ReviewAdvancing dental implant surface technology – From micron- to nanotopography
Introduction
Current dental implant success has evolved from modest results of the middle of the past century. Beginning in the late 1960s the focused efforts of PI Branemark led to the detailed microscopic characterization of interfacial bone formation at machined titanium endosseous implants [1], [2]. These concepts of osseointegration focused the profession on a proscribed surgical technique and the biocompatible nature of the machined titanium surface. Bone formation at the endosseous implant surface was considered a positive outcome that was contrasted to fibrous encapsulation, a negative and undesired result [3]. The main clinical advantage of osseointegration was the predictable clinical result that occurred when an osseous interface was reproducibly formed and maintained at the titanium surface of load bearing dental implants [4].
Over two decades later, osseointegration is widely accepted in clinical dentistry as the basis for dental implant success. The low rate of implant failure in dense bone of the parasymphyseal mandible [5], [6], [7], [8] has not been fully recapitulated by subsequent data from studies involving more challenging clinical situations [9], [10]. Anecdotal reports of difficulty in achieving high rates of implant success in selected patient populations (e.g. smokers and diabetics) were supported by initial reports [11], [12], [13]. The cause of these failures, while not precisely determined, was largely attributed to a failure in bone formation in support of osseointegration. Challenging osseointegration with new protocols such as immediate placement and immediate loading may require further control of bone formation and osseointegration [9].
Failure to achieve osseointegration at a high rate can be attributed to one or more implant, local anatomic, local biologic, systemic or functional factors [5], [8]. Clinical control of all of these factors is represented by multidisciplinary treatment planning procedures. While it is presently acknowledged that these, as well as clinician-related factors, are important determinants of endosseous implants success, a major interest in implant design factors is evident and clinical efforts to improve implant success have been focused on increasing the amount of bone that forms at the endosseous implant surface.
Implant surface character is one implant design factor affecting the rate and extent of osseointegration [14], [15], [16], [17], [18]. The process of osseointegration is now well described both histologically and at the cellular level. The adhesion of a fibrin blood clot and the population of the implant surface by blood-derived cells and mesenchymal stem cells is orchestrated in a manner that results in osteoid formation and its subsequent mineralization [19], [20], [21]. A seamless progression of changing cell populations and elaboration and modification of the tissue/implant interface eventually results in bone forming in direct contact with the implant surface. Precisely how much of the implant surface directly contacts bone, how rapidly this bone accrual occurs, and the mechanical nature of the bone/implant connection is influenced by the nature of the implant surface itself [22].
The character of the implant surface is implicated in this complex process of osseointegration in a number of different ways. Early investigations revealed the biocompatible nature of the cpTitanium implant [23], and revealed some pragmatic advantages for cpTitanium over other suitable materials [24]. Molecular investigations have contributed to defining cellular responses to titanium as “compatible” and advantageous. For example, Suska and colleagues [25] showed relatively low inflammatory signaling within cells in tissues adjacent to cpTitanium implants and suggested that this is a part of the osseointegration process. During the first 10–20 years of applied endosseous implant experience, the concept that cpTitanium implant biocompatibility supported clinical osseointegration success dominated clinical thinking. Subsequently, experiments with surface topography encouraged new considerations of improvements in bone formation at the implant surface.
Section snippets
Micron-scale surface topography
The significance of micron-scale topography was highlighted in an important report by Buser and colleagues [26] that compared various surface preparations of cpTitanium to an electropolished surface negative control and a hydroxyapatite coated positive control group. The observation that a micron-scale rough surface prepared by grit blasting and subsequent acid etching was capable of rapid and increased bone accrual reiterated an earlier report that a TiO2 grit blasted surface also supported
Nanotechnology and surface science
Nanotechnology has been defined as “the creation of functional materials, devices and systems through control of matter on the nanometer length scale (1–100 nm), and exploitation of novel phenomena and properties (physical, chemical, and biological) at that length scale” (National Aeronautics and Space Administration). Nanotechnology involves materials that have a nano-sized topography or are composed of nano-sized materials. These materials have a size range between 1 and 100 nm (10−9 m) (Fig. 1
Biomimetics and nanotechnology
The recapitulation of natural cellular environments can be achieved at the nanoscale. Nanoscale modification of an implant surface could contribute to the mimicry of cellular environments to favor the process of rapid bone accrual. For example, cell adhesion to basement membranes is an often cited example of nanoscale biomimetics. The structure of the epithelial basement membrane contains pores approximating 70–100 nm [91]. It is suggested that the surface roughness of bone is approximately 32 nm
Nanotopography alters cellular responses
Surface nanotopography appears to affect cell interactions at surfaces and alter cell behavior when compared to conventional sized topography (Fig. 3) [97], [98], [99]. Different physical relationships exist between cells and nano- vs cell and micron-scale surface features. Nanotopography specific effects on cellular behavior have been demonstrated using a wide range of different cell types including epithelial cells, fibroblasts, myocytes and osteoblasts. Nanostructured surfaces possess unique
Nanotechnology alters surface reactivity
Nanoscale modification of the implant surface may alter the endosseous implants surface reactivity. Existing reports suggest that little bone bonding occurs at endosseous titanium implants, particularly during the early phases of bone formation [150]. Nanoscale modifications of topography appear to change the chemical reactivity of bulk materials [151]. Ellingsen [152] demonstrated that the calcium phosphate precipitation on grit blasted titanium was dramatically altered by HF surface treatment
The relative value of nanoscale and micron-scale roughness
The development of an implant/bone interface may be influenced by both nanoscale and micron-scale parameters of topography. The role of surface parameters (both bulk chemistry and topography) requires consideration of molecular (ionic and biomolecular) interactions with the surface, cell adhesion phenomenon and local biomechanical features of the established interface. It is clear that nanoscale modification will affect the chemical reactivity of an endosseous implant surface and alter the
Nanostructured surfaces for implant dentistry
There are many different methods to impart nanoscale features to the implant surface (see Table 1). Several of these methods have already been used to modify implants available commercially. Others are advancing through the research and development process.
As indicated above, positive bone responses occur at nanostructured surfaces tested in vitro and in vivo. Presently, only a few nanoscale surface topography modifications have been used to enhance bone responses at clinical dental implants.
Conclusions
Nanoscale modification can alter the chemistry and/or topography of the implant surface. Different methods have been described to modify or to embellish titanium substrates with nanoscale features. Such changes alter the implant surface interaction with ions, biomolecules and cells. These interactions can favorably influence molecular and cellular activities and alter the process of osseointegration. Cell culture studies reveal that there exists a range of nanoscale topography that promotes the
Acknowledgements
The authors would like to thank CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), and UNC-Astra Tech contributions to this article by fellowship.
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