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implants - international magazine of oral implantology International Edition

I overview 28 I implants4_2013 withcellrecruitmentfromblood,bonemarrowandthe periosteum depending on the local anatomy (Fig. 3). Thiscellrecruitmentincludeslocalosteoblastprogeni- tors (human trabecular bone derived cells - HTBs), and blood-derivedCD-34-positiveembryogenicprogenitor cells (EPCs).18-21 Differentiation of local adult stem cells isnotwellunderstoodyet.However,thedifferentiation of EPC follows a stepwise change into an osteoblast while losing pluripotent capabilities from the EPC with high CD34-positive cells, followed by the stage of "cir- culating osteoblast lineage cell" towards the "blood mesenchymalprecursorcell"(BMPC)directdifferentia- tion into osteoblasts with continuously rising percent- ageofstromalcell-derivedfactor-1(SDF-1)CXCR4–re- ceptor.18 Vascular cells in newly formed bone tissue are alsoderivedfromEPCs.22,23 Osteoclasthoweverarepart ofthemononuclearphagocytesystemandderivedsyn- cytia formations of these cell lineages.24 Vicerocranial bone of the facial area is mesektodermal tissue derived fromthebranchialarchratherthanmesenchymaltissue derivedboneintheremainingskeleton.25-28 Thisissueis oftennotfullyconsideredwhendiscussionboneofthe skull. Cell induction in bone tissue: osteoinduction and neoangiogenesis The time course of bone healing is influenced by many factors. This includes the stability and morphol- ogyofthetransplantormeshgraftwithbiomaterialas well as biological factors. Those factors are osteocon- ductivity, osteoinductivity/vasoinductivity and os- teogenicity present in the bony implant bed and the graft or mesh. Osteoconductivity concerns macro and micromechanicalandmorphologicalpropertiesthatto promoteabonetissuespecificactivityanddifferentia- tion including matrix-cell-interactions, but also pore- size,surfacepropertiesandinterconnectivityofporesin biomaterials and bone grafts, and finally also external mechanical influences like shock waves.29-34 Osteoin- ductivity leads to bone growth and differentiation by specific ligand-dependent cell activation by growth factors and other molecules, that are mostly related to the TGF-ß family, like BMPs (bone morphogenic pro- teins) e.g. BMP-2 oder BMP-7, or basic FGF and VEGF.16, 35-44 The last ones are also strong vasoinductive factors. There are several approaches to apply growth factors in clinics.45-53 However, this application will re- main limited to specific problematic conditions due to some main reasons. On the one hand, tissue healing is limitedbycellactivity,includingthedescribedphasesof bone healing with cell recruitment, chemotaxis, differ- entiation and specific tissue matrix production that cannot be accelerated beyond biological limits and ex- ternalgrowthfactorsarethereforenotrelevantevenif certaineffectscanbeshowninvitroandinvivo.17, 54 On theotherhand,itisafactthatapplicationofgrowthfac- tors bears the immanent risk of carcinogenic transfor- mation.55, 56 Osteogenic properties concern the conti- nuityoflivingbonecellsaftertransplantationthatcan beachievedwithsmallbonepiecescombinedwithfast revascularisation(e.g.particulateboneaugmentation), press-fitincancellousbonehealing(clinicallynotrele- vant in oral- and maxillofacial surgery), and microvas- culartransplantslikefibulaflaps.Mainsignaltransduc- tion in bone cells includes classic general elements like ras und MAP-kinases, but also Smad-dependent path- ways and integrin-associated signaling.57-60 Smad-de- pendentsignallingincludesspecificsecondmessengers likeRunX2andconsistsofacomplexsystemofsub-el- ements depending on the associated ligand-system. BMPs activate Smad 1, 5 and 8 complex that binds Smad 4 and others as DNA-binding complex.58-60 Inte- grins on the other hand represent an element of signal transductionactivationbyspecificmatrixbinding.32-34 _II. Biomaterials and transplants Bonetransplantsandalloplasticbiomaterials We will not discuss the wide field of biomaterial products in depth as already published elsewhere.61, 62 Materials for bone augmentation are divided into allo- plastic (artificial) biomaterials and transplants along- side with their natural xenogenic, allogenic or autolo- gous derivatives.63-73 Most common alloplastic materi- als are: ß-tricalciumphosphate,72 bioactive glasses,71 and hydroxylapatite.73 Bone material can be further Fig. 2_Synopsis of the types of bone healing: contact healing (left), defect healing (right). Fig. 3_Synopsis of the subperiosteal implant healing. Contact healing Haversian canals bridge over the fracture line. CAVE: Here, remodelling is essential for com- plete stability as well. Defect healing A: Inflammation: haematoma ? chemotaxis/migration ? granulation tissue B: Soft callus. C: Hard callus. D: Remodelling. Lamellar healing in the matrix Appositional healing superiostal Contact healing at the interface, only in vaskularized material Periosteum Spongy bone Cortical bone Implant Fig. 2 Fig. 3