Prosthodontics: It,S Changing Trends ( Part 2)

DENTAL IMPLANTS

Implants are boon for older people who have lost most or all of their teeth and ordinarily people would replace them with a full or partial denture. Some people find dentures agonizingly uncomfortable, and the bone underneath a denture tends to resorbs, particularly in the lower jaw. Like real teeth, implants put weight on the jaw, preserving the bone.

For example, when a single tooth is missing, the conventional treatment is a three-unit bridge. However, that entails preparing down adjacent healthy teeth so that the crowns fit over them. Because they're connected, the crowned teeth can't be cleaned as effectively as other teeth and may decay over time. 15

A NANOTECHNOLOGY APPROACH FOR BETTER DENTAL IMPLANTS

Current trend in dental implants include use of endosseous dental implant surfaces coated with nano scale topographies. There is a strong belief that nano scale materials will produce a new generation of implant materials with high efficiency, low cost, and high volume.The nano scale in materials processing is truly a new frontier.

Metallic dental implants have been successfully used for decades but they have serious shortcomings related to their osseo integration and the fact that their mechanical properties do not match those of bone. Nano scale modification of titanium end osseous implant surfaces can alter cellular and tissue responses that may benefit Osseo integration and dental implant therapy. There are possible advantages of nanotechnology for further improvements.10

Dental implants have a long and successful history. The percentage of failure is very low, approximately 5%, most likely due to infection, rejection, accelerated bone loss, and poor osseo integration with loosening of the implant. The most frequent cause for failure is insufficient bone formation around the biomaterial immediately after implantation 5,10

Challenging osseo integration with new protocols such as immediate placement and immediate loading may require further control of bone formation and osseo integration . Consequently most modifications in the implant design are geared toward reducing the time needed to wait before loading. It is here that the implant surface and tissue interface are critical .

Current surface chemistries and morphologies are controlled at best at the micron level, but tissue response is mainly dictated by processes controlled at the nanoscale. Understanding and controlling interfacial reactions at the nano level is the key to developing new implant surfaces that will eliminate rejection and promote adhesion and integration to the surrounding tissue.10

NANOTECHNOLOGY AND SURFACE SCIENCE

Nano scale modification of the titanium endosseous implant surface may affect both the topography as well as the chemistry of the surface. Application of nanotechnology to the dental implant surface involves a two dimensional association of surface features. These nano features can be arranged in an organized manner (isotropic) or unorganized manner (anisotropic), often depending on the method of manufacture.

Anisotropic topography is mostly applied to a dental implant surface. When these concepts are applied to the endosseous implant surface, implied is the embellishment of the surface with nano meter scale features that lead to novel Physicochemical behavior (e.g. bone bonding) or biochemical events (e.g. altered protein adsorption, cell adhesion with changes in cell behavior).

SURFACE MODIFICATIONS OF DENTAL IMPLANTS

1. Ceramic Coatings-

This thin ceramic layer will bond both to the implant and to the surrounding tissue while promoting bone apposition. These materials are bioactive compounds able to promote cell attachment, differentiation, and bone formation. The most prevalent bioactive materials are calcium phosphates such as hydroxyapatite (HA) or tri-calcium phosphate (TCP), and bioactive glasses.

When implanted, these bioactive ceramics form a carbonated apatite (HCA) layer on their surfaces through dissolution and precipitation. This phase is equivalent in composition and structure to the mineral phase of osseous tissue. At the same time, collagen fibrils can be incorporated into the apatite agglomerates.9,10

Essential requirements of the new family of coatings include:

Coating/implant adherence:

Currently available coating techniques provide inadequate adherence of calcium phosphates coatings to Ti alloys. Quoted bond strengths, 15–30 MPa, are very low. Interfacial strength and toughness are needed to optimize the mechanical stability of the coatings. A need to develop standard procedures to test adhesion, and a clear understanding of the mechanisms of interfacial failure in the body environment is required.10

Fixation to bone—

Two primary modes of attachment are: (1)mechanical, in which the implant has a rough, porous surface into which bone grows (often supplemented by use of a bone cement) and (2)chemical, in which bone “bonds” to the implant material. Here it is difficult to decouple the effect of coating chemistry and topography. Dissolution of calcium and phosphorus from the coating may promote mineralization and bone formation but it is not clear how much coating solubility contributes to the best fixation, and excessive resorption can limit implant life.10

DRUG DELIVERY:

Inflammatory responses to implants are a significant problem. Medications that are given to a patient following surgery to suppress inflammation seem to be insufficient or entirely ineffective in many cases. A different approach is to provide a local dose of anti-inflammation agents gradually released from a coating on the surface of the implanted device. The main advantage of this approach over traditional means of administering the drug is that the drug can be directly released at the implant site without having to go through the bloodstream. This lowers the amount of drug needed, reducing the overall toxicity and side effects.10

Plasma Spraying Of HA: Plasma spraying of HA being the most popular method , offers only rudimentary control of the coating thickness and composition. The thickness of plasma-sprayed coatings typically exceeds 50 μm. The high process temperatures cause partial thermal decomposition of HA, leading to the formation of other calcium phosphatess, including 22–62% of highly soluble amorphous calcium phosphate , α- TCP, β-TCP, tetracalcium phosphate, and calcium oxide. The result is coatings with unacceptable heterogeneous properties.

Severe problems include:(1) unreliable adhesion—they can detach, yielding floating fragments ; (2) continued dissolution of the coating, leading to catastrophic failure of the implant at the coating-substrate interface—attempts to increase crystallinity and reduce solubility by post-coating thermal annealing markedly degrades the coating adherence ;(3) variability during processing in terms of resulting phases, stresses, and cracking ; (4)partial crystallization of the deposited coating, leading to the presence of more soluble calcium phosphates compounds ; (5) significant degradation in the fatigue resistance and endurance strength of the implant alloy ; (6) very irregular morphology of the coatings; (7) poor control of the coating thickness, with greater risk of fracture and (8) poor control of physicochemical properties, and hence the biological stability of the coating.10

Anodic Oxidation: Another technique worth further exploration is anodic oxidation, which has been used as a means to engineer the surface of Ti-based implants . This technique can be used to create an adherent oxide coating on the implant surface. By selecting different electrolytes and manipulating the conditions, it is possible to create oxide layers with a wide range of stoichiometries as well as micro- and nano porosities. It is also possible to incorporate Ca and P ions into the layers. This is a relatively simple and economical technique that can be easily adapted to fabrication, and anodized implants are commercially available.10

2. Surface functionalization:

An alternative or complementary strategy to the use of coatings for dental implants is the molecular grafting or chemical treatment of implant or coating surfaces to enhance cell adhesion and promote mineralization, and the production of matrix and marker proteins. Proteins used for molecular grafting are present in the extracellular matrix, and implant surfaces that have been functionalized with fibronectin (FN), vitronectin (VN), or laminin (LN), to name a few . Signaling Domains composed of several amino acids; are present along the chain of the extracellular matrix proteins and are the ones that interact with cell-membrane receptors.10,9

3. Surface topography of dental implants:

It has long been recognized that the chemical and morphological characteristics of implant surfaces affect the behavior of the surrounding tissues. The ability of the implants to support bone formation can be enhanced by modifying surface topography . The topography of the implant surfaces can now be manipulated at a wide range of length scales, down to the nano level.

Several implant designs use surfaces with large pores to promote bone ingrowth and favorable anchoring. Texturing of the implant surface at the micro level has been proposed as a feasible alternative to promote bone apposition. Different techniques to increase the surface roughness, including sandblasting, grinding, Ti plasma spraying, or laser texturing . It could be argued that the main advantage of microtextured surfaces is the contribution to mechanical interlocking and tissue attachment of the implant. It should also be noted that implants with surfaces too rough have a high possibility of peri-implantitis and that the roughness will affect the degree of hydrophilicity.9,10

4. Nanocomposites for bone regeneration:

Ti and its alloys have had considerable advantages over other metals because of their inertness, which yields excellent biocompatibility and non-sensitization of tissues. Issues concerning the release of Ti and alloying elements from implants and the formation of Ti debris due to wear during implantation still remain. Ceramic materials are known to have excellent aesthetics, corrosion resistance, and biocompatibility; several ceramic implants have been already commercialized. Unfortunately, in contrast to metallic materials, most ceramics suffer from almost a complete lack of plastic deformation; this tissue for which they substitute. Current hybrid organic/inorganic composites have significant problems related mostly to their mechanical performance and their degradation in vivo.

The tissues in teeth have very specific and sophisticated hierarchical architectures, with strength and toughening mechanisms built in at almost every dimension. Nano dimension is only one of them but we need to develop new technologies for the fabrication of materials with architectures designed at multiple length scales that, from the mechanical point of view, will exhibit extrinsic toughening mechanisms similar to those observed in natural composites. Recent advances in the processing of bio inspired materials may open new possibilities.

In our laboratory we have pioneered a new technique, freeze casting, that can be used to fabricate ceramic-based composites with complex architectures to assemble novel, bio inspired hierarchical structures modeled after natural composites such as bone or nacre, striking a balance between mechanical and functional responses. This technique uses the controlled freezing of ceramic suspensions to generate porous ceramic scaffolds whose architecture is template by the ice crystals.
By controlling the composition of the suspension and the freezing conditions, it is possible to generate materials with complex hierarchical architectures that mimic those of the inorganic component of nacre at multiple length scales, from the nano to the macro levels. These scaffolds can subsequently be infiltrated with a second “soft” phase (e.g., polymer) to prepare composites that exhibit unique combinations of strength and toughness.9,10

ZIRCONIA: A CHALLENGE TO TITANIUM

Replacement options for missing teeth include fixed bridges, removable partial dentures, and dental implants . The survival rates of dental implants is high. Wennerberg et al. suggested implant endurance varies between 96 and 97% and for prosthetic constructions, 87 and 97% are still in use after 5 years . There are many factors affecting the success rate of implants . Goodacre et al. reported the causes of implant failure which are divided into the following 6 categories: surgical, implant loss, bone loss, peri-implant soft tissue, mechanical, and esthetic/phonetic. Esthetic problems are a challenge for dentists.

Basically, titanium abutments respond well to gum tissue and have good mechanical properties. However, there remains an unesthetic bluish appearance through the soft tissues . This is of great concern when the maxillary incisors are treated, especially in patients with a high smile line or thin mucosal biotype. With the increase in demand for better esthetic outcomes, ceramic appears to be an alternative material because of its tooth like color. Because of the unsightly color of titanium, the first ceramic abutments were made in the 1990's.

The initial replacements were made of alumina , however, several clinical studies reported fractures on alumina abutments. Due to this weakness, yttrium oxide-stabilized zirconia was developed and widely used in the dentistry field . Vagkopoulou et al. demonstrated that yttrium oxide-stabilized zirconia has the characteristic of transformation toughening, which efficiently prevents crack propagation . Yildirim et al. indicated that yttrium oxide-stabilized partially stabilized zirconia ceramic has twice the strength of alumina ceramic. It is not clear whether zirconia abutments will entirely replace titanium abutments.

In addition to esthetic problems in implant failures, bone loss is another common issue . Goodacre et al. observed that a mean bone loss of 0.9 mm, with a range from 0.4 to 1.6 mm occurs in the first year, and ongoing annual bone loss was 0.1 mm (range from 0 to 0.2 mm). Roos-Jansaker and colleagues found that in 20.4% of the Branemark implants, the bone level was located 3 mm apical to the implant head after 9 to 14 years of use. They further claimed that 8% of the implants suffered from progressive bone loss (> or =1.8 mm) after an observation period of 9 to 14 years when compared with 1-year data. From 1997-2004, 3609 implants were reported as failures, the reasons included implant mobility, inflammation/bone loss, and breakage . There were also a tandem increase in the number of implant fracture reports .11

Although the study of titanium alloy and zirconia abutments are resistant to breakage, peri-implant marginal bone loss accounts for some of failures of dental implants. The two materials have different properties, but comparisons of the mechanical properties are few. Therefore, the objectives of this study were to evaluate the fracture resistance of the calcium phosphate titanium implants with titanium alloy abutments and zirconia abutments with 3.0 mm and 1.5 mm marginal bone loss. In addition, damage of the implant-abutment interface was also observed with scanning electron microscopy (SEM).

DIGITAL SHADE GUIDES

Visual shade matching is a common method, in which a color standard from a commercially available dental shade guide is compared to and matched with the target tooth . However, the visual shade selection varies, depending on the clinician’s color perception and experience, ambient light condition, background of the tooth, and the shade guide used . Communication with a dental laboratory is another problem . The technician does not usually see the patient and has to work on dentist’s written prescription as based on the shade guide used.17

Historically, determining the shade of a patient’s natural tooth has presented many clinical challenges. Early shade guides lacked order and clarity in the arrangement of shades.

Consequently, dental professionals often experienced difficulty when determining the proper hue, chroma, and value required in a case. Contributing to these challenges, individual perceptions of shades and user error when incorporating the shade guides led to further incidences of restorations appearing unnatural and needing to be remade. Additionally, dentists have experienced difficulty when attempting to communicate the proper shade to the dental laboratory, since shade guides provided in conclusive results.13

CLASSICAL SHADE GUIDE

Many dental professionals still use the VITA Classical Shade Guide, which incorporates the familiar “A” through “D” hue-based system . Although still a useful tool, the last significant modification to this system was made in 1954. However, in the 1960s the VITA Company incorporated a value-based system into the classical guide to aid dental professionals in determining brightness. B1 was listed as the highest value and C4 the lowest value, with in-between values also listed; these values were recommended for use in determining the value of the teeth to be matched. Although this modification provided dental professionals with further clarification of the value of particular shades, clinical challenges remained—the most significant of which continued to be human perception of the color of tooth structures.

In tests of visual perception, the authors found that when there are two objects that have nearly the same value (i.e, brightness), the object that demonstrates higher chroma and color intensity will be perceived by the viewer to have a lower value. For example, one of the most common mistakes when determining a tooth color occurs when selecting between A1 and B1 on the classic linear shade guide. When viewed, B1 typically appears brighter than A1.

However, A1 is actually the brighter of the two, and this can be seen when the two are photographed in black and white and compared. The incorrect perception is caused by the lower chroma of the B1 shade. Furthermore, the shades between the brightest and darkest values often create color confusion. These clinical challenges have remained because there was no scientific order to this type of shading system when it was developed.13

3D SHADE GUIDE

More than 10 years ago a new shading system was developed to better represent and communicate the existing shade of natural teeth. Based on the three dimensions of color hue, chroma, and value, the VITA 3D-Master®Shade Guide (Vident) addressed dental professionals’ concerns about conventional shade guides (ie, the ability to separately choose value from chroma). This shade guide system demonstrates improvements in accuracy and ease of use through the incorporation of a value numbering system of 0 to 5. In this system, 0 is the brightest shade and 5 is the darkest shade, with each number representing an equivalent change in brightness. Chroma is then numerically categorized from 1 to 3, with 1 being the lowest chroma and 3 being the highest.

The 3D-Master Shade Guide reduces the risk of improper shade selection from inconsistencies in the clinician’s perception of tooth structure shades. In addition to a separate value and chroma numbering system, 3-D guides also place specific hues in different categories, including left (L), middle (M), or right (R); L represents yellow, M more orange, and R the more reddish portion of the shade guide.

The biggest issue with the 3D-Master Shade Guide has been the lack of ease of removing the shade tabs from the unit’s holder, and the holder itself does not allow placing the individual tabs close to the teeth for evaluation. The authors have found the best way to use this guide is to place the M hues of the six different value ranges in the shade guide holder with the L and R shades removed. By then holding the guide arranged this way next to the teeth for value comparison it is easier to see value differences. Once the value is selected the chroma guide of the chosen value can be compared to the natural teeth to determine the chroma. The authors recommend using a quality digital single-lens reflex (SLR) camera to record the observed/chosen shade. It is also recommended to image one shade brighter and one shade darker for final comparison on a monitor.13

UPGRADED TAB HOLDER

The 3D-Master Shade Guide was reengineered with a new Linearguide 3D-Master®, offering a better, more efficient tab holder for evaluation next to teeth . The value guide’s tab holder has the 0 to 5 values in close approximation to each other and all on the same plane, allowing for quick and accurate evaluation against natural teeth. The chroma guide of the selected value is then used to choose the chroma. This guide is ideal for use with porcelains that use the 3-D color system.13

DIGITAL PHOTOGRAPHY FOR SHADE ANALYSIS

One of the most powerful tools in communicating shade information with both patients and dental ceramists is digital photography. Effectively executed digital photography along with the use of Adobe photoshop can significantly help in the shade determination process.

To use digital images to analyze or determine shade, images of the compared tooth and the shade guide are taken in the same vertical plane, ie, the shade guide can not be closer to or farther away than the tooth from the digital film plane . One way to help position and stabilize the shade tab is to flatten the neck and create a shoulder or edge on the neck of the guide. It is strongly recommended to do an image capture in RAW file format, as this is an uncompressed file type with the most color information and is also easily fixable if the camera settings were wrong.

The images are then opened in Photoshop. The images of the teeth and shade guide are then selected and the background outside the area of the teeth and shade guide are turned to black , which allows for a more accurate visual comparison. An area of the shade guide can be selected and then “moved” or dragged onto the image of the natural tooth . This makes it easier to see the difference between the guide and the tooth. A duplicate image is then made and turned to gray scale, making it easy to see the value differences between the shade guide and tooth.

The last Photoshop tool used in the shade determination process is to select the “HSB” color model in the “INFO” palette. When the cursor is placed on the shade guide or tooth, the hue will be given by the “H” number, the chroma by the “S” number (“S” stands for saturation) and the value by the “B” number (“B” stands for brightness). Thus, the difference in value, hue, and chroma between the tooth and guide can be quickly measured. The ceramist would then use this data to choose the correct porcelains to match the natural tooth.13

COMPUTERIZED DIGITAL SHADE TECHNOLOGY

The most current shade-taking tools available, digital shade technologies, and software are often used in combination with either conventional classical guides or newer linear and 3-D guides to reinforce shade selections. Used in conjunction with visual shade guides and digital photography, digital shade guides are an important tool in the shade determination process. It has been that when all three available shade-taking modalities (ie, visual shade analysis, digital photography shade analysis, and computerized digital shade-taking technology) are used, remakes for shade have been reduced by 80%, even when working with student dentists.

Among the various digital technologies, spectrophotometers adapted for dental shade technology (eg, VITA Easyshade® Compact, Vident; Crystaleye, Olympus America Inc )enable dental professionals to quickly determine the basic tooth shade and various color zones . Spectrophotometers are amongst the most accurate, useful and flexible instruments for overall color matching and color matching in dentistry. They measure the amount of light energy reflected from an object at 1–25 nm intervals along the visible spectrum. A spectrophotometer contains a source

of optical radiation, a means of dispersing light, an optical system for measuring, a detector and a means of converting light obtained to a signal that can be analysed. The data obtained from spectrophotometers must be manipulated and translated into a form useful for dental professionals. The measurements obtained by the instruments are frequently keyed to dental shade guides and converted to shade tab

equivivalent . However, to ensure optimum, error-free results, digital shade-taking devices should be used in combination with linear and 3-D guides.13,14

Crystaleye (Olympus, Tokyo, Japan) combines the benefits of a traditional spectrophotometer with digital photography. Through the development of optical and image processing technology, this product allows the practitioner to match tooth shade and color more accurately and simply compared with the traditional spectrophotometer.

The significant benefit of this system is that ‘virtual shade tabs’ in the computers database can be cross-referenced and superimposed visually onto the natural tooth image to be matched giving the technician the ability to visualize the correct shade tabs. The digital image produced by the Crystaleye uses a 7-band LED light source, which results in a more precise depiction of color than the conventional systems used with digital cameras. Moreover, the image produced by the Crystaleye is taken from inside the oral cavity and consequently is devoid of the external light that can cause discrepancies. 14

Vita Easyshade Compact (Vita Zahnfabrik, Bad Sa¨ckingen, Germany) is cordless, small, portable, cost efficient, battery operated, contact-type spectrophotometer that provides enough shade information to help aid in the color analysis process. Different measurement modes are possible with Easyshade Compact: tooth single mode, tooth area mode (cervical, middle and incisal shades), restoration color verification (includes lightness, chroma and hue comparison) and shade tab mode (practice/training mode).14

Shade-X (X-Rite, Grandville, MI) is also compact and cordless ‘‘spot’’ measurement’’ spectrophotometer with 3- mm probe diameter, and keyed to the majority of popular shade guides. Shade-X have two databases to match the color of the dentin (more opaque) and the incisal tooth regions (more translucent). 14

SpectroShade Micro (MHT Optic Research, Niederhasli, Switzerland) is an imaging spectrophotometer. It uses a digital camera/LED spectrophotometer combination. It has an internal computer with the analytical software. The tooth positioning guidance system, shown on the LCD touch screen, is used during color measurement. Images and spectral data can be saved on the internal memory and transferred to a computer.14

CONCLUSION

“Prosthodontists are innovation leaders and have formed new partnerships in the rapidly changing technology industry. Together we will bring clinical improvements to the dental community by careful testing and evaluation, documentation and, especially, education.” While thousands of dentists and technicians have integrated digital technologies into their practices and laboratories over the past decade, thousands more have yet to take more than a first step. Thus, today’s prosthodontist’s share a great responsibility, and a greater privilege, to lead the digital dentistry revolution both in research and in practice, and to continue to move dreams to the drawing table, and then to reality.

REFERENCES