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Extreme Magnification in Fixed Prosthodontics

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sujatabyahatti's picture
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The surgical operating microscope was introduced in the early 1920s, but it was largely ignored until 1950, when it was reintroduced by Richard A. Perritt for use in microsurgical operations in opthamology.The magnification and illumination provided by the microscope has resulted in its acceptance as an essential part of the surgical armamentarium. Microsurgical techniques gained widespread acceptance, primarily in otolaryngology, ophthalmology, neurology, and urology, and they play a critical role in surgery to reattach severed extremities.

The application of the operating microscope in clinical dentistry can be traced to Apotheker in 1981.He converted a medical operating microscope for use in endodontics. Reports by Carr, Arens, Buchanan, Kim, Ruddle, and others led to the routine use of the microscope in the 1990s for both surgical and standard endodontic therapy. During that time, specifically designed dental operating microscopes became an accepted part of endodontic therapy. Since 1998, competency in the use of a surgical microscope is required of all postdoctoral students in endodontics who complete an accredited training program.

During the mid-1990s, a number of periodontists began to advocate the use of the surgical microscope for certain periodontal procedures. Shanelec, Belcher, and Nordland developed new suturing techniques with ultrafine (7-0 to 10-0) sutures that relied upon the significantly enhanced vision made possible by the microscope. The microscope provided clinicians with improved precision during delicate surgical procedures and resulted in reduced postoperative discomfort. Until recently, the role of the operating microscope in general dental practice has been very limited, involving practitioners who performed a significant number of endodontic or periodontic procedures.

Use of the operating microscope has been shown to improve visual acuity, providing increased precision in surgical care as well as in certain other types of treatment. In addition, the operating microscope provides important ergonomic benefits. The balanced position of the clinician when using the microscope may help reduce musculoskeletal injuries that are common among members of the dental profession.

In addition to the ergonomic benefits, dentists using the operating microscope have discovered that the video camera attached to the microscope can be useful by providing both patients and the auxiliary staff with the ability to observe treatment in real time. The microscope, like intraoral cameras, allows for co-observation. In addition, this feature makes it possible for patients to observe treatment.

Mehrabian has shown that as much as 55% of the understanding that occurs in verbal communication is through visual cues, and that only 7% of the comprehension in communication comes from the words we use. Stated differently, patients remember more of what they see than what they hear. Clinicians have found that the images from the operating microscope are a benefit in educating patients about their treatment needs. The ability to easily document a procedure using digital microphotography and microvideography with cameras attached to the operating microscope opens up new possibilities for patient education, documentation for professional presentations, and medical/legal documentation.

In addition to the ergonomic benefits and documentation provided by the operating microscope, general dentists have found that it allows for significant enhancement in the precision of restorative and prosthodontic care. Martignoni, in his textbook on prosthodontics published in 1990, was among the first to discuss how the microscope might be useful in operative dentistry. Since that time, several clinicians have suggested that this device can be used to improve tooth preparation and the final restoration in fixed prosthodontics.

RESOLUTION OF THE HUMAN EYE AND MAGNIFICATION IN CLINICAL DENTISTRY

Carr reported that the human eye, when unaided by magnification, has the ability to resolve or distinguish 2 discrete lines or objects separated by a space of 200 µm (0.2 mm). If the lines are closer together than 0.2 mm, the eye will see them as a single line. Magnification improves the ability of the eye to resolve these objects, and allows the clinician to see greater detail than is possible with the eye alone. For example, 2x magnifiers such as telescopic loupes improve resolution to 100 µm, and 4x loupes improve the resolution of the human eye to 50 µm, or 0.05 mm .
Amsted or single-lens loupes are simple clip-on lenses that helped magnify the operating field. These single-lens systems have been replaced by multiple-lens telescopic loupes that are commonly used today . These magnifiers are available in either a flip-up variety or are directly mounted on the lenses of the glasses. Their magnification range is from 2x to 8x. Typically, loupes with magnification greater than 3.5x require some form of illumination from an accessory headlamp for adequate visualization of the operating field.
Figure 3. The magnification continuum.

Typically, dentists will initially use loupes with a magnification of 2.5x, and employ them for difficult procedures requiring enhanced vision such as endodontic therapy or veneer cementation. After an initial adjustment period, the dentist will begin to use the loupes for all procedures. In British Columbia the routine use of surgical telescopes by dentists has increased from approximately 20% in 1986 to 75% in 2000.38,39 Dentists may then change to higher-powered loupes used with illumination .

Dentists have realized that many details that are potentially important in the provision of care are beyond the resolution of the human eye. Incipient carious lesions, tooth fractures, preparation of crown margins, and crown margin evaluation during insertion are often determined by tactile means. At magnifications above 4x to 6x, visual enhancement provided by the microscope may lessen the reliance on tactile sense. One study has shown that an experienced clinician with a sharp, new explorer can determine marginal gaps in the range of 35 to 50 µm.40

Microscopes, in contrast to telescopic loupes, allow multiple steps of magnification between 2x to 40x. The microscope provides true stereoscopic vision through the binoculars, and the coaxial illumination from halogen, metal halide, or xenon bulbs creates shadow-free lighting.

THE ROLE OF THE OPERATING MICROSCOPE IN THE PREPARATION STAGE OF FIXED PROSTHETICS

The preparation phase of fixed prosthodontic care is technically demanding. Many factors determine the final design of the preparation, including the required reduction of tooth structure, detection of marginal caries, fractures, furcations, and the need for the margins of the restoration to be placed on solid tooth structure.

In addition, the clinician must consider the aesthetic, functional, and biologic principles regarding placement of the margins of the restoration (supragingival, gingival, or subgingival), and the type of margin best suited for the tooth (shoulder, bevel, or chamfer). While the successful provision of inlay, onlay, and crown restorations depends upon a solid understanding of the above, successful treatment is predicated on the ability of the clinician to clearly and distinctly view the operating field . Leiknius and Geissberger have shown that magnification (low-magnification telescopic loupes), when used by dental students, helped reduce errors in preparation design and laboratory processing by half when compared to a control group not using magnification.

The dental operating microscope can be used for the entire preparation of a tooth, but some dentists will use loupes for gross reduction of tooth structure before using the microscope to finish the preparation. Alternatively, the lower range of magnification of the microscope (2.5x to 4x) can be used for delivery of anesthesia and rubber dam placement. An entire quadrant of teeth can generally be seen at this level of magnification. Gross reduction of tooth structure is accomplished using medium magnification (6.4x to 10x), and margins are completed using 16x magnification. After finishing the margins, the preparation is examined at a lower magnification to ensure that no undercuts have been created.

Clinically, the use of higher magnification for preparation of margins appears to reduce the degree of taper of the preparations. Close attention must be given to the line of draw and parallelism when preparing multiple abutments for a fixed bridge. Otherwise, it is easy to create tooth preparations with divergent tapers, which can affect the path of insertion. The transition from low magnification (for gross tooth reduction) to high magnification (for final placement of the margin), and then back to low magnification (for the final evaluation), ensures that the clinician does not become so focused on the placement of the margin that unwanted undercuts are created.

An important benefit of higher magnification is the ideal placement of the crown or veneer margin. Finishing a margin at higher magnification can be accomplished with an electric handpiece. Finer control when finishing crown preparations is possible with an electric handpiece, where the rpm can be precisely lowered by using the dial on the front of the unit, not by a foot control as is employed with a high-speed air turbine. Or, an air turbine handpiece may be used without water spray, using a delicate “brushing” motion with a finishing bur and a gentle stream of air to avoid excessive heat generation. This brush cutting at high magnification polishes the margin, resulting in a smooth and discernable finish line.

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