Surface modifications of dental implants.

Dental implant surface technologies have been evolving rapidly to enhance a more rapid bone formation on their surface and hold a potential to increase the predictability of expedited implant therapy. While implant outcomes have become highly predictable, there are sites and conditions that result in elevated implant loss. This paper reviews the impact of macro-retentive features which includes approaches to surface oxide modification, thread design, press-fit and sintered-bead technologies to increase predictability of outcomes. Implant designs that lead to controlled lateral compression of the bone can improve primary stability as long as the stress does not exceed the localized yield strength of the cortical bone. Some implant designs have reduced crestal bone loss by use of multiple cutting threads that are closely spaced, smoothed on the tip but designed to create a hoop-stress stability of the implant as it is completely seated in the osteotomy. Following the placement of the implant, there is a predictable sequence of bone turnover and replacement at the interface that allows the newly formed bone to adapt to microscopic roughness on the implant surface, and on some surfaces, a nanotopography (<10(-9) m scale) that has been shown to preferably influence the formation of bone. Newly emerging studies show that bone cells are exquisitely sensitive to these topographical features and will upregulate the expression of bone related genes for new bone formation when grown on these surfaces. We live in an exciting time of rapid changes in the modalities we can offer patients for tooth replacement therapy. Given this, it is our responsibility to be critical when claims are made, incorporate into our practice what is proven and worthwhile, and to continue to support and provide the best patient care possible.

Issues and considerations in dental implant occlusion: what do we know, and what do we need to find out?

Implant dentistry continues to struggle with what are the appropriate occlusal concept(s) for implant-supported restorations. The biological and mechanical consequences of the loading environment leads to establishing and maintaining an implant interface in a wide variety of bone quality and quantity, implant and prosthesis designs. To the restorative dentist, the role of occlusion is more focused on extending the service life of the restoration and the connecting abutment(s) than protecting the osseous integration of the implant(s). This study reviews the relevant issues regarding implant occlusion along with implant and prosthesis design in order to provide optimal patient care.

Proliferative and phenotypic responses of bone-like cells to mechanical deformation.

Limited in vivo and in vitro experiments suggest that bone and bone-like cells respond to mechanical signals in a trigger-like rather than a dose-response fashion; i.e., they fail to respond until they have been stimulated with some given number of cycles of loading, and then once they respond, additional cycles produce little or no effect. To explore this notion, rat calvaria-derived osteoblast-like cells and the cell line MC3T3-E1 were plated at a high cell density (5,000 cells/mm2) on silicone membranes coated with type-I collagen and were allowed to attach for 24 hours. The membranes then were exposed to vacuum pressure (-1 kPa, 0.5 Hz) on a daily basis, and cultures were assayed every 2 days for 2 weeks. The proliferation of nontransformed cells increased 7-fold with as few as four daily cycles but not with one cycle per day. Furthermore, 1,800 cycles of vacuum did not result in a greater response than four cycles per day. We observed inverse phenotypic responses: the expression of osteocalcin was depressed compared with controls in the cultures of osteoblast-like cells that were strained with as few as four cycles per day. Alkaline phosphatase activity was depressed in the cultures of both the osteoblast-like cells and the MC3T3-E1 cells exposed to low vacuum pressures (-1 kPa) with four daily cycles of vacuum pressure. Increasing the vacuum magnitude did not affect the occurrence of a "trigger response" between one and four cycles of vacuum application.(ABSTRACT TRUNCATED AT 250 WORDS)

Bone cell expression on titanium surfaces is altered by sterilization treatments.

Phenotypic responses of rat calvarial osteoblast-like cells (RCOB) were evaluated on commercially pure titanium (cpTi) surfaces when cultured at high density (5100 cells/mm2). These surfaces were prepared to three different clinically relevant surface preparations (1-micron, 600-grit, and 50-microns-grit sand-blast), followed by sterilization with either ultraviolet light, ethylene oxide, argon plasma-cleaning, or routine clinical autoclaving. Osteocalcin and alkaline phosphatase, but not collagen expression, were significantly affected by surface roughness when these surfaces were altered by argon plasma-cleaning. In general, plasma-cleaned cpTi surfaces demonstrated an inverse relationship between surface roughness and phenotypic markers for a bone-like response. On a per-cell basis, levels of the bone-specific protein, osteocalcin, and the enzymatic activity of alkaline phosphatase were highest on the smooth 1-micron polished surface and lowest on the roughest surfaces for the plasma-cleaned cpTi. Detectable bone cell expression can be altered by clinically relevant surfaces prepared by standard dental implant preparation techniques.

Effects of backing reflectance and mold size on polymerization of photo-activated composite resin.

The in vitro depth of cure of a visible-light-activated microfilled composite, as influenced by backing reflectance and mold size, was investigated using microhardness measurements. More extensive polymerization was obtained with higher values of backing reflectance. Mold size larger than the cross-sectional area of the photo-activating beam also increased the degree of polymerization. Backing reflectance and mold size are influential factors and need to be defined in depth-of-cure evaluations if in vitro measurements are to be valid predictors of clinical performance.

Cellular deformation reversibly depresses RT-PCR detectable levels of bone-related mRNA.

Osteoblastic cells respond to mechanical stimuli with alterations in proliferation and/or phenotypic expression. In some cases, these responses occur within only a few applications of stimuli (i.e. 'cycle-dependent trigger response') rather than in a dose-dependent manner. To explore potential mechanisms of the cycle dependent trigger response, we raised the following questions: (1) Does strain of bone cells alter gene expression; if so, how quickly does it occur and how long does it last? (2) Are alterations in message level strain magnitude dependent? (3) Are alterations in steady-state message levels cycle dependent? Cultures were evaluated for osteocalcin mRNA one week following a daily stretch application at four stretch magnitudes and four cycle numbers and compared to nonstretched controls. Steady state mRNA message was ascertained prior to and at 10, 20, 30, 60, 120, 180, and 240 min following initiation of stretch. Following mRNA isolation, first strand cDNA synthesis was performed and fluorometrically quantitated. A reverse transcriptase based PCR (RT-PCR) approach allowed assessment of osteocalcin mRNA levels from microcultures (50,000 cells per 10 microliters culture or 5000 cells mm2) of rat calvarial osteoblasts. Optimized PCR was performed using primers to the bone specific protein, osteocalcin (OC) and two 'housekeeping' genes, beta-actin and GAP-DH. PCR products were separated on 4% agarose gels and band intensities digitized with relative quantitation based on internal standards in each gel. The lowest magnitude of stretch (- 1 KPa) at 1800 cycles per day reproducibly depressed message for osteocalcin, but not beta-actin when assayed immediately following the cessation of strain application. By three hours following the initiation of stretch, message levels returned to control values. At the time of stretch cessation, the 1800 cycle stretch regimen diminished (p < 0.0001) steady-state osteocalcin message independently of the four stretch magnitudes. Stretch for 300 cycles failed to depress (p = 0.05) osteocalcin message cultures at any time, but 600 cycles depressed message by 30 min. By one and two hours, cultures stretch 600, 900, and 1800 cycles showed similar levels of message depression. Four hours following the initiation of stretch, message levels returning to nonstrained levels in all groups. We conclude that alterations in cell response to strain are in part mediated by gene expression, that alterations last 3-4 h in this system, and that the message mechanism itself exhibits a trigger-response dependency to cycle number.

Primary human bone cultures from older patients do not respond at continuum levels of in vivo strain magnitudes.

Osteoporosis is characterized by excessive loss of bone mass, while exercise is believed to maintain or enhance bone mass. Since exercise marginally affects osteoporosis, we wondered whether bone cells from osteoporotic patients would fail to respond to strain. Primary human bone-like cultures were obtained from females over age 60 with hip arthroplasty procedures performed for either osteoporotic fracture (n = 8) or non-osteoporotic osteoarthrosis (n = 5). Cultures (96,000 cell/cm2) were strained in rectangular optically clear silastic wells. Three periods of uniaxial substratum strain (1000 micro-strain, 1 Hz, 10,000 cycles, sine wave) were provided every 24 h using a four-point bending, computer-controlled device. Results at a frequency of 1 Hz were compared to cultures exposed to 20 Hz with bone cells derived from one osteoarthritic subject. Alterations in protein level expression of bone-related proteins were determined using a semi-quantitative confocal approach along with enzyme (alkaline phosphatase) activity and enzyme mRNA copy number using cRNA RT-PCR. Strain did not alter levels of bone-related protein levels, enzyme activity, or steady state copy number per cell in response to strain in either group. Strained cultures from osteoporotic patients exhibited little variation from unstrained controls, while individual cultures from osteoarthritic patients exhibited increases in one protein or the other. The results suggest that bone cells from older individuals may not be responsive to continuum levels of strain anticipated with vigorous activity.

Osteoblastic cell attachment to hydroxyapatite-coated implant surfaces in vitro.

Hydroxyapatite (HA) used as a coating for implants can exhibit varying levels of interaction with the biologic environment. The crystallinity of the HA-based coating has been shown to control the rate of dissolution and appears to play a role in the initial cellular interaction with the implant surfaces. An osteoblastic cell attachment assay was employed to examine the cell attachment to untreated and pretreated (pH 5.2, 24 hours) titanium and HA coatings of low (50%), medium (75%), and high (90%) crystallinity. A slightly higher percentage of cell attachment (%CA) was found on untreated and pretreated HA surfaces as compared to the titanium surface. No significant difference could be found in the %CA between the 3 levels of crystallinity. However, higher levels of %CA were observed on pretreated HA surfaces than on untreated HA surfaces (t test, P < .05). Elevated calcium and phosphate levels in culture medium did not have any effect on cell attachment. Scanning electron microscopic examinations revealed surface degradation of the HA coating following pretreatment in the simulated inflammatory media (pH 5.2, 24 hours). The results suggest that the altered surface topography may influence the initial cell attachment to HA surfaces.

How connective tissues temporally process mechanical stimuli.

Connective tissues have the capacity to adapt to a changing mechanical environment. Considerable experimental evidence suggests, however, that tissues respond to only a small portion of mechanical loads they experience and thus ignore the majority of the load they experience. Evidence suggests that tissues 'temporally process' these external mechanical signals in specific ways: 1) they respond in a trigger-like manner after a relatively few events or cycles of loading; 2) they respond only to some window of strain magnitude; 3) they exhibit a refractory period after a response; 4) they have a memory for previous stimuli. These characteristics do not necessarily mirror distinct cellular phenomena but rather reflect typical features of experimental and theoretical models. The manner in which tissues temporally process mechanical signals can be explained by energy transfer over short periods of time from the matrix to the second messenger systems. Much of the energy input into connective tissues during deformation is recovered, and relatively little is 'lost' and thus potentially transferable to the cells. Since evolutionary principles suggest that a pathway of response needs to be defined by biologically optimal use of energy, it is logical that cells would have evolved mechanisms to respond to a small finite level of integrated stimuli in order to rapidly respond (fitness) especially in times of environmental duress.

Removal of supragingival plaque in an intraoral model by use of the Sonicare toothbrush.

This study was performed to evaluate plaque removal efficacy of a mechanical toothbrush in a controlled in vivo trial. The study used enamel sections, obtained from extracted human teeth, to evaluate the efficacy of supragingival plaque removal by a mechanical powered toothbrush in various modes of use. Enamel sections were positioned in milled depressions on metal extensions of a maxillary partial denture that bilaterally extended along the buccal corridors. Four sections were used per side, positioned zero, 2 or 3mm from the surface of the metal extension. The prosthesis was worn for 16 hours, whereupon four enamel sections (two per side) were removed. Intra-oral brushing was then performed for 5 or 15 seconds on the remaining four sections. The total of adherent bacteria was then assayed from all enamel sections. Ten to twenty trials were performed (n=4 pairs/trial) for each set of variables. When the electric toothbrush was used as a manual brush ('off'), it was equivalent in plaque removal to a conventional toothbrush (Oral-B 35; P = 0.49). However, when the powered brush was in its active mode ('on'), it removed more bacteria (P < 0.0001); this efficacy was observed even when the bristle tips were at a distance from the plaque surface.

Radiopacity of light-cured posterior composite resins.

Radiographic images of teeth and restorations were used to evaluate the radiopacity of 11 light-cured posterior composite resins. The radiopacity of these composite resins provided enough variation on radiographs so that clinicians distinguished the images of the restoration from adjacent tooth structure.

In vitro periodontal ligament fibroblast attachment to plasma-cleaned titanium surfaces.

The objective of this research was to characterize the in vitro cellular behavior of fibroblast-like cells derived from rat periodontal ligament on commercially pure titanium surfaces which were sterilized by a variety of treatments. Following standard surface preparation protocols, the Ti specimens were sterilized by either steam autoclaving, exposure to ethylene oxide gas, exposure to ultraviolet light, or plasma-cleaning in argon for either one min or five min. Fibroblast-like cells in serum-supplemented media were incubated on the various Ti specimens for up to two h. In general, the levels of cell attachment for plasma-cleaned surfaces were significantly higher than those for steam-sterilized surfaces, but were significantly lower than the attachment levels for both the ultraviolet-treated surfaces and the tissue culture plastic control. The duration of plasma cleaning itself did not have a significant effect on the percentage of cell attachment at any time period. SEM evaluations indicated that by two h, the cellular morphology was different on the variously treated specimens. These studies indicate that the method of sterilization following implant surface preparation can affect the initial in vitro biological events of cell attachment and spreading.

Short-term plasma-cleaning treatments enhance in vitro osteoblast attachment to titanium.

The purpose of this research was to characterize the in vitro cellular behavior of osteoblast-like cells on titanium surfaces prepared with argon plasma-cleaning (PC) treatments for various lengths of time. The highest levels of cell attachment were observed for surfaces which had been plasma-treated for one min. Surface analyses indicated that although PC treatments dramatically improved surface wettability, the presence of inorganic contaminants was observed with longer treatment times and may have interfered with cell attachment. Further work is suggested to investigate the longer-term phenotypic expression of osteoblasts when grown on implant surfaces.

Efficacy of the Sonicare toothbrush fluid dynamic action on removal of human supragingival plaque.

The fluid pressure and shear forces generated by the high frequency bristle motion of the Sonicare sonic toothbrush remove adherent colonies of cultured bacteria from model dental surfaces in vitro. These dynamic fluid effects can remove bacteria in vitro even at distances up to 4 mm beyond the tips of the bristles. To evaluate the efficacy of the Sonicare in removing actual human plaque deposits formed in vivo, an intraoral model was developed. Enamel sections were obtained from extracted human teeth and mounted on acrylic resin palatal prostheses, worn by two volunteers. Six enamel sections were arranged as three pairs at different locations on the prosthesis, and plaque was allowed to form overnight (approximately 16 h). The sections were removed, placed in phosphate-buffered saline, and exposed in vitro to the sonic toothbrush for 5, 10 or 15 seconds. The bristle tips were maintained at distances of 2 or 3 mm from the enamel surface. As a comparison, sections were also exposed to another electric toothbrush (Interplak) for 10 seconds using a distance of 3 mm between the bristles and the enamel. Following exposure to the toothbrushes, residual bacteria were removed from the sections by ultrasonication for 15 seconds, and total viable cell counts determined by serial dilution on blood agar plates. One section from each pair was used to measure total (baseline) microbial accumulation. At a distance of 3 mm between bristles and enamel, the sections exposed to Sonicare demonstrated significant (p < 0.001) plaque removal of 56-78% relative to non-treated controls. In contrast, the control electric brush did not demonstrate removal of plaque bacteria after 10 seconds exposure. These quantitative results were visually confirmed by scanning electron microscopy. The findings demonstrate that the fluid dynamic activity generated by the sonic vibrations of the Sonicare toothbrush removed microbial plaque formed in vivo, even at a distance of 3 mm beyond its bristle tips.

Biomechanical and functional behavior of implants.

The ability to achieve a long-term stable implant interface is not a significant clinical issue when sufficient uni- or bi-cortical stabilization is available. Clinical outcomes studies suggest that the higher-risk implants are those placed in compromised cortical bone (thin, porous, etc.) in anatomic sites with minimal existing trabecular bone (characterized as type IV bone). In establishing and maintaining an implant interface in such an environment, one needs to consider the impact of masticatory forces. These forces, in turn, have the potential to create localized changes in interfacial stiffness through the viscoelastic properties of bone. Changes in these properties will alter the communication between osteocytes and osteoblasts, leading to an increase in new bone growth, a maintenance of established bone, or a loss (potentially catastrophic) of either cortical or trabecular bone. Therefore, a key to understanding the biomechanical and functional behavior at an implant interface is to control the extent of anticipated modeling and remodeling behavior through an optimal implant design combined with a thorough understanding of how tissues respond to the mechanically active environment.

Development and application of a new abrasion testing device.

STATEMENT OF PROBLEM: Wear of gypsum materials is a significant problem in the fabrication of accurately fitting cast prosthetic devices. Unfortunately, there is little agreement on how to measure it. PURPOSE: This study was designed to evaluate the efficacy of a newly designed abrasion device and to develop a test methodology that provides a clinically relevant measure of material loss from gypsum material. MATERIAL AND METHODS: In this study, a unique benchtop microabrasion/microimpact device was created. The device consists of a vertical arm with a variably loaded stylus and a reciprocating table that moves the specimen under the stylus. Type IV gypsum samples (Silky Rock, Whip Mix Corp, Louisville, Ky.) were made with 1 mm vertical, 45-degree angled ridges used to represent crown margins. Samples (n = 30) were separated 1 hour after pouring and allowed to bench set for 24 hours or 7 days. Three loads (15, 50, and 75 g) were used, and the resulting defect was evaluated after 5, 10, 15, or 20 cycles of loading. Changes in mass and volume were recorded. RESULTS: At both 24 hours and 7 days, there was an increase in both mass and material volume loss with increasing load on the stylus (P<0.0001). There was no significant change in mass after 5 cycles of loading (P<0.05), but an increase in the volume loss occurred because of compaction of the walls of the defect (P<0.0001). CONCLUSION: Under these conditions, the increasing load had a greater effect than the number of load cycles on gypsum brittle fracture.

The concept of osseointegration and bone matrix expression.

Osseointegration has been defined as the direct structural and functional connection between ordered, living bone and the surface of a load-carrying implant. To date, this concept has been described by descriptive histological and ultrastructural criteria but not by biochemical means. This review evaluates the basic science work performed on this concept and then applies the concept to the principle of osseous healing. Specific studies are cited where alterations in the healing response are due to clinical management of implant placement and how studies of surface properties may lead to further insights on implant design and prognosis. In addition, a review of bone expression as a function of in vitro stress applications is given. This is followed by an indepth review of the collagens and noncollagenous proteins, described to date, within isolated bone matrix. It is this collagenous matrix (especially type I) that is described as being close to and oriented with a glycoprotein component next to the implant surface. In turn, the large family of noncollagenous proteins are important in mediating bone proliferation, matrix accumulation, orientation, mineralization, and turnover. This section is followed by a discussion of specific growth factors as they may relate to osseous healing around an implant.