On the importance of nanometer structures for implant incorporation in bone tissue

Introduction Replacing lost teeth with dental implants is today a reliable treatment method associated with good long-term clinical results. Different surface modifications alter the surface topography at micro- and nanometer level of resolution as well as chemical properties, which have shown to be of importance for osseointegration. Research within the field of implantology is still intense and aims at further improving the implant properties to achieve successful treatments for patients with compromised bone as well as developing a surface that provides faster integration to shorten the treatment period. Furthermore, more basic science data is needed to increase our understanding of the mechanisms involved in osseointegration. The significance of the surface topography on the micrometer level for implant integration is well known. However, the knowledge of how and to what extent nanostructures may be of importance in early bonehealing and osseointegration remains to be investigated. Aim The overall aim of this thesis was to describe a technique to characterize commercial oral implants on the nanometer level when nanostructures are applied on a microroughness and to investigate whether or not the nanometer surface roughness was correlated to the more well-known micrometer roughnesss; to study the real-time initial cellular interactions of human osteoblasts and fibroblasts to different implant surfaces with and without a coat of nanocrystalline hydroxyapatite; and to evaluate the early bone response to a nanocrystalline hydroxyapatite coating (nano-HA) applied on smooth cylindrical and moderately rough screw-shaped implants. Materials and Methods Twelve different commercial screw-shaped dental implants with different surface modifications were examined using optical interferometry together with Gaussian digital filters and scanning electron microscopy (SEM). Human osteoblasts and fibroblasts were used when investigating the initial cell-surface interaction to different surfaces modifications with optical tweezers (OT) and quartz crystal balance with dissipation monitoring (QCM-D). To evaluate the effect of nanocrystalline hydroxyapatite (nano-HA) compared to nanosized particles of titanium in early bone response, smooth cylindrical titanium implants with no microroughness were inserted in rabbit tibia. The implant surfaces were examined using atomic force microscopy (AFM) and interferometry. To evaluate the biological response, histological analyses including bone contact (BIC) and bone area (BA), as well as qualitative analysis were performed. Furthermore, screw-shaped sandblasted and acid etched titanium implants coated with nano-HA of different thicknesses and un-coated controls were evaluated in rabbit tibia as well as femur. Interferometry, SEM and X-ray Photoelectron Spectroscopy (XPS) were used to characterize the implant surface topography and chemical composition. Biomechanical and histological evaluations including BA, newly formed bone and qualitative evaluations were performed. Results The studies showed that it is possible to characterize the surface nanoroughness of commercially dental implants using interferometry. A 1x1μm Gaussian filter was found useful to identify nanoroughness in terms of height deviation. It was demonstrated that the implants do have distinct roughness on the nanometer level of resolution and that the nanoroughness is not correlated to the microroughness when comparing mean surface roughness (Sa). Significant differences in Sa on the nanometer scale were found among some of the implants investigated. However, to detect specific nanostructures an additional SEM examination is necessary. The results from optical trap experiments showed that both osteoblasts and fibroblasts responded in a similar way towards most of the surfaces. No difference in initial cell attachment could be detected between the surfaces when using the QCM-D technique. A nano-HA coating applied on smooth cylindrical implants did not enhance bone responses in terms of bone contact (BIC) and bone area (BA) values as compared to nano-titania. Screw-shaped sandblasted and acid etched titanium implants with applied nanothick (~20nm) coating of nano-HA with similar Sa values on both micro- and nanometer scale of resolution presented similar removal torque values, BA values and showed similar amounts of newly formed bone as compared to un-coated controls when placed in cortical bone. The same result was demonstrated in trabecular bone with a submicron thick coating of nano-HA onto sandblasted and acid etched screw-shaped implants. Conclusions Within the limits of the studies in this thesis, it was demonstrated that commercially available oral implants do have nanoroughness of various amounts and that the nanoroughness is not correlated to the microroughness. It was demonstrated possible to observe cell attachment using optical trapping and QCM-D, however no obvious differences between the surfaces could be detected. A nano-HA coating applied on cylindrical titanium implants did not enhance early bone response compared to a nano-titania coating when evaluated in cortical bone. Furthermore, sandblasted and acid etched screw-shaped implants with applied coatings of different thicknesses of nano-HA perform similar as un-coated controls when evaluated in cortical and trabecular bone.