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					                                                                       Dental Implants




                                             Advancements in Implant Surface Technology for Predictable
                                             Long-term Results
                                                                                                                                                          a report by
                                                                                                                          Clark M Stanford, DDS, PhD

                                                          Centennial Fund Professor for Clinical and Translational Research, College of Dentistry, University of Iowa




                                             Since the mid 1960s, the ongoing research with                   self-tapping); solid body press-fit designs; and/or sintered
                                             endosseous dental implants has revolutionized dental care.       bead technologies. Each of these respective approaches is
                                             The clinical success of implants is related to improvements      designed to achieve initial implant stability and/or create
                                             in surgical management, combined with greater                    large volumetric spaces for bone in-growth. Implant
                                             understanding of biological responses and engineering of         designs incorporate thread-cutting profiles to reduce
                                             dental implants.While high success rates of >95% hold for        interfacial shear stress such as a 15º thread profile
                                             many anatomic sites in the jaws, the bony response within        (ITI/Straumann, Institut Straumann AG, Waldenburg,
                                             the thin cortical plates and diminished cancellous bone          Switzerland) with a rounded tip to reduce shear forces at
   Clark Stanford, DDS, PhD, holds the
                                             characterizing the posterior maxilla is less successful with     the tip of the tread, which maintains bone in a favorable
             position of Centennial Fund     conventional machined surfaced implants (e.g. 65–85%).           compressive zone beneath the thread profile. Other thread
                Professor for Clinical and   In a meta-analysis, it was observed that implant success in      designs (Microthread™ Astra Tech AB, Mölndal, Sweden)
   Translational Research in the Dows
           Institute for Dental Research,
                                             the posterior maxillae is less than other regions of the         have focused on reducing the surrounding transcortical
      College of Dentistry, University of    mouth and is highly dependent on adequate bone                   shear forces by reducing the height of the thread profile
 Iowa. He also holds an appointment          volume in the area. Strategies to increase the quantity and      (thus reducing the contribution of any one thread) with
    in the Department of Orthopaedic
            Surgery, College of Medicine,
                                             quality of osseous tissue at the interface are an important      a rounded tip surface, along with an increase in the
         University of Iowa. Dr Stanford     means to provide predictable implant therapy for patients        number of threads per unit area of the implant surface (see
    received his BS, DDS, Certificate in     with poor bone quality, as well as greater predictability for    Figure 1). This has the added benefit of increasing the
     Prosthodontics and PhD from the
  University of Iowa. He has been on
                                             immediate loading of implants at the time of placement.          strength of the implant body by increasing the amount of
        the faculty at the University of                                                                      remaining wall thickness of the implant body. The
  Iowa since 1992. His research focus        As a means to develop initial implant stability, different       addition of microthreads to the upper portion of the
includes osteoblastic gene expression
 as a function of cell shape and the
                                             implant designs (lengths, thread features, surface               implant body has been copied by other implant designs
     role of substratum topography on        topography), as well as surgical technologies, are used to       on the market (BioLok™, Deerfield Beach FL and the
 gene expression. He is the Associate        assist the primary stability in poor quality bone. The           Groovy™ implant, Nobel BioCare, Göteborg, Sweden).
 Director for the National Center for
            Research Resources, National
                                             healing process of conversion from primary to secondary          Finally, orthopedic prostheses (e.g. femoral stems, pelvic
  Institutes of Health, General Clinical     bone has been reviewed elsewhere. Dental implant                 acetabular caps, knee prostheses, etc.) have used various
              Research Center (GCRC) at      surface technologies have used various approaches to             sintering technologies to create mesh or sintered beads as
       University of Iowa Hospitals and
 Clinics. Dr Stanford is the author of
                                             alter the surface micro– and nano-topography and                 a surface for bone to grow into. The application of this
        six book chapters, 68 published      recently the surface chemistry of the titanium dioxide           technology to dental implants has involved attempts to
             papers and more than 140        surface. Improved bone bonding and accelerated bone              improve the success rate of short implants (<10mm in
             published research abstracts
       primarily dealing with osteoblast
                                             formation appears to be possible with roughened surfaces         length), which are associated with the highest failure rates.
   cell and molecular biology, implant       modified with certain acid treatments. Sandblasted and
       surface technologies, and clinical    acid-etched surfaces have been demonstrated to enhance           Implant Micro-retentive Features
                           trial outcomes.
                                             bone apposition in histomorphometric and removal
                                             torque analyses. These studies indicate that a subtractive       Upon the placement of an implant into a surgical site,
                                             surface modification has an osteoconductive effect on            there is a cascade of molecular and cellular processes
                                             bone integration to the implant surface                          that provides for new bone growth and differentiation
                                             (osseointegration), and suggests a synergistic mechanism         along the biomaterial surface.The goal of a number of
                                             to enhance bone formation involved between the                   current strategies is to provide an enhanced osseous
                                             macro-topography (due to the blasting procedure) and             stability through micro- and nano-surface mediated
                                             micro-texture (due to the acid etching) of the implant.          events. These strategies can be divided into those that
                                                                                                              attempt to enhance the in-migration of new bone (e.g.
                                             Implant Macro-retentive Features                                 osteoconduction) through changes in surface
                                                                                                              topography (e.g. surface ‘roughness’) and the biological
                                             Implants used in the oral environment have three major           means to manipulate the type of cells that grow onto
                                             types of macro-retentive features: screw threads (tapped or      the surface.


30                                                                                                                                                    U S D E N T I S T RY 2 0 0 6
                           Advancements in Implant Surface Technology for Predictable Long-term Results

What is Surface Roughness?                                           Figure 1: Microthreads on the Upper Portion of the
                                                                     Implant Body Serve to Reduce Shear Stress in the
A common means to improve implant success is through                 Transcortical Region
alterations in the implant surface topography and surface
chemistry. In implant design, it is typically assumed that
a greater surface area (per unit of bulk metal surface) will
allow a greater area for load transfer of bone against the
implant surface. It should be noted that surface
roughness is often a poorly described characteristic of
implant surfaces, making comparisons between implant
systems difficult. One advantage of gentle acid etching is
to increase the roughness of a grit-blasted surface on a
nanometer-scale level without losing the macro-
retentive areas for bone tissue to grow into.

Various titanium surfaces have utilized surface roughness
created by a grit-blasting and etching procedure, or                 large grit-blasted and etched surfaces in a one-stage
blasting of the surface alone under tightly controlled               surgical procedure has documented greater than 10-year
conditions, to obtain a pre-defined optimal surface                  cumulative survival rates of 96.2%. An alternative
topography. One such optimization criterium has been                 commercial surface-etched implant design uses a
proposed. This suggests that an implant surface has an               combination of HCL/H2SO4 to create a surface
optimal balance between pore size on the surface (pore               topography (Osseotite™, 3i Implant innovations/BioMet
sizes of 1–5µm diameter and 1–5µm in depth) and                      Company, Palm Beach Gardens, Florida).The application
number of pores per unit area. This model relates the                of a roughened surface topography appears to have
relative shear strength of bone (τs) with a topography of            become the current ‘state of the art’ with multiple implant
interfacial load-carrying capacity for a surface                     systems coming out with new or modified products to
topographic feature (‘pit effectivity factor’) combined              reflect this increased interest.
with a value for the optimal number of pits per unit area
of the surface (‘pit density factor’). These topographic             Implant Micro-retentive Features:
features are then combined with macroscopic implant                  A l t e r a t i o n s o f S u r f a c e C h e m i s t r y.
thread profiles (pitch, angle, and position) to provide a
high compressive loading (low shear) implant interface.              Various attempts have been made to systematically alter
This surface topography is currently available from one              the surface topography and/or composition of the oxide
manufacturer and shows promise by maintaining crestal                surface on commercially pure and Ti-6Al-4V titanium
bone at the level of the head of the implant.                        alloy.These approaches have involved various techniques
                                                                     to increase the surface oxide thickness, create repeated
Implant Micro-retentive Features:                                    patterns (e.g. through additive or subtractive lithography)
S u r f a c e R o u g h n e s s by B l a s t i n g / E t c h i n g   or control oxide composition through the sterilization
                                                                     techniques used during or following manufacture.
Various studies have also addressed the issue of surface             Titanium, being a non-noble metal, spontaneously forms
roughness through various means of grit blasting followed            a 3–5nm thick oxide surface (primarily dioxide and
by a surface etching or coating procedure. This has                  trioxides) upon exposure to air. In vitro studies have been
included titanium plasma spray (TPS), abrasion (TiO2                 performed to characterize changes in the cell and
(titanium oxide) blasting or use of soluble abrasives)               molecular regulation of bone-related matrix proteins as a
combinations of blasting and etching (e.g. Al2O3 with                function of the oxide composition resulting from
H2SO4/HCL), thin apatite coating or sintered beads.                  sterilization procedures. These studies emphasize the
Commercially available roughened surfaces using the                  need for implants to be manufactured, packaged, and
large grit-blasted and acid etched surface                           clinically handled in a very careful manner. This then
(ITI/Straumann’s SLA surface) have shown both                        leads to a provocative question: Can the results of a
laboratory and clinical evidence of elevated success rates           simple cleaning and sterilization process impact
in areas of the posterior maxilla. The role of the                   subsequent biological healing?
roughened surface is complex, since the actual strength of
bone contact against a TiO2 surface is low (4MPa or less);           Interestingly, recent studies on surface sterilization have
weak enough that without macroscopic surface                         led to implant surfaces having a relatively enriched oxide
topography (e.g. resulting from a grit blasting process)             content of fluoride. Fluoride appears to play a significant
little bone contact occurs. Clinically, the combination of           role not only in modulating the physical chemistry of


U S D E N T I S T RY 2 0 0 6                                                                                                       31
                               Dental Implants



     Figure 2: Micro-CT 3-D Imaging of a Fluoride-modified         the bone morphogenetic protein-2 (BMP-2) expression,
     Implant at Eight Weeks Showing Uniform Adapted                as well as on the expression of other key genes involved
     Bone Around the Implant Surface                               in bone matrix expression (e.g. Bone sialoprotein).

                                                                   In order to assess the impact of the fluoride modification
                                                                   on tissue-level response, a series of animal studies were
                                                                   performed. Using a Sprague-Dawley rat tibiae model, two
                                                                   different threaded implants (1.5 x 2mm) were compared
                                                                   with either a TiO2 grit-blasted or blasted surface and
                                                                   treated with a fluoride-containing solution. Following 21
                                                                   days, a significantly greater amount of bone contact had
                                                                   occurred on the fluoride coated surfaces (55.5% versus
                                                                   34.2%; p<0.027). Histomorphometrically, the fluoride-
                                                                   exposed surface demonstrated greater osteoconductive
                                                                   growth of trabecular bone in the marrow cavity. In a
                                                                   rabbit tibial model, it was demonstrated that implant
     hydroxypatite formation, but also in stimulating new          surfaces that were exposed to this fluoride surface
     bone growth, matrix expression, and subsequent                treatment had enhanced pull-out strengths (57% increase
     mineralization. It is known that fluoride can stimulate the   from 54 to 85Ncm at three months) and a more rapid
     production of new bone, in part, by stimulating the           formation of bone contact. In a rabbit tibial model,
     proliferation of osteoblasts. The introduction of fluoride    greater bone-to-implant contact (BIC) on the fluoride-
     to the oxide surface can be performed by initially grit       modified surface as compared to a TiO2 grit-blasted
     blasting the surface with a known diameter of TiO2            surface (TiOblast) at one and three months was
     particles followed by etching in a dilute hydrofluric (HF)    demonstrated. As a measure of the interfacial strength,
     acid solution. The etching step is carefully controlled to    removal torque and interfacial shear strength values were
     create a moderately rough surface with potential              measured.Whereas the fluoride-modified surface did not
     biological effects on undifferentiated mesenchymal stem       result in any difference at one month of healing, by three
     cells that migrate onto the implant surface by way of the     months, the fluoride-modified surface had a significantly
     fibrin scaffold. In vivo research has indicated that          higher removal torque (85 ± 16Ncm) relative to the
     following a three-month healing period, fluoride-             TiOblast implants (54 ± 12Ncm). In a rat tibia model,
     modified implant surfaces demonstrated significantly          elevated BIC (55.45 ± 22.01%) relative to the non-
     higher bone-to-metal contact and retention to bone            modified TiOblast implants (34.21 ± 12.08%) were
     compared to implants with similar surface roughness.          observed three weeks following implantation, a difference
     Surface modification of titanium with fluoride changes        considered significant (p<0.027). In the oral cavity canine
     the surface chemical structures, resulting in an increased    dog model, enhanced bone formation around the
     affinity to the titanium dioxide surface for calcium and      fluoride-modified implant surface was observed using
     phosphate ions. The capability to adsorb calcium and          3-D micro-CT (computer tomography) imaging. This
     phosphate promotes bone formation and the process of          approach showed greater adaptive bone contact on the
     bone bonding in vitro and in vivo.                            walls of the implant surface (osteoconductive properties)
                                                                   (see Figure 2).
     In vitro studies have shown that differences in the
     microtopography of an implant surface can affect the          Conclusion
     expression of key bone matrix-related proteins and
     osteogenic transcription factors that will enhance            Fluoride surface conditioning is currently undergoing
     osteogenesis on implant surfaces. In a laboratory-based       human clinical trials and is available in European, North
     comparison study of the fluoride-modified implant             American, and some pan-Asian markets (OsseoSpeed™
     surface (OsseoSpeed™ Astra Tech AB, Mölndal, Sweden)          Astra Tech AB, Mölndal, Sweden).These studies serve to
     compared to a hydrophilic surface (SLActive™, Institute       emphasize how relatively minor changes to surface
     Straumann, Basel, Switzerland), it was found that the         technology can create the potential for significant
     fluoride-modified surface specifically lead to the up-        changes in bone contact and biomechanical strength, and
     regulation of a key transcription factor, RUNX-2/cbfa-1       which have the potential to allow more rapid clinical
     vital to the differentiation of osteoblasts. These            function of implants. ■
     observations were extended by an analysis of gene
     expression coupled with analysis of the changes in the        A longer version of this article containing references can be
     nano-topography on the oxide surface due to the HF            found in the Reference Section on the website supporting this
     exposure. Other laboratory studies have shown effects on      briefing (www.touchbriefings.com).


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