was observed in these patients, compared with variety of disorders, including Parkinson disease, stroke, predicted values of 0 to 4 g. The results of this study un- cardiomyopathy, hepatic failure, and renal disease. Mes- derscore the feasibility of using culture-expanded MSCs enchymal stem cells have also been identified and are cur- for the induction of osteogenesis for chronic bone disease; rently being developed for the repair and regeneration however, these techniques need further evaluation and bone, cartilage, muscle, tendon, and ligament.8 Mesenchy- refinement prior to widespread clinical application. mal stem cells are typically harvested from bone marrow Mesenchymal Stem Cells Predifferentiated Into Osteo- and have been isolated from rodents, canines, and hu- blasts. Several research groups have also studied the abil- mans. Interestingly, these cells can undergo extensive sub- ity of cultured marrow MSCs, which had been prediffer- cultivation in vitro without differentiation, magnifying entiated into osteoblasts, to induce bone regeneration.28,35 their potential clinical utility. Human MSCs can be direct- When undifferentiated MSCs are used to treat bone de- ed toward osteoblastic differentiation by adding dexa- fects, these cells must first differentiate into their osteo- methasone, ascorbic acid, and -glycerophosphate to the blastic phenotype for bone deposition to occur. If the tissue culture media. This osteoblastic commitment and MSCs are differentiated into osteoblasts with dexametha- differentiation can be documented by analyzing alkaline sone, ascorbic acid, and -glycerophosphate prior to phosphatase activity, the expression of bone matrix pro- implantation, the healing process should, at least theoreti- teins, and by the mineralization of the ECM. cally, be accelerated. This method has been investigated in Human MSCs express a variety of different cell surface both rodents and rabbits, demonstrating improved bone proteins, including numerous integrins (1, 2, 3, 5, healing with osteoblast-loaded scaffolds compared with 6, V, 1, 3, and 4), growth factor receptors (basic fi- cell-free control matrices. The obvious disadvantages to broblast growth factor, platelet-derived growth factor, this method are similar to those for allogeneic MSCs— interleukin-1, TGF-I receptors, and TGF-II receptors) namely problems with cellular rejection, graft contamina- and cell adhesion molecules (intracellular adhesion mole- tion, and batch-to-batch variability. These important ap- culeÐ1, vascular cell adhesion molecule, activated leu- proaches require additional investigation to determine kocyte cell adhesion molecule, and L-selection).7 They whether they can become clinically effective in the future. should therefore be highly responsive to osteogenic growth factors, as well as to osteoconductive matrices used as cellular delivery vehicles. Although human MSC CONCLUSIONS therapies are theoretically attractive, the development Future biosynthetic bone implants may obviate the need of allogenic grafts has several potential problems: graft for autologous bone grafts. Armed with the knowledge of rejection, transmittable diseases, and bacterial and fun- the basic mechanisms involved in bone repair and regen- gal contamination. If these issues are overcome, human eration, researchers can now maximize the osteoinductive, MSCs may be ideal for ex vivo BMP gene therapy, be- osteoconductive, and osteogenic properties of the graft cause the expressed proteins will not only stimulate the material. The optimal graft will most likely be composed osteogenic cascade by the grafted stem cells but also local of osteoprogenitor cells and osteoinductive growth factors host progenitor cells. that are delivered on an osteoconductive, resorbable scaf- Kadiyala, et al.,22 recently demonstrated that MSCs can fold. The major issues that need to be addressed include regenerate bone in a rat femoral defect model. These in- limiting the host immune responses directed against allo- vestigators seeded HA/-TCP scaffolds with syngeneic geneic mesenchymal cell implants, the optimum dose and

4 Neurosurg. Focus / Volume 10 / April, 2001

Unauthenticated | Downloaded 10/01/21 11:33 PM UTC Bone graft substitutes and spinal arthrodesis rate of protein release for each osteoinductive growth fac- genetic proteinÐ2 to enhance autologous bone lumbar spinal fu- tor, and the development of appropriate delivery vehicles sion. J Neurosurg 86:93Ð100, 1997 for each clinical situation. It is anticipated that construc- 18. Herr G, Wahl D, Kusswetter W: Osteogenic activity of bone tion of a material with these three key components should morphogenetic protein and hydroxyapatite composite implants. Ann Chir Gynaecol Suppl 207:99Ð107, 1993 produce a biosynthetic bone graft that possesses perfor- 19. Horwitz EM, Prockop DJ, Fitzpatrick LA, et al: Transplantabil- mance characteristics superior to autologous bone. ity and therapeutic effects of bone marrowÐderived mesenchymal cells in children with osteogenesis imperfecta. Nat Med References 5:309Ð313, 1999 20. Ishaug SL, Crane GM, Miller MJ, et al: Bone formation by 1. An HS, Simpson JM, Glover JM, et al: Comparison between al- three-dimensional stromal osteoblast culture in biodegradable lograft plus demineralized bone matrix versus autograft in ante- polymer scaffolds. J Biomed Mater Res 36:17Ð28, 1997 rior cervical fusion. A prospective multicenter study. Spine 20: 21. Jorgenson SS, Lowe TG, France J, et al: A prospective analysis 2211Ð2216, 1995 of autograft versus allograft in posterolateral lumbar fusion in 2. Andriano KP, Tabata Y, Ikada Y, et al: In vitro and in vivo com- the same patient. A minimum of 1-year follow-up in 144 pa- parison of bulk and surface hydrolysis in absorbable polymer tients. Spine 19:2048Ð2053, 1994 scaffolds for tissue engineering. J Biomed Mater Res 48: 22. Kadiyala S, Young RG, Thiede MA, et al: Culture expanded ca- 602Ð612, 1999 nine mesenchymal stem cells possess osteochondrogenic po- 3. Badylak S, Arnoczky S, Plouhar P, et al: Naturally occurring tential in vivo and in vitro. Cell Transplant 6:125Ð134, 1997 extracellular matrix as a scaffold for musculoskeletal repair. 23. Laursen M, Hoy K, Hansen ES, et al: Recombinant bone mor- Clin Orthop 367 (Suppl):S333ÐS343, 1999 phogenetic protein-7 as an intracorporal bone growth stimula- 4. Boden SD, Zdeblick TA, Sandhu HS, et al: The use of rhBMP- tor in unstable thoracolumbar burst fractures in humans: pre- 2 in interbody fusion cages. Definitive evidence of osteoinduc- liminary results. Eur Spine J 8:485Ð490, 1999 tion in humans: a preliminary report. Spine 25:376Ð381, 2000 24. Lee YM, Park YJ, Lee SJ, et al: Tissue engineered bone forma- 5. Boyan BD, Lohmann CH, Romero J, et al: Bone and cartilage tion using chitosan/tricalcium phosphate sponges. J Periodon- tissue engineering. Clin Plast Surg 26:629Ð645, 1999 tol 71:410Ð417, 2000 6. Boyne PJ, Marx RE, Nevins M, et al: A feasibility study evalu- 25. Maddox E, Zhan M, Mundy GR, et al: Optimizing human dem- ating rhBMP-2/absorbable collagen sponge for maxillary sinus ineralized bone matrix for clinical application. Tissue Eng 6: floor augmentation. Int J Periodontics Restorative Dent 17: 441Ð448, 2000 11Ð25, 1997 26. Massague J: TGFbeta signaling: receptors, transducers, and 7. Bruder SP, Fox BS: Tissue engineering of bone. Cell based Mad proteins. Cell 85:947Ð950, 1996 strategies. Clin Orthop 367 (Suppl):S68ÐS83, 1999 27. Oikarinen J: Experimental spinal fusion with decalcified bone 8. Bruder SP, Ricalton NS, Boynton RE, et al: Mesenchymal stem matrix and deepÐfrozen allogeneic bone in rabbits. Clin Or- cell surface antigen SB-10 corresponds to activated leukocyte thop 162:210-218, 1982 cell adhesion molecule and is involved in osteogenic differenti- 28. Okumura M, Ohgushi H, Dohi Y, et al: Osteoblastic phenotype ation. J Bone Miner Res 13:655Ð663, 1998 expression on the surface of hydroxyapatite ceramics. J Bio- 9. Brunski JB, Puleo DA, Nanci A: Biomaterials and biome- med Mater Res 37:122Ð129, 1997 chanics of oral and maxillofacial implants: current status and 29. Ragni P, Lindholm TS: Interaction of allogeneic demineralized future developments. Int J Oral Maxillofac Implants 15: bone matrix and porous hydroxyapatite bioceramics in lumbar 15Ð46, 2000 interbody fusion in rabbits. Clin Orthop 272:292Ð299, 1991 10. Bucholz RW, Carlton A, Holmes RE: Hydroxyapatite and tri- 30. Ragni PC, Lindholm TS: Bone formation and static changes in calcium phosphate bone graft substitutes. Orthop Clin North the thoracic spine at uni- or bilateral experimental spondylo- Am 18:323Ð334, 1987 desis with demineralized bone matrix (DBM). Ital J Orthop 11. Connolly J, Guse R, Lippiello L, et al: Development of an os- Traumatol 15:237Ð252, 1989 teogenic bone-marrow preparation. J Bone Joint Surg Am 71: 31. Reddi AH: BMPs: actions in flesh and bone. Nat Med 3: 684Ð691, 1989 837Ð839, 1997 12. Eggli PS, Muller W, Schenk RK: Porous hydroxyapatite and tri- 32. Russell JL, Block JE: Clinical utility of demineralized bone ma- calcium phosphate cylinders with two different pore size ranges trix for osseous defects, arthrodesis, and reconstruction: impact implanted in the cancellous bone of rabbits. A comparative his- of processing techniques and study methodology. Orthopedics tomorphometric and histologic study of bone ingrowth and im- 22:524Ð533, 1999 plant substitution. Clin Orthop 232:127Ð138, 1988 33. Sakou T: Bone morphogenetic proteins: from basic studies to 13. Farso Nielsen F, Karring T, Gogolewski S: Biodegradable clinical approaches. Bone 22:591Ð603, 1998 guide for bone regeneration. Polyurethane membranes tested in 34. Service RF: Tissue engineers build new bone. Science 289: rabbit radius defects. Acta Orthop Scand 63:66Ð69, 1992 1498Ð1500, 2000 14. Geesink RG, Hoefnagels NH, Bulstra SK: Osteogenic activity 35. Yoshikawa T, Ohgushi H, Tamai S: Immediate bone forming of OP-1 bone morphogenetic protein (BMP-7) in a human fibu- capability of prefabricated osteogenic hydroxyapatite. J Bio- lar defect. J Bone Joint Surg Br 81:710Ð718, 1999 med Mater Res 32:481Ð492, 1996 15. Healy KE, Rezania A, Stile RA: Designing biomaterials to direct biological responses. Ann NY Acad Sci 875:24Ð35, 1999 16. Helm GA, Alden TD, Sheehan JP, et al: Bone morphogenet- Manuscript received March 13, 2001. ic proteins and bone morphogenetic protein gene therapy in Accepted in final form March 21, 2001. neurological surgery: a review. Neurosurgery 46:1213Ð1222, Address reprint requests to: Gregory A. Helm, M.D., Ph.D., De- 2000 partment of Neurosurgery, Box 800212, University of Virginia 17. Helm GA, Sheehan JM, Sheehan JP, et al: Utilization of type I Health Sciences Center, Charlottesville, Virginia 22908. email: collagen gel, demineralized bone matrix, and bone morpho- [email protected].

Neurosurgical Focus / Volume 10 / April, 2001 5

Unauthenticated | Downloaded 10/01/21 11:33 PM UTC