Indian Journal of Ophthalmology

REVIEW ARTICLE
Year
: 2015  |  Volume : 1  |  Issue : 1  |  Page : 2--7

Clinical insight into tooth preparation: An update


Manisha Jayna1, Amit Jayna2, Bhupender Yadav1, Nupur Dabas1,  
1 Department of Prosthodontics, SGT Dental College and Hospital, Gurgaon, Haryana, India
2 Department of Prosthodontics, ITS Dental College, Greater Noida, Uttar Pradesh, India

Correspondence Address:
Dr. Manisha Jayna
Department of Prosthodontics, C134, Greater Kailash, Part 1, New Delhi - 110 048
India

Abstract

Dentistry has witnessed some major discoveries during the past two decades, to the extent that many routine procedures in modern dental practice vary considerably from the way in which they were practiced for over half a century. The purpose of this article is to provide an insight to the latest techniques of crown preparation. A sound tooth preparation is the foundation stone for the longevity of a restoration. This can only be achieved if the dentist has an adequate knowledge and understanding of the various factors, which enable one to achieve this goal.



How to cite this article:
Jayna M, Jayna A, Yadav B, Dabas N. Clinical insight into tooth preparation: An update.Saint Int Dent J 2015;1:2-7


How to cite this URL:
Jayna M, Jayna A, Yadav B, Dabas N. Clinical insight into tooth preparation: An update. Saint Int Dent J [serial online] 2015 [cited 2019 Oct 17 ];1:2-7
Available from: http://www.sidj.org/text.asp?2015/1/1/2/161793


Full Text

It takes a genius to create a masterpiece. Creators like Michelangelo are born once in a lifetime. However, every dentist can aim to be like Michelangelo creating perfect restorations, provided one follows the scientific principles, and blend them with art.

Tooth preparations must be based on fundamental principles for a predictable success of prosthodontic treatment. The principles of tooth preparation may be divided into the preservation of tooth structure and periodontium, retention and resistance form, marginal integrity, structural durability, and esthetic considerations. Improvement in one area often adversely affects another area, and striving for one may lead to failure in another. Careful attention to every detail is imperative during tooth preparation. It is important to remember that even though we as clinicians are meticulous in our preparations, impressions, provisionalization, and cementation these are traumatic events for the teeth. Teeth do not possess the regenerative ability found in most other tissues.

The various factors to be considered while tooth preparation depends on the type of restoration, type of luting cement used, optimal aesthetic requirement, retention and resistance form, position of the margins, and tooth alignment. [1]

In-depth knowledge and an understanding of the various criteria is a prerequisite to the development of optimum tooth preparation. This enables a clinician to find a best combination of compromises among the applicable biological, mechanical, and esthetic considerations.

 Preservation of Tooth Structure



Care must be taken to preserve the health of oral tissue. The adjacent teeth, soft tissue, and the pulp of the tooth being prepared can be easily damaged while tooth preparation.

Protection of adjacent teeth

Iatrogenic damage to the adjacent tooth is a common error in dentistry. The technique of tooth preparation must avoid and prevent damage to the adjacent tooth. This can also be achieved by using a matrix band or by using the proximal lip of enamel of the tooth that is being prepared for protection of the adjacent tooth. [2]

A damaged proximal contact area makes it more susceptible to dental caries in spite of reshaping and polishing, as compared to the intact tooth surface. This could be due to high fluoride concentration in undamaged tooth structure and also because the smooth surface is less prone to plaque retention. [2]

Protection of pulp

Pulpal degeneration can occur many years after tooth preparation. [2],[3] Extreme temperature, chemical irritation, and microorganisms can cause irreversible pulpitis, [2],[4] particularly when they occur on freshly sectioned dentinal tubules.

Minimizing heat production during the preparation

During tooth preparation, energy used in the cutting process is mostly transformed into heat, which primarily depends on type of bur-diamond or carbide, pressure applied, cutting time and rate, cooling technique, and last but not the least the speed and torque of the rotary instrument. [2],[5] Prevention of pulpal damage necessitates the selection of techniques and materials that reduce the risk of damage.

The handpiece pressure and speed affect the rise of intrapulpal temperature. It was established that doubling the rotating speed of the bur and/or the pressure applied on the handpiece produced a 50% increase in the tooth temperature. [6] Most dentists, when preparing teeth for fixed restorations with a high-speed handpiece, apply a force [7] that varies from 50 to 150 g/cm 2. Cutting force of 100 g/cm 2 is considered optimal for medium-grit (bur with blue-band) diamond burs. [8] The torque of the handpiece has a significant effect on the cutting force. Handpiece with higher torque requires a mean cutting force of 1.44 N compared to a lower torque handpiece, which requires a force of 1.2 N. [9]

One of the initial concerns about cutting with ultra-high-speed handpiece (200,000-400,000 RPM) is the temperature rise caused by the heat generated due to friction. Temperature rise causes thermal expansion of the dentinal fluid and its outward flow. Cooling during high-speed cutting diminishes the outward fluid flow, whereas cutting dry causes inward fluid flow, which stimulates the A-delta neurons of the pulp, eliciting a sharp pain. [10],[11] In vivo measurements of tooth temperature rise showed that water-cooled high-speed tooth preparation resulted in a 3-4°C rise in pulpal temperature, whereas dry, air cooled high-speed preparation caused a 14°C rise in temperature. [12],[13]

By increasing the water flow from minimal to 45 ml/min and using light pressure, slight or no changes in pulpal temperature have been observed. [14],[15] The water spray (which is directed at the area of contact between tooth and bur) not only cools the intrapulpal temperature, but also removes debris and prevents desiccation of dentin. Collection of debris causes clogging of the bur, which in turn reduces its cutting efficiency. [3],[16] The higher coolant flow rates [17] and handpiece with multiple spray ports help in clearing the cutting debris more efficiently. [18]

It is essential to reduce pulpal damage for the longevity of treatment. This can be achieved to some extent by having adequate patient chair time, using copious water spray and a feather-light touch while preparation and also by using new burs and avoiding "bur drag". Cutting with carbide burs is most efficient at ultra-high-speed in dentin, with intermittent cutting. [19] Whereas finishing of the preparation (using bur with red and yellow band) should be done using a slow-speed handpiece with light intermittent pressure.

Chemical irritation

The chemical action of certain luting cements can cause pulpal damage when used on a freshly cut tooth. Use of cavity varnish and dentin bonding agents form an effective barrier in most cases, but their effect on the retention of cemented crowns is controversial. [20],[21],[22] According to a study conducted in 2013, it was observed that though resin sealer can decrease the retention of the castings when zinc phosphate is used for permanent cementation, it increases the retention when used with glass-ionomer cement. [23] In 2009, Magne and Nielsen [24] concluded that air blocking and pumicing the existing resin coatings was necessary to obtain defect-free impressions, regardless of the type of dentin bonding agent used. Certain chemical agents used for cleaning and degreasing can also cause pulpal irritation. [25]

Bacterial action

It is of utmost importance to remove any carious lesion prior to making impressions for a restoration. Presence of any residual traces of infection at the site of restoration affects the success of restoration. [26],[27] The bacteria which gain access to dentin because of microleakage can cause pulpal damage. The highest anti-bacterial value of luting cement was found for zinc phosphate, [28] polycarboxylate cement, and glass ionomer cement. [29],[30],[31],[32] Application of a dentine bonding agent prior to making an impression provides immediate protection against bacterial leakage and sensitivity, improved patient comfort, and helps in tissue conservation. [33]

Conservation of tooth structure

As a dentist, it is our duty to conserve as much tooth structure as possible while tooth preparation. Thickness of the remaining dentin is inversely proportional to the pulpal response. [34] As good clinicians we should avoid extending the tooth preparation in close proximity to the pulp, should follow the occlusal anatomy during occlusal reduction and produce minimum taper. Partial veneer crown should be preferred whenever possible, provided patient maintains good oral hygiene.

Intracrevicular margins

The placement of finish line plays an important role in the success of the restoration. Margins should ideally be as smooth as possible, should be easily duplicated in the impression and on the die to produce good fit of the restoration and should be placed such that both the dentist and the patient can clean it. Finish lines should be placed in enamel and also supragingivally whenever possible. The dentin-enamel junction has fracture toughness approximately 5-10 times greater than the enamel, but about 75% lower than dentin. [35],[36] Some studies reported subgingival margins as the major cause of periodontitis. [37],[38],[39],[40],[41],[42],[43],[44] Whereas some suggested that margin location is not as crucial when placed by a highly skilled dentist in the mouth of a cooperative patient who understands the importance of oral hygiene. [45] Mechanical insults to supracrestal gingival attachment are reversible as long as restoration does not invade biological width. If junctional epithelium and gingival fibers are not disturbed, then the level of alveolar crest and gingival margin level remains stable. The intracrevicular margin should not be placed deeper than 0.5-0.7 mm.

The periodontal pocket depth can be tested by inserting a periodontal probe with defined force (0.2-0.3 N) in an apical direction parallel to the tooth axis between gingival and tooth surface. [46] In posterior teeth, the probing depth is up to 2 mm. In anterior teeth, the probing depth can be up to 1 mm, so it is easy to violate the biological width in anteriors. A buffer zone must exist between the finish line of prepared tooth and bottom of the gingival crevice to prevent apical migration of gingiva. This can be achieved by prepacking gingival sulcus with non-impregnated cords (for not more than 30 min) taking care that does not displace into the connective tissue. Furthermore, dentist should use end cutting burs to relocate the margin subgingivally, to prevent injury to gingival sulcus.

Flemmig et al. [47] advocated the use of 0.12% chlorhexidine gluconate for 2 weeks prior to tooth preparation. This helps in reducing gingival inflammation around the teeth and provides healthy working environment.

The dentist should advice the use of mouthwash with 0.2% chlorhexidine prior and also after crown preparation. The dentist should try to keep the margins of the preparation supragingival in areas where esthetics are not so much of a concern, for the ease of cleansing by the patient. In the esthetic zone where margins have to be placed subgingivally, patient needs to be motivated for good hygiene.

 Mechanical Considerations



Mechanical considerations would include factors affecting the integrity and durability of the restoration and would include resistance form, retention form, and deformation.

It is advised to use a flat end tapered fissure bur for preparing a shoulder finish line. The use of torpedo or round-end bur has been suggested for preparing chamfer finish lines. Mansueto [48] concluded in his study that preparation of a chamfer finish line was statistically better when a round-end bur type was used as compared to torpedo-shaped diamond bur of similar bur-tip size.

Structural durability

A restoration must have adequate bulk of material to withstand the occlusal forces without distortion. Furthermore, this bulk should be within the natural confines of the tooth being prepared to avoid any periodontal insult. Occlusal clearance required for a metal crown is 1.5 mm on the functional cusp and 0.7-1 mm on a nonfunctional cusp. Metal-ceramic crown requires occlusal reduction of 1.5-2 mm on the functional cusp and 1-1.5 mm on a nonfunctional cusp. For an all-ceramic restorations, there should be 2 mm of occlusal clearance. Furthermore, a functional cusp bevel plays an integral part in occlusal reduction. Adequate bulk of metal is needed on the functional cusp, it being an area of heavy occlusal loads. Clinicians should round off all the line angles of the tooth prepared to avoid stress concentration.

Creating resistance and retention form

More recently, resistance to lateral forces and not retention along the path of insertion has been advocated as the determining factor in a crown's resistance to dislodgement. Theoretically, the more parallel the opposing walls of a tooth preparation are, the greater the retention and the more conservative is the tooth preparation. However, it is difficult to achieve parallel walls under all clinical conditions without the risk of incorporating undercuts into the tooth preparation.

Total occlusal convergence

Historically the accepted total occlusal convergence (TOC) is 2-6° taper. However, it has been determined that dental students, general dentists, and prosthodontists are unable to routinely create this minimal taper. Weed et al. [49] reported that dental students produced a taper of 12.7° on typodonts and 22.8° during clinical preparations. Some studies suggested 16.5° as clinically achievable TOC without affecting the retention of crowns irrespective of the cements used. [50]

The best way for clinically observing TOC is by visualizing the preparation in a mirror rather than looking occlusally that is by using binocular vision. A tapered diamond bur imparts a taper of 2-3° to a tooth surface it cuts, if the shank of the instrument is held parallel to the intended path of insertion of the preparation. [51]

Various studies have been conducted to determine TOC. In recent years, recommended optimal taper for tooth preparations for full crowns have ranged from 3° to 5°, [52] 6°, [53] 7-9°, [54] 10-14°, [55] 11.3 ± 7.8°. [56]

Parker et al. [57] in his study evaluated the ratio of occluso-cervical (OC)/inciso-cervical dimension to facio-lingual (FL) dimension and its relation to TOC. He proposed a new guideline for preparation of taper. According to which adequate resistance form can be achieved when OC/FL ratio is 0.1 and TOC is 5.6°when OC/FL ratio is 0.2 and TOC is 11.6°, when OC/FL ratio is 0.3 and TOC is 17.6° and when OC/FL ratio is 0.4 and TOC is 23.6°. Maxwell concluded that for a crown with minimal of 3 mm OC dimension, 6° TOC was required to provide adequate resistance for crowns. Based on the results of newer testing methods and resistance data it has been proposed that TOC of 10-20 is acceptable.

Though the crown preparation modifications enhanced the resistance form, however, reduction in cervical TOC proved to be more effective than proximal grooves. According to a study, the crown retention in a tooth with adequate crown height was statistically higher, followed by retention in clinically short crowns with boxes then short crowns with grooves and finally conventional crown preparation for short crown height. [58] Short clinical crown height can be managed by decreasing TOC, by introducing auxiliary retentive features (like grooves and boxes) and lastly by surgically increasing the crown height. Proussaefs et al. [59] in 2004 and Roudsari and Satterthwaite [60] in 2011 advised use of resin cements for cementing crown on a tooth with short clinical height, though they also found that the resistance form plays a more important role in retention than resin cement. O'Kray et al. [61] in a study done in 2012 concluded that by placing one or two horizontal circumferential grooves into the internal surface of the metal complete crowns increased their retention on an optimal tooth preparation. He also concluded that grooves placed into the crown were as effective as or more effective than grooves placed into the tooth/die.

Thus, one can conclude that in a clinical situation, where crown height is adequate TOC can be >20°. However, in situations where crown height is inadequate TOC <20° should be planned, with some auxiliary retentive features in preparation or on the internal surface of the crown. Use of resin cements can be of some benefit. The best way for clinically observing TOC is by visualizing the preparation in mirror rather than looking occlusally that is by using binocular vision. A tapered diamond bur imparts a taper of 2-3° to a tooth surface.

Cementation

Luting cements, which bond to tooth and restoration may aid in the retention of restorations, while passive luting cements merely fill the gap between crown and tooth and mechanically lock the restoration. [62] Zinc phosphate cement has been used successfully for over a century to lute well-fitting metal and metal-ceramic definitive restorations, as it is a very inexpensive, rigid material and with very high early compressive strength. However, because of its high acidic nature and solubility it is no longer used. Polycarboxylate cement has lower compressive strength but high tensile strength and is less injurious to the pulp. Zinc oxide eugenol and zinc oxide noneugenol cements typically have good sealing abilities, but their relatively low compressive and tensile strengths, inherent brittleness, and high solubility limit usage to provisional restorations or implant supported crowns. [63]

Resin cements are typically formulated for a specific function or metal-free restorations [64] as they offer strength, aesthetics, flexible working times, and very low solubility though they are technique sensitive, expensive, and often hard to clean-up. Glass ionomer cements offer good strength, optical properties, and potential for fluoride release/recharge, but may have short working times, and are sensitive to moisture or dehydration and take time to fully set. Resin-modified glass ionomer cements are hybrid, dual-phase materials which are manipulated like glass ionomer, but set quicker and are stronger. [65],[66]

 New Methods of Crown Preparation



Ultrasonics

Ultrasonic instruments have recently been developed for finishing crown preparations. They are successful in accessing difficult areas on the preparation margin. Ultrasonic diamond coated tips have been created for sulcus penetration and intracrevicular finish line preparation and polishing. The use of ultrasonic instruments to prepare dentin resulted in comparable bond strengths to the use of diamond burs.

Recent study by Ellis et al. [67] states that the extremely precise margin preparation with ultrasonic instruments improves the quality and accuracy of crown preparations, which may lead to better impressions and closer adaptation of restorations. The margins finished with the ultrasonic instruments exhibited a better-defined axial wall/margin angle and a smoother marginal surface. Rotary instruments produced a sharper and more continuous external line angle. Two-dimensional surface roughness analysis showed that the margins produced with the ultrasonic instruments were approximately half as rough as the margins prepared with the conventional rotary instruments. [68],[69]

Lasers

Erbium: Yttrium-aluminium-garnet (Er: YAG) laser has been investigated for tooth preparation because of its mechanical ablation process by microexplosion. Geraldo-Martins et al. [70] offered new perspectives for enamel and dentin removal without significant adverse thermal effects. Er: YAG lasers have been introduced that cut dental hard tissues and most recently an Er, Cr 3+ :YSGG laser that cuts soft and hard tissues.

According to a study done by Mollica et al.,[71] the use of Er: YAG laser and high-speed handpiece for cavity preparation resulted in similar intrapulpal temperature rise and ultrasound tips generated significantly higher intrapulpal temperature rise. Though this rise in temperature due to ultrasonic tips did not reach the critical value and can be considered safe for use.

 Conclusion



Tooth preparation is an art, which is based on fundamental principles for a predictable success of the prosthesis. In-depth knowledge and an understanding of the various criteria is a prerequisite to the development of optimum tooth preparation. It is essential to reduce iatrogenic damage to the pulp and the adjacent tooth for the longevity of treatment. This can be achieved to some extent by having adequate patient chair time, using copious water spray, feather light touch while preparation, and by use of new burs. In a clinical situation, where crown height is adequate TOC can be more than 20°. However, in situations where crown height is inadequate TOC <20° should be planned, with some auxiliary retentive features in preparation or on the internal surface of the crown. Use of resin cements can be of some benefit. It is advised that one must use a mouthwash with 0.2% chlorhexidine prior and also after crown preparation for good gingival health. A dentist while preparing a tooth for a crown should try to keep the margins of the preparation supragingival, in areas where esthetics is not so much of a concern. Whereas in the esthetic zone, margins have to be placed subgingivally, so the patient needs to be motivated for good hygiene. Finishing of the preparation should be done using a slow-speed handpiece with light intermittent pressure.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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